The long-term goal of my research program is to reduce aflatoxin and sterigmatocystin contamination of food and feed crops. Aflatoxin (AF) and sterigmatocystin (ST) are toxic and carcinogenic secondary metabolites produced by the same biochemical pathway in several Aspergillus spp. My program focuses on identifying the molecular genetic processes controlling expression of ST/AF biosynthesis and host metabolites which effect fungal growth and ST/AF biosynthesis. Results from these areas of research are directed towards designing rational ST/AF control strategies and should contribute to control of other fungal pathogens.
We are using A. nidulans to dissect the molecular mechanisms governing genetic control of ST biosynthesis. We have isolated a ST gene cluster in A. nidulans and have identified gene functions within the cluster through gene disruption and sequence analysis. This cluster contains 26 genes encoding both enzymatic activities and regulatory proteins necessary for ST and AF biosynthesis. We have demonstrated that both regulatory and structural genes are conserved between the A. nidulans ST gene cluster and the A. flavus and A. parasiticus AF gene clusters. Most interestingly, we find that both sporulation and mycotoxin biosynthesis are inactivated by the same Ga protein-dependent signaling pathway thus suggesting the possibility of a strategy to control inoculum and ST/AF production simultaneously.
We are also interested in host factors which affect the regulation of mycotoxin production in Aspergillus spp. and have identified linoleic acid-derived compounds from the lipoxygenase pathway of plants which regulate both fungal sporulation and ST/AF production. One current goal involves transforming corn and peanut plants with a soybean lipoxygenase gene whose product inhibited AF/ST biosynthesis in laboratory studies. Ultimately, many of our studies should lead to creating transgenic crops where AF/ST-inducing or inhibiting traits will be engineered 'out' or 'into' plants respectively.
These studies will be complemented by functional genomic projects where we are involved in sequencing the Aspergillus genome and analyzing Aspergillus/seed ESTs. Microarray analysis of host seed and Aspergillus genes should reveal those set required for pathogenesis and aflatoxin production in situ.
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The blue mold fungus, Penicillium expansum, is a postharvest apple pathogen that contributes to food waste by rotting fruit and by producing harmful mycotoxins (e.g. patulin). To identify genes controlling pathogen virulence, a random T-DNA insertional library was created from wild-type P. expansum strain R19. One transformant, T625, had reduced virulence in apples, blistered mycelial hyphae, and a T-DNA insertion that abolished transcription of the single copy locus in which it was inserted. The gene, Blistering1, encodes a protein with a DnaJ domain, but otherwise has little homology outside the Aspergillaceae, a family of fungi known for producing antibiotics, mycotoxins, and cheese. Because protein secretion is critical for these processes and for host infection, mass spectrometry was used to monitor proteins secreted into liquid media during fungal growth. T625 failed to secrete a set of enzymes that degrade plant cell walls, along with ones that synthesize the three final biosynthetic steps of patulin. Consequently, the culture broth of T625 had significantly reduced capacity to degrade apple tissue and contained 30 times less patulin. Quantitative mass spectrometry of 3,282 mycelial proteins revealed that T625 had altered cellular networks controlling protein processing in the endoplasmic reticulum, protein export, vesicle-mediated transport, and endocytosis. T625 also had reduced proteins controlling mRNA surveillance and RNA processing. Transmission electron microscopy of hyphal cross sections confirmed that T625 formed abnormally enlarged endosomes or vacuoles. These data reveal that Blistering1 affects internal and external protein processing involving vesicle-mediated transport in a family of fungi with medical, commercial, and agricultural importance.
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Aspergillus fumigatus is a versatile fungus able to successfully exploit diverse environments from mammalian lungs to agricultural waste products. Among its many fitness attributes are dozens of genetic loci containing biosynthetic gene clusters (BGCs) producing bioactive small molecules (often referred to as secondary metabolites or natural products) that provide growth advantages to the fungus dependent on environment. Here we summarize the current knowledge of these BGCs-18 of which can be named to product-their expression profiles in vivo, and which BGCs may enhance virulence of this opportunistic human pathogen. Furthermore, we find extensive evidence for the presence of many of these BGCs, or similar BGCs, in distantly related genera including the emerging pathogen Pseudogymnoascus destructans, the causative agent of white-nose syndrome in bats, and suggest such BGCs may be predictive of pathogenic potential in other fungi.This article is part of the themed issue 'Tackling emerging fungal threats to animal health, food security and ecosystem resilience'.
Seed contamination with polyketide mycotoxins such as sterigmatocystin (ST) produced by Aspergilli is a worldwide issue. The ST biosynthetic pathway is well-characterized in A. nidulans, but regulatory aspects related to the carbon source are still enigmatic. This is particularly true for lactose, inasmuch as some ST production mutant strains still synthesize ST on lactose but not on other carbon substrates. Here, kinetic data revealed that on d-glucose, ST forms only after the sugar is depleted from the medium, while on lactose, ST appears when most of the carbon source is still available. Biomass-specified ST production on lactose was significantly higher than on d-glucose, suggesting that ST formation may either be mediated by a carbon catabolite regulatory mechanism, or induced by low specific growth rates attainable on lactose. These hypotheses were tested by d-glucose limited chemostat-type continuous fermentations. No ST formed at a high growth rate, while a low growth rate led to the formation of 0.4 mg·L-1 ST. Similar results were obtained with a CreA mutant strain. We concluded that low specific growth rates may be the primary cause of mid-growth ST formation on lactose in A. nidulans, and that carbon utilization rates likely play a general regulatory role during biosynthesis.
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Leukocyte motility is required for host defense responses. Rac-family Rho GTPases are implicated in leukocyte function; however, the distinct roles of different Rac isoforms in host defense in vivo have remained unclear. In this study, we generated Rac2-deficient zebrafish using transcription activator-like effector nucleases to directly compare the role of Rac2 in vivo in neutrophils and macrophages in motility and the response to infection. This zebrafish larval model is highly amenable to live imaging of leukocyte behavior, and we report that in rac2-/- larvae both neutrophils and macrophages are defective in basic motility, leading to impaired responses to localized wounds or infections. rac2-/- larvae are highly susceptible to infection with Pseudomonas aeruginosa, which can be almost fully rescued by ectopic expression of either Rac2 or Rac1 specifically in neutrophils, indicating that these isoforms have partially overlapping functions in vivo. Rescue of Rac2 expression specifically in macrophages also confers resistance to Pseudomonas infection, highlighting an important role for Rac2 in this leukocyte population as well. Surprisingly, in contrast to neutrophils expressing a Rac2 dominant inhibitory human disease mutation, rac2-/- neutrophils do not have altered polarity or mobilization from hematopoietic tissue, suggesting that a different Rac isoform, such as Rac1, also contributes to these phenotypes in vivo.
One mission of the Microbial Observatory Experiments on the International Space Station (ISS) is to examine the traits and diversity of fungal isolates to gain a better understanding of how fungi may adapt to microgravity environments and how this may affect interactions with humans in a closed habitat. Here, we report an initial characterization of two isolates, ISSFT-021 and IF1SW-F4, of Aspergillus fumigatus collected from the ISS and a comparison to the experimentally established clinical isolates Af293 and CEA10. Whole-genome sequencing of ISSFT-021 and IF1SW-F4 showed 54,960 and 52,129 single nucleotide polymorphisms, respectively, compared to Af293, which is consistent with observed genetic heterogeneity among sequenced A. fumigatus isolates from diverse clinical and environmental sources. Assessment of in vitro growth characteristics, secondary metabolite production, and susceptibility to chemical stresses revealed no outstanding differences between ISS and clinical strains that would suggest special adaptation to life aboard the ISS. Virulence assessment in a neutrophil-deficient larval zebrafish model of invasive aspergillosis revealed that both ISSFT-021 and IF1SW-F4 were significantly more lethal than Af293 and CEA10. Taken together, these genomic, in vitro, and in vivo analyses of two A. fumigatus strains isolated from the ISS provide a benchmark for future investigations of these strains and for continuing research on specific microbial isolates from manned space environments. IMPORTANCE As durations of manned space missions increase, it is imperative to understand the long-term consequence of microbial exposure on human health in a closed human habitat. To date, studies aimed at bacterial and fungal contamination of space vessels have highlighted species compositions biased toward hardy, persistent organisms capable of withstanding harsh conditions. In the current study, we assessed traits of two independent Aspergillus fumigatus strains isolated from the International Space Station. Ubiquitously found in terrestrial soil and atmospheric environments, A. fumigatus is a significant opportunistic fungal threat to human health, particularly among the immunocompromised. Using two well-known clinical isolates of A. fumigatus as comparators, we found that both ISS isolates exhibited normal in vitro growth and chemical stress tolerance yet caused higher lethality in a vertebrate model of invasive disease. These findings substantiate the need for additional studies of physical traits and biological activities of microbes adapted to microgravity and other extreme extraterrestrial conditions.
Metabolism of fatty acids is a critical requirement for the pathogenesis of oil seed pathogens including the fungus Aspergillus flavus Previous studies have correlated decreased ability to grow on fatty acids with reduced virulence of this fungus on host seed. Two fatty acid metabolism regulatory transcription factors, FarA and FarB, have been described in other filamentous fungi. Unexpectedly, we find A. flavus possesses three Far homologs, FarA, FarB, and FarC, with FarA and FarC showing a greater protein similarity to each other than FarB. farA and farB are located in regions of colinearity in all Aspergillus spp. sequenced to date, whereas farC is limited to a subset of species where it is inserted in an otherwise colinear region in Aspergillus genomes. Deletion and overexpression (OE) of farA and farB, but not farC, yielded mutants with aberrant growth patterns on specific fatty acids as well as altered expression of genes involved in fatty acid metabolism. Marked differences included significant growth defects of both ∆farA and ∆farB on medium-chain fatty acids and decreased growth of OE::farA on unsaturated fatty acids. Loss of farA diminished expression of mitochondrial β-oxidation genes whereas OE::farA inhibited expression of genes involved in unsaturated fatty acid catabolism. FarA also positively regulated the desaturase genes required to generate polyunsaturated fatty acids. Aflatoxin production on toxin-inducing media was significantly decreased in the ∆farB mutant and increased in the OE::farB mutant, with gene expression data supporting a role for FarB in tying β-oxidation processes with aflatoxin accumulation.
White-nose syndrome has recently emerged as one of the most devastating wildlife diseases recorded, causing widespread mortality in numerous bat species throughout eastern North America. Here, we present an improved reference genome of the fungal pathogen Pseudogymnoascus destructans for use in comparative genomic studies.
Unlocking the biochemical stores of fungi is key for developing future pharmaceuticals. Through reduced expression of a critical histone deacetylase in Aspergillus nidulans, increases of up to 100-fold were observed in the levels of 15 new aspercryptins, recently described lipopeptides with two noncanonical amino acids derived from octanoic and dodecanoic acids. In addition to two NMR-verified structures, MS/MS networking helped uncover an additional 13 aspercryptins. The aspercryptins break the conventional structural orientation of lipopeptides and appear "backward" when compared to known compounds of this class. We have also confirmed the 14-gene aspercryptin biosynthetic gene cluster, which encodes two fatty acid synthases and several enzymes to convert saturated octanoic and dodecanoic acid to α-amino acids.
Fungal genome projects are revealing thousands of cryptic secondary metabolism (SM) biosynthetic gene clusters that encode pathways that potentially produce valuable compounds. Heterologous expression systems should allow these clusters to be expressed and their products obtained, but approaches are needed to identify the most valuable target clusters. The inp cluster of Aspergillus nidulans contains a gene, inpE, that encodes a proteasome subunit, leading us to hypothesize that the inp cluster produces a proteasome inhibitor and inpE confers resistance to this compound. Previous efforts to express this cluster have failed, but by sequentially replacing the promoters of the genes of the cluster with a regulatable promotor, we have expressed them successfully. Expression reveals that the product of the inp cluster is the proteasome inhibitor fellutamide B, and our data allow us to propose a biosynthetic pathway for the compound. By deleting inpE and activating expression of the inp cluster, we demonstrate that inpE is required for resistance to internally produced fellutamide B. These data provide experimental validation for the hypothesis that some fungal SM clusters contain genes that encode resistant forms of the enzymes targeted by the compound produced by the cluster.
Natural product discovery efforts have focused primarily on microbial biosynthetic gene clusters (BGCs) containing large multimodular polyketide synthases and nonribosomal peptide synthetases; however, sequencing of fungal genomes has revealed a vast number of BGCs containing smaller NRPS-like genes of unknown biosynthetic function. Using comparative metabolomics, we show that a BGC in the human pathogen Aspergillus fumigatus named fsq, which contains an NRPS-like gene lacking a condensation domain, produces several new isoquinoline alkaloids known as the fumisoquins. These compounds derive from carbon-carbon bond formation between two amino acid-derived moieties followed by a sequence that is directly analogous to isoquinoline alkaloid biosynthesis in plants. Fumisoquin biosynthesis requires the N-methyltransferase FsqC and the FAD-dependent oxidase FsqB, which represent functional analogs of coclaurine N-methyltransferase and berberine bridge enzyme in plants. Our results show that BGCs containing incomplete NRPS modules may reveal new biosynthetic paradigms and suggest that plant-like isoquinoline biosynthesis occurs in diverse fungi.
The immune mechanisms that recognize inhaled Aspergillus fumigatus conidia to promote their elimination from the lungs are incompletely understood. FleA is a lectin expressed by Aspergillus fumigatus that has twelve binding sites for fucosylated structures that are abundant in the glycan coats of multiple plant and animal proteins. The role of FleA is unknown: it could bind fucose in decomposed plant matter to allow Aspergillus fumigatus to thrive in soil, or it may be a virulence factor that binds fucose in lung glycoproteins to cause Aspergillus fumigatus pneumonia. Our studies show that FleA protein and Aspergillus fumigatus conidia bind avidly to purified lung mucin glycoproteins in a fucose-dependent manner. In addition, FleA binds strongly to macrophage cell surface proteins, and macrophages bind and phagocytose fleA-deficient (∆fleA) conidia much less efficiently than wild type (WT) conidia. Furthermore, a potent fucopyranoside glycomimetic inhibitor of FleA inhibits binding and phagocytosis of WT conidia by macrophages, confirming the specific role of fucose binding in macrophage recognition of WT conidia. Finally, mice infected with ΔfleA conidia had more severe pneumonia and invasive aspergillosis than mice infected with WT conidia. These findings demonstrate that FleA is not a virulence factor for Aspergillus fumigatus. Instead, host recognition of FleA is a critical step in mechanisms of mucin binding, mucociliary clearance, and macrophage killing that prevent Aspergillus fumigatus pneumonia.
Regarding targeted disruption of epigenetic regulators, histone methyltransferase and deacetylase in a plant endophytic fungus Pestalotiopsis fici have been uncovered as an unexplored chemical repertoire. Manipulation of epigenetic regulators led to the isolation of 15 new polyketides, including pestaloficiols T-W (1-3 and 5), as well as 11 macrodiolide ficiolides A-K (6-16). Ficiolide K (16) was found to contain a very rare 1,6-anhydro-pyranose moiety. Finally, the biosynthetic origin of macrodiolide was characterized by isotope-labeling experiments.
Nitric oxide (NO) is known to be involved in modulating production of styrylpyrone polyphenols in the basidiomycete Inonotus obliquus. However, it remains unknown how NO orchestrates fungal styrylpyrone biosynthesis. Here, we show that a transient NO burst correlated with an enhanced expression of phenylalanine ammonia lyase (PAL), 4-coumarate CoA ligase (4CL), and styrylpyrone synthase (SPS), the key enzymes involved in styrylpyrone biosynthesis, and subsequently an increased production of styrylpyrone polyphenols. In parallel, the NO burst also resulted in S-nitrosylation of PAL, 4CL, and SPS, which compromised their enzymatic activities mediating a post-translational feedback mechanism that keeps NO-dependent transcriptional activation in check. Moreover, dysfunction of thioredoxin reductase (TrxR) further increased the formation of S-nitrosylated proteins, implicating the significance of the Trx system in maintaining a low level of protein-nitrosothiols. Three thioredoxin-like proteins (TrxLs) from I. obliquus show in vitro denitrosylation potential toward S-nitrosylated proteins via trans-denitrosylation or mixed disulfide intermediates. Thus, S-nitrosylation triggered by the NO burst limits over production of fungal styrylpyrone polyphenols, and denitrosylation by TrxLs that act in concert with TrxR play a key role in maintaining redox balance and orchestrating catalytic activities of the enzymes engaged in styrylpyrone synthetic metabolism.
Ralstonia solanacearum is a globally distributed soil-borne plant pathogenic bacterium, which shares a broad ecological range with many plant- and soil-associated fungi. We sought to determine if R. solanacearum chemical communication directs symbiotic development of polymicrobial consortia. R. solanacearum produced a diffusible metabolite that induced conserved morphological differentiation in 34 species of fungi across three diverse taxa (Ascomycetes, Basidiomycetes and Zygomycetes). Fungi exposed to this metabolite formed chlamydospores, survival structures with thickened cell walls. Some chlamydospores internally harbored R. solanacearum, indicating a newly described endofungal lifestyle for this important plant pathogen. Using imaging mass spectrometry and peptidogenomics, we identified an undescribed lipopeptide, ralsolamycin, produced by an R. solanacearum non-ribosomal peptide synthetase-polyketide synthase hybrid. Inactivation of the hybrid non-ribosomal peptide synthetase-polyketide synthase gene, rmyA, abolished ralsolamycin synthesis. R. solanacearum mutants lacking ralsolamycin no longer induced chlamydospore development in fungal coculture and invaded fungal hyphae less well than wild-type. We propose that ralsolamycin contributes to the invasion of fungal hyphae and that the formation of chlamydospores may provide not only a specific niche for bacterial colonization but also enhanced survival for the partnering fungus.
Oxylipins are a class of molecules derived from the incorporation of oxygen into polyunsaturated fatty acid substrates through the action of oxygenases. While extensively investigated in the context of mammalian immune responses, over the last decade it has become apparent that oxylipins are a common means of communication among and between plants, animals, and fungi to control development and alter host-microbe interactions. In fungi, some oxylipins are derived nonenzymatically while others are produced by lipoxygenases, cyclooxygenases, and monooxygenases with homology to plant and human enzymes. Recent investigations of numerous plant and human fungal pathogens have revealed oxylipins to be involved in the establishment and progression of disease. This review highlights oxylipin production by pathogenic fungi and their role in fungal development and pathogen/host interactions.
Ergosterol is a major and specific component of the fungal plasma membrane, and thus, the cytochrome P450 enzymes (Erg proteins) that catalyze ergosterol synthesis have been selected as valuable targets of azole antifungals. However, the opportunistic pathogen Aspergillus fumigatus has developed worldwide resistance to azoles largely through mutations in the cytochrome P450 enzyme Cyp51 (Erg11). In this study, we demonstrate that a cytochrome b5-like heme-binding damage resistance protein (Dap) family, comprised of DapA, DapB, and DapC, coordinately regulates the functionality of cytochrome P450 enzymes Erg5 and Erg11 and oppositely affects susceptibility to azoles. The expression of all three genes is induced in an azole concentration-dependent way, and the decreased susceptibility to azoles requires DapA stabilization of cytochrome P450 protein activity. In contrast, overexpression of DapB and DapC causes dysfunction of Erg5 and Erg11, resulting in abnormal accumulation of sterol intermediates and further accentuating the sensitivity of ΔdapA strains to azoles. The results of exogenous-hemin rescue and heme-binding-site mutagenesis experiments demonstrate that the heme binding of DapA contributes the decreased azole susceptibility, while DapB and -C are capable of reducing the activities of Erg5 and Erg11 through depletion of heme. In vivo data demonstrate that inactivated DapA combined with activated DapB yields an A. fumigatus mutant that is easily treatable with azoles in an immunocompromised mouse model of invasive pulmonary aspergillosis. Compared to the single Dap proteins found in Saccharomyces cerevisiae and Schizosaccharomyces pombe, we suggest that this complex Dap family regulatory system emerged during the evolution of fungi as an adaptive means to regulate ergosterol synthesis in response to environmental stimuli. Knowledge of the ergosterol biosynthesis route in fungal pathogens is useful in the design of new antifungal drugs and could aid in the study of antifungal-drug resistance mechanisms. In this study, we demonstrate that three cytochrome b5-like Dap proteins coordinately regulate the azole resistance and ergosterol biosynthesis catalyzed by cytochrome P450 proteins. Our new insights into the Dap regulatory system in fungal pathogens may have broad therapeutic ramifications beyond their usefulness for classic azole antifungals. Moreover, our elucidation of the molecular mechanism of Dap regulation of cytochrome P450 protein functionality through heme-binding activity may extend beyond the Kingdom Fungi with applicability toward Dap protein regulation of mammalian sterol synthesis.
