Aspergillus nidulans mutants defective in stc gene cluster regulation

The Role of Chromatin and Transcriptional Control in the Formation of Sexual Fruiting Bodies in Fungi

Fungal fruiting bodies are complex, three-dimensional structures that arise from a less complex vegetative mycelium. Their formation requires the coordinated action of many genes and their gene products, and fruiting body formation is accompanied by major changes in the transcriptome. In recent years, numerous transcription factor genes as well as chromatin modifier genes that play a role in fruiting body morphogenesis were identified, and through research on several model organisms, the underlying regulatory networks that integrate chromatin structure, gene expression, and cell differentiation are becoming clearer. This review gives a summary of the current state of research on the role of transcriptional control and chromatin structure in fruiting body development. In the first part, insights from transcriptomics analyses are described, with a focus on comparative transcriptomics. In the second part, examples of more detailed functional characterizations of the role of chromatin modifiers and/or transcription factors in several model organisms (Neurospora crassa, Aspergillus nidulans, Sordaria macrospora, Coprinopsis cinerea, and Schizophyllum commune) that have led to a better understanding of regulatory networks at the level of chromatin structure and transcription are discussed.

Upstream Regulation of Development and Secondary Metabolism in Aspergillus Species

In filamentous fungal Aspergillus species, growth, development, and secondary metabolism are genetically programmed biological processes, which require precise coordination of diverse signaling elements, transcription factors (TFs), upstream and downstream regulators, and biosynthetic genes. For the last few decades, regulatory roles of these controllers in asexual/sexual development and primary/secondary metabolism of Aspergillus species have been extensively studied. Among a wide spectrum of regulators, a handful of global regulators govern upstream regulation of development and metabolism by directly and/or indirectly affecting the expression of various genes including TFs. In this review, with the model fungus Aspergillus nidulans as the central figure, we summarize the most well-studied main upstream regulators and their regulatory roles. Specifically, we present key functions of heterotrimeric G proteins and G protein-coupled receptors in signal transduction), the velvet family proteins governing development and metabolism, LaeA as a global regulator of secondary metabolism, and NsdD, a key GATA-type TF, affecting development and secondary metabolism and provide a snapshot of overall upstream regulatory processes underlying growth, development, and metabolism in Aspergillus fungi.

StcU-2 Gene Mutation via CRISPR/Cas9 Leads to Misregulation of Spore-Cyst Formation in Ascosphaera apis

Ascosphaera apis is the causative agent of honey bee chalkbrood disease, and spores are the only known source of infections. Interference with sporulation is therefore a promising way to manage A. apis. The versicolorin reductase gene (StcU-2) is a ketoreductase protein related to sporulation and melanin biosynthesis. To study the StcU-2 gene in ascospore production of A. apis, CRISPR/Cas9 was used, and eight hygromycin B antibiotic-resistant transformants incorporating enhanced green fluorescent protein (EGFP) were made and analyzed. PCR amplification, gel electrophoresis, and sequence analysis were used for target gene editing analysis and verification. The CRISPR/Cas9 editing successfully knocked out the StcU-2 gene in A. apis. StcU-2 mutants had shown albino and non-functional spore-cyst development and lost effective sporulation. In conclusion, editing of StcU-2 gene has shown direct relation with sporulation and melanin biosynthesis of A. apis; this effective sporulation reduction would reduce the spread and pathogenicity of A. apis to managed honey bee. To the best of our knowledge, this is the first time CRISPR/Cas9-mediated gene editing has been efficiently performed in A. apis, a fungal honey bee brood pathogen, which offers a comprehensive set of procedural references that contributes to A. apis gene function studies and consequent control of chalkbrood disease.

Functional characterization of the sterigmatocystin secondary metabolite gene cluster in the filamentous fungus Podospora anserina: involvement in oxidative stress response, sexual development, pigmentation and interspecific competitions

Filamentous fungi are known as prolific untapped reservoirs of diverse secondary metabolites, where genes required for their synthesis are organized in clusters. The bioactive properties of these compounds are closely related to their functions in fungal biology, which are not well understood. In this study, we focused on the Podospora anserina gene cluster responsible for the biosynthesis of sterigmatocystin (ST). Deletion of the PaStcA gene encoding the polyketide synthase and overexpression (OE) of the PaAflR gene encoding the ST-specific transcription factor in P. anserina were performed. We showed that growth of PaStcA^Δ was inhibited in the presence of methylglyoxal, while OE-PaAflR showed a little inhibition, indicating that ST production may enhance oxidative stress tolerance in P. anserina. We also showed that the OE-PaAflR strain displayed an overpigmented thallus mediated by the melanin pathway. Overexpression of PaAflR also led to sterility. Interspecific confrontation assays showed that ST-overexpressed strains produced a high level of peroxides and possessed a higher competitiveness against other fungi. Comparative metabolite profiling demonstrated that PaStcA^Δ strain was unable to produce ST, while OE-PaAflR displayed a ST overproduction. This study contributes to a better understanding of ST in P. anserina, especially with regard to its involvement in fungal physiology.

Moulding the mould: understanding and reprogramming filamentous fungal growth and morphogenesis for next generation cell factories

Filamentous fungi are harnessed as cell factories for the production of a diverse range of organic acids, proteins, and secondary metabolites. Growth and morphology have critical implications for product titres in both submerged and solid-state fermentations. Recent advances in systems-level understanding of the filamentous lifestyle and development of sophisticated synthetic biological tools for controlled manipulation of fungal genomes now allow rational strain development programs based on data-driven decision making. In this review, we focus on Aspergillus spp. and other industrially utilised fungi to summarise recent insights into the multifaceted and dynamic relationship between filamentous growth and product titres from genetic, metabolic, modelling, subcellular, macromorphological and process engineering perspectives. Current progress and knowledge gaps with regard to mechanistic understanding of product secretion and export from the fungal cell are discussed. We highlight possible strategies for unlocking lead genes for rational strain optimizations based on omics data, and discuss how targeted genetic manipulation of these candidates can be used to optimise fungal morphology for improved performance. Additionally, fungal signalling cascades are introduced as critical processes that can be genetically targeted to control growth and morphology during biotechnological applications. Finally, we review progress in the field of synthetic biology towards chassis cells and minimal genomes, which will eventually enable highly programmable filamentous growth and diversified production capabilities. Ultimately, these advances will not only expand the fungal biotechnology portfolio but will also significantly contribute to a sustainable bio-economy.

