AcFKH1, a novel member of the forkhead family, associates with the RFX transcription factor CPCR1 in the cephalosporin C-producing fungus Acremonium chrysogenum

The Forkhead Gene fkhB is Necessary for Proper Development in Aspergillus nidulans

The forkhead domain genes are important for development and morphogenesis in fungi. Six forkhead genes fkhA-fkhF have been found in the genome of the model filamentous Ascomycete Aspergillus nidulans. To identify the fkh gene(s) associated with fungal development, we examined mRNA levels of these six genes and found that the level of fkhB and fkhD mRNA was significantly elevated during asexual development and in conidia. To investigate the roles of FkhB and FkhD, we generated fkhB and fkhD deletion mutants and complemented strains and investigated their phenotypes. The deletion of fkhB, but not fkhD, affected fungal growth and both sexual and asexual development. The fkhB deletion mutant exhibited decreased colony size with distinctly pigmented (reddish) asexual spores and a significantly lower number of conidia compared with these features in the wild type (WT), although the level of sterigmatocystin was unaffected by the absence of fkhB. Furthermore, the fkhB deletion mutant produced sexual fruiting bodies (cleistothecia) smaller than those of WT, implying that the fkhB gene is involved in both asexual and sexual development. In addition, fkhB deletion reduced fungal tolerance to heat stress and decreased trehalose accumulation in conidia. Overall, these results suggest that fkhB plays a key role in proper fungal growth, development, and conidial stress tolerance in A. nidulans.

Fungal BGCs for Production of Secondary Metabolites: Main Types, Central Roles in Strain Improvement, and Regulation According to the Piano Principle

Filamentous fungi are one of the most important producers of secondary metabolites. Some of them can have a toxic effect on the human body, leading to diseases. On the other hand, they are widely used as pharmaceutically significant drugs, such as antibiotics, statins, and immunosuppressants. A single fungus species in response to various signals can produce 100 or more secondary metabolites. Such signaling is possible due to the coordinated regulation of several dozen biosynthetic gene clusters (BGCs), which are mosaically localized in different regions of fungal chromosomes. Their regulation includes several levels, from pathway-specific regulators, whose genes are localized inside BGCs, to global regulators of the cell (taking into account changes in pH, carbon consumption, etc.) and global regulators of secondary metabolism (affecting epigenetic changes driven by velvet family proteins, LaeA, etc.). In addition, various low-molecular-weight substances can have a mediating effect on such regulatory processes. This review is devoted to a critical analysis of the available data on the "turning on" and "off" of the biosynthesis of secondary metabolites in response to signals in filamentous fungi. To describe the ongoing processes, the model of "piano regulation" is proposed, whereby pressing a certain key (signal) leads to the extraction of a certain sound from the "musical instrument of the fungus cell", which is expressed in the production of a specific secondary metabolite.

Physiological and metabolic traits of Taxol biosynthesis of endophytic fungi inhabiting plants: Plant-microbial crosstalk, and epigenetic regulators

Attenuating the Taxol productivity of fungi with the subculturing and storage under axenic conditions is the challenge that halts the feasibility of fungi to be an industrial platform for Taxol production. This successive weakening of Taxol productivity by fungi could be attributed to the epigenetic down-regulation and molecular silencing of most of the gene clusters encoding Taxol biosynthetic enzymes. Thus, exploring the epigenetic regulating mechanisms controlling the molecular machinery of Taxol biosynthesis could be an alternative prospective technology to conquer the lower accessibility of Taxol by the potent fungi. The current review focuses on discussing the different molecular approaches, epigenetic regulators, transcriptional factors, metabolic manipulators, microbial communications and microbial cross-talking approaches on restoring and enhancing the Taxol biosynthetic potency of fungi to be industrial platform for Taxol production.