The microbial secondary metabolome encompasses great synthetic diversity, empowering microbes to tune their chemical responses to changing microenvironments. Traditional metabolomics methods are ill-equipped to probe a wide variety of environments or environmental dynamics. Here we introduce a class of microscale culture platforms to analyse chemical diversity of fungal and bacterial secondary metabolomes. By leveraging stable biphasic interfaces to integrate microculture with small molecule isolation via liquid-liquid extraction, we enable metabolomics-scale analysis using mass spectrometry. This platform facilitates exploration of culture microenvironments (including rare media typically inaccessible using established methods), unusual organic solvents for metabolite isolation and microbial mutants. Utilizing Aspergillus, a fungal genus known for its rich secondary metabolism, we characterize the effects of culture geometry and growth matrix on secondary metabolism, highlighting the potential use of microscale systems to unlock unknown or cryptic secondary metabolites for natural products discovery. Finally, we demonstrate the potential for this class of microfluidic systems to study interkingdom communication between fungi and bacteria.
Filamentous fungi are historically known as rich sources for production of biologically active natural products, so-called secondary metabolites. One particularly pharmaceutically relevant chemical group of secondary metabolites is the nonribosomal peptides synthesized by nonribosomal peptide synthetases (NRPSs). As most of the fungal NRPS gene clusters leading to production of the desired molecules are not expressed under laboratory conditions, efforts to overcome this impediment are crucial to unlock the full chemical potential of each fungal species. One way to activate these silent clusters is by overexpressing and deleting global regulators of secondary metabolism. The conserved fungal-specific regulator of secondary metabolism, LaeA, was shown to be a valuable target for sleuthing of novel gene clusters and metabolites. Additionally, modulation of chromatin structures by either chemical or genetic manipulation has been shown to activate cryptic metabolites. Furthermore, NRPS-derived molecules seem to be affected by cross talk between the specific gene clusters and some of these metabolites have a tissue- or developmental-specific regulation. This chapter summarizes how this knowledge of different tiers of regulation can be combined to increase production of NRPS-derived metabolites in fungal species.
Small peptides formed from non-ribosomal peptide synthetases (NRPS) are bioactive molecules produced by many fungi including the genus Aspergillus. A subset of NRPS utilizes tryptophan and its precursor, the non-proteinogenic amino acid anthranilate, in synthesis of various metabolites such as Aspergillus fumigatus fumiquinazolines (Fqs) produced by the fmq gene cluster. The A. fumigatus genome contains two putative anthranilate synthases - a key enzyme in conversion of anthranilic acid to tryptophan - one beside the fmq cluster and one in a region of co-linearity with other Aspergillus spp. Only the gene found in the co-linear region, trpE, was involved in tryptophan biosynthesis. We found that site-specific mutations of the TrpE feedback domain resulted in significantly increased production of anthranilate, tryptophan, p-aminobenzoate and fumiquinazolines FqF and FqC. Supplementation with tryptophan restored metabolism to near wild type levels in the feedback mutants and suggested that synthesis of the tryptophan degradation product kynurenine could negatively impact Fq synthesis. The second putative anthranilate synthase gene next to the fmq cluster was termed icsA for its considerable identity to isochorismate synthases in bacteria. Although icsA had no impact on A. fumigatus Fq production, deletion and over-expression of icsA increased and decreased respectively aromatic amino acid levels suggesting that IcsA can draw from the cellular chorismate pool.
Filamentous fungi are renowned for the production of a diverse array of secondary metabolites (SMs) where the genetic material required for synthesis of a SM is typically arrayed in a biosynthetic gene cluster (BGC). These natural products are valued for their bioactive properties stemming from their functions in fungal biology, key among those protection from abiotic and biotic stress and establishment of a secure niche. The producing fungus must not only avoid self-harm from endogenous SMs but also deliver specific SMs at the right time to the right tissue requiring biochemical aid. This review highlights functions of BGCs beyond the enzymatic assembly of SMs, considering the timing and location of SM production and other proteins in the clusters that control SM activity. Specifically, self-protection is provided by both BGC-encoded mechanisms and non-BGC subcellular containment of toxic SM precursors; delivery and timing is orchestrated through cellular trafficking patterns and stress- and developmental-responsive transcriptional programs.
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Fungal polyketides are a diverse class of natural products, or secondary metabolites (SMs), with a wide range of bioactivities often associated with toxicity. Here, we focus on a group of non-reducing polyketide synthases (NR-PKSs) in the fungal phylum Ascomycota that lack a thioesterase domain for product release, group V. Although widespread in ascomycete taxa, this group of NR-PKSs is notably absent in the mycotoxigenic genus Fusarium and, surprisingly, found in genera not known for their secondary metabolite production (e.g., the mycorrhizal genus Oidiodendron, the powdery mildew genus Blumeria, and the causative agent of white-nose syndrome in bats, Pseudogymnoascus destructans). This group of NR-PKSs, in association with the other enzymes encoded by their gene clusters, produces a variety of different chemical classes including naphthacenediones, anthraquinones, benzophenones, grisandienes, and diphenyl ethers. We discuss the modification of and transitions between these chemical classes, the requisite enzymes, and the evolution of the SM gene clusters that encode them. Integrating this information, we predict the likely products of related but uncharacterized SM clusters, and we speculate upon the utility of these classes of SMs as virulence factors or chemical defenses to various plant, animal, and insect pathogens, as well as mutualistic fungi.
Filamentous fungi are renowned for the production of bioactive secondary metabolites. Typically, one distinct metabolite is generated from a specific secondary metabolite cluster. Here, we characterize the newly described trypacidin (tpc) cluster in the opportunistic human pathogen Aspergillus fumigatus. We find that this cluster as well as the previously characterized endocrocin (enc) cluster both contribute to the production of the spore metabolite endocrocin. Whereas trypacidin is eliminated when only tpc cluster genes are deleted, endocrocin production is only eliminated when both the tpc and enc non-reducing polyketide synthase-encoding genes, tpcC and encA, respectively, are deleted. EncC, an anthrone oxidase, converts the product released from EncA to endocrocin as a final product. In contrast, endocrocin synthesis by the tpc cluster likely results from incomplete catalysis by TpcK (a putative decarboxylase), as its deletion results in a nearly 10-fold increase in endocrocin production. We suggest endocrocin is likely a shunt product in all related non-reducing polyketide synthase clusters containing homologues of TpcK and TpcL (a putative anthrone oxidase), e.g. geodin and monodictyphenone. This finding represents an unusual example of two physically discrete secondary metabolite clusters generating the same natural product in one fungal species by distinct routes.
The bZIP transcription factors are conserved in all eukaryotes and play critical roles in organismal responses to environmental challenges. In filamentous fungi, several lines of evidence indicate that secondary metabolism (SM) is associated with oxidative stress mediated by bZIP proteins. Here we uncover a connection with a bZIP protein and oxidative stress induction of SM in the plant endophytic fungus Pestalotiopsis fici. A homology search of the P. fici genome with the bZIP protein RsmA, involved in SM and the oxidative stress response in Aspergillus nidulans, identified PfZipA. Deletion of PfzipA resulted in a strain that displayed resistant to the oxidative reagents tert-butylhydroperoxide (tBOOH), diamide, and menadione sodium bisulfite (MSB), but increased sensitivity to H2O2 as compared to wild type (WT). Secondary metabolite production presented a complex pattern dependent on PfzipA and oxidative reagents. Without oxidative treatment, the ΔPfzipA strain produced less isosulochrin and ficipyroneA than WT; addition of tBOOH further decreased production of iso-A82775C and pestaloficiol M in ΔPfzipA; diamide treatment resulted in equivalent production of isosulochrin and ficipyroneA in the two strains; MSB treatment further decreased production of RES1214-1 and iso-A82775C but increased pestaloficiol M production in the mutant; and H2O2 treatment resulted in enhanced production of isosulochrin, RES1214-1 and pestheic acid but decreased ficipyroneA and pestaloficiol M in ΔPfzipA compared to WT. Our results suggest that PfZipA regulation of SM is modified by oxidative stress pathways and provide insights into a possible role of PfZipA in mediating SM synthesis in the endophytic lifestyle of P. fici.
The microbial world offers a rich source of bioactive compounds for those able to sift through it. Technologies capable of quantitatively detecting natural products while simultaneously identifying known compounds would expedite the search for new pharmaceutical leads. Prior efforts have targeted histone deacetylases in fungi to globally activate the production of new secondary metabolites, yet no study has directly assessed its effects with minimal bias at the metabolomic level. Using untargeted metabolomics, we monitored changes in >1000 small molecules secreted from the model fungus, Aspergillus nidulans, following genetic or chemical reductions in histone deacetylase activity (HDACi). Through quantitative, differential analyses, we found that nearly equal numbers of compounds were up- and down-regulated by >100 fold. We detected products from both known and unknown biosynthetic pathways and discovered that A. nidulans is capable of producing fellutamides, proteasome inhibitors whose expression was induced by ∼100 fold or greater upon HDACi. This work adds momentum to an "omics"-driven resurgence in natural products research, where direct detection replaces bioactivity as the primary screen for new pharmacophores.
With thousands of fungal genomes being sequenced, each genome containing up to 70 secondary metabolite (SM) clusters 30-80 kb in size, breakthrough techniques are needed to characterize this SM wealth. Here we describe a novel system-level methodology for unbiased cloning of intact large SM clusters from a single fungal genome for one-step transformation and expression in a model host. All 56 intact SM clusters from Aspergillus terreus were individually captured in self-replicating fungal artificial chromosomes (FACs) containing both the E. coli F replicon and an Aspergillus autonomously replicating sequence (AMA1). Candidate FACs were successfully shuttled between E. coli and the heterologous expression host A. nidulans. As proof-of-concept, an A. nidulans FAC strain was characterized in a novel liquid chromatography-high resolution mass spectrometry (LC-HRMS) and data analysis pipeline, leading to the discovery of the A. terreus astechrome biosynthetic machinery. The method we present can be used to capture the entire set of intact SM gene clusters and/or pathways from fungal species for heterologous expression in A. nidulans and natural product discovery.
Fungal interspecific interactions enhance biosynthesis of phenylpropanoid metabolites (PM), and production of nitric oxide (NO) is known to be involved in this process. However, it remains unknown which signaling pathway(s) or regulator(s) mediate fungal PM biosynthesis. In this study, we cocultured two white-rot fungi, Inonotus obliquus and Phellinus morii, to examine NO production, expression of the genes involved in phenylpropanoid metabolism and accumulation of phenylpropanoid-derived polyphenols by I. obliquus. Coculture of the two fungi caused an enhanced NO biosynthesis followed by increased transcription of the genes encoding phenylalanine ammonia lyase (PAL) and 4-coumarate CoA ligase (4CL), as well as an upregulated biosynthesis of styrylpyrone polyphenols in I. obliquus. Addition of the NO synthase (NOS) selective inhibitor aminoguanidine (AG) inhibited NO production by more than 90% followed by cease in transcription of PAL and 4Cl. Treatment of guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one did not affect NO production but suppressed transcription of PAL and 4CL and reduced accumulation of total phenolic constituents. Genome-wide analysis of I. obliquus revealed two genes encoding a constitutive and an inducible NOS-like protein, respectively (cNOSL and iNOSL). Coculture of the two fungi did not increase the expression of the cNOSL gene but triggered expression of the iNOSL gene. Cloned iNOSL from Escherichia coli shows higher activity in transferring L-arginine to NO, and this activity is lost upon AG addition. Thus, iNOSL is more responsible for NO production in I. obliquus and may act as an important regulator governing PM production during fungal interspecific interactions.
Aspergillus fumigatus is an opportunistic human pathogenic fungus causing life-threatening infections in immunocompromised patients. Adaptation to different habitats and also virulence of the fungus depends on signal perception and transduction by modules such as the cyclic AMP-dependent protein kinase A (PKA) pathway. Here, by transcriptome analysis, 632 differentially regulated genes of this important signaling cascade were identified, including 23 putative transcriptional regulators. The highest upregulated transcription factor gene was located in a previously unknown secondary metabolite gene cluster, which we named fmp, encoding an incomplete non-ribosomal peptide synthetase, FmpE. Overexpression of the regulatory gene fmpR using the Tet(On) system led to the specific expression of the other six genes of the fmp cluster. Metabolic profiling of wild type and fmpR overexpressing strain by HPLC-DAD and HPLC-HRESI-MS and structure elucidation by NMR led to identification of 5-benzyl-1H-pyrrole-2-carboxylic acid, which we named fumipyrrole. Fumipyrrole was not described as natural product yet. Chemical synthesis of fumipyrrole confirmed its structure. Interestingly, deletion of fmpR or fmpE led to reduced growth and sporulation of the mutant strains. Although fmp cluster genes were transcribed in infected mouse lungs, deletion of fmpR resulted in wild-type virulence in a murine infection model.
Filamentous fungi and bacteria form mixed-species biofilms in nature and diverse clinical contexts. They secrete a wealth of redox-active small molecule secondary metabolites, which are traditionally viewed as toxins that inhibit growth of competing microbes. Here, we report that these "toxins" can act as interspecies signals, affecting filamentous fungal development via oxidative stress regulation. Specifically, in coculture biofilms, Pseudomonas aeruginosa phenazine-derived metabolites differentially modulated Aspergillus fumigatus development, shifting from weak vegetative growth to induced asexual sporulation (conidiation) along a decreasing phenazine gradient. The A. fumigatus morphological shift correlated with the production of phenazine radicals and concomitant reactive oxygen species (ROS) production generated by phenazine redox cycling. Phenazine conidiation signaling was conserved in the genetic model A. nidulans and mediated by NapA, a homolog of AP-1-like bZIP transcription factor, which is essential for the response to oxidative stress in humans, yeast, and filamentous fungi. Expression profiling showed phenazine treatment induced a NapA-dependent response of the global oxidative stress metabolome, including the thioredoxin, glutathione, and NADPH-oxidase systems. Conidiation induction in A. nidulans by another microbial redox-active secondary metabolite, gliotoxin, also required NapA. This work highlights that microbial redox metabolites are key signals for sporulation in filamentous fungi, which are communicated through an evolutionarily conserved eukaryotic stress response pathway. It provides a foundation for interspecies signaling in environmental and clinical biofilms involving bacteria and filamentous fungi.
No abstract available.
Chemical mutagenesis screens are useful to identify mutants involved in biological processes of interest. Identifying the mutation from such screens, however, often fails when using methodologies involving transformation of the mutant to wild type phenotype with DNA libraries. Here we analyzed Illumina sequence of a chemically derived mutant of Aspergillus nidulans and identified a gene encoding a C2H2 transcription factor termed RsrA for regulator of stress response. RsrA is conserved in filamentous fungal genomes, and upon deleting the gene in three Aspergillus species (A. nidulans, A. flavus and A. fumigatus), we found two conserved phenotypes: enhanced resistance to oxidative stress and reduction in sporulation processes. For all species, rsrA deletion mutants were more resistant to hydrogen peroxide treatment. In depth examination of this latter characteristic in A. nidulans showed that upon exposure to hydrogen peroxide, RsrA loss resulted in global up-regulation of several components of the oxidative stress metabolome including the expression of napA and atfA, the two bZIP transcription factors mediating resistance to reactive oxygen species (ROS) as well as NapA targets in thioredoxin and glutathione systems. Coupling transcriptional data with examination of ΔrsrAΔatfA and ΔrsrAΔnapA double mutants indicate that RsrA primarily operates through NapA-mediated stress response pathways. A model of RsrA regulation of ROS response in Aspergillus is presented. RsrA, found in a highly syntenic region in Aspergillus genomes, coordinates a NapA mediated oxidative response in Aspergillus fungi.
Iron plays a critical role in survival and virulence of the opportunistic pathogen Aspergillus fumigatus. Two transcription factors, the GATA-factor SreA and the bZip-factor HapX oppositely monitor iron homeostasis with HapX activating iron acquisition pathways (e.g., siderophores) and shutting down iron consumptive pathways (and SreA) during iron starvation conditions whereas SreA negatively regulates HapX and corresponding pathways during iron sufficiency. Recently the non-ribosomal peptide, hexadehydroastechrome (HAS; a tryptophan-derived iron (III)-complex), has been found important in A. fumigatus virulence. We found that HAS overproduction caused an iron starvation phenotype, from alteration of siderophore pools to regulation of iron homeostasis gene expression including sreA. Moreover, we uncovered an iron dependent secondary metabolism network where both SreA and HapX oppositely regulate multiple other secondary metabolites including HAS. This circuitry links iron-acquisition and consumption pathways with secondary metabolism-thus placing HAS as part of a metabolic feedback circuitry designed to balance iron pools in the fungus and presenting iron availability as one environmental trigger of secondary metabolism.
G protein-coupled receptors (GPCRs) are transmembrane receptors that relay signals from the external environment inside the cell, allowing an organism to adapt to its surroundings. They are known to detect a vast array of ligands, including sugars, amino acids, pheromone peptides, nitrogen sources, oxylipins, and light. Despite their prevalence in fungal genomes, very little is known about the functions of filamentous fungal GPCRs. Here we present the first full-genome assessment of fungal GPCRs through characterization of null mutants of all 15 GPCRs encoded by the aflatoxin-producing fungus Aspergillus flavus. All strains were assessed for growth, development, ability to produce aflatoxin, and response to carbon sources, nitrogen sources, stress agents, and lipids. Most GPCR mutants were aberrant in one or more response processes, possibly indicative of cross talk in downstream signaling pathways. Interestingly, the biological defects of the mutants did not correspond with assignment to established GPCR classes; this is likely due to the paucity of data for characterized fungal GPCRs. Many of the GPCR transcripts were differentially regulated under various conditions as well. The data presented here provide an extensive overview of the full set of GPCRs encoded by A. flavus and provide a framework for analysis in other fungal species. Importance: Aspergillus flavus is an opportunistic pathogen of crops and animals, including humans, and it produces a carcinogenic toxin called aflatoxin. Because of this, A. flavus accounts for food shortages and economic losses in addition to sickness and death. Effective means of combating this pathogen are needed to mitigate its deleterious effects. G protein-coupled receptors (GPCRs) are often used as therapeutic targets due to their signal specificity, and it is estimated that half of all drugs target GPCRs. In fungi such as A. flavus, GPCRs are likely necessary for sensing the changes in the environment, including food sources, developmental signals, stress agents, and signals from other organisms. Therefore, elucidating their functions in A. flavus could identify ideal receptors against which to develop antagonists.
Despite their oftentimes-elusive ecological role, fungal natural products have, for better or worse, impacted our daily lives tremendously owing to their diverse and potent bioactive properties. This Janus-faced nature of fungal natural products inevitably ushered in a field of research dedicated towards understanding the ecology, organisms, genes, enzymes, and biosynthetic pathways that give rise to this arsenal of diverse and complex chemistry. Ongoing research in fungal secondary metabolism has not only increased our appreciation for fungal natural products as an asset but also sheds light on the pivotal role that these once-regarded "metabolic wastes" play in fungal biology, defense, and stress response in addition to their potential contributions towards human mycoses. Full orchestration of secondary metabolism requires not only the seamless coordination between temporal and spatial control of SM-associated machineries (e.g. enzymes, cofactors, intermediates, and end-products) but also integration of these machineries into primary metabolic processes and established cellular mechanisms. An intriguing, but little known aspect of microbial natural product synthesis lies in the spatial organization of both pathway intermediates and enzymes responsible for the production of these compounds. In this highlight, we summarize some major breakthroughs in understanding the genes and regulation of fungal natural product synthesis and introduce the current state of knowledge on the spatial and temporal control of fungal natural product synthesis.
Aspergillus fumigatus is the most common filamentous fungal pathogen of immunocompromised hosts, resulting in invasive aspergillosis (IA) and high mortality rates. Innate immunity is known to be the predominant host defense against A. fumigatus; however, innate phagocyte responses to A. fumigatus in an intact host and their contributions to host survival remain unclear. Here, we describe a larval zebrafish A. fumigatus infection model amenable to real-time imaging of host-fungal interactions in live animals. Following infection with A. fumigatus, innate phagocyte populations exhibit clear preferences for different fungal morphologies: macrophages rapidly phagocytose conidia and form aggregates around hyphae, while the neutrophil response is dependent upon the presence of hyphae. Depletion of macrophages rendered host larvae susceptible to invasive disease. Moreover, a zebrafish model of human leukocyte adhesion deficiency with impaired neutrophil function also resulted in invasive disease and impaired host survival. In contrast, macrophage-deficient but not neutrophil-deficient larvae exhibited attenuated disease following challenge with a less virulent (ΔlaeA) strain of A. fumigatus, which has defects in secondary metabolite production. Taking these results together, we have established a new vertebrate model for studying innate immune responses to A. fumigatus that reveals distinct roles for neutrophils and macrophages in mediating host defense against IA.
