Specific binding sites in the alcR and alcA promoters of the ethanol regulon for the CREA repressor mediating carbon catabolite repression in Aspergillus nidulans

Overproduction of alpha-glucosidase in Aspergillus niger transformed with the cloned gene aglA

We have transformed an industrial strain, Aspergillus niger GN-3, with the alpha-glucosidase gene (aglA) from the same strain. Southern hybridization analysis revealed that transformants had multiple copies of the cloned DNA inserted into the host genome. An 11-fold improvement of enzyme production was achieved by transformation with a DNA fragment composed of 1.11 kb of the 5' noncoding region, 3.12 kb of the coding region containing three introns, and 1.2 kb of the 3' noncoding region. It was found that the 3' noncoding region (1.2 kb) was preferable for maximum production of the enzyme in the transformant.

The essential Aspergillus nidulans gene pmaA encodes an homologue of fungal plasma membrane H(+)-ATPases

pmaA, an Aspergillus nidulans gene encoding a P-ATPase, has been cloned by heterologous hybridization with the yeast PMA1 gene. The putative 990-residue PmaA polypeptide shows 50% identity to Saccharomyces cerevisiae and Neurospora crassa plasma membrane H(+)-ATPases and weak (19-26%) identity to other yeast P-type cation-translocating ATPases. PmaA contains all catalytic domains characterizing H(+)-ATPases. pmaA transcript levels are not regulated by PacC, the transcription factor mediating pH regulation, and were not significantly affected by an extreme creAd mutation resulting in carbon catabolite derepression. Deletion of pmaA causes lethality, but a single copy of the gene is sufficient to support normal growth rate in pmaA hemizygous diploids, even under acidic growth conditions. As compared to other fungal H(+)-ATPases, PmaA presents three insertions, 39, 7, and 16 residues long, in the conserved central region of the protein. Two of these insertions are predicted to be located in extracellular loops and might be of diagnostic value for the identification of Aspergillus species. Their absence from most mammalian P-type ATPases may have implications for antifungal therapy.

Molecular cloning and transcriptional regulation of the Aspergillus nidulans xlnD gene encoding a beta-xylosidase

The xlnD gene encoding the 85-kDa beta-xylosidase was cloned from Aspergillus nidulans. The deduced primary structure of the protein exhibits considerable similarity to the primary structures of the Aspergillus niger and Trichoderma reesei beta-xylosidases and some similarity to the primary structures of the class 3 beta-glucosidases. xlnD is regulated at the transcriptional level; it is induced by xylan and D-xylose and is repressed by D-glucose. Glucose repression is mediated by the product of the creA gene. Although several binding sites for the pH regulatory protein PacC were found in the upstream regulatory region, it was not clear from a Northern analysis whether PacC is involved in transcriptional regulation of xlnD.

Sequence, exon-intron organization, transcription and mutational analysis of prnA, the gene encoding the transcriptional activator of the prn gene cluster in Aspergillus nidulans

The prnA gene codes for a transcriptional activator that mediates proline induction of four other genes involved in proline utilization as a nitrogen and/or carbon source in Aspergillus nidulans. In this paper, we present the genomic and cDNA sequence and the transcript map of prnA. The PrnA protein belongs to the Zn binuclear cluster family of transcriptional activators. The gene shows a striking intron-exon organization, with the putative nuclear localization sequence and the Zn cluster domain in discrete exons. Although the protein sequence presents some interesting similarities with the isofunctional protein of Saccharomyces cerevisiae Put3p, a higher degree of similarity is found with a functionally unrelated protein Thi1 of Schizosaccharomyces pombe. A number of mutations mapping in the prnA gene were sequenced. This comprises a deletion that results in an almost complete loss of the prnA-specific mRNA, a mutation in the putative nuclear localization signal, a proline to leucine mutation in the second loop of the zinc cluster and a cold-sensitive mutation in the so-called 'central region'. Other complete or partial loss of function mutations map in regions of unknown function. We establish that the transcription of the gene is neither self-regulated nor significantly affected by carbon and/or nitrogen metabolite repression.