The arthrospore-related gene Acaxl2 is involved in cephalosporin C production in industrial Acremonium chrysogenum by the regulatory factors AcFKH1 and CPCR1

Cephalosporin C (CPC) production is often accompanied by a typical morphological differentiation of Acremonium chrysogenum, involving the fragmentation of its hyphae into arthrospores. The type I integral plasma membrane protein Axl2 is a central component of the bud site selection system (BSSS), which was identified as the regulatory factor involved in the hyphal septation process and arthrospore formation. Using CRISPR/Cas9 technology and homologous recombination (HR), we inserted an egfp donor DNA sequence into the Acaxl2 locus, causing the generation of the deletion strain Ac-ΔAcaxl2::eGFP from Acremonium chrysogenum FC³-5-23, the industrial producer of CPC. The mycelial morphology of the deletion strain Ac-ΔAcaxl2::eGFP was mainly composed of arthrospores with a characteristic diameter of 2-8 μm, which increased from 75% at 48h to 90% at 72h post culture and were maintained until the end of the fermentation process. However, the deletion strain showed accelerated production of CPC, and the final titer was 5573μg/ml, which was nearly three times higher than that of the control strain FC³-5-23. The up-regulation of genes related to the biosynthesis gene cluster in Ac-ΔAcaxl2::eGFP, especially the "late" genes, was one reason why its CPC production was higher than that of the original strain. Furthermore, compared with FC³-5-23, the more significant increase of genes involved in the BSSS (Acbud3 and Acbud4) in Ac-ΔAcaxl2::eGFP in the late stage of fermentation, may be responsible for this increase in arthrospore formation. Similarily, the transcription of the regulatory factors AcFKH1 and CPCR1 were also markedly increased at this time and may be the factors responsible for the regulation of CPC synthesis. These results indicated that Acaxl2 plays an important role in both arthrospore formation and CPC production, strongly implicating these regulatory factors as having pivotal links between mycelial morphology and secondary metabolite production in high-yielding A. chrysogenum. To the opposite, the axl2 gene knockout of wild strain CGMCC 3.3795 did not significantly influence the CPC production, which reflected the complexity of the secondary metabolic process and the differences in the function of axl2 gene in high- and low-yielding strains.

Endophytic Fungi: From Symbiosis to Secondary Metabolite Communications or Vice Versa?

Endophytic fungi (EF) are a group of fascinating host-associated fungal communities that colonize the intercellular or intracellular spaces of host tissues, providing beneficial effects to their hosts while gaining advantages. In recent decades, accumulated research on endophytic fungi has revealed their biodiversity, wide-ranging ecological distribution, and multidimensional interactions with host plants and other microbiomes in the symbiotic continuum. In this review, we highlight the role of secondary metabolites (SMs) as effectors in these multidimensional interactions, and the biosynthesis of SMs in symbiosis via complex gene expression regulation mechanisms in the symbiotic continuum and via the mimicry or alteration of phytochemical production in host plants. Alternative biological applications of SMs in modern medicine, agriculture, and industry and their major classes are also discussed. This review recapitulates an introduction to the research background, progress, and prospects of endophytic biology, and discusses problems and substantive challenges that need further study.

Omics Approaches Applied to Penicillium chrysogenum and Penicillin Production: Revealing the Secrets of Improved Productivity

Penicillin biosynthesis by Penicillium chrysogenum is one of the best-characterized biological processes from the genetic, molecular, biochemical, and subcellular points of view. Several omics studies have been carried out in this filamentous fungus during the last decade, which have contributed to gathering a deep knowledge about the molecular mechanisms underlying improved productivity in industrial strains. The information provided by these studies is extremely useful for enhancing the production of penicillin or other bioactive secondary metabolites by means of Biotechnology or Synthetic Biology.