Fungal attack and host defence pathways unveiled in near-avirulent interactions of Penicillium expansum creA mutants on apples

Amongst the universal diseases affecting apples, blue mould caused by Penicillium expansum is a major concern, resulting in yield and quality losses as a result of the production of the mycotoxin patulin. Despite the characterization of the patulin biosynthetic gene cluster at both the molecular and chemical levels, the underlying regulation of patulin biosynthesis in P. expansum and the mechanisms of apple colonization remain largely obscure. Recent work has indicated that sucrose, a carbon catabolite repressive metabolite, is a critical factor in the regulation of patulin synthesis. Here, CreA, the global carbon catabolite regulator, was assessed for virulence both in vitro and in vivo. We showed that loss-of-function creA strains were nearly avirulent and did not produce patulin in apples. On the basis of RNA-sequencing (RNA-seq) analysis and physiological experimentation, these mutants were unable to successfully colonize apples for a multitude of potential mechanisms including, on the pathogen side, a decreased ability to produce proteolytic enzymes and to acidify the environment and impaired carbon/nitrogen metabolism and, on the host side, an increase in the oxidative defence pathways. Our study defines CreA and its downstream signalling pathways as promising targets for the development of strategies to fight against the development and virulence of this post-harvest pathogen.

Revitalization of a Forward Genetic Screen Identifies Three New Regulators of Fungal Secondary Metabolism in the Genus Aspergillus

The study of aflatoxin in Aspergillus spp. has garnered the attention of many researchers due to aflatoxin's carcinogenic properties and frequency as a food and feed contaminant. Significant progress has been made by utilizing the model organism Aspergillus nidulans to characterize the regulation of sterigmatocystin (ST), the penultimate precursor of aflatoxin. A previous forward genetic screen identified 23 A. nidulans mutants involved in regulating ST production. Six mutants were characterized from this screen using classical mapping (five mutations in mcsA) and complementation with a cosmid library (one mutation in laeA). The remaining mutants were backcrossed and sequenced using Illumina and Ion Torrent sequencing platforms. All but one mutant contained one or more sequence variants in predicted open reading frames. Deletion of these genes resulted in identification of mutant alleles responsible for the loss of ST production in 12 of the 17 remaining mutants. Eight of these mutations were in genes already known to affect ST synthesis (laeA, mcsA, fluG, and stcA), while the remaining four mutations (in laeB, sntB, and hamI) were in previously uncharacterized genes not known to be involved in ST production. Deletion of laeB, sntB, and hamI in A. flavus results in loss of aflatoxin production, confirming that these regulators are conserved in the aflatoxigenic aspergilli. This report highlights the multifaceted regulatory mechanisms governing secondary metabolism in Aspergillus Additionally, these data contribute to the increasing number of studies showing that forward genetic screens of fungi coupled with whole-genome resequencing is a robust and cost-effective technique.IMPORTANCE In a postgenomic world, reverse genetic approaches have displaced their forward genetic counterparts. The techniques used in forward genetics to identify loci of interest were typically very cumbersome and time-consuming, relying on Mendelian traits in model organisms. The current work was pursued not only to identify alleles involved in regulation of secondary metabolism but also to demonstrate a return to forward genetics to track phenotypes and to discover genetic pathways that could not be predicted through a reverse genetics approach. While identification of mutant alleles from whole-genome sequencing has been done before, here we illustrate the possibility of coupling this strategy with a genetic screen to identify multiple alleles of interest. Sequencing of classically derived mutants revealed several uncharacterized genes, which represent novel pathways to regulate and control the biosynthesis of sterigmatocystin and of aflatoxin, a societally and medically important mycotoxin.

cpsA regulates mycotoxin production, morphogenesis and cell wall biosynthesis in the fungus Aspergillus nidulans

The model fungus Aspergillus nidulans synthesizes numerous secondary metabolites, including sterigmatocystin (ST). The production of this toxin is positively controlled by the global regulator veA. In the absence of veA (ΔveA), ST biosynthesis is blocked. Previously, we performed random mutagenesis in a ΔveA strain and identified revertant mutants able to synthesize ST, among them RM1. Complementation of RM1 with a genomic library revealed that the mutation occurred in a gene designated as cpsA. While in the ΔveA genetic background cpsA deletion restores ST production, in a veA wild-type background absence of cpsA reduces and delays ST biosynthesis decreasing the expression of ST genes. Furthermore, cpsA is also necessary for the production of other secondary metabolites, including penicillin, affecting the expression of PN genes. In addition, cpsA is necessary for normal asexual and sexual development. Chemical and microscopy analyses revealed that CpsA is found in cytoplasmic vesicles and it is required for normal cell wall composition and integrity, affecting adhesion capacity and oxidative stress sensitivity. The conservation of cpsA in Ascomycetes suggests that cpsA homologs might have similar roles in other fungal species.