The CreA repressor is the sole DNA-binding protein responsible for carbon catabolite repression of the alcA gene in Aspergillus nidulans via its binding to a couple of specific sites

Carbon catabolite repression is mediated in Aspergillus nidulans by the negative acting protein CreA. The CreA repressor plays a major role in the control of the expression of the alc regulon, encoding proteins required for the ethanol utilization pathway. It represses directly, at the transcriptional level, the specific transacting gene alcR, the two structural genes alcA and aldA, and other alc genes in all physiological growth conditions. Among the seven putative CreA sites identified in the alcA promoter region, we have determined the CreA functional targets in AlcR constitutive and derepressed genetic backgrounds. Two different divergent CreA sites, of which one overlaps a functional AlcR inverted repeat site, are largely responsible for alcA repression. Totally derepressed alcA expression is achieved when these two CreA sites are disrupted in addition to another single site, which overlaps the functional palindromic induction target. The fact that derepression is always associated with alcA overexpression is consistent with a competition model between AlcR and CreA for their cognate targets in the same region of the alcA promoter. Our results also indicate that the CreA repressor is necessary and sufficient for the total repression of the alcA gene.

Differential Gene Expression in the Laccase Gene Family from Basidiomycete I-62 (CECT 20197)

A family of genes encoding laccases has recently been described for the basidiomycete I-62 (CECT 20197). Transcript levels of genes lcc1, lcc2, and lcc3 were analyzed under four different culture conditions to study their expression patterns. Two of the laccase genes were clearly inducible by veratryl alcohol: the lcc1 gene is inducible in early stages of growth, and the lcc2 gene is also inducible but only when the organism reaches the stationary phase. Transcript levels for the third gene, lcc3, were uninduced by veratryl alcohol and repressed by glucose.

Cloning, expression, and role in pathogenicity of pg1 encoding the major extracellular endopolygalacturonase of the vascular wilt pathogen Fusarium oxysporum

pg1 encoding the major in vitro extracellular endopolygalacturonase of the tomato vascular wilt pathogen Fusarium oxysporum f. sp. lycopersici was cloned and sequenced. The deduced mature protein had a calculated molecular mass of 35.5 kDa and a pI of 6.2, and showed significant similarity with other fungal endoPGs. pg1 mRNA was induced in vitro by citrus pectin, tomato vascular tissue, 0.1% D-galacturonic acid, and polygalacturonic acid, and repressed by 1% D-galacturonic acid and 1% glucose. Reverse transcription-polymerase chain reaction revealed pg1 expression in roots and lower stems of tomato plants infected by F. oxysporum f. sp. lycopersici. Three naturally occurring F. oxysporum f. sp. melonis isolates deficient in PG1 were transformed with the cloned gene. The PG1 enzyme secreted by the transformants had the same molecular mass, pI, and glycosylation pattern as those of the donor isolate. Polygalacturonase activity in cultures of transformants grown in vitro on citrus pectin and on melon plants, but not on glucose, increased 10- to 20-fold, compared with the PG1-deficient wild-type isolate, whereas mycelial dry weight increased two- to three-fold. Transformants exhibited the same degree of virulence toward susceptible muskmelon cultivars as the wild-type isolate and were avirulent on a resistant cultivar.

aguA, the gene encoding an extracellular alpha-glucuronidase from Aspergillus tubingensis, is specifically induced on xylose and not on glucuronic acid