AcAxl2 and AcMst1 regulate arthrospore development and stress resistance in the cephalosporin C producer Acremonium chrysogenum

The filamentous fungus Acremonium chrysogenum is the primordial producer of the β-lactam antibiotic cephalosporin C. This antibiotic is of major biotechnological and medical relevance because of its antibacterial activity against Gram-positive and Gram-negative bacteria. Antibiotic production during the lag phase of fermentation is often accompanied by a typical morphological feature of A. chrysogenum, the fragmentation of the mycelium into arthrospores. Here, we sought to identify factors that regulate the hyphal septation process and present the first comparative functional characterization of the type I integral plasma membrane protein Axl2 (axial budding pattern protein 2), a central component of the bud site selection system (BSSS) and Mst1 (mammalian Sterile20-like kinase), a septation initiation network (SIN)-associated germinal center kinase (GCK). Although an Acaxl2 deletion strain showed accelerated arthrospore formation after 96 h in liquid culture, deletion of Acmst1 led to a 24 h delay in arthrospore development. The overexpression of Acaxl2 resulted in an arthrospore formation similar to the A3/2 strain. In contrast to this, A3/2::Acmst1 OE strain displayed an enhanced arthrospore titer. Large-scale stress tests revealed an involvement of AcAxl2 in controlling osmotic, endoplasmic reticulum, and cell wall stress response. In a similar approach, we found that AcMst1 plays an essential role in regulating growth under osmotic, cell wall, and oxidative stress conditions. Microscopic analyses and plating assays on media containing Calcofluor White and NaCl showed that arthrospore development is a stress-dependent process. Our results suggest the potential for identifying candidate genes for strain improvement programs to optimize industrial fermentation processes.

An atypical forkhead-containing transcription factor SsFKH1 is involved in sclerotial formation and is essential for pathogenicity in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum (Lib.) de Bary is a necrotrophic plant pathogen with a worldwide distribution. The sclerotia of S. sclerotiorum are pigmented multicellular structures formed from the aggregation of vegetative hyphae. These survival structures play a central role in the life and infection cycles of this pathogen. Here, we characterized an atypical forkhead (FKH)-box-containing protein, SsFKH1, involved in sclerotial development and virulence. To investigate the role of SsFkh1 in S. sclerotiorum, the partial sequence of SsFkh1 was cloned and RNA interference (RNAi)-based gene silencing was employed to alter the expression of SsFkh1. RNA-silenced mutants with significantly reduced SsFkh1 RNA levels exhibited slow hyphal growth and sclerotial developmental defects. In addition, the expression levels of a set of putative melanin biosynthesis-related laccase genes and a polyketide synthase-encoding gene were significantly down-regulated in silenced strains. Disease assays demonstrated that pathogenicity in RNAi-silenced strains was significantly compromised with the development of a smaller infection lesion on tomato leaves. Collectively, the results suggest that SsFkh1 is involved in hyphal growth, virulence and sclerotial formation in S. sclerotiorum.

Catalytic Subunit 1 of Protein Phosphatase 2A Is a Subunit of the STRIPAK Complex and Governs Fungal Sexual Development

The generation of complex three-dimensional structures is a key developmental step for most eukaryotic organisms. The details of the molecular machinery controlling this step remain to be determined. An excellent model system to study this general process is the generation of three-dimensional fruiting bodies in filamentous fungi like Sordaria macrospora Fruiting body development is controlled by subunits of the highly conserved striatin-interacting phosphatase and kinase (STRIPAK) complex, which has been described in organisms ranging from yeasts to humans. The highly conserved heterotrimeric protein phosphatase PP2A is a subunit of STRIPAK. Here, catalytic subunit 1 of PP2A was functionally characterized. The Δpp2Ac1 strain is sterile, unable to undergo hyphal fusion, and devoid of ascogonial septation. Further, PP2Ac1, together with STRIPAK subunit PRO22, governs vegetative and stress-related growth. We revealed in vitro catalytic activity of wild-type PP2Ac1, and our in vivo analysis showed that inactive PP2Ac1 blocks the complementation of the sterile deletion strain. Tandem affinity purification, followed by mass spectrometry and yeast two-hybrid analysis, verified that PP2Ac1 is a subunit of STRIPAK. Further, these data indicate links between the STRIPAK complex and other developmental signaling pathways, implying the presence of a large interconnected signaling network that controls eukaryotic developmental processes. The insights gained in our study can be transferred to higher eukaryotes and will be important for understanding eukaryotic cellular development in general.

