Functional organization of the Aspergillus nidulans trpC promoter

A novel polyketide synthase gene cluster in the plant pathogenic fungus Pseudocercospora fijiensis

Pseudocercospora fijiensis, causal agent of black Sigatoka of banana, produces polyketide synthase (PKS) pathways shown to be important in disease development by related Dothideomycete fungi. Genome analysis of the P. fijiensis PKS8-1 gene identified it as part of a gene cluster including genes encoding two transcription factors, a regulatory protein, a glyoxylase/beta-lactamase-like protein, an MFS transporter, a cytochrome P450, two aldo/keto reductases, a dehydrogenase, and a decarboxylase. Genome analysis of the related pathogens Pseudocercospora musae, Pseudocercospora eumusae, and Pseudocercospora pini-densiflorae, identified orthologous clusters containing a nearly identical combination of genes. Phylogenetic analysis of PKS8-1 identified homology to PKS proteins in the monodictyphenone and cladofulvin pathways in Aspergillus nidulans and Cladosporium fulvum, respectively. Analysis of clustered genes showed that the PKS8-1 cluster shares genes for enzymes involved in the production of the emodin intermediate in the monodictyphenone and cladofulvin pathways, but differs in many genes, suggesting production of a different metabolic product. Time course analysis of gene expression in infected banana showed up-regulation of PKS8-1 and four of eight clustered genes as early as 2 weeks post-inoculation and remaining high through 9 weeks. Overexpression of the pathway through constitutive expression of an aflR-like transcription factor gene in the cluster resulted in increased expression in culture of PKS8-1 as well as the four clustered genes that are up-regulated in infected plants. No differences were seen in timing or severity of disease symptoms with the overexpression strains relative to controls, however gene expression analysis showed no difference in expression in planta by an overexpression strain relative to controls. Thus constitutive expression of the aflR-like gene is not sufficient to upregulate the pathway above normal expression in planta. Pathway expression during all phases of disease development and conservation of the pathway in related Pseudocercospora species support a role for this pathway in disease.

The Aspergillus fumigatus Sialidase (Kdnase) Contributes to Cell Wall Integrity and Virulence in Amphotericin B-Treated Mice

Aspergillus fumigatus is a filamentous fungus that can cause a life-threatening invasive pulmonary aspergillosis (IPA) in immunocompromised individuals. We previously characterized an exo-sialidase from A. fumigatus that prefers the sialic acid substrate, 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (Kdn); hence it is a Kdnase. Sialidases are known virulence factors in other pathogens; therefore, the goal of our study was to evaluate the importance of Kdnase in A. fumigatus. A kdnase knockout strain (Δkdnase) was unable to grow on medium containing Kdn and displayed reduced growth and abnormal morphology. Δkdnase was more sensitive than wild type to hyperosmotic conditions and the antifungal agent, amphotericin B. In contrast, Δkdnase had increased resistance to nikkomycin, Congo Red and Calcofluor White indicating activation of compensatory cell wall chitin deposition. Increased cell wall thickness and chitin content in Δkdnase were confirmed by electron and immunofluorescence microscopy. In a neutropenic mouse model of invasive aspergillosis, the Δkdnase strain had attenuated virulence and a significantly lower lung fungal burden but only in animals that received liposomal amphotericin B after spore exposure. Macrophage numbers were almost twofold higher in lung sections from mice that received the Δkdnase strain, possibly related to higher survival of macrophages that internalized the Δkdnase conidia. Thus, A. fumigatus Kdnase is important for fungal cell wall integrity and virulence, and because Kdnase is not present in the host, it may represent a potential target for the development of novel antifungal agents.

The Aspergillus nidulans metZ gene encodes a transcription factor involved in regulation of sulfur metabolism in this fungus and other Eurotiales

In Aspergillus nidulans, expression of sulfur metabolism genes is activated by the MetR transcription factor containing a basic region and leucine zipper domain (bZIP). Here we identified and characterized MetZ, a new transcriptional regulator in A. nidulans and other Eurotiales. It contains a bZIP domain similar to the corresponding region in MetR and this similarity suggests that MetZ could potentially complement the MetR deficiency. The metR and metZ genes are interrupted by unusually long introns. Transcription of metZ, unlike that of metR, is controlled by the sulfur metabolite repression system (SMR) dependent on the MetR protein. Overexpression of metZ from a MetR-independent promoter in a ΔmetR background activates transcription of genes encoding sulfate permease, homocysteine synthase and methionine permease, partially complementing the phenotype of the ΔmetR mutation. Thus, MetZ appears to be a second transcription factor involved in regulation of sulfur metabolism genes.