Aerial spores, crucial for propagation and dispersal of the Kingdom Fungi, are commonly the initial inoculum of pathogenic fungi. Natural products (secondary metabolites) have been correlated with fungal spore development and enhanced virulence in the human pathogen Aspergillus fumigatus but mechanisms for metabolite deposition in the spore are unknown. Metabolomic profiling of A. fumigatus deletion mutants of fumiquinazoline (Fq) cluster genes reveal that the first two products of the Fq cluster, FqF and FqA, are produced to comparable levels in all fungal tissues but the final enzymatically derived product, FqC, predominantly accumulates in the fungal spore. Loss of the sporulation-specific transcription factor, BrlA, yields a strain unable to produce FqA or FqC. Fluorescence microscopy showed FmqD, the oxidoreductase required to generate FqC, was secreted via the Golgi apparatus to the cell wall in an actin-dependent manner. In contrast, all other members of the Fq pathway including the putative transporter, FmqE - which had no effect on Fq biosynthesis - were internal to the hyphae. The co-ordination of BrlA-mediated tissue specificity with FmqD secretion to the cell wall presents a previously undescribed mechanism to direct localization of specific secondary metabolites to spores of the differentiating fungus.
Eight-carbon (C8) volatiles, such as 1-octen-3-ol, are ubiquitous among fungi. They are the volatiles critical for aroma and flavor of fungi, and assumed to be signals controlling germination of several fungi. In this study, we found that intact Aspergillus flavus conidia scarcely synthesized C8 volatiles but repeated freeze-thaw treatment that made the cell membrane permeable promoted (R)-1-octen-3-ol formation. Loss or down regulation of any one of five fatty acid oxygenases (PpoA, PpoB, PpoC, PpoD or lipoxygenase) hypothesized contribute to 1-octen-3-ol formation had little impact on production of this volatile. This suggested that none of the oxygenases were directly involved in the formation of 1-octen-3-ol or that compensatory pathways exist in the fungus. Germination of the conidia was markedly inhibited at high density (1.0 × 10(9)spores mL(-1)). It has been postulated that 1-octen-3-ol is an autoinhibitor suppressing conidia germination at high density. 1-Octen-3-ol at concentration of no less than 10 mM was needed to suppress the germination while the concentration of 1-octen-3-ol in the suspension at 1.0 × 10(9) mL(-1) was under the detection limit (<1 µM). Thus, 1-octen-3-ol was not the principal component responsible for inhibition of germination. Instead, it was evident that the other heat-labile factor(s) suppressed conidial germination.
Microbes in the rhizosphere have a suite of extracellular compounds, both primary and secondary, that communicate with other organisms in their immediate environment. Here, we describe a two-way volatile interaction between two widespread and economically important soil-borne pathogens of peanut, Aspergillus flavus and Ralstonia solanacearum, a fungus and bacterium, respectively. In response to A. flavus volatiles, R. solanacearum reduced production of the major virulence factor extracellular polysaccharide (EPS). In parallel, A. flavus responded to R. solanacearum volatiles by reducing conidia production, both on plates and on peanut seeds and by increasing aflatoxin production on peanut. Volatile profiling of these organisms using solid-phase micro-extraction gas chromatography mass spectroscopy (SPME-GCMS) provided a first glimpse at the compounds that may drive these interactions.
The plant and human opportunistic fungus Aspergillus flavus is recognized for the production of the carcinogen aflatoxin. Although many reviews focus on the wealth of information known about aflatoxin biosynthesis, few articles describe other genes and molecules important for A. flavus development or secondary metabolism. Here we compile the most recent work on A. flavus secondary metabolite clusters, environmental response mechanisms (stress response pathways, quorum sensing and G protein signaling pathways) and the function of the transcriptional regulatory unit known as the Velvet Complex. A comparison to other Aspergilli reveals conservation in several pathways affecting fungal development and metabolism.
Fungi are well known for their ability to produce a multitude of natural products. On the one hand their potential to provide beneficial antibiotics and immunosuppressants has been maximized by the pharmaceutical industry to service the market with cost-efficient drugs. On the other hand identification of trace amounts of known mycotoxins in food and feed samples is of major importance to ensure consumer health and safety. Although several fungal natural products, their biosynthesis and regulation are known today, recent genome sequences of hundreds of fungal species illustrate that the secondary metabolite potential of fungi has been substantially underestimated. Since expression of genes and subsequent production of the encoded metabolites are frequently cryptic or silent under standard laboratory conditions, strategies for activating these hidden new compounds are essential. This review will cover the latest advances in fungal genome mining undertaken to unlock novel products.
No abstract available.
The outbreak of fungal meningitis associated with contaminated methylprednisolone acetate has thrust the importance of fungal infections into the public consciousness. The predominant pathogen isolated from clinical specimens, Exserohilum rostratum (teleomorph: Setosphaeria rostrata), is a dematiaceous fungus that infects grasses and rarely humans. This outbreak highlights the potential for fungal pathogens to infect both plants and humans. Most crossover or trans-kingdom pathogens are soil saprophytes and include fungi in Ascomycota and Mucormycotina phyla. To establish infection, crossover fungi must overcome disparate, host-specific barriers, including protective surfaces (e.g. cuticle, skin), elevated temperature, and immune defenses. This review illuminates the underlying mechanisms used by crossover fungi to cause infection in plants and mammals, and highlights critical events that lead to human infection by these pathogens. Several genes including veA, laeA, and hapX are important in regulating biological processes in fungi important for both invasive plant and animal infections.
The hallmark trait of fungal secondary-metabolite gene clusters is well established, consisting of contiguous enzymatic and often regulatory gene(s) devoted to the production of a metabolite of a specific chemical class. Unexpectedly, we have found a deviation from this motif in a subtelomeric region of Aspergillus fumigatus. This region, under the control of the master regulator of secondary metabolism, LaeA, contains, in its entirety, the genetic machinery for three natural products (fumitremorgin, fumagillin, and pseurotin), where genes for fumagillin and pseurotin are physically intertwined in a single supercluster. Deletions of 29 adjoining genes revealed that fumagillin and pseurotin are coregulated by the supercluster-embedded regulatory gene with biosynthetic genes belonging to one of the two metabolic pathways in a noncontiguous manner. Comparative genomics indicates the fumagillin/pseurotin supercluster is maintained in a rapidly evolving region of diverse fungal genomes. This blended design confounds predictions from established secondary-metabolite cluster search algorithms and provides an expanded view of natural product evolution.
A perplexing aspect of fungal secondary metabolite gene clusters is that most clusters remain 'silent' under common laboratory growth conditions where activation is obtained through gene manipulation or encounters with environmental signals. Few proteins have been found involved in repression of silent clusters. Through multicopy suppressor mutagenesis, we have identified a novel cluster suppressor in Aspergillus nidulans, MvlA (modulator of veA loss). Genetic assessment of MvlA mutants revealed the role of both itself and VeA (but not the VeA partner LaeA) in the suppression of the cryptic ors gene cluster producing orsellinic acid and its F9775 derivatives. Loss of veA upregulates F9775A and F9775B production and this increase is reduced 4-5-fold when an overexpression mvlA (OE:mvlA) allele is introduced into the ΔveA background. Previous studies have implicated a positive role for GcnE (H3K9 acetyltransferase of the SAGA/ADA complex) in ors cluster expression and here we find expression of gcnE is upregulated in ΔveA and suppressed by OE:mvlA in the ΔveA background. H3K9 acetylation levels of ors cluster genes correlated with gcnE expression and F9775 production in ΔveA and OE:mvlAΔveA strains. Finally, deletion of gcnE in the ΔveA background abolishes ors cluster activation and F9775 production. Together, this work supports a role for VeA and MvlA in modifying SAGA/ADA complex activity.
LaeA is a fungal specific virulence factor of both plant and human pathogenic fungi. Transcriptional profiles of laeA mutants have been successfully exploited to identify regulatory mechanisms of secondary metabolism in fungi; here we use laeA mutants as tools to elucidate virulence attributes in Aspergillus flavus. Microarray expression profiles of ΔlaeA and over-expression laeA (OE::laeA) were compared to wild type A. flavus. Strikingly, several nitrogen metabolism genes are oppositely mis-regulated in the ΔlaeA and OE::laeA mutants. One of the nitrogen regulatory genes, the bZIP encoding meaB, is up-regulated in ΔlaeA. Significantly, over-expression of meaB (OE::meaB) phenocopies the decreased virulence attributes of a ΔlaeA phenotype including decreased colonization of host seed, reduced lipase activity and loss of aflatoxin B1 production in seed. However, a double knock-down of laeA and meaB (KD::laeA,meaB) demonstrated that KD::laeA,meaB closely resembled ΔlaeA rather than wild type or ΔmeaB in growth, aflatoxin biosynthesis and sclerotia production thus suggesting that meaB does not contribute to the ΔlaeA phenotype. MeaB and LaeA appear to be part of regulatory networks that allow them to have both shared and distinct roles in fungal biology.
A wide diversity of pathogens and mutualists of plant and animal hosts, including oomycetes and fungi, produce effector proteins that enter the cytoplasm of host cells. A major question has been whether or not entry by these effectors can occur independently of the microbe or requires machinery provided by the microbe. Numerous publications have documented that oomycete RxLR effectors and fungal RxLR-like effectors can enter plant and animal cells independent of the microbe. A recent reexamination of whether the RxLR domain of oomycete RxLR effectors is sufficient for microbe-independent entry into host cells concluded that the RxLR domains of Phytophthora infestans Avr3a and of P. sojae Avr1b alone are NOT sufficient to enable microbe-independent entry of proteins into host and nonhost plant and animal cells. Here, we present new, more detailed data that unambiguously demonstrate that the RxLR domain of Avr1b does show efficient and specific entry into soybean root cells and also into wheat leaf cells, at levels well above background nonspecific entry. We also summarize host cell entry experiments with a wide diversity of oomycete and fungal effectors with RxLR or RxLR-like motifs that have been independently carried out by the seven different labs that coauthored this letter. Finally we discuss possible technical reasons why specific cell entry may have been not detected by Wawra et al. (2013).
Although the field of microfluidics has made significant progress in bringing new tools to address biological questions, the accessibility and adoption of microfluidics within the life sciences are still limited. Open microfluidic systems have the potential to lower the barriers to adoption, but the absence of robust design rules has hindered their use. Here, we present an open microfluidic platform, suspended microfluidics, that uses surface tension to fill and maintain a fluid in microscale structures devoid of a ceiling and floor. We developed a simple and ubiquitous model predicting fluid flow in suspended microfluidic systems and show that it encompasses many known capillary phenomena. Suspended microfluidics was used to create arrays of collagen membranes, mico Dots (μDots), in a horizontal plane separating two fluidic chambers, demonstrating a transwell platform able to discern collective or individual cellular invasion. Further, we demonstrated that μDots can also be used as a simple multiplexed 3D cellular growth platform. Using the μDot array, we probed the combined effects of soluble factors and matrix components, finding that laminin mitigates the growth suppression properties of the matrix metalloproteinase inhibitor GM6001. Based on the same fluidic principles, we created a suspended microfluidic metabolite extraction platform using a multilayer biphasic system that leverages the accessibility of open microchannels to retrieve steroids and other metabolites readily from cell culture. Suspended microfluidics brings the high degree of fluidic control and unique functionality of closed microfluidics into the highly accessible and robust platform of open microfluidics.
The filamentous fungi in the genus Aspergillus are opportunistic plant and animal pathogens that can adapt to their environment by producing various secondary metabolites, including lovastatin, penicillin, and aflatoxin. The synthesis of these small molecules is dependent on gene clusters that are globally regulated by the LaeA protein. Null mutants of LaeA in all pathogenic fungi examined to date show decreased virulence coupled with reduced secondary metabolism. Although the amino acid sequence of LaeA contains the motifs characteristic of seven-β-strand methyltransferases, a methyl-accepting substrate of LaeA has not been identified. In this work we did not find a methyl-accepting substrate in Aspergillus nidulans with various assays, including in vivo S-adenosyl-[methyl-(3)H]methionine labeling, targeted in vitro methylation experiments using putative protein substrates, or in vitro methylation assays using whole cell extracts grown under different conditions. However, in each experiment LaeA was shown to self-methylate. Amino acid hydrolysis of radioactively labeled LaeA followed by cation exchange and reverse phase chromatography identified methionine as the modified residue. Point mutations show that the major site of modification of LaeA is on methionine 207. However, in vivo complementation showed that methionine 207 is not required for the biological function of LaeA. LaeA is the first protein to exhibit automethylation at a methionine residue. These findings not only indicate LaeA may perform novel chemistry with S-adenosylmethionine but also provide new insights into the physiological function of LaeA.
Secondary metabolite (SM) production in filamentous fungi is mechanistically associated with chromatin remodeling of specific SM clusters. One locus recently shown to be involved in SM suppression in Aspergillus nidulans was CclA, a member of the histone 3 lysine 4 methylating COMPASS complex. Here we examine loss of CclA and a putative H3K4 demethylase, HdmA, in the human pathogen Aspergillus fumigatus. Although deletion of hdmA showed no phenotype under the conditions tested, the cclA deletant was deficient in tri- and di-methylation of H3K4 and yielded a slowly growing strain that was rich in the production of several SMs, including gliotoxin. Similar to deletion of other chromatin modifying enzymes, ΔcclA was sensitive to 6-azauracil indicating a defect in transcriptional elongation. Despite the poor growth, the ΔcclA mutant had wild-type pathogenicity in a murine model and the Toll-deficient Drosophila model of invasive aspergillosis. These data indicate that tri- and di-methylation of H3K4 is involved in the regulation of several secondary metabolites in A. fumigatus, however does not contribute to pathogenicity under the conditions tested.
Dimeric basic leucine zipper (bZIP) proteins are conserved transcriptional enhancers found in all eukaryotes. A recently reported and novel function for bZIPs is association of these proteins with secondary metabolite production in filamentous fungi. In particular a Yap-like bZIP termed RsmA (restorer of secondary metabolism A) was identified in Aspergillus nidulans that positively regulates the carcinogen sterigmatocystin. To assess for conserved function for RsmA, we examined a role of this protein in secondary metabolism in the pathogen A. fumigatus. RsmA was found to positively regulate gliotoxin where overexpression (OE) of rsmA led to 2-100 fold increases of twelve gli cluster metabolites in culture medium including the newly identified gli metabolite cyclo(L-Phe-L-Ser). Lungs from both wild type and OErsmA infected mice contained gliotoxin (2.3 fold higher in OErsmA treatment) as well as the gliotoxin precursor cyclo(L-Phe-L-Ser) (3.2 fold higher in OErsmA treatment). The data here presents a conserved role for RsmA in secondary metabolite cluster activation and suggests cyclo(L-Phe-L-Ser) may serve as an alternative marker for diagnosis of invasive aspergillosis.
The secondary metabolome provides pathogenic fungi with a plethoric and versatile panel of molecules that can be deployed during host ingress. While powerful genetic and analytical chemistry methods have been developed to identify fungal secondary metabolites (SMs), discovering the biological activity of SMs remains an elusive yet critical task. Here, we describe a process for identifying the immunosuppressive properties of Aspergillus SMs developed by coupling a cost-effective microfluidic neutrophil chemotaxis assay with an in vivo zebrafish assay. The microfluidic platform allows the identification of metabolites inhibiting neutrophil recruitment with as little as several nano-grams of compound in microliters of fluid. The zebrafish assay demonstrates a simple and accessible approach for performing in vivo studies without requiring any manipulation of the fish. Using this methodology we identify the immunosuppressive properties of a fungal SM, endocrocin. We find that endocrocin is localized in Aspergillus fumigatus spores and its biosynthesis is temperature-dependent. Finally, using the Drosophila toll deficient model, we find that deletion of encA, encoding the polyketide synthase required for endocrocin production, yields a less pathogenic strain of A. fumigatus when spores are harvested from endocrocin permissive but not when harvested from endocrocin restrictive conditions. The tools developed here will open new "function-omic" avenues downstream of the metabolomics, identification, and purification phases.
Microscale methods for cell-based assays typically rely on macroscopic reagent handling and fluidic loading protocols that are technically challenging and do not scale with the number of assays favorably. Here, we demonstrate a microfluidic platform technology called "Kit-On-A-Lid-Assay" (KOALA), that enables the creation of self-contained microfluidic cell-based assays, integrating all the steps required to perform cell-based assays. The KOALA platform allows the pre-packaging of reagents, cryopreservation of cell suspensions, thawing of cell suspensions, culture of cells, and operation of whole cell-based assays. The operation of the KOALA platform is user-friendly and consists of bringing together a lid containing the microchannels, and a base containing the pre-packaged reagents, thereby causing fluidic exchange in all the channels simultaneously. We demonstrate that the KOALA cell-based assays can be simply operated from start to finish without any external laboratory equipment.
Small molecules (SMs) play central roles as virulence factors of pathogenic fungi and bacteria; however, genomic analyses suggest that the majority of microbial SMs have remained uncharacterized. Based on microarray analysis followed by comparative metabolomics of overexpression/knockout mutants, we identified a tryptophan-derived iron(III)-complex, hexadehydro-astechrome (HAS), as the major product of the cryptic has nonribosomal peptide synthetase (NRPS) gene cluster in the human pathogen Aspergillus fumigatus. Activation of the has cluster created a highly virulent A. fumigatus strain that increased mortality of infected mice. Comparative metabolomics of different mutant strains allowed to propose a pathway for HAS biosynthesis and further revealed cross-talk with another NRPS pathway producing the anticancer fumitremorgins.
The putative methyltransferase LaeA is a global regulator that affects the expression of multiple secondary metabolite gene clusters in several fungi, and it can modify heterochromatin structure in Aspergillus nidulans. We have recently shown that the LaeA ortholog of Trichoderma reesei (LAE1), a fungus that is an industrial producer of cellulase and hemicellulase enzymes, regulates the expression of cellulases and polysaccharide hydrolases. To learn more about the function of LAE1 in T. reesei, we assessed the effect of deletion and overexpression of lae1 on genome-wide gene expression. We found that in addition to positively regulating 7 of 17 polyketide or nonribosomal peptide synthases, genes encoding ankyrin-proteins, iron uptake, heterokaryon incompatibility proteins, PTH11-receptors, and oxidases/monoxygenases are major gene categories also regulated by LAE1. chromatin immunoprecipitation sequencing with antibodies against histone modifications known to be associated with transcriptionally active (H3K4me2 and -me3) or silent (H3K9me3) chromatin detected 4089 genes bearing one or more of these methylation marks, of which 75 exhibited a correlation between either H3K4me2 or H3K4me3 and regulation by LAE1. Transformation of a laeA-null mutant of A. nidulans with the T. reesei lae1 gene did not rescue sterigmatocystin formation and further impaired sexual development. LAE1 did not interact with A. nidulans VeA in yeast two-hybrid assays, whereas it interacted with the T. reesei VeA ortholog, VEL1. LAE1 was shown to be required for the expression of vel1, whereas the orthologs of velB and VosA are unaffected by lae1 deletion. Our data show that the biological roles of A. nidulans LaeA and T. reesei LAE1 are much less conserved than hitherto thought. In T. reesei, LAE1 appears predominantly to regulate genes increasing relative fitness in its environment.
The eukaryotic basic leucine zipper (bZIP) transcription factors play critical roles in the organismal response to the environment. Recently, a novel YAP-like bZIP, restorer of secondary metabolism A (RsmA), was found in a suppressor screen of an Aspergillus nidulans secondary metabolism (SM) mutant in which overexpression of rsmA was found to partially remediate loss of SM in Velvet Complex mutants. The Velvet Complex is a conserved fungal transcriptional heteromer that couples SM with sexual development in fungi. Here we characterized and contrasted SM in mutants of RsmA and four other A. nidulans bZIP proteins (NapA, ZipA, ZipB and ZipC) with predicted DNA binding motifs similar to RsmA. Only two overexpression mutants exhibited both SM and sexual abnormalities that were noteworthy: OE : : rsmA resulted in a 100-fold increase in sterigmatocystin and a near loss of meiotic spore production. OE : : napA displayed decreased production of sterigmatocystin, emericellin, asperthecin, shamixanthone and epishamixanthone, coupled with a shift from sexual to asexual development. Quantification of bZIP homodimer and heterodimer formation using fluorescence resonance energy transfer (FRET) suggested that these proteins preferentially self-associate.
No abstract available.
Secondary metabolism and development are linked in Aspergillus through the conserved regulatory velvet complex composed of VeA, VelB, and LaeA. The founding member of the velvet complex, VeA, shuttles between the cytoplasm and nucleus in response to alterations in light. Here we describe a new interaction partner of VeA identified through a reverse genetics screen looking for LaeA-like methyltransferases in Aspergillus nidulans. One of the putative LaeA-like methyltransferases identified, LlmF, is a negative regulator of sterigmatocystin production and sexual development. LlmF interacts directly with VeA and the repressive function of LlmF is mediated by influencing the localization of VeA, as over-expression of llmF decreases the nuclear to cytoplasmic ratio of VeA while deletion of llmF results in an increased nuclear accumulation of VeA. We show that the methyltransferase domain of LlmF is required for function; however, LlmF does not directly methylate VeA in vitro. This study identifies a new interaction partner for VeA and highlights the importance of cellular compartmentalization of VeA for regulation of development and secondary metabolism.