Antibiotic dialogues: induction of silent biosynthetic gene clusters by exogenous small molecules

Natural products have traditionally served as a dominant source of therapeutic agents. They are produced by dedicated biosynthetic gene clusters that assemble complex, bioactive molecules from simple precursors. Recent genome sequencing efforts coupled with advances in bioinformatics indicate that the majority of biosynthetic gene clusters are not expressed under normal laboratory conditions. Termed 'silent' or 'cryptic', these gene clusters represent a treasure trove for discovery of novel small molecules, their regulatory circuits and their biosynthetic pathways. In this review, we assess the capacity of exogenous small molecules in activating silent secondary metabolite gene clusters. Several approaches that have been developed are presented, including coculture techniques, ribosome engineering, chromatin remodeling and high-throughput elicitor screens. The rationale, applications and mechanisms attendant to each are discussed. Some general conclusions can be drawn from our analysis: exogenous small molecules comprise a productive avenue for the discovery of cryptic metabolites. Specifically, growth-inhibitory molecules, in some cases clinically used antibiotics, serve as effective inducers of silent biosynthetic gene clusters, suggesting that old antibiotics may be used to find new ones. The involvement of natural antibiotics in modulating secondary metabolism at subinhibitory concentrations suggests that they represent part of the microbial vocabulary through which inter- and intraspecies interactions are mediated.

Further characterization of the role of the mitochondrial high-mobility group box protein in the intracellular redox environment of Aspergillus nidulans

HmbB, a predominantly mitochondrial high-mobility group box (HMGB) protein, of Aspergillus nidulans affects diverse biological activities, such as sterigmatocystin production, the maintenance of mitochondrial DNA copy number, germination of asexual and sexual spores, and protection against oxidative stress agents. We hypothesized that the latter correlates with an unbalanced intracellular redox state, in which case, a not yet fully characterized physiological function could be attributed to this mitochondrial HMGB protein. Here, we studied the intracellular redox environment and oxidative stress tolerance in hmbB+ and hmbBΔ strains under normal and oxidative stress conditions by measuring glutathione redox couple, intracellular reactive oxygen species (ROS) content and ROS-protecting enzyme activities. Our results revealed that the intracellular redox environment is different in hmbBΔ conidia and mycelia from that of hmbB+, and shed light on the seemingly contradictory difference in the tolerance of hmbBΔ mycelia to diamide and menadione oxidative stressors.

One Juliet and four Romeos: VeA and its methyltransferases

Fungal secondary metabolism has become an important research topic with great biomedical and biotechnological value. In the postgenomic era, understanding the diversity and the molecular control of secondary metabolites (SMs) are two challenging tasks addressed by the research community. Discovery of the LaeA methyltransferase 10 years ago opened up a new horizon on the control of SM research when it was found that expression of many SM gene clusters is controlled by LaeA. While the molecular function of LaeA remains an enigma, discovery of the velvet family proteins as interaction partners further extended the role of the LaeA beyond secondary metabolism. The heterotrimeric VelB–VeA–LaeA complex plays important roles in development, sporulation, secondary metabolism, and pathogenicity. Recently, three other methyltransferases have been found to associate with the velvet complex, the LaeA-like methyltransferase F and the methyltransferase heterodimers VipC–VapB. Interaction of VeA with at least four methyltransferase proteins indicates a molecular hub function for VeA that questions: Is there a VeA supercomplex or is VeA part of a highly dynamic cellular control network with many different partners?

Illumina identification of RsrA, a conserved C2H2 transcription factor coordinating the NapA mediated oxidative stress signaling pathway in Aspergillus

Background

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.

Results

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.

Conclusion

RsrA, found in a highly syntenic region in Aspergillus genomes, coordinates a NapA mediated oxidative response in Aspergillus fungi.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1011) contains supplementary material, which is available to authorized users.

A dually located multi-HMG-box protein of Aspergillus nidulans has a crucial role in conidial and ascospore germination

Seven HMG-box proteins of Aspergillus nidulans have been identified in the genomic databases. Three of these have the characteristics of non-specific DNA-binding proteins. One of these, AN1267 (HmbB), comprises one canonical HMG-box in its C-terminus and upstream of the canonical box two structurally related boxes, to be called Shadow-HMG-boxes. This protein defines, together with the Podospora anserina mtHMG1, a clade of proteins present in the Pezizomycotina, with orthologues in some of the Taphrinomycotina. HmbB localizes primarily to the mitochondria but occasionally in nuclei. The deletion of the cognate gene results in a number of pleiotropic effects, including those on hyphal morphology, sensitivity to oxidative stress, absence of sterigmatocystin production and changes in the profile of conidial metabolites. The most striking phenotype of deletion strains is a dramatic decrease in conidial and ascospore viability. We show that this is most likely due to the protein being essential to maintain mitochondrial DNA in spores.

Comprehensive annotation of secondary metabolite biosynthetic genes and gene clusters of Aspergillus nidulans, A. fumigatus, A. niger and A. oryzae

Background

Secondary metabolite production, a hallmark of filamentous fungi, is an expanding area of research for the Aspergilli. These compounds are potent chemicals, ranging from deadly toxins to therapeutic antibiotics to potential anti-cancer drugs. The genome sequences for multiple Aspergilli have been determined, and provide a wealth of predictive information about secondary metabolite production. Sequence analysis and gene overexpression strategies have enabled the discovery of novel secondary metabolites and the genes involved in their biosynthesis. The Aspergillus Genome Database (AspGD) provides a central repository for gene annotation and protein information for Aspergillus species. These annotations include Gene Ontology (GO) terms, phenotype data, gene names and descriptions and they are crucial for interpreting both small- and large-scale data and for aiding in the design of new experiments that further Aspergillus research.

Results

We have manually curated Biological Process GO annotations for all genes in AspGD with recorded functions in secondary metabolite production, adding new GO terms that specifically describe each secondary metabolite. We then leveraged these new annotations to predict roles in secondary metabolism for genes lacking experimental characterization. As a starting point for manually annotating Aspergillus secondary metabolite gene clusters, we used antiSMASH (antibiotics and Secondary Metabolite Analysis SHell) and SMURF (Secondary Metabolite Unknown Regions Finder) algorithms to identify potential clusters in A. nidulans, A. fumigatus, A. niger and A. oryzae, which we subsequently refined through manual curation.

Conclusions

This set of 266 manually curated secondary metabolite gene clusters will facilitate the investigation of novel Aspergillus secondary metabolites.