The MAT locus genes play different roles in sexual reproduction and pathogenesis in Fusarium graminearum

Sexual reproduction plays a critical role in the infection cycle of Fusarium graminearum because ascospores are the primary inoculum. As a homothallic ascomycete, F. graminearum contains both the MAT1-1 and MAT1-2-1 loci in the genome. To better understand their functions and regulations in sexual reproduction and pathogenesis, in this study we assayed the expression, interactions, and mutant phenotypes of individual MAT locus genes. Whereas the expression of MAT1-1-1 and MAT12-1 rapidly increased after perithecial induction and began to decline after 1 day post-perithecial induction (dpi), the expression of MAT1-1-2 and MAT1-1-3 peaked at 4 dpi. MAT1-1-2 and MAT1-1-3 had a similar expression profile and likely are controlled by a bidirectional promoter. Although none of the MAT locus genes were essential for perithecium formation, all of them were required for ascosporogenesis in self-crosses. In outcrosses, the mat11-1-2 and mat11-1-3 mutants were fertile but the mat1-1-1 and mat1-2-1 mutants displayed male- and female-specific defects, respectively. The mat1-2-1 mutant was reduced in FgSO expression and hyphal fusion. Mat1-1-2 interacted with all other MAT locus transcription factors, suggesting that they may form a protein complex during sexual reproduction. Mat1-1-1 also interacted with FgMcm1, which may play a role in controlling cell identity and sexual development. Interestingly, the mat1-1-1 and mat1-2-1 mutants were reduced in virulence in corn stalk rot assays although none of the MAT locus genes was important for wheat infection. The MAT1-1-1 and MAT1-2-1 genes may play a host-specific role in colonization of corn stalks.

Transcription factor Amr1 induces melanin biosynthesis and suppresses virulence in Alternaria brassicicola

Alternaria brassicicola is a successful saprophyte and necrotrophic plant pathogen. Several A. brassicicola genes have been characterized as affecting pathogenesis of Brassica species. To study regulatory mechanisms of pathogenesis, we mined 421 genes in silico encoding putative transcription factors in a machine-annotated, draft genome sequence of A. brassicicola. In this study, targeted gene disruption mutants for 117 of the transcription factor genes were produced and screened. Three of these genes were associated with pathogenesis. Disruption mutants of one gene (AbPacC) were nonpathogenic and another gene (AbVf8) caused lesions less than half the diameter of wild-type lesions. Unexpectedly, mutants of the third gene, Amr1, caused lesions with a two-fold larger diameter than the wild type and complementation mutants. Amr1 is a homolog of Cmr1, a transcription factor that regulates melanin biosynthesis in several fungi. We created gene deletion mutants of Δamr1 and characterized their phenotypes. The Δamr1 mutants used pectin as a carbon source more efficiently than the wild type, were melanin-deficient, and more sensitive to UV light and glucanase digestion. The AMR1 protein was localized in the nuclei of hyphae and in highly melanized conidia during the late stage of plant pathogenesis. RNA-seq analysis revealed that three genes in the melanin biosynthesis pathway, along with the deleted Amr1 gene, were expressed at low levels in the mutants. In contrast, many hydrolytic enzyme-coding genes were expressed at higher levels in the mutants than in the wild type during pathogenesis. The results of this study suggested that a gene important for survival in nature negatively affected virulence, probably by a less efficient use of plant cell-wall materials. We speculate that the functions of the Amr1 gene are important to the success of A. brassicicola as a competitive saprophyte and plant parasite.

A split luciferase complementation assay for studying in vivo protein-protein interactions in filamentous ascomycetes

Protein-protein interactions play important roles in controlling many cellular events. To date, several techniques have been developed for detection of protein-protein interactions in living cells, among which split luciferase complementation has been applied in animal and plant cells. Here, we examined whether the split luciferase assay could be used in filamentous ascomycetes, such as Gibberella zeae and Cochliobolus heterostrophus. The coding sequences of two strongly interacting proteins (the F-box protein, FBP1, and its partner SKP1) in G. zeae, under the control of the cryparin promoter from Cryphonectria parasitica, were translationally fused to the C- and N-terminal fragments of firefly luciferase (luc), respectively. Each fusion product inserted into a fungal transforming vector carrying the gene for resistance to either geneticin or hygromycin B, was transformed into both fungi. We detected complementation of split luciferase proteins driven by interaction of the two fungal proteins with a high luminescence intensity-to-background ratio only in the fungal transformants expressing both N-luc and C-luc fusion constructs. Using this system, we also confirmed a novel protein interaction between transcription factors, GzMCM1 and FST12 in G. zeae, which could hardly be proven by the yeast two-hybrid method. This is the first study demonstrating that monitoring of split luciferase complementation is a sensitive and efficient method of studying in vivo protein-protein interactions in filamentous ascomycetes.