Aspergillus fumigatus is an increasingly serious pathogen of immunocompromised patients, causing the often fatal disease invasive aspergillosis (IA). One A. fumigatus virulence determinant of IA is LaeA, a conserved virulence factor in pathogenic fungi. To further understand the role of LaeA in IA, the expression profile of ΔlaeA was compared to wild type, and several transcription factors were found significantly misregulated by LaeA loss. One of the transcription factors up-regulated over 4-fold in the ΔlaeA strain was Afu4g09710, similar in sequence to Aspergillus nidulans NosA, which is involved in sexual development. Here we assessed loss of nosA (ΔnosA) and overexpression of nosA (OE::nosA) on A. fumigatus in both a wild type and ΔlaeA background. Based on the multiple alterations of physiological development of single and double mutants, we suggest that NosA mediates the decreased radial growth and delayed conidial germination observed in ΔlaeA strains, the former in a light dependent manner. The ΔnosA mutant showed increased virulence in the Galleria mellonella larvae model of disseminated aspergillosis, potentially due to its increased growth and germination rate. Furthermore, the A. fumigatus nosA allele was able to partially remediate sexual development in an A. nidulans ΔnosA background. Likewise, the A. nidulans nosA allele was able to restore the menadione sensitivity defect of the A. fumigatus ΔnosA strain, suggesting conservation of function of the NosA protein in these two species.
Plasmid vector construction is an essential step for molecular microbiology yet often a time-consuming process. Manipulation of the fungal genome to express genes to activate secondary metabolite production often requires creation of plasmid constructs in a reiterative fashion. Here we introduce a modified Quick-change site-directed mutagenesis method that allows for rapid and accurate construction of fungal transformation vectors.
Although the high affinity Ca(2+) channel, Cch1, and its subunit Mid1 have been investigated and evaluated in yeast and some of filamentous fungi, little is known about the function of their homologs in the Aspergilli. Here, we have functionally characterized the yeast homologs, CchA and MidA, in Aspergillus nidulans using conditional and null deletion mutants. CchA and MidA not only have functional benefits of fast growth, which is consistent with Cch1 and Mid1 in yeast, but also have unique and complex roles in regulating conidiation, hyphal polarity and cell wall components in low-calcium environments. The defect of CchA or MidA resulted in a sharp reduction in the number of conidiospores, accompanied by abnormal metulae, and undeveloped-phialides at a higher density of inoculum. Most interestingly, these conidiation defects in mutants can, remarkably, be rescued either by extra-cellular Ca(2+) in a calcineurin-dependent way or by osmotic stress in a calcineurin-independent way. Moreover, the fact that the phenotypic defects are not exacerbated by the presence of the double deletion, together with the Y2H assay, indicates that CchA and MidA may form a complex to function together. Our findings suggest that the high-affinity Ca(2+) channel may represent a viable and completely unexplored avenue to reduce conidiation in the Aspergilli.
Mining for novel natural compounds is of eminent importance owing to the continuous need for new pharmaceuticals. Filamentous fungi are historically known to harbor the genetic capacity for an arsenal of natural compounds, both beneficial and detrimental to humans. The majority of these metabolites are still cryptic or silent under standard laboratory culture conditions. Mining for these cryptic natural products can be an excellent source for identifying new compound classes. Capitalizing on the current knowledge on how secondary metabolite gene clusters are regulated has allowed the research community to unlock many hidden fungal treasures, as described in this chapter.
Regulation of secondary metabolite (SM) gene clusters in Aspergillus nidulans has been shown to occur through cluster-specific transcription factors or through global regulators of chromatin structure such as histone methyltransferases, histone deacetylases, or the putative methyltransferase LaeA. A multicopy suppressor screen for genes capable of returning SM production to the SM deficient ΔlaeA mutant resulted in identification of the essential histone acetyltransferase EsaA, able to complement an esa1 deletion in Saccharomyces cereviseae. Here we report that EsaA plays a novel role in SM cluster activation through histone 4 lysine 12 (H4K12) acetylation in four examined SM gene clusters (sterigmatocystin, penicillin, terrequinone and orsellinic acid), in contrast to no increase in H4K12 acetylation of the housekeeping tubA promoter. This augmented SM cluster acetylation requires LaeA for full effect and correlates with both increased transcript levels and metabolite production relative to wild type. H4K12 levels may thus represent a unique indicator of relative production potential, notably of SMs.
Oxylipins regulate Aspergillus development and mycotoxin production and are also involved in Aspergillus quorum sensing mechanisms. Despite extensive knowledge of how these oxylipins are synthesized and what processes they regulate, nothing is known about how these signals are detected and transmitted by the fungus. G protein-coupled receptors (GPCR) have been speculated to be involved as they are known oxylipin receptors in mammals, and many putative GPCRs have been identified in the Aspergilli. Here, we present evidence that oxylipins stimulate a burst in cAMP in A. nidulans, and that loss of an A. nidulans GPCR, gprD, prevents this cAMP accumulation. A. flavus undergoes an oxylipin-mediated developmental shift when grown at different densities, and this regulates spore, sclerotial and aflatoxin production. A. flavus encodes two putative GprD homologs, GprC and GprD, and we demonstrate here that they are required to transition to a high-density development state, as well as to respond to spent medium of a high-density culture. The finding of GPCRs that regulate production of survival structures (sclerotia), inoculum (spores) and aflatoxin holds promise for future development of anti-fungal therapeutics.
Endocrocin is a simple anthraquinone frequently identified in extracts of numerous fungi. Several biosynthetic schemes for endocrocin synthesis have been hypothesized, but to date, no dedicated secondary metabolite gene cluster that produces this polyketide as its major metabolite has been identified. Here we describe our biosynthetic and regulatory characterization of the endocrocin gene cluster in Aspergillus fumigatus. This is the first report of this anthraquinone in this species. The biosynthetic genes required for endocrocin production are regulated by the global regulator of secondary metabolism, LaeA, and encode an iterative nonreducing polyketide synthase (encA), a physically discrete metallo-β-lactamase type thioesterase (encB), and a monooxygenase (encC). Interestingly, the deletion of a gene immediately adjacent to encC, termed encD and encoding a putative 2-oxoglutarate-Fe(II) type oxidoreductase, resulted in higher levels of endocrocin production than in the wild-type strain, whereas overexpression of encD eliminated endocrocin accumulation. We found that overexpression of the encA transcript resulted in higher transcript levels of encA-D and higher production of endocrocin. We discuss a model of the enc cluster as one evolutionary origin of fungal anthraquinones derived from a nonreducing polyketide synthase and a discrete metallo-β-lactamase-type thioesterase.
This review studies the impact of whole genome sequencing on Aspergillus secondary metabolite research. There has been a proliferation of many new, intriguing discoveries since sequencing data became widely available. What is more, the genomes disclosed the surprising finding that there are many more secondary metabolite biosynthetic pathways than laboratory research had suggested. Activating these pathways has been met with some success, but many more dormant genes remain to be awakened.
The eukaryotic bZIP transcription factors are critical players in organismal response to environmental challenges. In fungi, the production of secondary metabolites (SMs) is hypothesized as one of the responses to environmental insults, e.g. attack by fungivorous insects, yet little data to support this hypothesis exists. Here we establish a mechanism of bZIP regulation of SMs through RsmA, a recently discovered YAP-like bZIP protein. RsmA greatly increases SM production by binding to two sites in the Aspergillus nidulans AflR promoter region, a C6 transcription factor known for activating production of the carcinogenic and anti-predation SM, sterigmatocystin. Deletion of aflR in an overexpression rsmA (OE:rsmA) background not only eliminates sterigmatocystin production but also significantly reduces asperthecin synthesis. Furthermore, the fungivore, Folsomia candida, exhibited a distinct preference for feeding on wild type rather than an OE:rsmA strain. RsmA may thus have a critical function in mediating direct chemical resistance against predation. Taken together, these results suggest RsmA represents a bZIP pathway hardwired for defensive SM production.
An RNA silencing construct was used to alter mycotoxin production in the plant pathogenic fungus Fusarium culmorum, the incitant of crown and foot rot on wheat. The transformation of a wild-type strain and its nitrate reductase-deficient mutant with inverted repeat transgenes (IRTs) containing sequences corresponding to the trichothecene regulatory gene TRI6 was achieved using hygromycin B resistance as a selectable marker. Southern analysis revealed a variety of integration patterns of the TRI6 IRT. One transformant underwent homologous recombination with deletion of the endogenous TRI6 gene, whereas, in another transformant, the TRI6 IRT was not integrated into the genome. The TRI6 IRT did not alter the physiological characteristics, such as spore production, pigmentation or growth rate, on solid media. In most transformants, a high TRI6 amplification signal was detected by quantitative reverse transcription-polymerase chain reaction, corresponding to a TRI6-hybridizing smear of degraded fragments by Northern analysis, whereas TRI5 expression decreased compared with the respective nontransformed strain. Four transformants showed increased TRI5 expression, which was correlated with a dramatic (up to 28-fold) augmentation of deoxynivalenol production. Pathogenicity assays on durum wheat seedlings confirmed that impairment of deoxynivalenol production in the TRI6 IRT transformants correlated with a loss of virulence, with decreased disease indices ranging from 40% to 80% in nine silenced strains, whereas the overproducing transformants displayed higher virulence compared with the wild-type.
Aspergillus flavus is saprophytic soil fungus that infects and contaminates preharvest and postharvest seed crops with the carcinogenic secondary metabolite aflatoxin. The fungus is also an opportunistic animal and human pathogen causing aspergillosis diseases with incidence increasing in the immunocompromised population. Whole genome sequences of A. flavus have been released and reveal 55 secondary metabolite clusters that are regulated by different environmental regimes and the global secondary metabolite regulators LaeA and VeA. Characteristics of A. flavus associated with pathogenicity and niche specialization include secondary metabolite production, enzyme elaboration, and a sophisticated oxylipin host crosstalk associated with a quorum-like development program. One of the more promising strategies in field control involves the use of atoxic strains of A. flavus in competitive exclusion studies. In this review, we discuss A. flavus as an agricultural and medical threat and summarize recent research advances in genomics, elucidation of parameters of pathogenicity, and control measures.
Secondary metabolite (SM) production by fungi is hypothesized to provide some fitness attribute for the producing organisms. However, most SM clusters are "silent" when fungi are grown in traditional laboratory settings, and it is difficult to ascertain any function or activity of these SM cluster products. Recently, the creation of a chromatin remodeling mutant in Aspergillus nidulans induced activation of several cryptic SM gene clusters. Systematic testing of nine purified metabolites from this mutant identified an emodin derivate with efficacy against both human fungal pathogens (inhibiting both spore germination and hyphal growth) and several bacteria. The ability of catalase to diminish this antimicrobial activity implicates reactive oxygen species generation, specifically, the generation of hydrogen peroxide, as the mechanism of emodin hydroxyl activity.
Filamentous fungi produce a variety of secondary metabolites of diverse beneficial and detrimental activities to humankind. The genes required for a given secondary metabolite are typically arranged in a gene cluster. There is considerable evidence that secondary metabolite gene regulation is, in part, by transcriptional control through hierarchical levels of transcriptional regulatory elements involved in secondary metabolite cluster regulation. Identification of elements regulating secondary metabolism could potentially provide a means of increasing production of beneficial metabolites, decreasing production of detrimental metabolites, aid in the identification of 'silent' natural products and also contribute to a broader understanding of molecular mechanisms by which secondary metabolites are produced. This review summarizes regulation of secondary metabolism associated with transcriptional regulatory elements from a broad view as well as the tremendous advances in discovery of cryptic or novel secondary metabolites by genomic mining.
Gliotoxin, a major product of the gli non-ribosomal peptide synthetase gene cluster, is strongly associated with virulence of the opportunistic human pathogen Aspergillus fumigatus. Despite identification of the gli cluster, the pathway of gliotoxin biosynthesis has remained elusive, in part because few potential intermediates have been identified. In addition, previous studies suggest that knowledge of gli-dependent metabolites is incomplete. Here we use differential analysis by 2D NMR spectroscopy (DANS) of metabolite extracts derived from gli knock-out and wild-type (WT) strains to obtain a detailed inventory of gli-dependent metabolites. DANS-based comparison of the WT metabolome with that of ΔgliZ, a knock-out strain devoid of the gene encoding the transcriptional regulator of the gli cluster, revealed nine novel gliZ-dependent metabolites including unexpected structural motifs. Their identification provides insight into gliotoxin biosynthesis and may benefit studies of the role of the gli cluster in A. fumigatus virulence. Our study demonstrates the utility of DANS for correlating gene expression and metabolite biosynthesis in microorganisms.
No abstract available.
The Aspergillus nidulans sterigmatocystin (ST) gene cluster contains both regulatory (aflR) and biosynthetic genes (stc genes) required for ST production. A total of 26 genes are in the cluster, 13 of which have been assigned a known function in the biosynthetic pathway. This complex secondary pathway represents a physiological cost to the fungus. We tested the amount of asexual spore production using a series of isogenic lines of A. nidulans, differing only in a mutation in aflR (resulting in a strain containing no ST intermediates) or a mutation in three stc genes that produced either no ST intermediates (ΔstcJ), an early ST intermediate, norsoloroinic acid (ΔstcE) or a late ST intermediate, versicolorin A (ΔstcU). In two independently replicated experiments we compared the numbers of conidia produced by each of these mutant strains and a wild type ST producer in a neutral (growth media) and a host (corn seed) environment. A stepwise increase in asexual spore production was observed with each progressive step in the ST pathway. Thus, the data suggest that recruitment or loss of these secondary metabolite pathway genes has a selective advantage apart from the physiological activity of the metabolite.
Gene-silencing mechanisms are being shown to be associated with an increasing number of fungal developmental processes. Telomere position effect (TPE) is a eukaryotic phenomenon resulting in gene repression in areas immediately adjacent to telomere caps. Here, TPE is shown to regulate expression of transgenes on the left arm of chromosome III and the right arm of chromosome VI in Aspergillus nidulans. Phenotypes found to be associated with transgene repression included reduction in radial growth and the absence of sexual spores; however, these pleiotropic phenotypes were remedied when cultures were grown on media with appropriate supplementation. Simple radial growth and ascosporogenesis assays provided insights into the mechanism of TPE, including a means to determine its extent. These experiments revealed that the KU70 homologue (NkuA) and the heterochromatin-associated proteins HepA, ClrD and HdaA were partially required for transgene silencing. This study indicates that TPE extends at least 30 kb on chromosome III, suggesting that this phenomenon may be important for gene regulation in subtelomeric regions of A. nidulans.
Fungal secondary metabolites (SM) are bioactive compounds that are important in fungal ecology and, moreover, both harmful and useful in human endeavors (e.g., as toxins and pharmaceuticals). Recently a nuclear heterocomplex termed the Velvet complex, characterized in the model ascomycete Aspergillus nidulans, was found to be critical for SM production. Deletion of two members of the Velvet complex, laeA and veA, results in near loss of SM and defective sexual spore production in A. nidulans and other species. Using a multicopy-suppressor genetics approach, we have isolated an Aspergillus nidulans gene named rsmA (remediation of secondary metabolism) based upon its ability to remediate secondary metabolism in ΔlaeA and ΔveA backgrounds. Overexpression of rsmA (OE::rsmA) restores production of sterigmatocystin (ST) (a carcinogenic SM) via transcriptional activation of ST biosynthetic genes. However, defects in sexual reproduction in either ΔlaeA or ΔveA strains cannot be overcome by OE::rsmA. An intact Velvet complex coupled with an OE::rsmA allele increases SM many fold over the wild-type level, but loss of rsmA does not decrease SM. RsmA encodes a putative bZIP basic leucine zipper-type transcription factor.
Filamentous fungi produce a vast array of small molecules called secondary metabolites, which include toxins as well as antibiotics. Coregulated gene clusters are the hallmark of fungal secondary metabolism, and there is a growing body of evidence that suggests regulation is at least, in part, epigenetic. Chromatin-level control is involved in several silencing phenomena observed in fungi including mating type switching, telomere position effect (TPE), silencing of ribosomal DNA, regulation of genes involved in nutrient acquisition, and as presented here, secondary metabolite cluster expression. These phenomena are tied together by the underlying theme of chromosomal location, often near centromeres and telomeres, where facultative heterochromatin plays a role in transcription. Secondary metabolite gene clusters are often located subtelomerically and recently it has been shown that proteins involved in chromatin remodeling, such as LaeA, ClrD, CclA, and HepA mediate cluster regulation.
Besides industrially produced gibberellins (GAs), Fusarium fujikuroi is able to produce additional secondary metabolites such as the pigments bikaverin and neurosporaxanthin and the mycotoxins fumonisins and fusarin C. The global regulation of these biosynthetic pathways is only poorly understood. Recently, the velvet complex containing VeA and several other regulatory proteins was shown to be involved in global regulation of secondary metabolism and differentiation in Aspergillus nidulans. Here, we report on the characterization of two components of the F. fujikuroi velvet-like complex, FfVel1 and FfLae1. The gene encoding this first reported LaeA orthologue outside the class of Eurotiomycetidae is upregulated in DeltaFfvel1 microarray-studies and FfLae1 interacts with FfVel1 in the nucleus. Deletion of Ffvel1 and Fflae1 revealed for the first time that velvet can simultaneously act as positive (GAs, fumonisins and fusarin C) and negative (bikaverin) regulator of secondary metabolism, and that both components affect conidiation and virulence of F. fujikuroi. Furthermore, the velvet-like protein FfVel2 revealed similar functions regarding conidiation, secondary metabolism and virulence as FfVel1. Cross-genus complementation studies of velvet complex component mutants between Fusarium, Aspergillus and Penicillium support an ancient origin for this complex, which has undergone a divergence in specific functions mediating development and secondary metabolism.
Aspergillus species have the ability to produce a wide range of secondary metabolites including polyketides that are generated by multi-domain polyketide synthases (PKSs). Recent biochemical studies using dissected single or multiple domains from PKSs have provided deep insight into how these PKSs control the structural outcome. Moreover, the recent genome sequencing of several species has greatly facilitated the understanding of the biosynthetic pathways for these secondary metabolites. In this review, we will highlight the current knowledge regarding polyketide biosynthesis in Aspergillus based on the domain architecture of non-reducing, highly reducing, and partially reducing PKSs, and PKS-non-ribosomal peptide synthetases.
Subtelomeric secondary metabolite (SM) gene clusters are frequently surrounded by DNA repeats and transposon-like elements. The Aspergillus nidulans penicillin cluster, 30kb from the telomere of chromosome VI, is bordered by such elements. Deletions of penicillin telomere proximal and distal border regions resulted in decreased penicillin production. A 3.7kb distal region called PbIa, consisting of the putative transposable element DNA-2, was examined further where its replacement by a pyrG marker presented a similar phenotype as loss of the global SM regulator LaeA, resulting in a decrease in penicillin gene expression and product formation. In contrast, placement of the pyrG marker on either side of PbIa had no effect on penicillin synthesis. A requirement for PbIa in penicillin production was also apparent in a histone deacetylase mutant, DeltahdaA, enhanced for penicillin production. Trans-complementation of the PbIa element near and within the terrequinone A cluster on chromosome V did not restore penicillin biosynthesis or increase production of terrequinone A. Taken together, this data provides evidence for transposon involvement in SM cluster regulation.
Deletion of cclA, a component of the COMPASS complex of Aspergillus nidulans, results in the production of monodictyphenone and emodin derivatives. Through a set of targeted deletions in a cclA deletion strain, we have identified the genes required for monodictyphenone and emodin analog biosynthesis. Identification of an intermediate, endocrocin, from an mdpHDelta strain suggests that mdpH might encode a decarboxylase. Furthermore, by replacing the promoter of mdpA (a putative aflJ homolog) and mdpE (a putative aflR homolog) with the inducible alcA promoter, we have confirmed that MdpA functions as a coactivator. We propose a biosynthetic pathway for monodictyphenone and emodin derivatives based on bioinformatic analysis and characterization of biosynthetic intermediates.
Species of Aspergillus produce a diverse array of secondary metabolites, and recent genomic analysis has predicted that these species have the capacity to synthesize many more compounds. It has been possible to infer the presence of 55 gene clusters associated with secondary metabolism in Aspergillus flavus; however, only three metabolic pathways-aflatoxin, cyclopiazonic acid (CPA) and aflatrem-have been assigned to these clusters. To gain an insight into the regulation of and to infer the ecological significance of the 55 secondary metabolite gene clusters predicted in A. flavus, we examined their expression over 28 diverse conditions. Variables included culture medium and temperature, fungal development, colonization of developing maize seeds and misexpression of laeA, a global regulator of secondary metabolism. Hierarchical clustering analysis of expression profiles allowed us to categorize the gene clusters into four distinct clades. Gene clusters for the production of aflatoxins, CPA and seven other unknown compound(s) were identified as belonging to one clade. To further explore the relationships found by gene expression analysis, aflatoxin and CPA production were quantified under five different cell culture environments known to be conducive or nonconducive for aflatoxin biosynthesis and during the colonization of developing maize seeds. Results from these studies showed that secondary metabolism gene clusters have distinctive gene expression profiles. Aflatoxin and CPA were found to have unique regulation, but are sufficiently similar that they would be expected to co-occur in substrates colonized with A. flavus.