IMPORTANCE

The striatin-interacting phosphatase and kinase (STRIPAK) complex is highly conserved from yeasts to humans and is an important regulator of numerous eukaryotic developmental processes, such as cellular signaling and cell development. Although functional insights into the STRIPAK complex are accumulating, the detailed molecular mechanisms of single subunits are only partially understood. The first fungal STRIPAK was described in Sordaria macrospora, which is a well-established model organism used to study the formation of fungal fruiting bodies, three-dimensional organ-like structures. We analyzed STRIPAK subunit PP2Ac1, catalytic subunit 1 of protein phosphatase PP2A, to study the importance of the catalytic activity of this protein during sexual development. The results of our yeast two-hybrid analysis and tandem affinity purification, followed by mass spectrometry, indicate that PP2Ac1 activity connects STRIPAK with other signaling pathways and thus forms a large interconnected signaling network.

The Sclerotinia sclerotiorum FoxE2 Gene Is Required for Apothecial Development

Sclerotinia sclerotiorum is a widely dispersed plant pathogenic fungus causing many diseases such as white mold, Sclerotinia stem rot, stalk rot, and Sclerotinia head rot on many varieties of broadleaf crops worldwide. Previous studies have shown that the Forkhead-box transcription factors (FOX TFs) play key regulatory roles in the sexual reproduction of some fungi. Ss-FoxE2 is one of four FOX TF family member genes in S. sclerotiorum. Based on ortholog function in other fungi it is hypothesized to function in S. sclerotiorum sexual reproduction. In this study, the role of Ss-FoxE2 in S. sclerotiorum was identified with a gene knock-out strategy. Following transformation and screening, strains having undergone homologous recombination in which the hygromycin resistance gene replaced the gene Ss-FoxE2 from the genomic DNA were identified. No difference in hyphae growth, number, and weight of sclerotia and no obvious change in virulence was observed among the wild type Ss-FoxE2 knock-out mutant and genetically complemented mutant; however, following induction of sclerotia for sexual development, apothecia were not formed in Ss-FoxE2 knock-out mutant. The Ss-FoxE2 gene expressed significantly higher in the apothecial stages than in other developmental stages. These results indicate that Ss-FoxE2 appears to be necessary for the regulation of sexual reproduction, but may not affect the pathogenicity and vegetative development of S. sclerotiorum significantly.

Modulation of genetic clusters for synthesis of bioactive molecules in fungal endophytes: A review

Novel drugs with unique and targeted mode of action are very much need of the hour to treat and manage severe multidrug infections and other life-threatening complications. Though natural molecules have proved to be effective and environmentally safe, the relative paucity of discovery of new drugs has forced us to lean towards synthetic chemistry for developing novel drug molecules. Plants and microbes are the major resources that we rely upon in our pursuit towards discovery of novel compounds of pharmacological importance with less toxicity. Endophytes, an eclectic group of microbes having the potential to chemically bridge the gap between plants and microbes, have attracted the most attention due to their relatively high metabolic versatility. Since continuous large scale supply of major metabolites from microfungi and especially endophytes is severely impeded by the phenomenon of attenuation in axenic cultures, the major challenge is to understand the regulatory mechanisms in operation that drive the expression of metabolic gene clusters of pharmaceutical importance. This review is focused on the major regulatory elements that operate in filamentous fungi and various combinatorial multi-disciplinary approaches involving bioinformatics, molecular biology, and metabolomics that could aid in large scale synthesis of important lead molecules.