Chromosomal location plays a role in regulation of aflatoxin gene expression in Aspergillus parasiticus

The nor-1 gene in the filamentous fungus Aspergillus parasiticus encodes a ketoreductase involved in aflatoxin biosynthesis. To study environmental influences on nor-1 expression, we generated plasmid pAPGUSNNB containing a nor-1 promoter-beta-glucuronidase (GUS) (encoded by uidA) reporter fusion with niaD (encodes nitrate reductase) as a selectable marker. niaD transformants of A. parasiticus strain NR-1 (niaD) carried pAPGUSNNB integrated predominantly at the nor-1 or niaD locus. Expression of the native nor-1 and nor-1::GUS reporter was compared in transformants grown under aflatoxin-inducing conditions by Northern and Western analyses and by qualitative and quantitative GUS activity assays. The timing and level of nor-1 promoter function with pAPGUSNNB integrated at nor-1 was similar to that observed for the native nor-1 gene. In contrast, nor-1 promoter activity in pAPGUSNNB and a second nor-1::GUS reporter construct, pBNG3.0, was not detectable when integration occurred at niaD. Because niaD-dependent regulation could account for the absence of expression at niaD, a third chromosomal location was analyzed using pAPGUSNP, which contained nor-1::GUS plus pyrG (encodes OMP decarboxylase) as a selectable marker. GUS expression was detectable only when pAPGUSNP integrated at nor-1 and was not detectable at pyrG, even under growth conditions that required pyrG expression. nor-1::GUS is regulated similarly to the native nor-1 gene when it is integrated at its homologous site within the aflatoxin gene cluster but is not expressed at native nor-1 levels at two locations outside of the aflatoxin gene cluster. We conclude that the GUS reporter system can be used effectively to measure nor-1 promoter activity and that nor-1 is subject to position-dependent regulation in the A. parasiticus chromosome.

Transcriptional autoregulation and inhibition of mRNA translation of amino acid regulator gene cpcA of filamentous fungus Aspergillus nidulans

The CPCA protein of the filamentous fungus Aspergillus nidulans is a member of the c-Jun-like transcriptional activator family. It acts as central transcription factor of the cross-pathway regulatory network of amino acid biosynthesis and is functionally exchangeable for the general control transcriptional activator Gcn4p of Saccharomyces cerevisiae. In contrast to GCN4, expression of cpcA is strongly regulated by two equally important mechanisms with additive effects that lead to a fivefold increased CPCA protein amount under amino acid starvation conditions. One component of cpcA regulation involves a transcriptional autoregulatory mechanism via a CPCA recognition element (CPRE) in the cpcA promoter that causes a sevenfold increased cpcA mRNA level when cells are starved for amino acids. Point mutations in the CPRE cause a constitutively low mRNA level of cpcA and a halved protein level when amino acids are limited. Moreover, two upstream open reading frames (uORFs) in the 5' region of the cpcA mRNA are important for a translational regulatory mechanism. Destruction of both short uORFs results in a sixfold increased CPCA protein level under nonstarvation conditions and a 10-fold increase under starvation conditions. Mutations in both the CPRE and uORF regulatory elements lead to an intermediate effect, with a low cpcA mRNA level but a threefold increased CPCA protein level independent of amino acid availability. These data argue for a combined regulation of cpcA that includes a translational regulation like that of yeast GCN4 as well as a transcriptional regulation like that of the mammalian jun and fos genes.

Characterization of the Ustilago maydis sid2 gene, encoding a multidomain peptide synthetase in the ferrichrome biosynthetic gene cluster