Aspergillus fumigatus is the causal agent of the life-threatening disease invasive aspergillosis. A. fumigatus laeA deletants, aberrant in toxin biosynthesis and spore development, are decreased in virulence. Among other characteristics, the decreased virulence is associated with increased spore susceptibility to macrophage phagocytosis. Three characteristics, cell wall microbe-associated molecular patterns (MAMPs), secreted metabolites, and rodlet content, thought to be important in macrophage-Aspergillus spore interactions were examined. Flow cytometry analysis of wild-type and DeltalaeA spores did not reveal any differences in surface-accessible MAMPs, including beta-(1,3)-glucan, alpha-mannose, chitin, and other carbohydrate ligands. Blocking experiments with laminarin and mannan supported the conclusion that differences in cell wall carbohydrates were not responsible for enhanced DeltalaeA spore phagocytosis. Aspergillus spores have been reported to secrete metabolites affecting phagocytosis. Neither spent culture exchange, transwell, nor coincubation internalization experiments supported a role for secreted metabolites in the differential uptake of wild-type and DeltalaeA spores. However, sonication assays implicated a role for surface rodlet protein/hydrophobin (RodAp) in differential spore phagocytosis. A possible role of RodAp in enhanced DeltalaeA spore uptake was further assessed by RodAp extraction and quantification, where wild-type spores were found to contain 60% more RodAp than DeltalaeA spores. After removal of the surface rodlet layer, wild-type spores were phagocytosed at similar rates as DeltalaeA spores. We conclude that increased uptake of DeltalaeA resting spores is not associated with changes in secreted metabolite production of this mutant or surface carbohydrate availability but, rather, due to a decrease in the surface RodAp content of DeltalaeA spores. We theorize that RodAp acts as an antiphagocytic molecule, possibly via physicochemical means and/or by impeding MAMP recognition by macrophage receptors.
Recent published sequencing of fungal genomes has revealed that these microorganisms have a surprisingly large number of secondary metabolite pathways that can serve as potential sources for new and useful natural products. Most of the secondary metabolites and their biosynthesis pathways are currently unknown, possibly because they are produced in very small amounts and are thus difficult to detect or are produced only under specific conditions. Elucidating these fungal metabolites will require new molecular genetic tools, better understanding of the regulation of secondary metabolism, and state of the art analytical methods. This review describes recent strategies to mine the cryptic natural products and their biosynthetic pathways in fungi.
Histone deacetylases (HDACs) play an important role in regulation of gene expression through histone modifications. Here we show that the Aspergillus fumigatus HDAC HdaA is involved in regulation of secondary metabolite production and is required for normal germination and vegetative growth. Deletion of the hdaA gene increased the production of several secondary metabolites but decreased production of gliotoxin whereas over-expression hdaA increased production of gliotoxin. RT-PCR analysis of 14 nonribosomal peptide synthases indicated HdaA regulation of up to nine of them. A mammalian cell toxicity assay indicated increased activity in the over-expression strain. Neither mutant affected virulence of the fungus as measured by macrophage engulfment of conidia or virulence in a neutropenic mouse model.
Cryptococcus neoformans was first described as a human fungal pathogen more than a century ago. One aspect of the C. neoformans infectious life cycle that has been the subject of earnest debate is whether the spores are pathogenic. Despite much speculation, no direct evidence has been presented to resolve this outstanding question. We present evidence that C. neoformans spores are pathogenic in a mouse intranasal inhalation model of infection. In addition, we provide mechanistic insights into spore-host interactions. We found that C. neoformans spores were phagocytosed by alveolar macrophages via interactions between fungal beta-(1,3)-glucan and the host receptors Dectin-1 and CD11b. Moreover, we discovered an important link between spore survival and macrophage activation state: intracellular spores were susceptible to reactive oxygen-nitrogen species. We anticipate these results will serve as the basis for a model to further investigate the pathogenic implications of infections caused by fungal spores.
Aspergillus species are globally ubiquitous saprophytes found in a variety of ecological niches. Almost 200 species of aspergilli have been identified, less than 20 of which are known to cause human disease. Among them, Aspergillus fumigatus is the most prevalent and is largely responsible for the increased incidence of invasive aspergillosis (IA) in the immunocompromised patient population. IA is a devastating illness, with mortality rates in some patient groups reaching as high as 90%. Studies identifying and assessing the roles of specific factors of A. fumigatus that contribute to the pathogenesis of IA have traditionally focused on single-gene deletion and mutant characterization. In combination with recent large-scale approaches analyzing global fungal responses to distinct environmental or host conditions, these studies have identified many factors that contribute to the overall pathogenic potential of A. fumigatus. Here, we provide an overview of the significant findings regarding A. fumigatus pathogenesis as it pertains to invasive disease.
Oxylipins, a class of oxygenase-derived unsaturated fatty acids, are important signal molecules in many biological systems. Recent characterization of an Aspergillus flavus lipoxygenase gene, lox, revealed its importance in maintaining a density-dependent morphology switch from sclerotia to conidia as population density increased. Here, we present evidence for the involvement of four more oxylipin-generating dioxygenases (PpoA, PpoB, PpoC, and PpoD) in A. flavus density-dependent phenomena and the effects of loss of these genes on aflatoxin production and seed colonization. Although several single mutants showed alterations in the sclerotia-to-conidia switch, the major effect was observed in a strain downregulated for all five oxygenases (invert repeat transgene [IRT] strain IRT4 = ppoA, ppoB, ppoC, ppoD, and lox). In strain IRT4, sclerotia production was increased up to 500-fold whereas conidiation was decreased down to 100-fold and the strain was unable to switch into conidial production. Aflatoxin (AF) production for all mutant strains and the wild type was greatest at low population densities and absent in high populations except for strain IRT4, which consistently produced high levels of the mycotoxin. Growth on host seed by both IRT4 and IRT2 (downregulated in ppoA, ppoB, and ppoD) was marked by decreased conidial but increased AF production. We propose that A. flavus oxygenases and the oxylipins they produce act in a highly interdependent network with some redundancy of biological function. These studies provide substantial evidence for oxylipin-based mechanisms in governing fungus-seed interactions and in regulating a coordinated quorum-sensing mechanism in A. flavus.
Loss-of-function Aspergillus nidulans CclA, a Bre2 ortholog involved in histone H3 lysine 4 methylation, activated the expression of cryptic secondary metabolite clusters in A. nidulans. One new cluster generated monodictyphenone, emodin and emodin derivatives, whereas a second encoded two anti-osteoporosis polyketides, F9775A and F9775B. Modification of the chromatin landscape in fungal secondary metabolite clusters allows for a simple technological means to express silent fungal secondary metabolite gene clusters.
Aspergillus flavus, a mycotoxigenic filamentous fungus, colonizes several important agricultural crops, such as maize and peanuts. Two proteins, VeA and LaeA, known to form a nuclear complex in Aspergillus nidulans have been found to positively regulate developmental processes in several Aspergillus species. Here, an examination of near-isogenic A. flavus mutants differing in copy number of veA and laeA alleles (0, 1, or at least 2 each) revealed critical roles for VeA and LaeA in A. flavus development and seed colonization. In contrast to the wild type, both null mutants were unable to metabolize host cell lipid reserves and were inhibited by oleic acid in growth assays. The copy number of LaeA but not VeA appeared critical for a density-dependent sclerotial-to-conidial shift, since the multicopy laeA (MClaeA) strain produced relatively constant sclerotial numbers with increasing population size rather than showing the decrease in sclerotia seen in both the wild-type and MCveA strains. The MCveA-laeA strain yielded an intermediate phenotype. This study revealed unique roles of VeA and LaeA in seed pathogenesis and fungal biology, distinct from their cooperative regulatory functions in aflatoxin and sclerotial development.
Plant and fungal lipoxygenases (LOX) catalyze the oxidation of polyunsaturated fatty acids, creating fatty-acid hydroperoxides (oxylipins). Fungal oxylipins are required for normal fungal development and secondary metabolism, and plant host-derived oxylipins interfere with these processes in fungi, presumably by signal mimicry. The maize LOX gene ZmLOX3 has been implicated previously in seed-Aspergillus interactions, so we tested the interactions of a mutant maize line (lox3-4, in which ZmLOX3 is disrupted) with the mycotoxigenic seed-infecting fungi Aspergillus flavus and Aspergillus nidulans. The lox3-4 mutant was more susceptible than wild-type maize to both Aspergillus species. All strains of A. flavus and A. nidulans produced more conidia and aflatoxin (or the precursor sterigmatocystin) on lox3-4 kernels than on wild-type kernels, in vitro and under field conditions. Although oxylipins did not differ detectably between A. flavus-infected kernels of the lox3-4 and wild-type (WT) maize, oxylipin precursors (free fatty acids) and a downstream metabolite (jasmonic acid) accumulated to greater levels in lox3-4 than in WT kernels. The increased resistance of the lox3-4 mutant to other fungal pathogens (Fusarium, Colletotrichum, Cochliobolus, and Exserohilum spp.) is in sharp contrast to results described herein for Aspergillus spp., suggesting that outcomes of LOX-governed host-pathogen interactions are pathogen-specific.
Aspergillus species are infamous for causing several plant and animal diseases that directly (e.g., invasive aspergillosis) or indirectly (e.g., consumption of toxic food supplies) can lead to high rates of morbidity in humans and animals worldwide. Despite progress in molecular information and manipulation of Aspergillus spp., including genome sequence availability and suitable transformation methodologies, efforts to control Aspergillus diseases are still far from satisfactory, due in part to lack of knowledge of fungal virulence attributes. In order to obtain meaningful insights on the disease mechanism(s), it is essential to detect virulence gene expression during host invasion. Here, we describe two PCR-based detection methods of Aspergillus gene expression in both plant and mammalian tissues. Moreover, these techniques can be employed for routine screening of large numbers of aspergilli to improve diagnosis, disease monitoring, and therapy of fungal disease.
In most species, chromatin remodeling mediates critical biological processes ranging from development to disease states. In fungi within the genus Aspergillus, chromatin remodeling may regulate expression of metabolic gene clusters, but other processes regulated by chromatin structure remain to be elucidated. In many eukaryotic species, methylation of lysine 9 of histone 3 (H3K9) is a hallmark of heterochromatin formation and subsequent gene silencing. The sole H3K9 methyltransferase in Schizosaccharomyces pombe is Clr4. We report that disruption of the Clr4 homolog in the pathogenic mold Aspergillus fumigatus (ClrD), which is involved in both mono- and trimethylation of H3K9, results in several growth abnormalities. Developmental defects in DeltaAfclrD include reduction in radial growth, reduction in conidial production, and delayed conidiation after developmental competence mediated by delayed expression of brlA, the master regulator of conidiophore development. Sensitivity of DeltaAfclrD to 6-azauracil suggests that ClrD influences transcriptional processing in A. fumigatus. Despite growth abnormalities, macrophage assays suggest ClrD may be dispensable for host interactions.
The nuclear regulator LaeA has been shown to govern production of multiple secondary metabolites in Aspergillus nidulans and Aspergillus fumigatus. Herein we examine the role of this protein in Aspergillus flavus. Similarly as in other Aspergilli, LaeA had a major effect on A. flavus secondary metabolism where DeltalaeA and over-expression laeA (OE::laeA) strains yielded opposite phenotypes resulting in decreased (increased) secondary metabolite production. The two mutant strains also exhibited striking morphological phenotypes in the loss (increase) of sclerotial production in comparison to wildtype. Growth on seed was marked by decreased (increased) conidial and aflatoxin production of the respective mutants; this was accompanied by decreased lipase activity in DeltalaeA, an enzymatic process correlated with seed maceration. Transcriptional examination of the mutants showed LaeA negatively regulates expression of its recently identified nuclear partner VeA, another global regulator of A. flavus secondary metabolites and sclerotia.
Aspergillus flavus differentiates to produce asexual dispersing spores (conidia) or overwintering survival structures called sclerotia. Results described here show that these two processes are oppositely regulated by density-dependent mechanisms and that increasing the cell density (from 10(1) to 10(7) cells/plate) results in the lowest numbers of sclerotial and the highest numbers of conidial. Extract from spent medium of low-cell-density cultures induced a high-sclerotium-number phenotype, whereas high-cell-density extract increased conidiation. Density-dependent development is also modified by changes in lipid availability. Exogenous linoleic acid increased sclerotial production at intermediate cell densities (10(4) and 10(5) cells/plate), whereas oleic and linolenic acids inhibited sclerotium formation. Deletion of Aflox encoding a lipoxygenase (LOX) greatly diminished density-dependent development of both sclerotia and conidia, resulting in an overall increase in the number of sclerotia and a decrease in the number of conidia at high cell densities (>10(5) cells/plate). Aflox mutants showed decreased linoleic acid LOX activity. Taken together, these results suggest that there is a quorum-sensing mechanism in which a factor(s) produced in dense cultures, perhaps a LOX-derived metabolite, activates conidium formation, while a factor(s) produced in low-density cultures stimulates sclerotium formation.
Oxygenated fatty acids, or oxylipins, play an essential role in physiological signaling and developmental processes in animals, plants, and fungi. Previous characterization of three Aspergillus fumigatus dioxygenases (PpoA, PpoB, and PpoC), similar in sequence to mammalian cyclooxygenases, showed that PpoA is responsible for the production of the oxylipins 8R-hydroperoxyoctadecadienoic acid and 5S,8R-dihydroxy-9Z,12Z-octadecadienoic acid and that PpoC is responsible for 10R-hydroxy-8E,12Z-hydroperoxyoctadecadienoic acid. Here, Delta ppo mutants were characterized to elucidate the role of fungal dioxygenases in A. fumigatus development and host interactions. The Delta ppoC strain displayed distinct phenotypes compared to those of other Delta ppo mutants and the wild type, including altered conidium size, germination, and tolerance to oxidative stress as well as increased uptake and killing by primary alveolar macrophages. These experiments implicate oxylipins in pathogen development and suggest that Delta ppoC represents a useful model for studying the A. fumigatus-host interaction.
Differentiation and secondary metabolism are correlated processes in fungi that respond to light. In Aspergillus nidulans, light inhibits sexual reproduction as well as secondary metabolism. We identified the heterotrimeric velvet complex VelB/VeA/LaeA connecting light-responding developmental regulation and control of secondary metabolism. VeA, which is primarily expressed in the dark, physically interacts with VelB, which is expressed during sexual development. VeA bridges VelB to the nuclear master regulator of secondary metabolism, LaeA. Deletion of either velB or veA results in defects in both sexual fruiting-body formation and the production of secondary metabolites.
A gene, sirZ, encoding a Zn(II)(2)Cys(6) DNA binding protein is present in a cluster of genes responsible for the biosynthesis of the epipolythiodioxopiperazine (ETP) toxin, sirodesmin PL in the ascomycete plant pathogen, Leptosphaeria maculans. RNA-mediated silencing of sirZ gives rise to transformants that produce only residual amounts of sirodesmin PL and display a decrease in the transcription of several sirodesmin PL biosynthetic genes. This indicates that SirZ is a major regulator of this gene cluster. Proteins similar to SirZ are encoded in the gliotoxin biosynthetic gene cluster of Aspergillus fumigatus (gliZ) and in an ETP-like cluster in Penicillium lilacinoechinulatum (PlgliZ). Despite its high level of sequence similarity to gliZ, PlgliZ is unable to complement the gliotoxin-deficiency of a mutant of gliZ in A. fumigatus. Putative binding sites for these regulatory proteins in the promoters of genes in these clusters were predicted using bioinformatic analysis. These sites are similar to those commonly bound by other proteins with Zn(II)(2)Cys(6) DNA binding domains.
An acyl-CoA dehydrogenase has been identified as part of the mitochondrial beta-oxidation pathway in the ascomycete fungus Aspergillus nidulans. Disruption of the scdA gene prevented use of butyric acid (C(4)) and hexanoic acid (C(6)) as carbon sources and reduced cellular butyryl-CoA dehydrogenase activity by 7.5-fold. While the mutant strain exhibited wild-type levels of growth on erucic acid (C(22:1)) and oleic acid (C(18:1)), some reduction in growth was observed with myristic acid (C(14)). The DeltascdA mutation was found to be epistatic to a mutation downstream in the beta-oxidation pathway (disruption of enoyl-CoA hydratase). The DeltascdA mutant was also unable to use isoleucine or valine as a carbon source. Transcription of scdA was observed in the presence of either fatty acids or amino acids. When the mutant was grown in medium containing either isoleucine or valine, organic acid analysis of culture supernatants showed accumulation of 2-oxo acid intermediates of branched chain amino acid catabolism, suggesting feedback inhibition of the upstream branched-chain alpha-keto acid dehydrogenase.
The 8th International Mycological Congress (IMC8; Cairns, Australia) hosted several plenary lectures, poster presentations, and even entire symposia dedicated to fungal secondary metabolites (extrolites). These advances, presented in this special issue, together demonstrated how the impact of molecular biology and genomics and the availability of sophisticated methods of analytical chemistry has resulted in paradigm shifts in our understanding of fungal secondary metabolism and its key role in fungal biology. Rather than focus on classical topics such as discovery of novel drug candidates and identification of toxins, here we address two major themes in this special issue: (1) the utility and importance of secondary metabolites and their genes in polyphasic taxonomy, phylogeny, and evolutionary history of kingdom Fungi (syn. Eumycota); and (2) the genetic processes regulating secondary metabolite biosynthesis. The history of fungal chemotaxonomy and some important classes of secondary metabolites are reviewed.
Fungi are renowned for their ability to produce bioactive small molecules otherwise known as secondary metabolites. These molecules have attracted much attention due to both detrimental (e.g. toxins) and beneficial (e.g. pharmaceuticals) effects on human endeavors. Once the topic only of chemical and biochemical studies, secondary metabolism research has reached a sophisticated level in the realm of genetic regulation. This review covers the latest insights into the processes regulating secondary metabolite production in filamentous fungi.
The genus Aspergillus is ideally suited for the investigation of RNA silencing evolution because it includes species that have experienced a variety of RNA silencing gene changes. Our work on this subject begins here with the model species Aspergillus nidulans. Filamentous ascomycete fungi generally each encode two of the core RNA silencing proteins, Dicer and Argonaute, but A. nidulans appears to have lost one of each to gene truncation events. Although a role in growth, development, or RNA silencing was not detected for the truncated genes, they do produce spliced and poly(A)-tailed transcripts, suggesting that they may have an undetermined biological function. Population analysis demonstrates that the truncated genes are fixed at the species level and that their full-length orthologs in a closely related species are also unstable. With these gene truncation events, A. nidulans encodes only a single intact Dicer and Argonaute. Their deletion results in morphologically and reproductively normal strains that are incapable of experimental RNA silencing. Thus, our results suggest that the remaining A. nidulans RNA silencing genes have a "nonhousekeeping" function, such as defense against viruses and transposons.
RNA silencing can function as a virus defense mechanism in a diverse range of eukaryotes, and many viruses are capable of suppressing the silencing machinery targeting them. However, the extent to which this occurs between fungal RNA silencing and mycoviruses is unclear. Here, three Aspergillus dsRNA mycoviruses were partially characterized, and their relationship to RNA silencing was investigated. Aspergillus virus 1816 is related to Agaricus bisporus white button mushroom virus 1 and suppresses RNA silencing through a mechanism that alters the level of small interfering RNA. Aspergillus virus 178 is related to RNA virus L1 of Gremmeniella abietina and does not appear to affect RNA silencing. The third virus investigated, Aspergillus virus 341, is distantly related to Sphaeropsis sapinea RNA virus 2. Detection of mycovirus-derived siRNA from this mycovirus demonstrates that it is targeted for degradation by the Aspergillus RNA silencing machinery. Thus, our results indicate that Aspergillus mycoviruses are both targets and suppressors of RNA silencing. In addition, they suggest that the morphological and physiological changes associated with some mycoviruses could be a result of their antagonistic relationship with RNA silencing.
In Aspergilli, mycotoxin production and sporulation are governed, in part, by endogenous oxylipins (oxygenated, polyunsaturated fatty acids and metabolites derived therefrom). In Aspergillus nidulans, oxylipins are synthesized by the dioxygenase enzymes PpoA, PpoB and PpoC. Structurally similar oxylipins are synthesized in seeds via the action of lipoxygenase (LOX) enzymes. Previous reports have shown that exogenous application of seed oxylipins to Aspergillus cultures alters sporulation and mycotoxin production. Herein, we explored whether a plant oxylipin biosynthetic gene (ZmLOX3) could substitute functionally for A. nidulans ppo genes. We engineered ZmLOX3 into wild-type A. nidulans, and into a DeltappoAC strain that was reduced in production of oxylipins, conidia and the mycotoxin sterigmatocystin. ZmLOX3 expression increased production of conidia and sterigmatocystin in both backgrounds. We additionally explored whether A. nidulans oxylipins affect seed LOX gene expression during Aspergillus colonization. We observed that peanut seed pnlox2-3 expression was decreased when infected by A. nidulansDeltappo mutants compared with infection by wild type. This result provides genetic evidence that fungal oxylipins are involved in plant LOX gene expression changes, leading to possible alterations in the fungal/host interaction. This report provides the first genetic evidence for reciprocal oxylipin cross-talk in the Aspergillus-seed pathosystem.