PcFKH1, a novel regulatory factor from the forkhead family, controls the biosynthesis of penicillin in Penicillium chrysogenum

Penicillin biosynthesis in Penicillium chrysogenum (re-identified as Penicillium rubens) is a good example of a biological process subjected to complex global regulatory networks and serves as a model to study fungal secondary metabolism. The winged-helix family of transcription factors recently described, which includes the forkhead type of proteins, is a key type of regulatory proteins involved in this process. In yeasts and humans, forkhead transcription factors are involved in different processes (cell cycle regulation, cell death control, pre-mRNA processing and morphogenesis); one member of this family of proteins has been identified in the P. chrysogenum genome (Pc18g00430). In this work, we have characterized this novel transcription factor (named PcFKH1) by generating knock-down mutants and overexpression strains. Results clearly indicate that PcFKH1 positively controls antibiotic biosynthesis through the specific interaction with the promoter region of the penDE gene, thus regulating penDE mRNA levels. PcFKH1 also binds to the pcbC promoter, but with low affinity. In addition, it also controls other ancillary genes of the penicillin biosynthetic process, such as phlA (encoding phenylacetyl CoA ligase) and ppt (encoding phosphopantetheinyl transferase). PcFKH1 also plays a role in conidiation and spore pigmentation, but it does not seem to be involved in hyphal morphology or cell division in the improved laboratory reference strain Wisconsin 54-1255. A genome-wide analysis of processes putatively coregulated by PcFKH1 and PcRFX1 (another winged-helix transcription factor) in P. chrysogenum provided evidence of the global effect of these transcription factors in P. chrysogenum metabolism.

A new transcription factor for mitosis: in Schizosaccharomyces pombe, the RFX transcription factor Sak1 works with forkhead factors to regulate mitotic expression

Mitotic genes are one of the most strongly oscillating groups of genes in the eukaryotic cell cycle. Understanding the regulation of mitotic gene expression is a key issue in cell cycle control but is poorly understood in most organisms. Here, we find a new mitotic transcription factor, Sak1, in the fission yeast Schizosaccharomyces pombe. Sak1 belongs to the RFX family of transcription factors, which have not previously been connected to cell cycle control. Sak1 binds upstream of mitotic genes in close proximity to Fkh2, a forkhead transcription factor previously implicated in regulation of mitotic genes. We show that Sak1 is the major activator of mitotic gene expression and also confirm the role of Fkh2 as the opposing repressor. Sep1, another forkhead transcription factor, is an activator for a small subset of mitotic genes involved in septation. From yeasts to humans, forkhead transcription factors are involved in mitotic gene expression and it will be interesting to see whether RFX transcription factors may also be involved in other organisms.

Disruption of the nitrogen regulatory gene AcareA in Acremonium chrysogenum leads to reduction of cephalosporin production and repression of nitrogen metabolism

AcareA, encoding a homologue of the fungal nitrogen regulatory GATA zinc-finger proteins, was cloned from Acremonium chrysogenum. Gene disruption and genetic complementation revealed that AcareA was required for nitrogen metabolism and cephalosporin production. Disruption of AcareA resulted in growth defect in the medium using nitrate, uric acid and low concentration of ammonium, glutamine or urea as sole nitrogen source. Transcriptional analysis showed that the transcription of niaD/niiA was increased drastically when induced with nitrate in the wild-type and AcareA complemented strains but not in AcareA disruption mutant. Consistent with the reduction of cephalosporin production, the transcription of pcbAB, cefD2, cefEF and cefG encoding the enzymes for cephalosporin production was reduced in AcareA disruption mutant. Band shift assays showed that AcAREA bound to the promoter regions of niaD, niiA and the bidirectional promoter region of pcbAB-pcbC. Sequence analysis showed that all the AcAREA binding sites contain the consensus GATA elements. These results indicated that AcAREA plays an important role both in the regulation of nitrogen metabolism and cephalosporin production in A. chrysogenum.