An extracellular alpha-glucuronidase was purified and characterized from a commercial Aspergillus preparation and from culture filtrate of Aspergillus tubingensis. The enzyme has a molecular mass of 107 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and 112 kDa as determined by mass spectrometry, has a determined pI just below 5.2, and is stable at pH 6.0 for prolonged times. The pH optimum for the enzyme is between 4.5 and 6.0, and the temperature optimum is 70 degrees C. The alpha-glucuronidase is active mainly on small substituted xylo-oligomers but is also able to release a small amount of 4-O-methylglucuronic acid from birchwood xylan. The enzyme acts synergistically with endoxylanases and beta-xylosidase in the hydrolysis of xylan. The enzyme is N glycosylated and contains 14 putative N-glycosylation sites. The gene encoding this alpha-glucuronidase (aguA) was cloned from A. tubingensis. It consists of an open reading frame of 2,523 bp and contains no introns. The gene codes for a protein of 841 amino acids, containing a eukaryotic signal sequence of 20 amino acids. The mature protein has a predicted molecular mass of 91,790 Da and a calculated pI of 5.13. Multiple copies of the gene were introduced in A. tubingensis, and expression was studied in a highly overproducing transformant. The aguA gene was expressed on xylose, xylobiose, and xylan, similarly to genes encoding endoxylanases, suggesting a coordinate regulation of expression of xylanases and alpha-glucuronidase. Glucuronic acid did not induce the expression of aguA and also did not modulate the expression on xylose. Addition of glucose prevented expression of aguA on xylan but only reduced the expression on xylose.

Isolation and analysis of xlnR, encoding a transcriptional activator co-ordinating xylanolytic expression in Aspergillus niger

Complementation by transformation of an Aspergillus niger mutant lacking xylanolytic activity led to the isolation of the xlnR gene. The xlnR gene encodes a polypeptide of 875 amino acids capable of forming a zinc binuclear cluster domain with similarity to the zinc clusters of the GAL4 superfamily of transcription factors. The XlnR-binding site 5'-GGCTAAA-3' was deduced after electrophoretic mobility shift assays, DNase I footprinting and comparison of various xylanolytic promoters. The importance of the second G within the presumed XlnR binding site 5'-GGCTAAA-3' was confirmed in vitro and in vivo. The 5'-GGCTAAA-3' consensus sequence is found within several xylanolytic promoters of various Aspergillus species and Penicillium chrysogenum. Therefore, this sequence may be an important and conserved cis-acting element in induction of xylanolytic genes in filamentous fungi. Our results indicate that XlnR is a transcriptional activator of the xylanolytic system in A. niger.

Glucose repression of maltase and methanol-oxidizing enzymes in the methylotrophic yeast Hansenula polymorpha: isolation and study of regulatory mutants

Regulation of the synthesis of maltase and methanol-oxidizing enzymes by the carbon source has been analyzed in the methylotrophic yeast Hansenula polymorpha. Maltase was shown to be responsible for the growth of H. polymorpha not only on maltose, but also on sucrose. The affinity of maltase towards maltase substrates decreased in the order: 4-nitrophenyl glucoside (PNPG) < sucrose < maltose. Mutants with glucose repression-insensitive synthesis of alcohol oxidase and maltase were obtained from H. polymorpha by mutagenesis and subsequent selection on methanol medium in the presence of 2-deoxy-D-glucose. One of the isolated mutants, L63, was studied in more detail. Mutant L63 was recessive and monogenic and it was not deficient in hexokinase. Its analysis revealed that H. polymorpha most probably has a repressor protein that in the presence of glucose can down-regulate expression of both maltase and enzymes of methanol oxidation.

The zinc binuclear cluster activator AlcR is able to bind to single sites but requires multiple repeated sites for synergistic activation of the alcA gene in Aspergillus nidulans