Aspergillus sp. contain ppoA, ppoB, and ppoC genes, which code for fatty acid oxygenases with homology to fungal linoleate 7,8-diol synthases (7,8-LDS) and cyclooxygenases. Our objective was to identify these enzymes, as ppo gene replacements show critical developmental aberrancies in sporulation and pathogenicity in the human pathogen Aspergillus fumigatus and the genetic model Aspergillus nidulans. The PpoAs of A. fumigatus and A. nidulans were identified as (8R)-dioxygenases with hydroperoxide isomerase activity, designated 5,8-LDS. 5,8-LDS transformed 18:2n-6 to (8R)-hydroperoxyoctadecadienoic acid ((8R)-HPODE) and (5S,8R)-dihydroxy-9Z,12Z-octadecadienoic acid ((5S,8R)-DiHODE). We also detected 8,11-LDS in A. fumigatus and (10R)-dioxygenases in both Aspergilli. The diol synthases oxidized [(8R)-(2)H]18:2n-6 to (8R)-HPODE with retention of the deuterium label, suggesting antarafacial hydrogen abstraction and insertion of molecular oxygen. Experiments with stereospecifically deuterated 18:2n-6 showed that (8R)-HPODE was isomerized by 5,8- and 8,11-LDS to (5S,8R)-DiHODE and to (8R,11S)-dihydroxy-9Z,12Z-octadecadienoic acid, respectively, by suprafacial hydrogen abstraction and oxygen insertion at C-5 and C-11. PpoCs were identified as (10R)-dioxygenases, which catalyzed abstraction of the pro-S hydrogen at C-8 of 18:2n-6, double bond migration, and antafacial insertion of molecular oxygen with formation of (10R)-hydroxy-8E,12Z-hydroperoxyoctadecadienoic acid ((10R)-HPODE). Deletion of ppoA led to prominent reduction of (8R)-H(P)ODE and complete loss of (5S,8R)-DiHODE biosynthesis, whereas biosynthesis of (10R)-HPODE was unaffected. Deletion of ppoC caused biosynthesis of traces of racemic 10-HODE but did not affect the biosynthesis of other oxylipins. We conclude that ppoA of Aspergillus sp. may code for 5,8-LDS with catalytic similarities to 7,8-LDS and ppoC for linoleate (10R)-dioxygenases. Identification of these oxygenases and their products will provide tools for analyzing the biological impact of oxylipin biosynthesis in Aspergilli.
LaeA is a regulatory protein of the genus Aspergillus which controls global expression of secondary metabolism gene clusters. During a previous genome-wide transcriptional profiling screen, numerous novel gene clusters were found to be regulated by LaeA including a cluster required for the production of the antitumor agent terrequinone A. Beginnings and ends of secondary metabolite gene clusters are difficult to predict by gene sequence alone, but sharp demarcation of transcriptional control by LaeA suggested this protein might present a tool to identify cluster ends. To address this possibility in a first test case, we created null mutants of three genes, one regulated and two not regulated by LaeA, lying at the first, sixth and seventh position of a contiguous seven gene region where the third gene in this continuum was already shown to be required for terrequinone A production. Only the gene regulated by LaeA, tdiE, sixth in the series, was required for terrequinone A. In this pilot experiment, these findings experimentally validate LaeA-based predictions of the terrequinone biosynthetic locus boundaries. Further, this study identifies TdiE as a protein of unknown yet essential function for terrequinone A biosynthesis.
The vast repertoire of toxic fungal secondary metabolites has long been assumed to have an evolved protective role against fungivory. It still remains elusive, however, whether fungi contain these compounds as an anti-predator adaptation. We demonstrate that loss of secondary metabolites in the soil mould Aspergillus nidulans causes, under the attack of the fungivorous springtail Folsomia candida, a disadvantage to the fungus. Springtails exhibited a distinct preference for feeding on a mutant deleted for LaeA, a global regulator of Aspergillus secondary metabolites. Consumption of the mutant yielded a reproductive advantage to the arthropod but detrimental effects on fungal biomass compared with a wild-type fungus capable of producing the entire arsenal of secondary metabolites. Our results demonstrate that fungal secondary metabolites shape food choice behaviour, can affect population dynamics of fungivores, and suggest that fungivores may provide a selective force favouring secondary metabolites synthesis in fungi.
Bioactive small molecules are critical in Aspergillus species during their development and interaction with other organisms. Genes dedicated to their production are encoded in clusters that can be located throughout the genome. We show that deletion of hdaA, encoding an Aspergillus nidulans histone deacetylase (HDAC), causes transcriptional activation of two telomere-proximal gene clusters--and subsequent increased levels of the corresponding molecules (toxin and antibiotic)--but not of a telomere-distal cluster. Introduction of two additional HDAC mutant alleles in a DeltahdaA background had minimal effects on expression of the two HdaA-regulated clusters. Treatment of other fungal genera with HDAC inhibitors resulted in overproduction of several metabolites, suggesting a conserved mechanism of HDAC repression of some secondary-metabolite gene clusters. Chromatin regulation of small-molecule gene clusters may enable filamentous fungi to successfully exploit environmental resources by modifying chemical diversity.
Secondary metabolites, including toxins and melanins, have been implicated as virulence attributes in invasive aspergillosis. Although not definitively proved, this supposition is supported by the decreased virulence of an Aspergillus fumigatus strain, DeltalaeA, that is crippled in the production of numerous secondary metabolites. However, loss of a single LaeA-regulated toxin, gliotoxin, did not recapitulate the hypovirulent DeltalaeA pathotype, thus implicating other toxins whose production is governed by LaeA. Toward this end, a whole-genome comparison of the transcriptional profile of wild-type, DeltalaeA, and complemented control strains showed that genes in 13 of 22 secondary metabolite gene clusters, including several A. fumigatus-specific mycotoxin clusters, were expressed at significantly lower levels in the DeltalaeA mutant. LaeA influences the expression of at least 9.5% of the genome (943 of 9,626 genes in A. fumigatus) but positively controls expression of 20% to 40% of major classes of secondary metabolite biosynthesis genes such as nonribosomal peptide synthetases (NRPSs), polyketide synthases, and P450 monooxygenases. Tight regulation of NRPS-encoding genes was highlighted by quantitative real-time reverse-transcription PCR analysis. In addition, expression of a putative siderophore biosynthesis NRPS (NRPS2/sidE) was greatly reduced in the DeltalaeA mutant in comparison to controls under inducing iron-deficient conditions. Comparative genomic analysis showed that A. fumigatus secondary metabolite gene clusters constitute evolutionarily diverse regions that may be important for niche adaptation and virulence attributes. Our findings suggest that LaeA is a novel target for comprehensive modification of chemical diversity and pathogenicity.
We review the literature on the enzymes, genes, and whole gene clusters underlying natural product biosyntheses and their regulation in filamentous fungi. We have included literature references from 1958, yet the majority of citations are between 1995 and the present. A total of 295 references are cited.
Pathogenic microbes and their hosts have acquired complex signalling mechanisms to appraise themselves of the environmental milieu in the ongoing battle for survival. Several recent studies have implicated oxylipins as a novel class of host-microbe signalling molecules. Oxylipins represent a vast and diverse family of secondary metabolites that originate from the oxidation or further conversion of polyunsaturated fatty acids. Among the microbial oxylipins, the fungal oxylipins are best characterized and function as hormone-like signals that modulate the timing and balance between asexual and sexual spore development in addition to toxin production. Coupled with other studies that implicate a role for fungal oxylipins in pathogenesis by Aspergillus and Candida spp., these results suggest that host and microbial oxylipins might interfere with the metabolism, perception or signalling processes of each other.
Gliotoxin is a nonribosomal peptide produced by Aspergillus fumigatus. This compound has been proposed as an A. fumigatus virulence factor due to its cytotoxic, genotoxic, and apoptotic properties. Recent identification of the gliotoxin gene cluster identified several genes (gli genes) likely involved in gliotoxin production, including gliZ, encoding a putative Zn(2)Cys(6) binuclear transcription factor. Replacement of gliZ with a marker gene (DeltagliZ) resulted in no detectable gliotoxin production and loss of gene expression of other gli cluster genes. Placement of multiple copies of gliZ in the genome increased gliotoxin production. Using endpoint survival data, the DeltagliZ and a multiple-copy gliZ strain were not statistically different from the wild type in a murine pulmonary model; however, both the wild-type and the multiple-copy gliZ strain were more virulent than DeltalaeA (a mutant reduced in production of gliotoxin and other toxins). A flow-cytometric analysis of polymorphonuclear leukocytes (PMNs) exposed to supernatants from wild-type, DeltagliZ, complemented DeltagliZ, and DeltalaeA strains supported a role for gliotoxin in apoptotic but not necrotic PMN cell death. This may indicate that several secondary metabolites are involved in A. fumigatus virulence.
In contrast to most primary metabolism genes, the genes involved in secondary metabolism and certain nutrient utilization pathways are clustered in fungi. Recently a nuclear protein, LaeA, was found to be required for the transcription of several secondary metabolite gene clusters in Aspergillus nidulans. Here we show that LaeA regulation does not extend to nutrient utilization or the spoC1 sporulation clusters. One of the secondary metabolite clusters regulated by LaeA contains the positive regulatory (i.e. aflR) and biosynthetic genes required for biosynthesis of sterigmatocystin (ST), a carcinogenic toxin. Analysis of ST gene cluster expression indicates LaeA regulation of the cluster is location specific as transcription of genes bordering the ST cluster are unaffected in a DeltalaeA mutant and placement of a primary metabolic gene, argB, in the ST cluster resulted in argB silencing in the DeltalaeA background. ST cluster gene expression was remediated when an additional copy of aflR was placed outside of the cluster but not when placed in the cluster. Site-specific mutation of an s-adenosyl methionine (AdoMet) binding site in LaeA generated a DeltalaeA phenotype suggesting the protein to be a methyltransferase.
SUMMARY Mycotoxin contamination of food and feed presents a serious food safety issue on a global scale, causing tremendous yield and economic losses. These toxins, produced largely by members of the genera Aspergillus and Fusarium, represent a subset of the impressive array of secondary metabolites produced by filamentous fungi. Some secondary metabolites are associated temporally and functionally with sporulation. In Aspergillus and Fusarium, sporulation and mycotoxin production are both regulated by G protein signalling pathways. G protein signalling pathways commonly regulate fungal development, stress response and expression of virulence traits. In addition, fungal development is influenced by external factors. Among these are lipids, and in particular, oxylipin signals, which may be derived from either the fungus or infected seeds. Regardless of origin, oxylipins have the potential to elicit profound changes in both sporulation and mycotoxin production in the fungus. Signal transduction via G protein signalling pathways represents one mechanism by which oxylipin signals might elicit these changes. Therefore, in this review we integrate discussion of oxylipin signals and of G protein signalling cascades as regulators of fungal development.
Secreted, hormone-like lipogenic molecules, called oxylipins, mediate the balance of asexual to sexual spore ratio in Aspergillus nidulans. Oxylipin production in this fungus is dependent on developmental regulation of three conserved fatty acid oxygenases, PpoA, PpoB and PpoC. Here, we show that in addition to altering spore ratios, loss of ppo genes affect natural product biosynthesis and seed colonization. DeltappoA;DeltappoC and DeltappoA;DeltappoB;DeltappoC mutants were unable to produce the mycotoxin sterigmatocystin (ST) in vitro or in planta but in contrast overproduced the antibiotic penicillin (PN). These findings were correlated with decreased expression of genes involved in ST biosynthesis and increased expression of a PN biosynthetic gene, thus suggesting that oxylipin species regulate secondary metabolites at the transcriptional level. Additionally, the DeltappoA;DeltappoC and the DeltappoA;DeltappoB;DeltappoC mutants were defective in colonization of peanut seeds as reflected by a decrease in conidiation and production of the seed degradative enzyme lipase. These results indicate that oxylipin production is important for host colonization and mycotoxin production and may provide a promising target for future control strategies.
The genus Aspergillus is renowned for its ability to produce a myriad of bioactive secondary metabolites. Although the propensity of biosynthetic genes to form contiguous clusters greatly facilitates assignment of putative secondary metabolite genes in the completed Aspergillus genomes, such analysis cannot predict gene expression and, ultimately, product formation. To circumvent this deficiency, we have examined Aspergillus nidulans microarrays for expressed secondary metabolite gene clusters by using the transcriptional regulator LaeA. Deletion or overexpression of laeA clearly identified numerous secondary metabolite clusters. A gene deletion in one of the clusters eliminated the production of the antitumor compound terrequinone A, a metabolite not described, from A. nidulans. In this paper, we highlight that LaeA-based genome mining helps decipher the secondary metabolome of Aspergilli and provides an unparalleled view to assess secondary metabolism gene regulation.
Much of natural product chemistry concerns a group of compounds known as secondary metabolites. These low-molecular-weight metabolites often have potent physiological activities. Digitalis, morphine and quinine are plant secondary metabolites, whereas penicillin, cephalosporin, ergotrate and the statins are equally well known fungal secondary metabolites. Although chemically diverse, all secondary metabolites are produced by a few common biosynthetic pathways, often in conjunction with morphological development. Recent advances in molecular biology, bioinformatics and comparative genomics have revealed that the genes encoding specific fungal secondary metabolites are clustered and often located near telomeres. In this review, we address some important questions, including which evolutionary pressures led to gene clustering, why closely related species produce different profiles of secondary metabolites, and whether fungal genomics will accelerate the discovery of new pharmacologically active natural products.
Oxylipins recently have been implicated as signaling molecules for cross-kingdom communication in plant-pathogen interactions. Linoleic acid and its two plant lipoxygenase (LOX) oxylipin products 9- and 13-hydroperoxy fatty acids (9S- and 13S-HPODE) have been shown to have a significant effect on differentiation processes in the mycotoxigenic seed pathogens Aspergillus spp. Whereas both fatty acids promote sporulation, 9S-HPODE stimulates and 13S-HPODE inhibits mycotoxin production. Additionally, Aspergillus flavus infection of seed promotes linoleate 9-LOX expression and 9S-HPODE accumulation. Here, we describe the characterization of two peanut seed lipoxygenase alleles (PnLOX2 and PnLOX3) highly expressed in mature seed. PnLOX2 and PnLOX3 both are 13S-HPODE producers (linoleate 13-LOX) and, in contrast to previously characterized 9-LOX or mixed function LOX genes, are repressed between 5-fold and 250-fold over the course of A. flavus infection. The results of these studies suggest that 9S-HPODE and 13S-HPODE molecules act as putative susceptibility and resistance factors respectively, in Aspergillus seed-aflatoxin interactions.
Opportunistic animal and plant pathogens, well represented by the genus Aspergillus, have evolved unique mechanisms to adapt to and avoid host defenses. Aspergillus fumigatus, an increasingly serious pathogen owing to expanding numbers of immunocompromised patients, causes the majority of human infections; however, an inability to identify bona fide virulence factors has impeded therapeutic advances. We show that an A. fumigatus mutation in a developmentally expressed transcriptional regulator (deltalaeA) coordinating morphological and chemical differentiation reduces virulence in a murine model; impaired virulence is associated with decreased levels of pulmonary gliotoxin and multiple changes in conidial and hyphal susceptibility to host phagocytes ex vivo. LaeA, a conserved protein in filamentous fungi, is a developmental regulator of virulence genes and, possibly, the first antimicrobial target specific to filamentous fungi that are pathogenic to plants and animals.
Seed contamination with polyketide mycotoxins, including aflatoxin (AF) and sterigmatocystin (ST) produced by Aspergillus spp., is an agricultural, economic, and medical issue worldwide. Acetyl-CoA, the fundamental building block of all known fungal polyketides, is generated by a large number of biochemical pathways, including beta-oxidation of fatty acids and glycolysis of sugars. We present several lines of evidence to support a major role for seed fatty acids in formation of AF and ST in A. flavus, A. parasiticus, and A. nidulans. Aspergillus strains exhibiting canonical signs of oleic acid-induced peroxisome proliferation, including increased catalase activity, beta-oxidation gene expression, and peroxisomal clustering, also exhibited a marked increase in toxin gene expression and biosynthesis. Furthermore, microscopic observations showed that the ST and AF precursor norsolorinic acid accumulated in peroxisomes of all three Aspergilli. While a peroxisomal beta-oxidation mutation eliminated oleic acid-induced increases in ST in A. nidulans, a mitochondrial beta-oxidation mutation played a larger role in eliminating ST formation on oatmeal medium and on live corn kernels, implicating a fundamental role for both peroxisomal and mitochondrial beta-oxidation in toxin production.
Oxylipins comprise a family of oxygenated fatty acid-derived signaling molecules that initiate critical biological activities in animals, plants, and fungi. Mammalian oxylipins, including the prostaglandins (PGs), mediate many immune and inflammation responses in animals. PG production by pathogenic microbes is theorized to play a role in pathogenesis. We have genetically characterized three Aspergillus genes, ppoA, ppoB, and ppoC, encoding fatty acid oxygenases similar in sequence to specific mammalian prostaglandin synthases, the cyclooxygenases. Enzyme-linked immunosorbent assay analysis showed that production of PG species is decreased in both Aspergillus nidulans and A. fumigatus ppo mutants, implicating Ppo activity in generating PGs. The A. fumigatus triple-ppo-silenced mutant was hypervirulent in the invasive pulmonary aspergillosis murine model system and showed increased tolerance to H(2)O(2) stress relative to that of the wild type. We propose that Ppo products, PG, and/or other oxylipins may serve as activators of mammalian immune responses contributing to enhanced resistance to opportunistic fungi and as factors that modulate fungal development contributing to resistance to host defenses.
Mycotoxins are natural fungal products that are defined by their harmful effects on humans and animals. Aflatoxin contamination of maize by Aspergillus species and trichothecene contamination of small grains by Fusarium species are two of the most severe mycotoxin problems in the United States. We are investigating RNA silencing in an effort to identify novel ways to control mycotoxin contamination of crops. Transformation of two Aspergilli (A. flavus and A. parasiticus) and a Fusarium (F. graminearum) with inverted repeat transgenes (IRT) containing sequences of mycotoxin-specific regulatory genes suppressed mycotoxin production in all three plant-pathogenic fungi. This atoxigenic phenotype was stable during infection on corn and wheat, and importantly, F. graminearum IRT strains were less virulent on wheat than were wild type. The IRT did not alter physiological characteristics of the fungi, such as spore production and growth rate on solid media. These results indicate that RNA silencing exists in Aspergillus and Fusarium plant pathogens and suggest that RNA silencing technology may be a useful tool for eliminating mycotoxin contamination of agricultural products.
Oxylipins called psi factors have been shown to alter the ratio of asexual to sexual sporulation in the filamentous fungus Aspergillus nidulans. Analysis of the A. nidulans genome has led to the identification of three fatty acid oxygenases (PpoA, PpoB and PpoC) predicted to produce psi factors. Here, it is reported that deletion of ppoB (DeltappoB) reduced production of the oleic-acid-derived oxylipin psiBbeta and increased the ratio of asexual to sexual spore development. Generation of the triple mutant Delta ppoA Delta ppoB Delta ppoC resulted in a strain deficient in producing oleic- and linoleic-acid-derived 8'-hydroxy psi factor and caused increased and mis-scheduled activation of sexual development. Changes in asexual to sexual spore development were positively correlated to alterations in the expression of brlA and veA, respectively. PpoB and/or its products antagonistically mediate the expression levels of ppoA and ppoC, thus revealing regulatory feedback loops among these three genes. Phylogenetic analyses showed that ppo genes are present in both saprophytic and pathogenic Ascomycetes and Basidiomycetes, suggesting a conserved role for Ppo enzymes in the life cycle of fungi.
The versatility of RNA-dependent RNA polymerases (RDRPs) in eukaryotic gene silencing is perhaps best illustrated in the kingdom Fungi. Biochemical and genetic studies of Schizosaccharomyces pombe and Neurospora crassa show that these types of enzymes are involved in a number of fundamental gene-silencing processes, including heterochromatin regulation and RNA silencing in S. pombe and meiotic silencing and RNA silencing in N. crassa. Here we show that Aspergillus nidulans, another model fungus, does not require an RDRP for inverted repeat transgene (IRT)-induced RNA silencing. However, RDRP requirements may vary within the Aspergillus genus as genomic analysis indicates that A. nidulans, but not A. fumigatus or A. oryzae, has lost a QDE-1 ortholog, an RDRP associated with RNA silencing in N. crassa. We also provide evidence suggesting that 5' --> 3' transitive RNA silencing is not a significant aspect of A. nidulans IRT-RNA silencing. These results indicate a lack of conserved kingdom-wide requirements for RDRPs in fungal RNA silencing.