Members of the Penicillium chrysogenum velvet complex play functionally opposing roles in the regulation of penicillin biosynthesis and conidiation

A velvet multisubunit complex was recently detected in the filamentous fungus Penicillium chrysogenum, the major industrial producer of the β-lactam antibiotic penicillin. Core components of this complex are P. chrysogenum VelA (PcVelA) and PcLaeA, which regulate secondary metabolite production, hyphal morphology, conidiation, and pellet formation. Here we describe the characterization of PcVelB, PcVelC, and PcVosA as novel subunits of this velvet complex. Using yeast two-hybrid analysis and bimolecular fluorescence complementation (BiFC), we demonstrate that all velvet proteins are part of an interaction network. Functional analyses using single- and double-knockout strains clearly indicate that velvet subunits have opposing roles in the regulation of penicillin biosynthesis and light-dependent conidiation. PcVelC, together with PcVelA and PcLaeA, activates penicillin biosynthesis, while PcVelB represses this process. In contrast, PcVelB and PcVosA promote conidiation, while PcVelC has an inhibitory effect. Our genetic analyses further show that light-dependent spore formation depends not only on PcVelA but also on PcVelB and PcVosA. The results provided here contribute to our fundamental understanding of the function of velvet subunits as part of a regulatory network mediating signals responsible for morphology and secondary metabolism and will be instrumental in generating mutants with newly derived properties that are relevant to strain improvement programs.

The regulatory factor PcRFX1 controls the expression of the three genes of β-lactam biosynthesis in Penicillium chrysogenum

Penicillin biosynthesis is subjected to a complex regulatory network of signalling molecules that may serve as model for other secondary metabolites. The information provided by the new "omics" era about Penicillium chrysogenum and the advances in the knowledge of molecular mechanisms responsible for improved productivity, make this fungus an excellent model to decipher the mechanisms controlling the penicillin biosynthetic pathway. In this work, we have characterized a novel transcription factor PcRFX1, which is an ortholog of the Acremonium chrysogenum CPCR1 and Penicillium marneffei RfxA regulatory proteins. PcRFX1 DNA binding sequences were found in the promoter region of the pcbAB, pcbC and penDE genes. We show in this article that these motifs control the expression of the β-galactosidase lacZ reporter gene, indicating that they may direct the PcRFX1-mediated regulation of the penicillin biosynthetic genes. By means of Pcrfx1 gene knock-down and overexpression techniques we confirmed that PcRFX1 controls penicillin biosynthesis through the regulation of the pcbAB, pcbC and penDE transcription. Morphology and development seemed not to be controlled by this transcription factor under the conditions studied and only sporulation was slightly reduced after the silencing of the Pcrfx1 gene. A genome-wide analysis of processes putatively regulated by this transcription factor was carried out in P. chrysogenum. Results suggested that PcRFX1, in addition to regulate penicillin biosynthesis, is also involved in the control of several pathways of primary metabolism.

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.

Stimulation of cephalosporin C production in Acremonium chrysogenum M35 by glycerol

In this study, the effects of glycerol on cephalosporin C production by Acremonium chrysogenum M35 were evaluated. The addition of glycerol increased cephalosporin production by up to 12-fold. Glycerol caused the upregulation of the transcription of the isopenicillin synthase (pcbC) and transporter (cefT) genes in early exponential phase, and affected the cell morphology since hyphal fragments differentiated into arthrospores. These results indicate that glycerol effectively enhances cephalosporin C production via stimulation of cell differentiation.