The alcA gene which is part of the recently identified ethanol regulon, is one of the most strongly inducible genes in Aspergillus nidulans. Its transcriptional activation is mediated by the AlcR transactivator which contains a DNA-binding domain belonging to the C6 zinc binuclear cluster family. AlcR differs from the other members of this family by several features, the most striking characteristic being its binding to both symmetric and asymmetric DNA sites with the same apparent affinity. However, AlcR is also able to bind to a single site with high affinity, suggesting that unlike the other C6 proteins, AlcR binds as a monomer. In this report, we show that AlcR targets, to be functional in vivo, have to be organized as inverted or direct repeats. In addition, we show a strong synergistic activation of alcA transcription in which the number and the position of the AlcR-binding sites are crucial. The fact that the AlcR unit for in vitro binding is a single site whereas the in vivo functional unit is a repeat opens the question of the mechanism of the strong alcA transactivation. These results show that AlcR displays both in vitro and in vivo a new range of binding specificity and provides a novel example in the C6 zinc cluster protein family.

Isolation of Aspergillus niger creA mutants and effects of the mutations on expression of arabinases and L-arabinose catabolic enzymes

Aspergillus niger mutants relieved of carbon repression were isolated from an areA parental strain by selection of colonies that exhibited improved growth on a combination of 4-aminobutanoic acid (GABA) and D-glucose. In addition to derepression of the utilization of GABA as a nitrogen source in the presence of D-glucose, three of the four mutants also showed derepression of L-alanine and L-proline utilization. Transformation of the mutants with the A. niger creA gene, encoding the repressor protein CREA, re-established the areA phenotype on GABA/D-glucose, identifying the mutations as creAd. The creA gene mapped on chromosome IV by linkage analysis and contour-clamped homogeneous electric field hybridization. The creA mutants obtained were used to study the involvement of CREA in repression by D-glucose of arabinases and L-arabinose catabolism in A. niger. In wild-type A. niger, alpha-L-arabinofuranosidase A, alpha-L-arabinofuranosidase B, endo-arabinase, L-arabinose reductase and L-arabitol dehydrogenase were induced on L-arabinose, but addition of D-glucose prevented this induction. Repression was relieved to varying degrees in the creA mutants, showing that biosynthesis of arabinases and L-arabinose catabolic enzymes is under control of CREA.

Cloning and characterization of two rhamnogalacturonan hydrolase genes from Aspergillus niger

A rhamnogalacturonan hydrolase gene of Aspergillus aculeatus was used as a probe for the cloning of two rhamnogalacturonan hydrolase genes of Aspergillus niger. The corresponding proteins, rhamnogalacturonan hydrolases A and B, are 78 and 72% identical, respectively, with the A. aculeatus enzyme. In A. niger cultures which were shifted from growth on sucrose to growth on apple pectin as a carbon source, the expression of the rhamnogalacturonan hydrolase A gene (rhgA) was transiently induced after 3 h of growth on apple pectin. The rhamnogalacturonan hydrolase B gene was not induced by apple pectin, but the rhgB gene was derepressed after 18 h of growth on either apple pectin or sucrose. Gene fusions of the A. niger rhgA and rhgB coding regions with the strong and inducible Aspergillus awamori exlA promoter were used to obtain high-producing A. awamori transformants which were then used for the purification of the two A. niger rhamnogalacturonan hydrolases. High-performance anion-exchange chromatography of oligomeric degradation products showed that optimal degradation of an isolated highly branched pectin fraction by A. niger rhamnogalacturonan hydrolases A and B occurred at pH 3.6 and 4.1, respectively. The specific activities of rhamnogalacturonan hydrolases A and B were then 0.9 and 0.4 U/mg, respectively, which is significantly lower than the specific activity of A. aculeatus rhamnogalacturonan hydrolase (2.5 U/mg at an optimal pH of 4.5). Compared to the A enzymes, the A. niger B enzyme appears to have a different substrate specificity, since additional oligomers are formed.

Carbon repression in Aspergilli

Many microorganisms prefer easily metabolizable carbon sources over alternative, less readily metabolized carbon sources. One of the mechanisms to achieve this is repression of the synthesis of enzymes related to catabolism of the alternative carbon sources, i.e. carbon repression. It is now clear that in Aspergillus nidulans and Aspergillus niger the repressor protein CREA plays a major role in carbon repression. CREA inhibits transcription of many target genes by binding to specific sequences in the promoter of these genes. Unfortunately there is little information on other components of the signalling pathway that triggers repression by CREA. In this review we summarize the current understanding of carbon repression in Aspergilli.