Ustilago maydis, the causal agent of corn smut disease, acquires and transports ferric ion by producing the extracellular, cyclic peptide, hydroxamate siderophores ferrichrome and ferrichrome A. Ferrichrome biosynthesis likely proceeds by hydroxylation and acetylation of L-ornithine, and later steps likely involve covalently bound thioester intermediates on a multimodular, nonribosomal peptide synthetase. sid1 encodes L-ornithine N(5)-oxygenase, which catalyzes hydroxylation of L-ornithine, the first committed step of ferrichrome and ferrichrome A biosynthesis in U. maydis. In this report we characterize sid2, another biosynthetic gene in the pathway, by gene complementation, gene replacement, DNA sequence, and Northern hybridization analysis. Nucleotide sequencing has revealed that sid2 is located 3.7 kb upstream of sid1 and encodes an intronless polypeptide of 3,947 amino acids with three iterated modules of an approximate length of 1,000 amino acids each. Multiple motifs characteristic of the nonribosomal peptide synthetase protein family were identified in each module. A corresponding iron-regulated sid2 transcript of 11 kb was detected by Northern hybridization analysis. By contrast, constitutive accumulation of this large transcript was observed in a mutant carrying a disruption of urbs1, a zinc finger, GATA family transcription factor previously shown to regulate siderophore biosynthesis in Ustilago. Multiple GATA motifs are present in the intergenic region between sid1 and sid2, suggesting bidirectional transcription regulation by urbs1 of this pathway. Indeed, mutation of two of these motifs, known to be important to regulation of sid1, altered the differential regulation of sid2 by iron.

Posttranscriptional control mediates cell type-specific localization of catalase A during Aspergillus nidulans development

Two differentially regulated catalase genes have been identified in the fungus Aspergillus nidulans. The catA gene belongs to a class whose transcripts are specifically induced during asexual sporulation (conidiation) and encodes a catalase accumulated in conidia. Using a developmental mutant affected in the brlA gene, which is unable to form conidia but capable of producing sexual spores (ascospores), we demonstrated that the catA mRNA accumulated during induction of conidiation but did not produce CatA protein. In contrast, high levels of catalase A activity were detected in the ascospores produced by this mutant, indicating that the catA gene is posttranscriptionally regulated. The same type of regulation was observed for a catA::lacZ translational gene fusion, suggesting that the catA message 5' untranslated region could be involved in translational control during development. In a wild-type strain, beta-galactosidase activity driven from the catA::lacZ gene fusion was low in hyphae and increased 50-fold during conidiation and 620-fold in isolated conidia. Consistent with this finding spatial expression of the reporter gene was restricted to metulae, phialides, and conidia. Conidium-associated expression was maintained in a stuA mutant, in which the conidiophore cell pattern is severely deranged. catA mRNA accumulation was also observed when vegetative mycelia was subject to oxidative, osmotic, and nitrogen or carbon starvation stress. Nevertheless, catalase A activity was restricted to the conidia produced under nutrient starvation. Our results provide support for a model in which translation of the catA message, accumulated during conidiation or in response to different types of stress, is linked to the morphogenetic processes involved in asexual and sexual spore formation. Our findings also indicate that brlA-independent mechanisms regulate the expression of genes encoding spore-specific products.

Aspergillus asexual reproduction and sexual reproduction are differentially affected by transcriptional and translational mechanisms regulating stunted gene expression

The Stunted protein (StuAp) is a member of a family of transcription factors that regulate fungal development and cell cycle progression. Regulated stuA gene expression is required for correct cell pattern formation during asexual reproduction (conidiation) and for initiation of the sexual reproductive cycle in Aspergillus nidulans. Transcriptional initiation from two different promoters yields overlapping mRNAs (stuA alpha and stuAbeta) that upon translation yield the same protein. Here we show that multiple regulatory mechanisms interact to control (i) developmental competence-dependent expression of both transcripts and (ii) induction-dependent expression of stuA alpha, but not stuAbeta, by the conidiation-specific Bristle (BrlAp) transcriptional activator. Quantitative levels of both mRNAs are further modulated by (i) an activator(s) located at a far-upstream upstream activation sequence, (ii) feedback regulation by StuAp, and (iii) positive translational regulation that requires the peptide product of a micro-open reading frame unique to the stuA alpha mRNA 5' untranslated region. Gradients in stuA alpha expression were most important for correct cell and tissue type development. Threshold requirements were as follows: metula-phialide differentiation < ascosporogenesis < cleistothecial shell-Hülle cell differentiation. Altered stuA expression affected conidiophore morphology and conidial yields quantitatively but did not alter the temporal development of cell types or conidiophore density. By contrast, the sexual cycle showed both temporal delay and quantitative reduction in the number of cleistothecial initials but normal morphogenesis of tissue types.