Aflatoxins, fumonisins, and trichothecenes: a convergence of knowledge

Plant pathogenic fungi Aspergillus flavus, Fusarium verticillioides, and Fusarium graminearum infect seeds of the most important food and feed crops, including maize, wheat, and barley. More importantly, these fungi produce aflatoxins, fumonisins, and trichothecenes, respectively, which threaten health and food security worldwide. In this review, we examine the molecular mechanisms and environmental factors that regulate mycotoxin biosynthesis in each fungus, and discuss the similarities and differences in the collective body of knowledge. Whole-genome sequences are available for these fungi, providing reference databases for genomic, transcriptomic, and proteomic analyses. It is well recognized that genes responsible for mycotoxin biosynthesis are organized in clusters. However, recent research has documented the intricate transcriptional and epigenetic regulation that affects these gene clusters. Significantly, molecular networks that respond to environmental factors, namely nitrogen, carbon, and pH, are connected to components regulating mycotoxin production. Furthermore, the developmental status of seeds and specific tissue types exert conditional influences during fungal colonization. A comparison of the three distinct mycotoxin groups provides insight into new areas for research collaborations that will lead to innovative strategies to control mycotoxin contamination of grain.

Overexpression of the Aspergillus nidulans histone 4 acetyltransferase EsaA increases activation of secondary metabolite production

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.

veA-dependent RNA-pol II transcription elongation factor-like protein, RtfA, is associated with secondary metabolism and morphological development in Aspergillus nidulans

In Aspergillus nidulans the global regulatory gene veA is necessary for the biosynthesis of several secondary metabolites, including the mycotoxin sterigmatocystin (ST). In order to identify additional veA-dependent genetic elements involved in regulating ST production, we performed a mutagenesis on a deletion veA (ΔveA) strain to obtain revertant mutants (RM) that regained the capability to produce toxin. Genetic analysis and molecular characterization of one of the revertant mutants, RM3, revealed that a point mutation occurred at the coding region of the rtfA gene, encoding a RNA-pol II transcription elongation factor-like protein, similar to Saccharomyces cerevisiae Rtf1. The A. nidulans rtfA gene product accumulates in nuclei. Deletion of rtfA gene in a ΔveA background restored mycotoxin production in a medium-dependent manner. rtfA also affects the production of other metabolites including penicillin. Biosynthesis of this antibiotic decreased in the absence of rtfA. Furthermore, rtfA is necessary for normal morphological development. Deletion of the rtfA gene in wild-type strains (veA+) resulted in a slight decrease in growth rate, drastic reduction in conidiation, and complete loss of sexual development. This is the first study of an Rtf1 like gene in filamentous fungi. We found rtfA putative orthologues extensively conserved in numerous fungal species.

Lae1 regulates expression of multiple secondary metabolite gene clusters in Fusarium verticillioides

The filamentous fungus Fusarium verticillioides can cause disease of maize and is capable of producing fumonisins, a family of toxic secondary metabolites linked to esophageal cancer and neural tube defects in humans and lung edema in swine and leukoencephalomalacia in equines. The expression of fumonisin biosynthetic genes is influenced by broad-domain transcription factors (global regulators) and Fum21, a pathway-specific transcription factor. LaeA is a global regulator that in Aspergillus nidulans, affects the expression of multiple secondary metabolite gene clusters by modifying heterochromatin structure. Here, we employed gene deletion analysis to assess the effect of loss of a F. verticillioides laeA orthologue, LAE1, on genome-wide gene expression and secondary metabolite production. Loss of Lae1 resulted in reduced expression of gene clusters responsible for synthesis of the secondary metabolites bikaverin, fumonisins, fusaric acid and fusarins as well as two clusters for which the corresponding secondary metabolite is unknown. Analysis of secondary metabolites revealed that, in contrast to a previously described Fusarium fujikuroi lae1 mutant, bikaverin production is reduced. Fumonisin production is unchanged in the F. verticillioides lae1 mutant. Complementation of the F. verticillioides mutant resulted in increased fumonisin production.

ChLae1 and ChVel1 regulate T-toxin production, virulence, oxidative stress response, and development of the maize pathogen Cochliobolus heterostrophus

LaeA and VeA coordinate secondary metabolism and differentiation in response to light signals in Aspergillus spp. Their orthologs, ChLae1 and ChVel1, were identified in the maize pathogen Cochliobolus heterostrophus, known to produce a wealth of secondary metabolites, including the host selective toxin, T-toxin. Produced by race T, T-toxin promotes high virulence to maize carrying Texas male sterile cytoplasm (T-cms). T-toxin production is significantly increased in the dark in wild type (WT), whereas Chvel1 and Chlae1 mutant toxin levels are much reduced in the dark compared to WT. Correspondingly, expression of T-toxin biosynthetic genes (Tox1) is up-regulated in the dark in WT, while dark-induced expression is much reduced/minimal in Chvel1 and Chlae1 mutants. Toxin production and Tox1 gene expression are increased in ChVEL1 overexpression (OE) strains grown in the dark and in ChLAE1 strains grown in either light or dark, compared to WT. These observations establish ChLae1 and ChVel1 as the first factors known to regulate host selective toxin production. Virulence of Chlae1 and Chvel1 mutants and OE strains is altered on both T-cms and normal cytoplasm maize, indicating that both T-toxin mediated super virulence and basic pathogenic ability are affected. Deletion of ChLAE1 or ChVEL1 reduces tolerance to H₂O₂. Expression of CAT3, one of the three catalase genes, is reduced in the Chvel1 mutant. Chlae1 and Chvel1 mutants also show decreased aerial hyphal growth, increased asexual sporulation and female sterility. ChLAE1 OE strains are female sterile, while ChVEL1 OE strains are more fertile than WT. ChLae1 and ChVel1 repress expression of 1,8-dihydroxynaphthalene (DHN) melanin biosynthesis genes, and, accordingly, melanization is enhanced in Chlae1 and Chvel1 mutants, and reduced in OE strains. Thus, ChLae1 and ChVel1 positively regulate T-toxin biosynthesis, pathogenicity and super virulence, oxidative stress responses, sexual development, and aerial hyphal growth, and negatively control melanin biosynthesis and asexual differentiation.