In vitro recognition of specific DNA targets by AlcR, a zinc binuclear cluster activator different from the other proteins of this class

AlcR is the transactivator mediating transcriptional induction of the alc gene cluster in Aspergillus nidulans. The AlcR DNA-binding domain consists of a zinc binuclear cluster different from the other members of the Zn2Cys6 family by several features. In particular, it is able to bind to symmetric and asymmetric sites with the same affinity, with both sites being functional in A. nidulans. Here, we show that unlike the other proteins of the Zn2Cys6 binuclear cluster family, AlcR binds most probably as a monomer to its cognate targets. Two molecules of the AlcR protein can simultaneously bind in a noncooperative manner to inverted repeats. The consensus core has been determined precisely (5'-CCGCN-3'), and the AlcR-binding site in the aldA promoter has been localized. The sequence downstream of the zinc cluster is necessary for high affinity binding. Furthermore, our data show that the use of the carrier protein glutathione S-transferase in AlcR binding experiments introduces an important bias in the recognition of DNA sites due to its tertiary dimeric structure.

Induction by (alpha)-L-Arabinose and (alpha)-L-Rhamnose of Endopolygalacturonase Gene Expression in Colletotrichum lindemuthianum

The production of endopolygalacturonase (endoPG) by Colletotrichum lindemuthianum, a fungal pathogen causing anthracnose on bean seedlings, was enhanced when the fungus was grown in liquid medium with L-arabinose or L-rhamnose as the sole carbon source. These two neutral sugars are present in plant cell wall pectic polysaccharides. The endolytic nature of the enzyme was demonstrated by its specific interaction with the polygalacturonase-inhibiting protein of the host plant as well as by sugar analysis of the products released from its action on oligogalacturonides. Additional characterization of the protein was achieved with an antiserum raised against the pure endoPG of the fungus. Induction by arabinose and rhamnose was more prolonged and led to a level of enzyme activity at least five times higher than that on pectin. Northern blot experiments showed that this effect was correlated to the induction of a 1.6-kb transcript. A dose-response study indicated that the endoPG transcript level was already increased at a concentration of each sugar as low as 2.75 mM in the medium and was maximum at 55 mM arabinose and 28 mM rhamnose. Glucose, the main plant cell wall sugar residue which is also present in the apoplast, prevented endoPG gene expression, partially when added to pectin at concentrations ranging from 5 to 110 mM and totally when added at 55 mM to arabinose. Inhibition by glucose of the rhamnose-induced endoPG was correlated to nonuptake of rhamnose. This is the first report that arabinose and rhamnose stimulate endoPG gene expression in a fungus. The possible involvement of these various sugars on endoPG gene expression during pathogenesis is discussed.

The Aspergillus nidulans transcription factor AlcR forms a stable complex with its half-site DNA: a NMR study

The Aspergillus nidulans transcription factor AlcR is shown by NMR and gel retardation assay to form a stable complex with oligonucleotide sequences comprising the consensus half-site 5'-TGCGG-3'. Apparent microM dissociation constants are evaluated by both methods. The measured lifetime of the complex is 74+/-7 ms at 20 degrees C with the following DNA sequence: 5'-C1G2T3G4C5G6G7A8T9C10-3'. The major chemical shift variations upon binding involve both the two adjacent GC pairs (G6 and G7) and, clearly, the AT pairs at both ends of the consensus sequence (T3 and A8), suggesting additional contacts of the protein with the DNA. This extensive and strong interaction with the half-site is another example of the variability in contacts of the fungal DNA-binding proteins containing Zn2Cys6 domains with their consensus sites. It is the first demonstration that a binuclear cluster protein can bind to DNA as a monomer with strong affinity.