Analysis of mechanisms regulating expression of the ver-1 gene, involved in aflatoxin biosynthesis

Previous studies have shown that ver-1A encodes an enzyme which is directly involved in the conversion of versicolorin A to demethylsterigmatocystin during aflatoxin B1 (AFB1) biosynthesis in the filamentous fungus Aspergillus parasiticus. In this study, two different tools were utilized to study the regulation of ver-1A expression at the level of transcription and protein accumulation. First, a ver-1A cDNA was expressed in Escherichia coli with the vector pMAL-c2. The resulting maltose-binding protein-Ver-1A fusion protein was purified and used to generate polyclonal antibodies. Western blot analyses showed that these antibodies specifically recognized the Ver-1 protein (approximately 28 kDa) in cell extracts of Aspergillus parasiticus SU1. Second, a GUS (uidA; encodes beta-glucuronidase) reporter system was developed by fusing the ver-1A promoter and transcription terminator to the GUS gene. Reporter constructs were transformed into A. parasiticus, resulting in a single copy of the ver-1A-GUS reporter integrated adjacent to the wild-type ver-1A gene (3' end) in the chromosome. Western blot analysis, Northern hybridization analysis, and a GUS activity assay were used to analyze transformants. The timing of appearance and pattern of accumulation of GUS transcript and GUS protein in transformants were consistent with the timing of appearance and pattern of accumulation of ver-1 transcript and Ver-1 protein. These data suggested that the GUS gene was under the same regulatory control as the wild-type ver-1 gene and confirmed that transcriptional regulation plays an important role in ver-1A expression. Integration of the ver-1A-GUS reporter construct at the niaD locus resulted in 500-fold-lower GUS activity, but the temporal pattern of accumulation of GUS activity was not affected. Therefore, chromosomal location can play a role in determining the level of gene expression in A. parasiticus and should be an important consideration when analyzing promoter function in this organism.

The Aspergillus nidulans abaA gene encodes a transcriptional activator that acts as a genetic switch to control development

The Aspergillus nidulans abaA gene encodes a protein containing an ATTS DNA-binding motif and is required for the terminal stages of conidiophore development. Results from gel mobility shift and protection, missing-contact, and interference footprint assays showed that AbaA binds to the sequence 5'-CATTCY-3', where Y is a pyrimidine, making both major- and minor-groove contacts. Multiple AbaA binding sites are present in the cis-acting regulatory regions of several developmentally controlled structural genes as well as those of the upstream regulatory gene brlA, the downstream regulatory gene wetA, and abaA itself. These cis-acting regulatory regions confer AbaA-dependent transcriptional activation in a heterologous Saccharomyces cerevisiae gene expression system. From these observations, we propose that the AbaA transcription factor establishes a novel set of feedback regulatory loops responsible for determination of conidiophore development.

The Aspergillus nidulans abaA gene encodes a transcriptional activator that acts as a genetic switch to control development

The Aspergillus nidulans abaA gene encodes a protein containing an ATTS DNA-binding motif and is required for the terminal stages of conidiophore development. Results from gel mobility shift and protection, missing-contact, and interference footprint assays showed that AbaA binds to the sequence 5'-CATTCY-3', where Y is a pyrimidine, making both major- and minor-groove contacts. Multiple AbaA binding sites are present in the cis-acting regulatory regions of several developmentally controlled structural genes as well as those of the upstream regulatory gene brlA, the downstream regulatory gene wetA, and abaA itself. These cis-acting regulatory regions confer AbaA-dependent transcriptional activation in a heterologous Saccharomyces cerevisiae gene expression system. From these observations, we propose that the AbaA transcription factor establishes a novel set of feedback regulatory loops responsible for determination of conidiophore development.

Expression of fungal genes involved in penicllin biosynthesis

Carbon catabolite repression and pH regulation are regulatory circuits with a wide domain of action in the Plectomycetes. Penicillin biosynthesis is one of the pathways which are under their control. The conclusions obtained so far, which are based on studies of the genetic and molecular regulation of the penicillin pathway of Aspergillus nidulans, would have been much harder to produce using an organism such as Penicillium chrysogenum (the industrial penicillin producer). However, A. nidulans and P. chrysogenum are close in terms of their phylogeny and one can reasonably predict that the conclusions about A. nidulans, which are summarized in this review and which are of unquestionable biotechnological relevance, will be extrapolable to the industrial organism.