The ability of fungi to produce both meiospores and mitospores has provided adaptive advantages in survival and dispersal of these organisms. Here we provide evidence of an endogenous mechanism that balances meiospore and mitospore production in the model filamentous fungus Aspergillus nidulans. We have discovered a putative dioxygenase, PpoC, that functions in association with a previously characterized dioxygenase, PpoA, to integrate fatty acid derived oxylipin and spore production. In contrast to PpoA, deletion of ppoC significantly increased meiospore production and decreased mitospore development. Examination of the PpoA and PpoC mutants indicate that this ratio control is associated with two apparent feedback loops. The first loop shows ppoC and ppoA expression is dependent upon, and regulates the expression of, nsdD and brlA, genes encoding transcription factors required for meiospore or mitospore production, respectively. The second loop suggests Ppo oxylipin products antagonistically signal the generation of Ppo substrates. These data support a case for a fungal "oxylipin signature-profile" indicative of relative sexual and asexual spore differentiation.
Beta-oxidation (beta-ox) occurs exclusively in the peroxisomes of Saccharomyces cerevisiae and other yeasts, leading to the supposition that fungi lack mitochondrial beta-ox. Here we present unequivocal evidence that the filamentous fungus Aspergillus nidulans houses both peroxisomal and mitochondrial beta-ox. While growth of a peroxisomal beta-ox disruption mutant (DeltafoxA) was eliminated on a very long-chain fatty acid (C(22:1)), growth was only partially impeded on a long-chain fatty acid (C(18:1)) and was not affected at all on short chain (C4-C6) fatty acids. In contrast, growth of a putative enoyl-CoA hydratase mutant (DeltaechA) was abolished on short-chain and severely restricted on long- and very long-chain fatty acids. Furthermore fatty acids inhibited growth of the DeltaechA mutant but not the DeltafoxA mutant in the presence of an alternate carbon source (lactose). Disruption of echA led to a 28-fold reduction in 2-butenoyl-CoA hydratase activity in a preparation of organelles. EchA was also required for growth on isoleucine and valine. The subcellular localization of the FoxA and EchA proteins was confirmed through the use of red and green fluorescent protein fusions.
To investigate how linoleic acid affects conidial production and sclerotial development in a strictly mitotic Aspergillus parasiticus field isolate as related to improving biocompetitivity of atoxigenic Aspergillus species. We disrupted A. parasiticusDelta12-oleic acid desaturase gene (odeA) responsible for the conversion of oleic acid to linoleic acid. We examined conidiation and sclerotial development of SRRC 2043 and three isogenic mutant strains deleted for the odeA gene (DeltaodeA), either with or without supplementing linoleic acid, on one complex potato dextrose agar (PDA) medium and on two defined media: nitrate-containing Czapek agar (CZ) and Cove's ammonium medium (CVN). The DeltaodeA mutants produced less conidia than the parental strain on all media. Linoleic acid supplementation (as sodium linoleate at 0.3 and 1.2 mg ml(-1)) restored the DeltaodeA conidial production comparable to or exceeding the unsupplemented parental level, and the effect was medium dependent, with the highest increase on CVN and the least on PDA. SRRC 2043 and the DeltaodeA mutants were unable to produce sclerotia on CVN. On unsupplemented PDA and CZ, DeltaodeA sclerotial mass was comparable to that of SRRC 2043, but sclerotial number increased significantly to two- to threefold. Supplementing linoleic acid to media, in general, tended to decrease wild type and DeltaodeA sclerotial mass and sclerotial number. Linoleic acid stimulates conidial production but has an inhibitory effect on sclerotial development. The relationship between the two processes in A. parasiticus is complex and affected by multiple factors, such as fatty acid composition and nitrogen source. Conditions that promote sclerotial development differ from those required to promote maximum conidial production. Manipulation of content and availability of linoleic acid at different fungal growth phases might optimize conidial and sclerotial production hence increasing the efficacy of biocompetitive Aspergillus species.
Propionyl-CoA is an intermediate metabolite produced through a variety of pathways including thioesterification of propionate and catabolism of odd chain fatty acids and select amino acids. Previously, we found that disruption of the methylcitrate synthase gene, mcsA, which blocks propionyl-CoA utilization, as well as growth on propionate impaired production of several polyketides-molecules typically derived from acetyl-CoA and malonyl-CoA-including sterigmatocystin (ST), a potent carcinogen, and the conidiospore pigment. Here we describe three lines of evidence that demonstrate that excessive propionyl-CoA levels in the cell can inhibit polyketide synthesis. First, inactivation of a putative propionyl-CoA synthase, PcsA, which converts propionate to propionyl-CoA, restored polyketide production and reduced cellular propionyl-CoA content in a DeltamcsA background. Second, inactivation of the acetyl-CoA synthase, FacA, which is also involved in propionate utilization, restored polyketide production in the DeltamcsA background. Third, fungal growth on several compounds (e.g., heptadecanoic acid, isoleucine, and methionine) whose catabolism includes the formation of propionyl-CoA, were found to inhibit ST and conidiospore pigment production. These results demonstrate that excessive propionyl-CoA levels in the cell can inhibit polyketide synthesis.
In the mycotoxigenic oilseed pathogens Aspergillus flavus and Aspergillus parasiticus and the model filamentous fungus Aspergillus nidulans, unsaturated fatty acids and their derivatives act as important developmental signals that affect asexual conidiospore, sexual ascospore and/or sclerotial development. To dissect the relationship between lipid metabolism and fungal development, an A. parasiticus Delta(12)-desaturase mutant that was unable to convert oleic acid to linoleic acid and was thus impaired in polyunsaturated fatty acid biosynthesis was generated. The Delta(12)-desaturase mutant demonstrates delayed spore germination, a twofold reduction in growth, a reduced level of conidiation and complete loss of sclerotial development, compared to the wild-type. Host colonization is impaired, as reflected by a decrease in conidial production on live peanut and corn seed by the mutant compared to the wild-type. Similarly, the previously isolated A. nidulans Delta(12)-desaturase mutant has reduced colonization capabilities compared to the wild-type. Therefore, desaturation mutants display a key requisite that affords a genetic solution to oilseed crop contamination by mycotoxigenic Aspergillus species: a reduction in the production of conidia, the infectious particle of the pathogenic aspergilli.
Analysis of fadA and pkaA mutants in the filamentous fungus Aspergillus nidulans demonstrated that FadA (Galpha) stimulates cyclic AMP (cAMP)-dependent protein kinase A (PKA) activity resulting, at least in part, in inhibition of conidiation and sterigmatocystin (ST) biosynthesis. In contrast, cAMP added to the growth medium stimulates aflatoxin (AF) synthesis in Aspergillus parasiticus. Our goal was to explain these conflicting reports and to provide mechanistic detail on the role of FadA, cAMP, and PKA in regulation of AF synthesis and conidiation in A. parasiticus. cAMP or dibutyryl-cAMP (DcAMP) were added to a solid growth medium and intracellular cyclic nucleotide levels, PKA activity, and nor-1 promoter activity were measured in A. parasiticus D8D3 (nor1::GUS reporter) and TJYP1-22 (fadAGA2R, activated allele). Similar to Tice and Buchanan [34], cAMP or DcAMP stimulated AF synthesis (and conidiation) associated with an AflR-dependent increase in nor-1 promoter activity. However, treatment resulted in a 100-fold increase in intracellular cAMP/DcAMP accompanied by a 40 to 80 fold decrease in total PKA activity. ThefadAG42R allele in TJYP1-22 decreased AF synthesis and conidiation, increased basal PKA activity 10 fold, and decreased total PKA activity 2 fold. In TJYP1-22, intracellular cAMP increased 2 fold without cAMP or DcAMP treatment; treatment did not stimulate conidiation or AF synthesis. Based on these data, we conclude that: (1) FadA/PKA regulate toxin synthesis and conidiation via similar mechanisms in Aspergillus spp.; and (2) intracellular cAMP levels, at least in part, mediate a PKA-dependent regulatory influence on conidiation and AF synthesis.
It was long been noted that secondary metabolism is associated with fungal development. In Aspergillus nidulans, conidiation and mycotoxin production are linked by a G protein signaling pathway. Also in A. nidulans, cleistothecial development and mycotoxin production are controlled by a gene called veA. Here we report the characterization of a veA ortholog in the aflatoxin-producing fungus A. parasiticus. Cleistothecia are not produced by Aspergillus parasiticus; instead, this fungus produces spherical structures called sclerotia that allow for survival under adverse conditions. Deletion of veA from A. parasiticus resulted in the blockage of sclerotial formation as well as a blockage in the production of aflatoxin intermediates. Our results indicate that A. parasiticus veA is required for the expression of aflR and aflJ, which regulate the activation of the aflatoxin gene cluster. In addition to these findings, we observed that deletion of veA reduced conidiation both on the culture medium and on peanut seed. The fact that veA is necessary for conidiation, production of resistant structures, and aflatoxin biosynthesis makes veA a good candidate gene to control aflatoxin biosynthesis or fungal development and in this way to greatly decrease its devastating impact on health and the economy.
Unsaturated fatty acids are important constituents of all cell membranes and are required for normal growth. In the filamentous fungus Aspergillus nidulans, unsaturated fatty acids and their derivatives also influence asexual (conidial) and sexual (ascospore) sporulation processes. To investigate the relationship between fatty acid metabolism and fungal development, we disrupted the A. nidulans sdeA and sdeB genes, both encoding Delta9-stearic acid desaturases responsible for the conversion of palmitic acid (16:0) and stearic acid (18:0) to palmitoleic acid (16:1) and oleic acid (18:1). The effects of sdeA deletion on development were profound, such that growth, conidial and ascospore production were all reduced at 22 and 37 degrees C. Total fatty acid content was increased over 3-fold in the DeltasdeA strain, reflected in up-regulation of the expression of the fasA gene encoding the alpha chain of the fatty acid synthase, compared to wild type. Stearic acid accumulated approximately 3-fold compared to wild type in the DeltasdeA strain, while unsaturated fatty acid production was decreased. In contrast, disruption of sdeB reduced fungal growth and conidiation at 22 degrees C, but did not affect these processes at 37 degrees C compared to wild type. Interestingly, ascospore production was increased at 37 degrees C for DeltasdeB compared to wild type. Total fatty acid content was not increased in this strain, although stearic acid accumulated 2-fold compared to wild type, and unsaturated fatty acid production was decreased. Combining the DeltasdeA and DeltasdeB alleles created a synthetic lethal strain requiring the addition of oleic acid to the medium for a modicum of growth. Taken together, our results suggest a role for sdeA in growth and development at all temperatures, while sdeB is involved in growth and development at lower temperatures.
Secondary metabolites, or biochemical indicators of fungal development, are of intense interest to humankind due to their pharmaceutical and/or toxic properties. We present here a novel Aspergillus nuclear protein, LaeA, as a global regulator of secondary metabolism in this genus. Deletion of laeA (DeltalaeA) blocks the expression of metabolic gene clusters, including the sterigmatocystin (carcinogen), penicillin (antibiotic), and lovastatin (antihypercholesterolemic agent) gene clusters. Conversely, overexpression of laeA triggers increased penicillin and lovastatin gene transcription and subsequent product formation. laeA expression is negatively regulated by AflR, a sterigmatocystin Zn2Cys6 transcription factor, in a unique feedback loop, as well as by two signal transduction elements, protein kinase A and RasA. Although these last two proteins also negatively regulate sporulation, DeltalaeA strains show little difference in spore production compared to the wild type, indicating that the primary role of LaeA is to regulate metabolic gene clusters.
Aspergillus nidulans produces the polyketide toxin sterigmatocystin (ST) of which the biosynthetic and pathway specific regulatory genes compose a stc gene cluster. A previous mutagenesis screen identified 23 mutants defective in production of ST. Five mutants constitute a single locus. Genetic complementation and sequencing analysis revealed the mutant locus to be mcsA encoding methylcitrate synthase that converts propionyl-CoA to methylcitrate. Feeding downstream products of methylcitrate synthase, methylcitrate and pyruvate, did not restore ST production in mcsA mutants, indicating that loss of methylcitrate cycle products is not the cause of the ST defect. However, propionate, a precursor for propionyl-CoA, inhibited ST production and induced transcription of mcsA in the wild type. Furthermore, propionate impaired formation of two polyketide spore pigments whereas overexpression of mcsA relieved inhibition of ST production by propionate. Transcription analyses revealed that disruption of mcsA did not affect expression of the specialized fatty acid synthase genes (stcJ and stcK) or polyketide synthase gene (stcA) required for formation of norsolorinic acid (NOR), the first stable intermediate in the ST biosynthetic pathway. Feeding studies showed that NOR but not hexanoic acid (the fatty acid produced by StcJ/StcK and primer unit of StcA) or malonate (source of the extender unit of StcA) restored ST production in the mcsA mutant. We hypothesize that excess buildup of propionyl-CoA in mcsA mutants interferes with polyketide synthase activity.
The coexistence of sexual and asexual reproductive cycles within the same individual is a striking phenomenon in numerous fungi. In the fungus Aspergillus nidulans (teleomorph: Emericella nidulans) endogenous oxylipins, called psi factor, serve as hormone-like signals that modulate the timing and balance between sexual and asexual spore development. Here, we report the identification of A. nidulans ppoA, encoding a putative fatty acid dioxygenase, involved in the biosynthesis of the linoleic acid derived oxylipin psiBalpha. PpoA is required for balancing anamorph and teleomorph development. Deletion of ppoA significantly reduced the level of psiBalpha and increased the ratio of asexual to sexual spore numbers 4-fold. In contrast, forced expression of ppoA resulted in elevated levels of psiBalpha and decreased the ratio of asexual to sexual spore numbers 6-fold. ppoA expression is mediated by two developmental regulators, VeA and the COP9 signalosome, such that ppoA transcript levels are correlated with the initiation of asexual and sexual fruiting body formation. PpoA localizes in lipid bodies in these tissues. These data support an important role for oxylipins in integrating mitotic and meiotic spore development.
Sterigmatocystin (ST) is a carcinogenic polyketide produced by several filamentous fungi including Aspergillus nidulans. Expression of ST biosynthetic genes (stc genes) requires activity of a Zn(II)2Cys6 transcription factor, AflR. aflR is transcriptionally and post-transcriptionally regulated by a G-protein/cAMP/protein kinase A (PkaA) signaling pathway involving FlbA, an RGS (regulator of G-protein signaling) protein. Prior genetic data showed that FlbA transcriptional regulation of aflR was PkaA dependent. Here we show that mutation of three PkaA phosphorylation sites in AflR allows resumption of stc expression in an overexpression pkaA background but does not remediate stc expression in a deltaflbA background. This demonstrates negative regulation of AflR activity by phosphorylation and shows that FlbA post-transcriptional regulation of aflR is PkaA independent. AflR nucleocytoplasmic location further supports PkaA-independent regulation of AflR by FlbA. GFP-tagged AflR is localized to the cytoplasm when pkaA is overexpressed but nuclearly located in a deltaflbA background. aflR is also transcriptionally and post-transcriptionally regulated by RasA. RasA transcriptional control of aflR is PkaA independent but RasA post-transcriptional control of AflR is partially mediated by PkaA.
Deletion of the spermidine synthase gene in the fungus Aspergillus nidulans results in a strain, deltaspdA, which requires spermidine for growth and accumulates putrescine as the sole polyamine. Vegetative growth but not sporulation or sterigmatocystin production is observed when deltaspdA is grown on media supplemented with 0.05-0.10 mM exogenous spermidine. Supplementation of deltaspdA with >/= 0.10 mM spermidine restores sterigmatocystin production and >/= 0.50 mM spermidine produces a phenotype with denser asexual spore production and decreased radial hyphal growth compared with the wild type. DeltaspdA spores germinate in unsupplemented media but germ tube growth ceases after 8 h upon which time the spores swell to approximately three times their normal diameter. Hyphal growth is resumed upon addition of 1.0 mM spermidine. Suppression of a G protein signalling pathway could not force asexual sporulation and sterigmatocystin production in deltaspdA strains grown in media lacking spermidine but could force both processes in deltaspdA strains supplemented with 0.05 mM spermidine. These results show that increasing levels of spermidine are required for the transitions from (i) germ tube to hyphal growth and (ii) hyphal growth to tissue differentiation and secondary metabolism. Suppression of G protein signalling can over-ride the spermidine requirement for the latter but not the former transition.
Filamentous fungi are unique organisms-rivaled only by actinomycetes and plants-in producing a wide range of natural products called secondary metabolites. These compounds are very diverse in structure and perform functions that are not always known. However, most secondary metabolites are produced after the fungus has completed its initial growth phase and is beginning a stage of development represented by the formation of spores. In this review, we describe secondary metabolites produced by fungi that act as sporogenic factors to influence fungal development, are required for spore viability, or are produced at a time in the life cycle that coincides with development. We describe environmental and genetic factors that can influence the production of secondary metabolites. In the case of the filamentous fungus Aspergillus nidulans, we review the only described work that genetically links the sporulation of this fungus to the production of the mycotoxin sterigmatocystin through a shared G-protein signaling pathway.
Maize kernels are highly susceptible to Aspergillus spp. infection and aflatoxin (AF) contamination. Fatty acid signaling molecules appear to mediate the plant-fungal interaction by affecting the growth, development, and AF production of the fungus. In particular, fatty acid derivatives of the plant lipoxygenase (LOX) pathway are implicated in the Aspergillus spp.-seed interaction. The 9(S)-hydroperoxide derivative of linoleic acid promotes transcription of AF genes, whereas the 13(S)-hydroperoxide derivative decreases AF gene expression and production; both are sporulation factors. Our goal was to identify LOX genes responsive to Aspergillus spp. colonization and determine their specificities, 9(S)- or 13(S)-. Screening maize LOX expressed sequence tags (ESTs) identified one clone, cssap 92, which is highly expressed in Aspergillus spp.-infected seed susceptible to AF contamination and repressed in lines with resistance to AF contamination. The accumulation of cssap 92 transcript was similar during Fusarium spp. infection. The cDNA clone has 94% identity to the previously described L2 LOX gene from maize. Product-specificity analysis of the CSSAP 92 protein shows that it preferentially adds oxygen to carbon 9 of linoleic acid. Because 9(S)-hydroperoxy linoleic acid has been implicated as an aflatoxin-signaling molecule, it is possible that cssap 92 could be used as a biomarker that is indicative of AF resistance in maize lines.
In the Ascomycete fungus Aspergillus nidulans, the ratio of conidia (asexual spores) to ascospores (sexual spores) is affected by linoleic acid moieties including endogenous sporogenic factors called psi factors. Deletion of odeA (Delta odeA), encoding a Delta-12 desaturase that converts oleic acid to linoleic acid, resulted in a strain depleted of polyunsaturated fatty acids (18:2 and 18:3) but increased in oleic acid (18:1) and total percent fatty acid content. Linoleic acid-derived psi factors were absent in this strain but oleic acid-derived psi factors were increased relative to wild type. The Delta odeA strain was reduced in conidial production and mycelial growth; these effects were most noticeable when cultures were grown at 26 degrees C in the dark. Under these environmental conditions, the Delta odeA strain was delayed in ascospore production but produced more ascospores than wild type over time. This suggests a role for oleic acid-derived psi factors in affecting the asexual to sexual spore ratio in A. nidulans. Fatty acid composition and spore development were also affected by veA, a gene previously shown to control light driven conidial and ascospore development. Taken together our results indicate an interaction between veA and odeA alleles for fatty acid metabolism and spore development in A. nidulans.
In the filamentous fungus Aspergillus nidulans, a heterotrimeric G protein alpha-subunit and an RGS domain protein, encoded by fadA and flbA, respectively, regulate production of the carcinogenic metabolite sterigmatocystin (ST) and asexual spores (i.e., conidia). We investigated the genetic involvement of the cAMP-dependent protein kinase catalytic subunit (PkaA), a potential downstream target of FadA activity, in ST production and conidiation. Relative to wild type, sporulation was decreased in the pkaA overexpression strain but was not totally absent, as occurs in DeltaflbA or fadA(G42R) (fadA-dominant active) strains. Deletion of pkaA resulted in a hyper-conidiating strain with limited radial growth. This phenotype was epistatic to mutation in flbA or fadA; the double mutants DeltapkaA; DeltaflbA and DeltapkaA; fadA(G42R) recovered sporulation and their radial growth was severely restricted. PkaA overexpression also negatively regulated AflR, the ST biosynthesis-specific transcription factor, both transcriptionally and post-transcriptionally. Deletion of pkaA restored ST production in the DeltaflbA background but not in the fadA(G42R) background. These data provide genetic evidence that the FlbA/FadA signaling pathway regulating ST production and morphological development is partially mediated through PkaA.