The regulation of the vanadium chloroperoxidase from Curvularia inaequalis

The effects of carbon and nitrogen source on the regulation of the vanadium chloroperoxidase secreted by the fungus Curnularia inaequalis were investigated. The addition of glucose showed a repressing effect on both the observed messenger RNA level and the measured enzyme activities, whereas the addition of glutamate as nitrogen source and the addition of both glutamate and glycerol had no effect. Addition of vanadate had no effect on the level of mRNA. Eight hundred base pairs of the upstream promoter region of vCPO were sequenced and various features of interest are highlighted. Closer inspection of the mycelium revealed that once secreted, vCPO probably remains tightly associated with the hyphae in two forms, one of which may be a proform of the enzyme. A possible cleavage event at the C-terminus may lower its potential for hyphal association and permit its disassociation into the growth medium. A putative role for the vanadium chloroperoxidase is put forward.

Regulation of cellulase gene expression in the filamentous fungus Trichoderma reesei

Basic features of regulation of expression of the genes encoding the cellulases of the filamentous fungus Trichoderma reesei QM9414, the genes cbh1 and cbh2 encoding cellobiohydrolases and the genes egl1, egl2 and egl5 encoding endoglucanases, were studied at the mRNA level. The cellulase genes were coordinately expressed under all conditions studied, with the steady-state mRNA levels of cbh1 being the highest. Solka floc cellulose and the disaccharide sophorose induced expression to almost the same level. Moderate expression was observed when cellobiose or lactose was used as the carbon source. It was found that glycerol and sorbitol do not promote expression but, unlike glucose, do not inhibit it either, because the addition of 1 to 2 mM sophorose to glycerol or sorbitol cultures provokes high cellulase expression levels. These carbon sources thus provide a useful means to study cellulase regulation without significantly affecting the growth of the fungus. RNA slot blot experiments showed that no expression could be observed on glucose-containing medium and that high glucose levels abolish the inducing effect of sophorose. The results clearly show that distinct and clear-cut mechanisms of induction and glucose repression regulate cellulase expression in an actively growing fungus. However, derepression of cellulase expression occurs without apparent addition of an inducer once glucose has been depleted from the medium. This expression seems not to arise simply from starvation, since the lack of carbon or nitrogen as such is not sufficient to trigger significant expression.

Cloning and sequencing of an alpha-glucosidase gene from Aspergillus niger and its expression in A. nidulans

We have cloned an extracellular alpha-glucosidase gene from Aspergillus niger with oligonucleotide probes synthesized on the basis of the determined peptide sequences. The nucleotide sequence revealed an open reading frame of 985 amino acids split with three introns, and the deduced amino acid sequence was nearly identical to that of the alpha-glucosidase previously determined. The cloned gene was introduced into Aspergillus nidulans, and its expression in the transformants was shown to be regulated by the carbon sources in the medium, suggesting that a common regulatory expression system is shared by these two species as is the case of other starch-degrading enzymes of Aspergillus species.

The amdA regulatory gene of Aspergillus nidulans: characterization of gain-of-function mutations and identification of binding sites for the gene product

Genetic evidence suggests that the amdA gene of Aspergillus nidulans encodes a protein which controls the expression of the amdS and aciA structural genes. The amd166 and amd1666 mutations in the 5' regulatory region of amdS lead to higher levels of amdA-dependent amdS expression. We show here that the putative DNA binding domain of amdA is capable of binding specific regions of the amdS and aciA promoters in vitro and this region includes sequences duplicated and triplicated in the amd166 and amd1666 mutations, respectively. Footprinting analysis has shown that AmdA binds to two sites in this region represented by the sequences 5'-GCGGGG-3' and 5'-GAGGGG-3'. A number of gain-of-function mutations in amdA were localized to a region rich in acidic and hydrophobic amino acid residues and shown to be involved in transcriptional activation by studies of fusions with the GAL4 DNA binding domain in Saccharomyces cerevisiae. Therefore, both an increased probability of AmdA binding to the amdS promoter and an increased activation potential of AmdA can result in higher levels of expression.