A homologous gene-reporter system for the basidiomycete Schizophyllum commune based on internally deleted homologous genes

Problems encountered in our attempts to achieve expression of heterologous genes, driven by ascomycetous regulatory sequences, in homobasidiomycetes led us to develop a gene-reporter system based on the expression of homologous genes in Schizophyllum commune. Internal deletions were made in the coding sequences of the regulated Sc4 gene and the constitutively expressed GPD gene. After introduction of these constructs into S. commune it was found that the expected truncated transcripts were produced. The internally deleted Sc4 gene, containing 1140bp of upstream and 200 bp of downstream sequences, was only expressed in dikaryons at the time of fruiting (as was the resident Sc4 gene) but not at all in monokaryons, indicating that the construct contained all regulatory sequences necessary and sufficient to confer control by the mating-type genes and expression during fruiting. The internally deleted GPD gene, containing 1300 bp of upstream and 150 bp of downstream sequences, was expressed both in monokaryons and dikaryons at levels similar to those of the resident GPD gene, indicating that all sequences necessary for proper expression were present. This reporter-gene system may be applicable to the analysis of cis-regulatory sequences of these genes. Furthermore, heterologous genes may be inserted into the well-expressed GPD deletion construct to obtain expression of such genes in S. commune and possibly in other homobasidiomycetes.

Characterization of the Trichoderma reesei cbh2 promoter

A 613-bp fragment of the 5' upstream region of the Trichoderma reesei cbh2 gene (coding for the cellulolytic enzyme cellobiohydrolase II) has been isolated and sequenced. Fusion of this fragment to the E. coli uidA gene (coding for beta-glucuronidase) leads to--albeit low--expression of beta-glucuronidase activity in the presence of cellulose and upon the addition of low molecular weight inducers (sophorose, lactose) of cellobiohydrolase II. It also governed the formation of beta-glucuronidase activity during sporulation and its transport to the conidial surface. However, despite the presence of a signal peptide in the cbh2:uidA fusion, beta-glucuronidase was not secreted in T. reesei. Defined fragments of the 613-bp promoter region were isolated and used to identify areas involved in the regulation of cbh2 expression by protein-DNA binding assays. At least two binding areas--between -443/-363 and -363/-173, respectively--were identified. In both areas, the DNA-protein complex observed was appreciably larger when cell-free extracts from sophorose-induced mycelia were used. This suggests that at least one of the proteins regulating cbh2 transcription is itself induced by cellulose.

Coordinate expression of ribosomal protein genes in Neurospora crassa and identification of conserved upstream sequences

The relative levels of rRNAs and ribosomal proteins are coordinately regulated by growth rate and carbon nutrition in Neurospora crassa. However, little is known about the mechanisms involved. To investigate the transcriptional regulation of ribosomal protein genes in N. crassa, we cloned and sequenced a ribosomal protein gene (crp-3). The inferred crp-3 protein sequence shares 89% and 83% homology at its N-terminus with the yeast rp51 and the human S17 ribosomal proteins respectively. The crp-3 gene contains two introns, neither of which are conserved in position with the RP51 or the S17 genes. The crp-3 gene is present in a single copy and was mapped by RFLP analysis to the right arm of linkage group IV, near the cot-1 locus. Sequence comparisons of the upstream regions of the three sequenced crp genes revealed several common features. These include a 'Taq box' (consensus: ARTTYGACTT) at -39, a CG repeat (consensus: CCCRCCRRR) at -65, and a major transcription initiation site embedded in a purine rich region flanked by an upstream pyrimidine rich sequence. Using four N.crassa ribosomal protein genes as probes, we demonstrated that the levels of the four ribosomal protein mRNAs were closely coordinated during a nutritional downshift from sucrose to quinic acid.

Aspergillus nidulans wetA activates spore-specific gene expression

The Aspergillus nidulans wetA gene is required for synthesis of cell wall layers that make asexual spores (conidia) impermeable. In wetA mutant strains, conidia take up water and autolyze rather than undergoing the final stages of maturation. wetA is activated during conidiogenesis by sequential expression of the brlA and abaA regulatory genes. To determine whether wetA regulates expression of other sporulation-specific genes, its coding region was fused to a nutritionally regulated promoter that permits gene activation in vegetative cells (hyphae) under conditions that suppress conidiation. Expression of wetA in hyphae inhibited growth and caused excessive branching. It did not lead to activation of brlA or abaA but did cause accumulation of transcripts from genes that are normally expressed specifically during the late stages of conidiation and whose mRNAs are stored in mature spores. Thus, wetA directly or indirectly regulates expression of some spore-specific genes. At least one gene (wA), whose mRNA does not occur in spores but rather accumulates in the sporogenous phialide cells, was activated by wetA, suggesting that wetA may have a regulatory function in these cells as well as in spores. We propose that wetA is responsible for activating a set of genes whose products make up the final two conidial wall layers or direct their assembly and through this activity is responsible for acquisition of spore dormancy.