Filamentous fungi produce chemically diverse metabolites that broker positive and negative interactions with other organisms, manage host pathogenicity/virulence, nutritional and environmental stresses, and differentiation of the fungus. The maize pathogen Cochliobolus heterostrophus is notorious as the causal agent of the most economically devastating epidemic to date, in 1970. Disease severity was associated with appearance of a new race, producing T-toxin, a host selective toxin promoting high virulence to Texas male sterile cytoplasm maize, widely planted at the time. LaeA and VeA are central regulators of secondary metabolism in Aspergillus, coordinating metabolite production and differentiation in response to light. Given the significance of effector-type host selective toxins in pathogenic interactions, we characterized ChLae1 and ChVel1 and found that deletion and overexpression affect T-toxin production in planta and in vitro. Both chlorosis due to T-toxin and necrotic lesion formation are altered, establishing these as the first factors known to regulate both super virulence conferred by T-toxin, and basic pathogenicity, due to unknown factors. The mutants are also altered in oxidative stress responses, key to success in the infection court, asexual and sexual development, essential for fungal dissemination in the field, aerial hyphal growth, and pigment biosynthesis, essential for survival in the field.

Advances in Aspergillus secondary metabolite research in the post-genomic era

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.

Transcriptional regulatory elements in fungal secondary metabolism

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.

Control of aflatoxin production of Aspergillus flavus and Aspergillus parasiticus using RNA silencing technology by targeting aflD (nor-1) gene

Aspergillus flavus and Aspergillus parasiticus are important pathogens of cotton, corn, peanuts and other oil-seed crops, producing toxins both in the field and during storage. We have designed three siRNA sequences (Nor-Ia, Nor-Ib, Nor-Ic) to target the mRNA sequence of the aflD gene to examine the potential for using RNA silencing technology to control aflatoxin production. Thus, the effect of siRNAs targeting of two key genes in the aflatoxin biosynthetic pathway, aflD (structural) and aflR (regulatory gene) and on aflatoxin B₁ (AFB₁), and aflatoxin G₁ (AFG₁) production was examined. The study showed that Nor-Ib gave a significant decrease in aflD mRNA, aflR mRNA abundance, and AFB₁ production (98, 97 and 97% when compared to the controls) in A. flavus NRRL3357, respectively. Reduction in aflD and aflR mRNA abundance and AFB₁ production increased with concentration of siRNA tested. There was a significant inhibition in aflD and AFB₁ production by A. flavus EGP9 and AFG₁ production by A. parasiticus NRRL 13005. However, there was no significant decrease in AFG₁ production by A. parasiticus SSWT 2999. Changes in AFB₁ production in relation to mRNA levels of aflD showed a good correlation (R = 0.88; P = 0.00001); changes in aflR mRNA level in relation to mRNA level of aflD also showed good correlation (R = 0.82; P = 0.0001). The correlations between changes in aflR and aflD gene expression suggests a strong relationship between these structural and regulatory genes, and that aflD could be used as a target gene to develop efficient means for aflatoxin control using RNA silencing technology.

Willow volatiles influence growth, development, and secondary metabolism in Aspergillus parasiticus

Aflatoxin is a mycotoxin and the most potent naturally occurring carcinogen in many animals. Aflatoxin contamination of food and feed crops causes a significant global burden on human and animal health. However, available methods to eliminate aflatoxin from food and feed are not fully effective. Our goal is to discover novel, efficient, and practical methods to control aflatoxin contamination in crops during storage. In the present study, we tested the effect of volatiles produced by willow (Salix acutifolia and Salix babylonica) and maple (Acer saccharinum) bark on fungal growth, development, and aflatoxin production by the fungus Aspergillus parasiticus, one economically important aflatoxin producer. S. acutifolia bark volatiles nearly eliminated aflatoxin accumulation (>90% reduction) by A. parasiticus grown on a minimal agar medium. The decrease in aflatoxin accumulation correlated with a twofold reduction in ver-1 (encodes a middle aflatoxin pathway enzyme) transcript level. Expression data also indicate that one histone H4 acetyltransferase, MYST3, may play a role in epigenetic control of aflatoxin gene transcription in response to volatile exposure. Volatiles derived from wood bark samples also increased fungal growth up to 20% and/or enhanced conidiospore development. Solid-phase microextraction-gas chromatographic-mass spectrometric analysis of bark samples identified sets of shared and unique volatile compounds that may mediate the observed regulatory effects on growth, development, and aflatoxin synthesis. This work provides an experimental basis for the use of willow industry by-products to control aflatoxin contamination in food and feed crops.

Aspergillus flavus

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.

Suppressor mutagenesis identifies a velvet complex remediator of Aspergillus nidulans secondary metabolism

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.

Volatile profiling reveals intracellular metabolic changes in Aspergillus parasiticus: veA regulates branched chain amino acid and ethanol metabolism

Background

Filamentous fungi in the genus Aspergillus produce a variety of natural products, including aflatoxin, the most potent naturally occurring carcinogen known. Aflatoxin biosynthesis, one of the most highly characterized secondary metabolic pathways, offers a model system to study secondary metabolism in eukaryotes. To control or customize biosynthesis of natural products we must understand how secondary metabolism integrates into the overall cellular metabolic network. By applying a metabolomics approach we analyzed volatile compounds synthesized by Aspergillus parasiticus in an attempt to define the association of secondary metabolism with other metabolic and cellular processes.