The filamentous fungus Aspergillus niger contains two "differentially regulated" trehalose-6-phosphate synthase-encoding genes, tpsA and tpsB

Two genes encoding trehalose-6-phosphate synthase were cloned from Aspergillus niger. tpsA was cloned using the Saccharomyces cerevisiae GGS1/TPS1 gene as a probe. It encodes a 517-amino acid polypeptide with 64-70% similarity to trehalose-6-phosphate synthase of S. cerevisiae, Kluyveromyces lactis, and Schizosaccharomyces pombe. Its transcription occurs constitutively and is enhanced on carbon-derepressing carbon sources, coinciding with the presence of a CreA-binding nucleotide motif in the 5'-noncoding region of tpsA. Disruption of tpsA only weakly reduces growth on glucose, and neither influences the glucose induction of a low affinity glucose permease nor interferes with the catabolite repression of a pectinase; it causes reduced the heat tolerance of conidia. tpsB was cloned by a polymerase chain reaction-based strategy. Its 480 amino acid sequence showed 76.5% identity to tpsA. Its transcription was hardly detectable at ambient temperatures but was enhanced strongly upon heat shock, which agrees with the presence of several copies of a C4T stress-responsive element in its 5'-upstream sequences. Hence the function of yeast GGS1/TPS1 has been split into two differentially regulated genes in A. niger, of which none appears to be involved in glucose sensing.

Molecular analysis of the isocitrate lyase gene (acu-7) of the mushroom Coprinus cinereus

The nucleotide sequence of the structural gene for isocitrate lyase (acu-7) is presented and features of its coding sequence and predicted protein are described. Several motifs were identified within the promoter region which are potentially involved in transcriptional regulation. Surprisingly, some of these occur within the coding sequence of an adjacent gene of unrelated function that terminates within 371 bp upstream from acu-7. The sequence of this second gene identified an N-acetylglucosamine-1-phosphate transferase.

Analysis of Cre1 binding sites in the Trichoderma reesei cbh1 upstream region

A 1.5-kb XbaI-SacII fragment containing the upstream region of the Trichoderma reesei cellobiohydrolase I gene (cbh1) has been sequenced. The 1.5-kb fragment contains eight 6-bp sites having an identical or similar sequence to the consensus sequence for binding a catabolite repressor, Aspergillus nidulans CreA. Results of binding assays with the maltose-binding protein::Cre1(10-131) fusion protein (Cre1 is a catabolite repressor of T. reesei) and the cbh1 upstream region revealed that a 504-bp XbaI-NspV fragment (nucleotide position -1496 to -993) bearing three 6-bp sites, A1, A2, and A3, and a 356-bp NspV-MunI fragment (nucleotide position -994 to -639) bearing three 6-bp sites, B1, B2, and B3, were shifted in the electrophoretic mobility shift assay. DNase I footprinting experiments showed that the 6-bp sites A2, B1, B2, and B3 were protected from DNase I digestion.

Mycoparasitic interaction relieves binding of the Cre1 carbon catabolite repressor protein to promoter sequences of the ech42 (endochitinase-encoding) gene in Trichoderma harzianum