Echinocandin B (ECB), a lipopolypeptide used as a starting material for chemical manufacture of the anti-Candida agent LY303366, is produced by fermentation using a strain of Aspergillus nidulans. In addition to ECB, the wild-type strain also produces a significant level of sterigmatocystin (ST), a potent carcinogen structurally related to the aflatoxins. Characterization of a mutant designated A42355-OC-1 (OC-1), which is blocked in ST biosynthesis, was the result of a chromosomal translocation. The chromosomal regions containing the breakpoints of the translocation were isolated and DNA sequencing and PCR analysis of the chromosomal breakpoints demonstrated the translocation occurred within the stcW gene of the ST biosynthetic pathway, resulting in disruption of the open reading frame for this gene. Biochemical feeding studies indicate the involvement of this gene product in the conversion of averufin to 1-hydroxy versicolorone. This work demonstrates an effective synergy between classical strain improvement methods and molecular genetics.
Several lines of evidence have indicated that lipoxygenase enzymes (LOX) and their products, especially 9S- and 13S-hydroperoxy fatty acids, could play a role in the Aspergillus/seed interaction. Both hydroperoxides exhibit sporogenic effects on Aspergillus spp. (Calvo, A., Hinze, L., Gardner, H.W. and Keller, N.P. 1999. Appl. Environ. Microbiol. 65: 3668-3673) and differentially modulate aflatoxin pathway gene transcription (Burow, G.B., Nesbitt, T.C., Dunlap, J. and Keller, N.P. 1997. Mol. Plant-Microbe Interact. 10: 380-387). To examine the role of seed LOXs at the molecular level, a peanut (Arachis hypogaea L.) seed gene, PnLOX1, was cloned and characterized. Analysis of nucleotide sequence suggests that PnLOX1 encodes a predicted 98 kDa protein highly similar in sequence and biochemical properties to soybean LOX2. The full-length PnLOX1 cDNA was subcloned into an expression vector to determine the type(s) of hydroperoxide products the enzyme produces. Analysis of the oxidation products of PnLOX1 revealed that it produced a mixture of 30% 9S-HPODE (9S-hydroperoxy-10E, 12Z-octadecadienoic acid) and 70% 13S-HPODE (13S-hydroperoxy-9Z, 11E-octadecadienoic acid) at pH 7. PnLOX1 is an organ-specific gene which is constitutively expressed in immature cotyledons but is highly induced by methyl jasmonate, wounding and Aspergillus infections in mature cotyledons. Examination of HPODE production in infected cotyledons suggests PnLOX1 expression may lead to an increase in 9S-HPODE in the seed.
Sterigmatocystin (ST) and aflatoxin B(1) (AFB(1)) are two polyketide-derived Aspergillus mycotoxins synthesized by functionally identical sets of enzymes. ST, the compound produced by Aspergillus nidulans, is a late intermediate in the AFB(1) pathway of A. parasiticus and A. flavus. Previous biochemical studies predicted that five oxygenase steps are required for the formation of ST. A 60-kb ST gene cluster in A. nidulans contains five genes, stcB, stcF, stcL, stcS, and stcW, encoding putative monooxygenase activities. Prior research showed that stcL and stcS mutants accumulated versicolorins B and A, respectively. We now show that strains disrupted at stcF, encoding a P-450 monooxygenase similar to A. parasiticus avnA, accumulate averantin. Disruption of either StcB (a putative P-450 monooxygenase) or StcW (a putative flavin-requiring monooxygenase) led to the accumulation of averufin as determined by radiolabeled feeding and extraction studies.
The genes involved in the biosynthesis of sterigmatocystin (ST), a toxic secondary metabolite produced by Aspergillus nidulans and an aflatoxin (AF) precursor in other Aspergillus spp., are clustered on chromosome IV of A. nidulans. The sterigmatocystin gene cluster (stc gene cluster) is regulated by the pathway-specific transcription factor aflR. The function of aflR appears to be conserved between ST- and AF-producing aspergilli, as are most of the other genes in the cluster. We describe a novel screen for detecting mutants defective in stc gene cluster activity by use of a genetic block early in the ST biosynthetic pathway that results in the accumulation of the first stable intermediate, norsolorinic acid (NOR), an orange-colored compound visible with the unaided eye. We have mutagenized this NOR-accumulating strain and have isolated 176 Nor(-) mutants, 83 of which appear to be wild type in growth and development. Sixty of these 83 mutations are linked to the stc gene cluster and are likely defects in aflR or known stc biosynthetic genes. Of the 23 mutations not linked to the stc gene cluster, 3 prevent accumulation of NOR due to the loss of aflR expression.
Aspergillus spp. are frequently occurring seed-colonizing fungi that complete their disease cycles through the development of asexual spores, which function as inocula, and through the formation of cleistothecia and sclerotia. We found that development of all three of these structures in Aspergillus nidulans, Aspergillus flavus, and Aspergillus parasiticus is affected by linoleic acid and light. The specific morphological effects of linoleic acid include induction of precocious and increased asexual spore development in A. flavus and A. parasiticus strains and altered sclerotium production in some A. flavus strains in which sclerotium production decreases in the light but increases in the dark. In A. nidulans, both asexual spore production and sexual spore production were altered by linoleic acid. Spore development was induced in all three species by hydroperoxylinoleic acids, which are linoleic acid derivatives that are produced during fungal colonization of seeds. The sporogenic effects of these linoleic compounds on A. nidulans are similar to the sporogenic effects of A. nidulans psi factor, an endogenous mixture of hydroxylinoleic acid moieties. Light treatments also significantly increased asexual spore production in all three species. The sporogenic effects of light, linoleic acid, and linoleic acid derivatives on A. nidulans required an intact veA gene. The sporogenic effects of light and linoleic acid on Aspergillus spp., as well as members of other fungal genera, suggest that these factors may be significant environmental signals for fungal development.
The Aspergillus nidulans aflR gene is found within a 60 kb gene cluster that includes approximately 24 other genes that putatively function in the production of the aflatoxin-related mycotoxin sterigmatocystin. Previous work showed that AflR is a C6 zinc binuclear cluster protein that is conserved across Aspergillus spp. and functions as a pathway-specific transcription factor in activating expression of other cluster genes. In this report, we demonstrate that A. nidulans AflR (AnAflR) is a 45kDa protein that binds to the palindromic sequence 5'-TCG(N5)CGA-3' found in the promoter regions of several aflatoxin and sterigmatocystin cluster genes (stc genes). The in vivo relevance of this AnAflR binding site was assessed by examining the contribution of the three TCG(N5)CGA elements in the 1.1 kb promoter region of stcU using gene fusions with the bacterial uidA gene encoding beta-glucuronidase (GUS). By mutating one, two or all three of the AnAflR-binding elements and examining GUS activity in wild-type aflR or delta aflR A. nidulans strains, we found that stc gene activation required both AnAflR and at least one TCG(N5)CGA AflR binding site.
The filamentous fungus Aspergillus nidulans contains a cluster of 25 genes that encode enzymes required to synthesize a toxic and carcinogenic secondary metabolite called sterigmatocystin (ST), a precursor of the better known fungal toxin aflatoxin (AF). One ST Cluster (stc) gene, aflR, functions as a pathway-specific transcriptional regulator for activation of other genes in the ST pathway. However, the mechanisms controlling activation of aflR and synthesis of ST and AF are not understood. Here we show that one important level for control of stc gene expression requires genes that were first identified as early acting regulators of asexual sporulation. Specifically, we found that loss-of-function mutations in flbA, which encodes a RGS domain protein, or dominant activating mutations in fadA, which encodes the alpha subunit of a heterotrimeric G protein, block both ST production and asexual sporulation. Moreover, overexpression of flbA or dominant interfering fadA mutations cause precocious stc gene expression and ST accumulation, as well as unscheduled sporulation. The requirement for flbA in sporulation and ST production could be suppressed by loss-of-function fadA mutations. The ability of flbA to activate stc gene expression was dependent upon another early acting developmental regulator, fluG, and AflR, the stc gene-specific transcription factor. These results are consistent with a model in which both asexual sporulation and ST production require inactivation of proliferative growth through inhibition of FadA-dependent signaling. This regulatory mechanism is conserved in AF-producing fungi and could therefore provide a means of controlling AF contamination.
ABSTRACT Aflatoxin (AF) and sterigmatocystin (ST) are toxic secondary metabolites produced by the same biochemical pathway found in several Aspergillus spp. The expression of the homologous ST/AF structural gene, stcU in A. nidulans and ver-1 in A. parasiticus, was affected by external pH of liquid growth media. Both stcU and ver-1 mRNAs appeared earlier and were expressed at higher levels in cultures grown in acidic media (pH 4 to 6) versus neutral (pH 7) and alkali (pH 8) media. Transcript levels correlated with ST/AF production. Visual and spectrophotometric analysis of production of the orange ST/AF intermediate, norsolorinic acid (NOR), also paralleled transcript patterns and indicated that the pH effects were operative in different nitrogen- and carbon-based solid growth media. Five- to 10-fold increases in ST, AF, and NOR were measured in cultures grown in pH 4 or 5 versus pH 8 media. An A. nidulans strain carrying a mutation resulting in constitutive activity of the pH regulatory factor, PacC, produced 10-fold less ST than did wild type. The stcU transcript was not noticeably affected by pH in this strain. The results suggest a general pattern of pH regulation of ST/AF biosynthesis that may override previously noted carbon and nitrogen effects.
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The Aspergillus nidulans stcL gene is predicted to encode a cytochrome P-450 monooxygenase and is located within a cluster of other genes that are required for synthesis of sterigmatocystin. Inactivation of stcL resulted in strains that accumulate dihydrosterigmatocystin, a tetrahydrobisfuran containing molecule that is very similar to the unsaturated product of the wild-type pathway, sterigmatocystin. This observation led us to hypothesize that the A. nidulans sterigmatocystin biosynthetic pathway is branched similarly to the aflatoxin pathway in Aspergillus parasiticus and Aspergillus flavus and that StcL is required for the desaturation of the bisfuran moiety in the sterigmatocystin/aflatoxin precursor versicolorin B. This prediction was confirmed by feeding the stcL mutant with the subsequent pathway intermediate, versicolorin A, which resulted in accumulation of both sterigmatocystin and dihydrosterigmatocystin, indicating that StcL functions before versicolorin A synthesis. A. nidulans stcU was shown previously to encode a ketoreductase required to convert versicolorin A to demethylsterigmatocystin and an stcL, stcU double mutant strain was shown here to accumulate only versicolorin B. These results indicate that both versicolorin A and versicolorin B can serve as substrates for StcU, resulting in a branched pathway. The final product of each branch are sterigmatocystin and dihydrosterigmatocystin, respectively.
Aspergillus nidulans contains two functionally distinct fatty acid synthases (FASs): one required for primary fatty acid metabolism (FAS) and the other required for secondary metabolism (sFAS). FAS mutants require long-chain fatty acids for growth, whereas sFAS mutants grow normally but cannot synthesize sterigmatocystin (ST), a carcinogenic secondary metabolite structurally and biosynthetically related to aflatoxin. sFAS mutants regain the ability to synthesize ST when provided with hexanoic acid, supporting the model that the ST polyketide synthase uses this short-chain fatty acid as a starter unit. The characterization of both the polyketide synthase and FAS may provide novel means for modifying secondary metabolites.
The Aspergillus nidulans stcP gene was previously identified as a transcribed region associated with a cluster of genes proposed to be involved in sterigmatocystin biosynthesis (D. W. Brown, J.-H. Yu, H. S. Kelkar, M. Fernandes, T. C. Nesbitt, N. P. Keller, T. H. Adams, and T. J. Leonard, Proc. Natl. Acad. Sci. USA 93:1418-1422, 1996). stcP was predicted to encode a methyltransferase responsible for conversion of demethylsterig-matocystin to sterigmatocystin. Here we demonstrate that disruption of stcP in A. nidulans results in strains that accumulate demethylsterigmatocystin.
Under limiting growth conditions, Aspergillus nidulans produces a carcinogenic secondary metabolite related to aflatoxin and called sterigmatocystin (ST). The genes for ST biosynthesis are co-ordinately regulated and are all found within an approximately 60-kilobase segment of DNA. One of the genes within this region is predicted to encode a CX2CX6CX6CX2CX6CX2 zinc binuclear cluster DNA-binding protein that is related to the Aspergillus flavus and Aspergillus parasiticus aflatoxin regulatory gene aflR. Deletion of the A. nidulans aflR homolog resulted in an inability to induce expression of genes within the ST gene cluster and a loss of ST production. Because A. nidulans aflR mRNA accumulates specifically under conditions that favor ST production we expect that activation of ST biosynthetic genes is determined by A. nidulans aflR. In support of this hypothesis, we demonstrated that induced expression of the A. flavus aflR gene in A. nidulans, under conditions that normally suppress ST gene expression, resulted in activation of genes in the ST biosynthetic pathway. This result demonstrates that AflR function is conserved between Aspergillus spp. and that aflR expression is sufficient to activate genes in the ST pathway.
Sterigmatocystin (ST) and the aflatoxins (AFs), related fungal secondary metabolites, are among the most toxic, mutagenic, and carcinogenic natural products known. The ST biosynthetic pathway in Aspergillus nidulans is estimated to involve at least 15 enzymatic activities, while certain Aspergillus parasiticus, Aspergillus flavus, and Aspergillus nomius strains contain additional activities that convert ST to AF. We have characterized a 60-kb region in the A. nidulans genome and find it contains many, if not all, of the genes needed for ST biosynthesis. This region includes verA, a structural gene previously shown to be required for ST biosynthesis, and 24 additional closely spaced transcripts ranging in size from 0.6 to 7.2 kb that are coordinately induced only under ST-producing conditions. Each end of this gene cluster is demarcated by transcripts that are expressed under both ST-inducing and non-ST-inducing conditions. Deduced polypeptide sequences of regions within this cluster had a high percentage of identity with enzymes that have activities predicted for ST/AF biosynthesis, including a polyketide synthase, a fatty acid synthase (alpha and beta subunits), five monooxygenases, four dehydrogenases, an esterase, an 0-methyltransferase, a reductase, an oxidase, and a zinc cluster DNA binding protein. A revised system for naming the genes of the ST pathway is presented.
Sterigmatocystin (ST) and aflatoxin are carcinogenic end point metabolites derived from the same biochemical pathway, which is found in several Aspergillus spp. Recently, an ST gene cluster, containing approximately 25 distinct genes that are each proposed to function specifically in ST biosynthesis, has been identified in Aspergillus nidulans. Each of these structural genes is named stc (sterigmatocystin) followed by a consecutive letter of the alphabet. We have previously described stcU (formerly verA) as encoding a keto-reductase required for the conversion of versicolorin A to ST. We now describe a second A. nidulans gene, stcS (formerly verB), that is located within 2 kb of stcU in the ST gene cluster. An stcS-disrupted strain of A. nidulans, TSS17, was unable to produce ST and converted ST/aflatoxin precursors to versicolorin A rather than ST, indicating that stcS functions at the same point in the pathway as stcU. Genomic sequence analysis of stcS shows that it encodes a cytochrome P-450 monooxygenase and constitutes a novel P-450 family, CYP59. Assuming that StcU activity mimics that of similar P-450s, it is likely that StcU catalyzes one of the proposed oxidation steps necessary to convert versicolorin A to ST. These results constitute the first genetic proof that the conversion of versicolorin A to ST requires more than one enzymatic activity.
Aspergillus nidulans has functioned as a model system for the study of fungal genetics since the 1950s. Application of methodologies ranging from Mendelian genetics to the most sophisticated molecular biological techniques have resulted in a detailed understanding of genes and pathways involved in primary metabolism, secondary metabolism and development in A. nidulans. We have taken advantage of this background in developing A. nidulans as a genetic system to study the molecular mechanisms regulating aflatoxin biosynthesis. Aflatoxin, a carcinogenic polyketide, is the product of a lengthy biochemical pathway found in the asexual spp., A. flavus and A. parasiticus. A. nidulans possesses most if not all of this pathway and produces sterigmatocystin, the penultimate precursor of the aflatoxin pathway. We have identified a approximately 60 kb cluster of genes in A. nidulans whose products are involved in sterigmatocystin biosynthesis. This cluster contains at least 20 genes proposed to encode both enzymatic activities and regulatory proteins. Our results have shown that at least some of these genes are functionally conserved between A. nidulans, A. flavus and A. parasiticus, and that they are regulated in similar ways. Further studies of sterigmatocystin regulation in A. nidulans should yield information transferable to studies of (i) secondary metabolism in other filamentous fungi and (ii) aflatoxin regulation in A. flavus and A. parasiticus in particular.
Aspergillus nidulans produces the carcinogenic mycotoxin sterigmatocystin (ST), the next-to-last precursor in the aflatoxin (AF) biosynthetic pathway found in the closely related fungi Aspergillus flavus and Aspergillus parasiticus. We identified and characterized an A. nidulans gene, verA, that is required for converting the AF precursor versicolorin A to ST. verA is closely related to several polyketide biosynthetic genes involved in polyketide production in Streptomyces spp. and exhibits extended sequence similarity to A. parasiticus ver-1, a gene proposed to encode an enzyme involved in converting versicolorin A to ST. By performing a sequence analysis of the region 3' to verA, we identified two additional open reading frames, designated ORF1 and ORF2. ORF2 is closely related to a number of cytochrome P-450 monooxygenases, while ORF1 shares identity with the gamma subunit of translation elongation factor 1. Given that several steps in the ST-AF pathway may require monooxygenase activity and that AF biosynthetic genes are clustered in A. flavus and A. parasiticus, we suggest that verA may be part of a cluster of genes required for ST biosynthesis. We disrupted the verA coding region by inserting the A. nidulans argB gene into the center of the coding region and transformed an A. nidulans argB2 mutant to arginine prototrophy. Seven transformants that produced DNA patterns indicative of a verA disruption event were grown under ST-inducing conditions, and all of the transformants produced versicolorin A but negligible amounts of ST (200-fold to almost 1,000-fold less than the wild type), confirming the hypothesis that verA encodes an enzyme necessary for converting versicolorin A to ST.
Aflatoxins are polyketide-derived secondary metabolites produced by the fungi Aspergillus flavus and Aspergillus parasiticus. Among the catalytic steps in the aflatoxin biosynthetic pathway, the conversion of sterigmatocystin to O-methylsterigmatocystin and the conversion of dihydrosterigmatocystin to dihydro-O-methylsterigmatocystin are catalyzed by an S-adenosylmethionine-dependent O-methyltransferase. A cDNA library was constructed by using RNA isolated from a 24-h-old culture of wild-type A. parasiticus SRRC 143 and was screened by using polyclonal antiserum raised against a purified 40-kDa O-methyltransferase protein. A clone that harbored a full-length cDNA insert (1,460 bp) containing the 1,254-bp coding region of the gene omt-1 was identified by the antiserum and isolated. The complete cDNA sequence was determined, and the corresponding 418-amino-acid sequence of the native enzyme with a molecular weight of 46,000 was deduced. This 46-kDa native enzyme has a leader sequence of 41 amino acids, and the mature form of the enzyme apparently consists of 377 amino acids and has a molecular weight of 42,000. Direct sequencing of the purified mature enzyme from A. parasiticus SRRC 163 showed that 19 of 22 amino acid residues were identical to the amino acid residues in an internal region of the deduced amino acid sequence of the mature protein. The 1,460-bp omt-1 cDNA was cloned into an Escherichia coli expression system; a Western blot (immunoblot) analysis of crude extracts from this expression system revealed a 51-kDa fusion protein (fused with a 5-kDa beta-galactosidase N-terminal fragment).(ABSTRACT TRUNCATED AT 250 WORDS)
The penultimate step in the aflatoxin biosynthetic pathway of the filamentous fungi Aspergillus flavus and A. parasiticus involves conversion of sterigmatocystin to O-methylsterigmatocystin. An S-adenosylmethionine-dependent methyltransferase that catalyzes this reaction was purified to homogeneity (> 90%) from 78-h-old mycelia of A. parasiticus SRRC 163. Purification of this soluble enzyme was carried out by five soft-gel chromatographic steps: cell debris remover treatment, QMA ACELL chromatography, hydroxylapatite-Ultrogel chromatography, DEAE-Spherodex chromatography, and Octyl Avidgel chromatography, followed by MA7Q high-performance liquid chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the protein peak from this step on silver staining identified a single band of approximately 40 kDa. This purified protein was distinct from the dimeric 168-kDa methyltransferase purified from the same fungal strain under identical growth conditions (D. Bhatnagar, A. H. J. Ullah, and T. E. Cleveland, Prep. Biochem. 18:321-349, 1988). The chromatographic behavior and N-terminal sequence of the 40-kDa enzyme were also distinct from those of the 168-kDa methyltransferase. The molar extinction coefficient of the 40-kDa enzyme at 278 nm was estimated to be 4.7 x 10(4) M-1 cm-1 in 50 mM potassium phosphate buffer (pH 7.5).