Results

Volatile compounds were examined using solid phase microextraction - gas chromatography/mass spectrometry. In the wild type strain Aspergillus parasiticus SU-1, the largest group of volatiles included compounds derived from catabolism of branched chain amino acids (leucine, isoleucine, and valine); we also identified alcohols, esters, aldehydes, and lipid-derived volatiles. The number and quantity of the volatiles produced depended on media composition, time of incubation, and light-dark status. A block in aflatoxin biosynthesis or disruption of the global regulator veA affected the volatile profile. In addition to its multiple functions in secondary metabolism and development, VeA negatively regulated catabolism of branched chain amino acids and synthesis of ethanol at the transcriptional level thus playing a role in controlling carbon flow within the cell. Finally, we demonstrated that volatiles generated by a veA disruption mutant are part of the complex regulatory machinery that mediates the effects of VeA on asexual conidiation and sclerotia formation.

Conclusions

1) Volatile profiling provides a rapid, effective, and powerful approach to identify changes in intracellular metabolic networks in filamentous fungi. 2) VeA coordinates the biosynthesis of secondary metabolites with catabolism of branched chain amino acids, alcohol biosynthesis, and β-oxidation of fatty acids. 3) Intracellular chemical development in A. parasiticus is linked to morphological development. 4) Understanding carbon flow through secondary metabolic pathways and catabolism of branched chain amino acids is essential for controlling and customizing production of natural products.

Aspergillus flavus genomics as a tool for studying the mechanism of aflatoxin formation

Aspergillus flavus is a weak pathogen that infects plants, animals and humans. When it infects agricultural crops, however, it produces one of the most potent carcinogens known (aflatoxins). To devise strategies to control aflatoxin contamination of pre-harvest agricultural crops and post-harvest grains during storage, we launched the A. flavus genomics program. The major objective of this program is the identification of genes involved in aflatoxin biosynthesis and regulation, as well as in pathogenicity, to gain a better understanding of the mechanism of aflatoxin formation. The sequencing of A. flavus whole genome has been completed. Initial annotation of the sequence revealed that there are about 13,071 genes in the A. flavus genome. Genes which potentially encode for enzymes involved in secondary metabolite production in the A. flavus genome have been identified. Preliminary comparative genome analysis of A. flavus with A. oryzae is summarized here.

Genetic regulation of aflatoxin biosynthesis: from gene to genome

Aflatoxins are notorious toxic secondary metabolites known for their impacts on human and animal health, and their effects on the marketability of key grain and nut crops. Understanding aflatoxin biosynthesis is the focus of a large and diverse research community. Concerted efforts by this community have led not only to a well-characterized biosynthetic pathway, but also to the discovery of novel regulatory mechanisms. Common to secondary metabolism is the clustering of biosynthetic genes and their regulation by pathway specific as well as global regulators. Recent data show that arrangement of secondary metabolite genes in clusters may allow for an important global regulation of secondary metabolism based on physical location along the chromosome. Available genomic and proteomic tools are now allowing us to examine aflatoxin biosynthesis more broadly and to put its regulation in context with fungal development and fungal ecology. This review covers our current understanding of the biosynthesis and regulation of aflatoxin and highlights new and emerging information garnered from structural and functional genomics. The focus of this review will be on studies in Aspergillus flavus and Aspergillus parasiticus, the two agronomically important species that produce aflatoxin. Also covered will be the important contributions gained by studies on production of the aflatoxin precursor sterigmatocystin in Aspergillus nidulans.

RNA silencing gene truncation in the filamentous fungus Aspergillus nidulans

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.

Aspergillus volatiles regulate aflatoxin synthesis and asexual sporulation in Aspergillus parasiticus

Aspergillus parasiticus is one primary source of aflatoxin contamination in economically important crops. To prevent the potential health and economic impacts of aflatoxin contamination, our goal is to develop practical strategies to reduce aflatoxin synthesis on susceptible crops. One focus is to identify biological and environmental factors that regulate aflatoxin synthesis and to manipulate these factors to control aflatoxin biosynthesis in the field or during crop storage. In the current study, we analyzed the effects of aspergillus volatiles on growth, development, aflatoxin biosynthesis, and promoter activity in the filamentous fungus A. parasiticus. When colonies of Aspergillus nidulans and A. parasiticus were incubated in the same growth chamber, we observed a significant reduction in aflatoxin synthesis and asexual sporulation by A. parasiticus. Analysis of the headspace gases demonstrated that A. nidulans produced much larger quantities of 2-buten-1-ol (CA) and 2-ethyl-1-hexanol (EH) than A. parasiticus. In its pure form, EH inhibited growth and increased aflatoxin accumulation in A. parasiticus at all doses tested; EH also stimulated aflatoxin transcript accumulation. In contrast, CA exerted dose-dependent up-regulatory or down-regulatory effects on aflatoxin accumulation, conidiation, and aflatoxin transcript accumulation. Experiments with reporter strains carrying nor-1 promoter deletions and mutations suggested that the differential effects of CA were mediated through separate regulatory regions in the nor-1 promoter. The potential efficacy of CA as a tool for analysis of transcriptional regulation of aflatoxin biosynthesis is discussed. We also identify a novel, rapid, and reliable method to assess norsolorinic acid accumulation in solid culture using a Chroma Meter CR-300 apparatus.

Histone deacetylase activity regulates chemical diversity in Aspergillus

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.