The RFX protein RfxA is an essential regulator of growth and morphogenesis in Penicillium marneffei

Fungi are small eukaryotes capable of undergoing multiple complex developmental programs. The opportunistic human pathogen Penicillium marneffei is a dimorphic fungus, displaying vegetative (proliferative) multicellular hyphal growth at 25 degrees C and unicellular yeast growth at 37 degrees C. P. marneffei also undergoes asexual development into differentiated multicellular conidiophores bearing uninucleate spores. These morphogenetic processes require regulated changes in cell polarity establishment, cell cycle dynamics, and nuclear migration. The RFX (regulatory factor X) proteins are a family of transcriptional regulators in eukaryotes. We sought to determine how the sole P. marneffei RFX protein, RfxA, contributes to the regulation of morphogenesis. Attempts to generate a haploid rfxA deletion strain were unsuccessful, but we did isolate an rfxA(+)/rfxADelta heterozygous diploid strain. The role of RfxA was assessed using conditional overexpression, RNA interference (RNAi), and the production of dominant interfering alleles. Reduced RfxA function resulted in defective mitoses during growth at 25 degrees C and 37 degrees C. This was also observed for the heterozygous diploid strain during growth at 37 degrees C. In contrast, overexpression of rfxA caused growth arrest during conidial germination. The data show that rfxA must be precisely regulated for appropriate nuclear division and to maintain genome integrity. Perturbations in rfxA expression also caused defects in cellular proliferation and differentiation. The data suggest a role for RfxA in linking cellular division with morphogenesis, particularly during conidiation and yeast growth, where the uninucleate state of these cell types necessitates coupling of nuclear and cellular division tighter than that observed during multinucleate hyphal growth.

Asexual cephalosporin C producer Acremonium chrysogenum carries a functional mating type locus

Acremonium chrysogenum, the fungal producer of the pharmaceutically relevant beta-lactam antibiotic cephalosporin C, is classified as asexual because no direct observation of mating or meiosis has yet been reported. To assess the potential of A. chrysogenum for sexual reproduction, we screened an expressed sequence tag library from A. chrysogenum for the expression of mating type (MAT) genes, which are the key regulators of sexual reproduction. We identified two putative mating type genes that are homologues of the alpha-box domain gene, MAT1-1-1 and MAT1-1-2, encoding an HPG domain protein defined by the presence of the three invariant amino acids histidine, proline, and glycine. In addition, cDNAs encoding a putative pheromone receptor and pheromone-processing enzymes, as well as components of a pheromone response pathway, were found. Moreover, the entire A. chrysogenum MAT1-1 (AcMAT1-1) gene and regions flanking the MAT region were obtained from a genomic cosmid library, and sequence analysis revealed that in addition to AcMAT1-1-1 and AcMAT1-1-2, the AcMAT1-1 locus comprises a third mating type gene, AcMAT1-1-3, encoding a high-mobility-group domain protein. The alpha-box domain sequence of AcMAT1-1-1 was used to determine the phylogenetic relationships of A. chrysogenum to other ascomycetes. To determine the functionality of the AcMAT1-1 locus, the entire MAT locus was transferred into a MAT deletion strain of the heterothallic ascomycete Podospora anserina (the PaDeltaMAT strain). After fertilization with a P. anserina MAT1-2 (MAT(+)) strain, the corresponding transformants developed fruiting bodies with mature ascospores. Thus, the results of our functional analysis of the AcMAT1-1 locus provide strong evidence to hypothesize a sexual cycle in A. chrysogenum.

Natural products of filamentous fungi: enzymes, genes, and their regulation

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.

A high throughput targeted gene disruption method for Alternaria brassicicola functional genomics using linear minimal element (LME) constructs

Alternaria brassicicola causes black spot disease of cultivated Brassicas and has been used consistently as a necrotrophic fungal pathogen for studies with Arabidopsis. In A. brassicicola, mutant generation has been the most rate-limiting step for the functional analysis of individual genes due to low efficiency of both transformation and targeted integration. To improve the targeted gene disruption efficiency as well as to expedite gene disruption construct production, we used a short linear construct with minimal elements, an antibiotic resistance selectable marker gene, and a 250- to 600-bp-long partial target gene. The linear minimal element (LME) constructs consistently produced stable transformants for diverse categories of genes. Typically, 100% of the transformants were targeted gene disruption mutants when using the LME constructs, compared with inconsistent transformation and usually less than 10% targeted gene disruption with circular plasmid disruption constructs. Each mutant displayed a unique molecular signature thought to originate from endogenous exonuclease activities in fungal cells. Our data suggests that a DNA double-stranded break repair mechanism (DSBR) functions to increase targeting efficiency. This method is advantageous for high throughput gene disruption, overexpression, and reporter gene introduction within target genes, especially for asexual filamentous fungi where genetic approaches are unfavorable.