Aspergillus nidulans wetA activates spore-specific gene expression

The Aspergillus nidulans wetA gene is required for synthesis of cell wall layers that make asexual spores (conidia) impermeable. In wetA mutant strains, conidia take up water and autolyze rather than undergoing the final stages of maturation. wetA is activated during conidiogenesis by sequential expression of the brlA and abaA regulatory genes. To determine whether wetA regulates expression of other sporulation-specific genes, its coding region was fused to a nutritionally regulated promoter that permits gene activation in vegetative cells (hyphae) under conditions that suppress conidiation. Expression of wetA in hyphae inhibited growth and caused excessive branching. It did not lead to activation of brlA or abaA but did cause accumulation of transcripts from genes that are normally expressed specifically during the late stages of conidiation and whose mRNAs are stored in mature spores. Thus, wetA directly or indirectly regulates expression of some spore-specific genes. At least one gene (wA), whose mRNA does not occur in spores but rather accumulates in the sporogenous phialide cells, was activated by wetA, suggesting that wetA may have a regulatory function in these cells as well as in spores. We propose that wetA is responsible for activating a set of genes whose products make up the final two conidial wall layers or direct their assembly and through this activity is responsible for acquisition of spore dormancy.

Regulation of the glaA gene of Aspergillus niger

The glucoamylase gene of Aspergillus niger, glaA, is expressed at high levels in the presence of starch. We have determined the nucleotide sequence of 1966 bp of the 5' flanking region of the glaA gene and have begun to identify sequences important for the control of glaA expression by deletion analysis. Constructs containing deletions extending into the glaA gene promoter were introduced into an A. niger host whose own glaA gene had been disrupted by a gene replacement event. Secreted levels of glucoamylase, expressed from each of the recombinant glaA genes, were measured by enzyme immunoassay. The effect of each deletion on the expression of glaA, when grown on differing carbon sources, was used to determine the limits of sequences upstream of glaA responsible for gene regulation. A region between -562 and -318 appears to direct high-level expression, whereas only 214 bp of 5' flanking sequence is required to initiate the start to transcription.

Upstream elements repress premature expression of an Aspergillus developmental regulatory gene

The Aspergillus nidulans abaA gene regulates intermediate steps in asexual reproductive development and is itself developmentally regulated. An 822-base-pair DNA fragment from the abaA 5'-flanking region is sufficient to drive developmentally appropriate expression of the Escherichia coli lacZ gene. Deletion analysis showed that this fragment contains elements that repress transcription in vegetative cells and immature conidiophores and that activate transcription later during development. A 45-base-pair region encompassing the major and minor abaA transcription initiation sites contains directly repeated sequences related to the mammalian initiator (Inr) element (S. T. Smale and D. Baltimore, Cell 57:103-113, 1989). This element or sequences in the untranslated leader were sufficient for correct transcription initiation and for measurable developmental induction. Similar elements were present at or near the initiation sites of other developmentally regulated genes. We propose that the temporal and spatial specificity of expression of these genes results from modulation of the activity of Inr elements.

Upstream elements repress premature expression of an Aspergillus developmental regulatory gene

The Aspergillus nidulans abaA gene regulates intermediate steps in asexual reproductive development and is itself developmentally regulated. An 822-base-pair DNA fragment from the abaA 5'-flanking region is sufficient to drive developmentally appropriate expression of the Escherichia coli lacZ gene. Deletion analysis showed that this fragment contains elements that repress transcription in vegetative cells and immature conidiophores and that activate transcription later during development. A 45-base-pair region encompassing the major and minor abaA transcription initiation sites contains directly repeated sequences related to the mammalian initiator (Inr) element (S. T. Smale and D. Baltimore, Cell 57:103-113, 1989). This element or sequences in the untranslated leader were sufficient for correct transcription initiation and for measurable developmental induction. Similar elements were present at or near the initiation sites of other developmentally regulated genes. We propose that the temporal and spatial specificity of expression of these genes results from modulation of the activity of Inr elements.

brlA requires both zinc fingers to induce development

Expression of the Aspergillus nidulans brlA gene induces a developmental pathway leading to the production of asexual spores. We have introduced mutations into brlA that are expected to disrupt either or both Cys2-His2 Zn(II) coordination sites postulated to exist in the BrlA polypeptide. The resultant brlA alleles fail to induce either the asexual reproductive pathway or the expression of development-specific genes. These data support the hypothesis that brlA encodes a nucleic acid-binding protein whose activity requires each of two zinc fingers.

brlA requires both zinc fingers to induce development

Expression of the Aspergillus nidulans brlA gene induces a developmental pathway leading to the production of asexual spores. We have introduced mutations into brlA that are expected to disrupt either or both Cys2-His2 Zn(II) coordination sites postulated to exist in the BrlA polypeptide. The resultant brlA alleles fail to induce either the asexual reproductive pathway or the expression of development-specific genes. These data support the hypothesis that brlA encodes a nucleic acid-binding protein whose activity requires each of two zinc fingers.