Phosphopantetheinyl transferase CfwA/NpgA is required for Aspergillus nidulans secondary metabolism and asexual development

Polyketide synthases (PKSs) and/or nonribosomal peptide synthetases (NRPSs) are central components of secondary metabolism in bacteria, plants, and fungi. In filamentous fungi, diverse PKSs and NRPSs participate in the biosynthesis of secondary metabolites such as pigments, antibiotics, siderophores, and mycotoxins. However, many secondary metabolites as well as the enzymes involved in their production are yet to be discovered. Both PKSs and NRPSs require activation by enzyme members of the 4'-phosphopantetheinyl transferase (PPTase) family. Here, we report the isolation and characterization of Aspergillus nidulans strains carrying conditional (cfwA2) and null (DeltacfwA) mutant alleles of the cfwA gene, encoding an essential PPTase. We identify the polyketides shamixanthone, emericellin, and dehydroaustinol as well as the sterols ergosterol, peroxiergosterol, and cerevisterol in extracts from A. nidulans large-scale cultures. The PPTase CfwA/NpgA was required for the production of these polyketide compounds but dispensable for ergosterol and cerevisterol and for fatty acid biosynthesis. The asexual sporulation defects of cfwA, DeltafluG, and DeltatmpA mutants were not rescued by the cfwA-dependent compounds identified here. However, a cfwA2 mutation enhanced the sporulation defects of both DeltatmpA and DeltafluG single mutants, suggesting that unidentified CfwA-dependent PKSs and/or NRPSs are involved in the production of hitherto-unknown compounds required for sporulation. Our results expand the number of known and predicted secondary metabolites requiring CfwA/NpgA for their biosynthesis and, together with the phylogenetic analysis of fungal PPTases, suggest that a single PPTase is responsible for the activation of all PKSs and NRPSs in A. nidulans.

Understanding the genetics of regulation of aflatoxin production and Aspergillus flavus development

Aflatoxins are polyketide-derived, toxic, and carcinogenic secondary metabolites produced primarily by two fungal species, Aspergillus flavus and A. parasiticus, on crops such as corn, peanuts, cottonseed, and treenuts. Regulatory guidelines issued by the U.S. Food and Drug Administration (FDA) prevent sale of commodities if contamination by these toxins exceeds certain levels. The biosynthesis of these toxins has been extensively studied. About 15 stable precursors have been identified. The genes involved in encoding the proteins required for the oxidative and regulatory steps in the biosynthesis are clustered in a 70 kb portion of chromosome 3 in the A. flavus genome. With the characterization of the gene cluster, new insights into the cellular processes that govern the genes involved in aflatoxin biosynthesis have been revealed, but the signaling processes that turn on aflatoxin biosynthesis during fungal contamination of crops are still not well understood. New molecular technologies, such as gene microarray analyses, quantitative polymerase chain reaction (PCR), and chromatin immunoprecipitation are being used to understand how physiological stress, environmental and soil conditions, receptivity of the plant, and fungal virulence lead to episodic outbreaks of aflatoxin contamination in certain commercially important crops. With this fundamental understanding, we will be better able to design improved non-aflatoxigenic biocompetitive Aspergillus strains and develop inhibitors of aflatoxin production (native to affected crops or otherwise) amenable to agricultural application for enhancing host-resistance against fungal invasion or toxin production. Comparisons of aflatoxin-producing species with other fungal species that retain some of the genes required for aflatoxin formation is expected to provide insight into the evolution of the aflatoxin gene cluster, and its role in fungal physiology. Therefore, information on how and why the fungus makes the toxin will be valuable for developing an effective and lasting strategy for control of aflatoxin contamination.

Signalling pathways connecting mycotoxin production and sporulation

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.

Aspergillus flavus genomics: gateway to human and animal health, food safety, and crop resistance to diseases

Aspergillus flavus is an imperfect filamentous fungus that is an opportunistic pathogen causing invasive and non-invasive aspergillosis in humans, animals, and insects. It also causes allergic reactions in humans. A. flavus infects agricultural crops and stored grains and produces the most toxic and potent carcinogic metabolites such as aflatoxins and other mycotoxins. Breakthroughs in A. flavus genomics may lead to improvement in human health, food safety, and agricultural economy. The availability of A. flavus genomic data marks a new era in research for fungal biology, medical mycology, agricultural ecology, pathogenicity, mycotoxin biosynthesis, and evolution. The availability of whole genome microarrays has equipped scientists with a new powerful tool for studying gene expression under specific conditions. They can be used to identify genes responsible for mycotoxin biosynthesis and for fungal infection in humans, animals and plants. A. flavus genomics is expected to advance the development of therapeutic drugs and to provide information for devising strategies in controlling diseases of humans and other animals. Further, it will provide vital clues for engineering commercial crops resistant to fungal infection by incorporating antifungal genes that may prevent aflatoxin contamination of agricultural harvest.

TmpA, a member of a novel family of putative membrane flavoproteins, regulates asexual development in Aspergillus nidulans

Asexual reproduction (conidiation) in Aspergillus nidulans is induced by environmental signals like exposure to air or nutrient starvation, and depends on brlA gene activation. The study of 'fluffy' mutants showing delayed asexual development and reduced brlA expression has defined the fluG pathway, involved in regulation of this differentiation process. Genetic characterization of a 'fluffy' mutant identified tmpA as a new gene involved in regulation of conidiation. TmpA defines a new family of putative transmembrane proteins of unknown function, widespread in filamentous fungi and plants, with homologues showing similarity to non-ribosomal peptide synthetases. The deletion of tmpA resulted in decreased brlA expression and conidiation in air-exposed colonies. This defect was suppressed when DeltatmpA mutants were grown next to wild-type or DeltafluG mutant colonies, even without direct contact between hyphae. In liquid culture, tmpA was essential for conidiation induced by nitrogen but not by carbon starvation, whereas the overexpression of different tmpA tagged alleles resulted in conidiation. The overexpression of fluG-induced conidiation independently of tmpA and DeltatmpADeltafluG double mutants showed an additive 'fluffy' phenotype, indicating that tmpA and fluG regulate asexual sporulation through different pathways. TmpA and its homologues appear to have diverged from the ferric reductase family, retaining overall transmembrane architecture, NAD(P), flavin adenine dinucleotide (FAD) and possibly haem-binding domains. Based on our results, we propose that TmpA is a membrane oxidoreductase involved in the synthesis of a developmental signal.