Fungal secondary metabolism — from biochemistry to genomics

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.

CPCR1, but not its interacting transcription factor AcFKH1, controls fungal arthrospore formation in Acremonium chrysogenum

Fungal morphogenesis and secondary metabolism are frequently associated; however, the molecular determinants connecting both processes remain largely undefined. Here we demonstrate that CPCR1 (cephalosporin C regulator 1 from Acremonium chrysogenum), a member of the winged helix/regulator factor X (RFX) transcription factor family that regulates cephalosporin C biosynthesis, also controls morphological development in the beta-lactam producer A. chrysogenum. The use of a disruption strain, multicopy strains as well as several recombinant control strains revealed that CPCR1 is required for hyphal fragmentation, and thus the formation of arthrospores. In a DeltacpcR1 disruption strain that exhibits only hyphal growth, the wild-type cpcR1 gene was able to restore arthrospore formation; a phenomenon not observed for DeltacpcR1 derivatives or non-related genes. The intracellular expression of cpcR1, and control genes (pcbC, egfp) was determined by in vivo monitoring of fluorescent protein fusions. Further, the role of the forkhead transcription factor AcFKH1, which directly interacts with CPCR1, was studied by generating an Acfkh1 knockout strain. In contrast to CPCR1, AcFKH1 is not directly involved in the fragmentation of hyphae. Instead, the presence of AcFKH1 seems to be necessary for CPCR1 function in A. chrysogenum morphogenesis, as overexpression of a functional cpcR1 gene in a DeltaAcfkh1 background has no effect on arthrospore formation. Moreover, strains lacking Acfkh1 exhibit defects in cell separation, indicating an involvement of the forkhead transcription factor in mycelial growth of A. chrysogenum. Our data offer the potential to control fungal growth in biotechnical processes that require defined morphological stages for optimal production yields.

Regulation of secondary metabolism in filamentous fungi

Fungal secondary metabolites are of intense interest to humankind due to their pharmaceutical (antibiotics) and/or toxic (mycotoxins) properties. In the past decade, tremendous progress has been made in understanding the genes that are associated with production of various fungal secondary metabolites. Moreover, the regulatory mechanisms controlling biosynthesis of diverse groups of secondary metabolites have been unveiled. In this review, we present the current understanding of the genetic regulation of secondary metabolism from clustering of biosynthetic genes to global regulators balancing growth, sporulation, and secondary metabolite production in selected fungi with emphasis on regulation of metabolites of agricultural concern. Particularly, the roles of G protein signaling components and developmental regulators in the mycotoxin sterigmatocystin biosynthesis in the model fungus Aspergillus nidulans are discussed in depth.

Use of bimolecular fluorescence complementation to demonstrate transcription factor interaction in nuclei of living cells from the filamentous fungus Acremonium chrysogenum

Using bimolecular fluorescence complementation assays, we were able to demonstrate protein-protein interaction of the transcription factors AcFKH1 and CPCR1 in living cells from the filamentous fungus Acremonium chrysogenum. This was accomplished by splitting the gene for the enhanced yellow fluorescent protein (EYFP) into two parts encoding the N- and C-terminus. Both fragments were fused to different gene derivatives of the fungal transcription factors. The recombinant plasmids were used to generate transgenic fungal strains for subsequent confocal laser microscopy. Only when the full-length transcription factors were fused to EYFP fragments yellow fluorescence was observed due to the bimolecular complementation of both chimeric proteins. The nuclear localization of the protein-protein interaction was verified by staining fungal cells with the nucleic acid dye TOTO-3.