Isolation and analysis of the acetate regulatory gene, facB, from Aspergillus nidulans

The facB gene of Aspergillus nidulans is thought to be involved in acetate induction of enzymes required for acetate utilization and of the acetamidase encoded by the multiply regulated amdS gene. In addition, some evidence suggests that the facB gene has a structural as well as a regulatory role in acetate metabolism. The facB gene was cloned from a cosmid library by complementation of the facB101 loss-of-function mutation. Transformants receiving multiple copies of facB displayed stronger growth on acetamide media, indicating increased amdS expression, while growth on acetate was inhibited in these multicopy transformants. A 3.1-kilobase acetate-inducible facB transcript was detected by Northern (RNA) blot analysis. Examination of message levels in wild-type and mutant strains indicated that the facB gene is subject to carbon catabolite repression. Previous work has indicated that the presence of multiple copies of the 5' end of the amdS gene can result in titration of regulatory proteins. Additional copies of the facB gene were shown to specifically overcome the effect of facB product titration.

Isolation and analysis of the acetate regulatory gene, facB, from Aspergillus nidulans

The facB gene of Aspergillus nidulans is thought to be involved in acetate induction of enzymes required for acetate utilization and of the acetamidase encoded by the multiply regulated amdS gene. In addition, some evidence suggests that the facB gene has a structural as well as a regulatory role in acetate metabolism. The facB gene was cloned from a cosmid library by complementation of the facB101 loss-of-function mutation. Transformants receiving multiple copies of facB displayed stronger growth on acetamide media, indicating increased amdS expression, while growth on acetate was inhibited in these multicopy transformants. A 3.1-kilobase acetate-inducible facB transcript was detected by Northern (RNA) blot analysis. Examination of message levels in wild-type and mutant strains indicated that the facB gene is subject to carbon catabolite repression. Previous work has indicated that the presence of multiple copies of the 5' end of the amdS gene can result in titration of regulatory proteins. Additional copies of the facB gene were shown to specifically overcome the effect of facB product titration.

Structure of the cutinase gene and detection of promoter activity in the 5'-flanking region by fungal transformation

The cutinase gene from Fusarium solani f. sp. pisi (Nectria hematococa) was cloned and sequenced. Sau3A fragments of genomic DNA from the fungus were cloned in a lambda Charon 35 vector. When restriction fragments generated from the inserts were screened with 5' and 3' probes from cutinase cDNA, a 5.5-kilobase SstI fragment hybridized with both probes, suggesting the presence of the entire cutinase gene. A 2,818-base pair segment was sequenced, revealing a 690-nucleotide open reading frame that was identical to that found in the cutinase cDNA with a single 51-base pair intron. Transformation vectors were constructed containing a promoterless gene for hygromycin resistance, which was translationally fused to flanking sequences of the cutinase gene. When protoplasts and mycelia were transformed with these vectors, hygromycin-resistant transformants were obtained. Successful transformation was assessed by Southern blot analysis by using radiolabeled probes for the hygromycin resistance gene and the putative promoter. The results of Southern blot analysis indicated that the plasmid had integrated into the Fusarium genome and that the antibiotic resistance was a manifestation of the promoter activity of the cutinase flanking sequences. Transformation of Colletotrichum capsici with the same construct confirmed the promoter activity of the flanking region and the integration of the foreign DNA. Transformation and deletion analysis showed that promoter activity resided within the 360 nucleotides immediately 5' to the cutinase initiation codon.

Expression of Tn5-derived kanamycin resistance in the fungus Phycomyces blakesleeanus

A plasmid, carrying the Tn5 gene for kanamycin resistance lacking its own promoter, has successfully been used in the selection of DNA sequences of the fungus Phycomyces blakesleeanus having promoter activity in Escherichia coli. Many of these sequences were also effective in promoting resistance to kanamycin when the corresponding chimeric plasmids were introduced in the fungus via spheroplast transformation. The selected phenotype was easily propagated through vegetative spores and behaved as a stable character since it was not appreciably lost in the absence of selection.