Aflatoxin conducive and non-conducive growth conditions reveal new gene associations with aflatoxin production

Research on aflatoxin (AF) production has traditionally focused on defining the AF biosynthetic pathway with the goal of identifying potential targets for intervention. To understand the effect of nitrogen source, carbon source, temperature, and pH on the regulation of AF biosynthesis, a targeted cDNA microarray consisting of genes associated with AF production over time was employed. Expression profiles for genes involved in AF biosynthesis grouped into five clades. A putative regulon was identified consisting of 20 genes that were induced in the conducive nitrogen and pH treatments and the non-conducive carbon and temperature treatments, as well as four other putative regulons corresponding to each of the four variables studied. Seventeen genes exhibited consistent induction/repression profiles across all the experiments. One of these genes was consistently downregulated with AF production. Overexpression of this gene resulted in repression of AF biosynthesis. The cellular function of this gene is currently unresolved.

LaeA, a regulator of secondary metabolism in Aspergillus spp

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.

RNA silencing in Aspergillus nidulans is independent of RNA-dependent RNA polymerases

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 expression of sterigmatocystin and penicillin genes in Aspergillus nidulans is controlled by veA, a gene required for sexual development

Secondary metabolism is commonly associated with morphological development in microorganisms, including fungi. We found that veA, a gene previously shown to control the Aspergillus nidulans sexual/asexual developmental ratio in response to light, also controls secondary metabolism. Specifically, veA regulates the expression of genes implicated in the synthesis of the mycotoxin sterigmatocystin and the antibiotic penicillin. veA is necessary for the expression of the transcription factor aflR, which activates the gene cluster that leads to the production of sterigmatocystin. veA is also necessary for penicillin production. Our results indicated that although veA represses the transcription of the isopenicillin synthetase gene ipnA, it is necessary for the expression of acvA, the key gene in the first step of penicillin biosynthesis, encoding the delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase. With respect to the mechanism of veA in directing morphological development, veA has little effect on the expression of the known sexual transcription factors nsdD and steA. However, we found that veA regulates the expression of the asexual transcription factor brlA by modulating the alpha/beta transcript ratio that controls conidiation.

Blockage of methylcitrate cycle inhibits polyketide production in Aspergillus nidulans

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.

Ethylene modulates development and toxin biosynthesis in aspergillus possibly via an ethylene sensor-mediated signaling pathway

Ethylene, a biologically active natural compound, inhibited aflatoxin accumulation by Aspergillus parasiticus on a solid growth medium in a dose-dependent manner at concentrations of 0.1 to 150 ppm. The activity of the nor-1 promoter (an early aflatoxin gene) was reduced to nondetectable levels by similar quantities of ethylene, suggesting that the inhibitory effect on toxin synthesis occurred, at least in part, at the level of transcription. The inhibitory effect of ethylene on aflatoxin accumulation was also observed when A. parasiticus was grown on raw peanuts. Under similar growth conditions and doses, ethylene strongly inhibited development of asci and ascospores in Aspergillus nidulans, with no detectable effect on Hülle cell formation, conidiation, or sterigmatocystin accumulation. During early growth, A. parasiticus and A. nidulans produced ethylene with approximately twofold higher quantities measured in continuous light than in the dark. 1-Methylcyclopropene (an inhibitor of ethylene receptors in plants), light, CO2, temperature, and growth medium composition altered the effect of ethylene on A. nidulans and A. parasiticus. These observations are consistent with the existence of an ethylene sensor molecule that mediates the function of an ethylene-responsive signaling pathway(s) in Aspergillus.

Genetic analysis of morphological variants of Aspergillus parasiticus deficient in secondary metabolite production

Aflatoxins (AFs) are secondary metabolites produced mainly by Aspergillus parasiticus and A. flavus. To study AF regulation, previously isolated non-toxigenic A. parasiticus sec- (for secondary metabolism minus) variants were genetically analysed. In parasexual crossing, the sec- strains failed to form heterokaryons and diploids with other sec- strains. Heterokaryon test results suggested that involvement of cytoplasmic elements in the formation of sec- phenotype was unlikely. At the molecular level, the coding sequence of the sec- aflR (the only known positive regulator of AF pathway) was identical to that of their toxigenic sec+ (for secondary metabolism plus) parents. However, the sec- aflR expression was 5- to 10-fold lower compared to that in the sec+ forms. RT-PCR analysis demonstrated that the AF pathway genes were expressed in the sec- forms but in trace amounts and in their unprocessed forms. Combined, these results suggest that aflR is necessary but not sufficient for AF production and that elements involved in fungal development directly or indirectly influence its proper function.

RcoA has pleiotropic effects on Aspergillus nidulans cellular development

Aspergillus nidulans rcoA encodes a member of the WD repeat family of proteins. The RcoA protein shares sequence similarity with other members of this protein family, including the Saccharomyces cerevisiae Tup1p and Neurospora crassa RCO1. Tup1p is involved in negative regulation of an array of functions including carbon catabolite repression. RCO1 functions in regulating pleiotropic developmental processes, but not carbon catabolite repression. In A. nidulans, deletion of rcoA (DeltarcoA), a recessive mutation, resulted in gross defects in vegetative growth, asexual spore production and sterigmatocystin (ST) biosynthesis. Expression of the asexual and ST pathway-specific regulatory genes, brlA and aflR, respectively, but not the signal transduction genes (i.e. flbA, fluG or fadA) regulating brlA and aflR expression was delayed (brlA) or eliminated (aflR) in a DeltarcoA strain. Overexpression of aflR in a DeltarcoA strain could not rescue normal expression of downstream targets of AflR. CreA-dependent carbon catabolite repression of starch and ethanol utilization was only weakly affected in a DeltarcoA strain. The strong role of RcoA in development, vegetative growth and ST production, compared with a relatively weak role in carbon catabolite repression, is similar to the role of RCO1 in N. crassa.

Genetic involvement of a cAMP-dependent protein kinase in a G protein signaling pathway regulating morphological and chemical transitions in Aspergillus 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.