The fungus Trichoderma harzianum is a potent mycoparasite of various plant pathogenic fungi. We have studied the molecular regulation of mycoparasitism in the host/mycoparasite system Botrytis cinerea/T. harzianum. Protein extracts, prepared from various stages of mycoparasitism, were used in electrophoretic mobility-shift assays (EM-SAs) with two promoter fragments of the ech-42 (42-kDa endochitinase-encoding) gene of T. harzianum. This gene was chosen as a model because its expression is triggered during mycoparasitic interaction [Carsolio, C., Gutierrez, A., Jimenez, B., van Montagu, M. & Herrera-Estrella, A. (1994) Proc. Natl. Acad. Sci. USA 91, 10903-10907]. All cell-free extracts formed high-molecular weight protein-DNA complexes, but those obtained from mycelia activated for mycoparasitic attack formed a complex with greater mobility. Competition experiments, using oligonucleotides containing functional and nonfunctional consensus sites for binding of the carbon catabolite repressor Cre1, provided evidence that the complex from nonmycoparasitic mycelia involves the binding of Cre1 to both fragments of the ech-42 promoter. The presence of two and three consensus sites for binding of Cre1 in the two ech-42 promoter fragments used is consistent with these findings. In contrast, the formation of the protein-DNA complex from mycoparasitic mycelia is unaffected by the addition of the competing oligonucleotides and hence does not involve Cre1. Addition of equal amounts of protein of cell-free extracts from nonmycoparasitic mycelia converted the mycoparasitic DNA-protein complex into the nonmycoparasitic complex. The addition of the purified Cre1::glutathione S-transferase protein to mycoparasitic cell-free extracts produced the same effect. These findings suggest that ech-42 expression in T. harzianum is regulated by (i) binding of Cre1 to two single sites in the ech-42 promoter, (ii) binding of a "mycoparasitic" protein-protein complex to the ech-42 promoter in vicinity of the Cre1 binding sites, and (iii) functional inactivation of Cre1 upon mycoparasitic interaction to enable the formation of the mycoparasitic protein-DNA complex.

Identification, isolation and sequence of the Aspergillus nidulans xlnC gene encoding the 34-kDa xylanase

The xlnC gene encoding the 34-kDa xylanase (X34) of Aspergillus nidulans (An) has been cloned and sequenced, as has its corresponding cDNA. xlnC contains nine introns and shows considerable similarity to the xynA and xylP xylanase-encoding genes of A. kawachii (Ak) and Penicillium chrysogenum (Pc), respectively. Analysis of xylanase production in An multicopy transformants showed elevated levels of X34 and increased total xylanase activity, but no elevated production of other xylanases. Northern analysis demonstrated transcriptional induction by xylan and repression by glucose.

Primary structure and characterization of an exopolygalacturonase from Aspergillus tubingensis

From the culture fluid of the hyphal fungus Aspergillus tubingensis, an exopolygalacturonase with a molecular mass of 78 kDa, an isoelectric point in the pH-range 3.7-4.4 and a pH optimum of 4.2 was purified. The enzyme has been characterized as an exopolygalacturonase [poly(1,4-alpha-D-galacturonide)galacturonohydrolase] that cleaves monomer units from the non-reducing end of the substrate molecule. K(m) and Vmax for polygalacturonic acid hydrolysis were 3.2 mg ml-1 and 3.1 mg ml-1 and 255 U mg-1 and 262 U mg-1 for the wild-type and recombinant enzymes, respectively. The kinetic data of exopolygalacturonase on oligogalacturonates of different degree of polymerization (2-7) were interpreted in terms of a subsite model to obtain more insight into catalysis and substrate binding. On oligogalacturonates of different degrees of polymerization (2-7), the Michaelis constant (K(m)) decreased with increasing chain length (n). The Vmax value increased with chain length up to n = 4, then reached a plateau value. The enzyme was competitively inhibited by galacturonic acid (Ki = 0.3 mM) as well as by reduced digalacturonate (Ki = 0.4 mM). The exopolygalacturonase gene (pgaX) was cloned by reverse genetics and shows only 13% overall amino acid sequence identity with A. niger endopolygalacturonases. The exopolygalacturonase is most related to plant polygalacturonases. Only four small stretches of amino acids are conserved between all known endogalacturonases and exopolygalacturonases. Expression of the pgaX gene is inducible with galacturonic acid and is subject to catabolite repression. A fusion between the promoter of the A. niger glycolytic gene encoding pyruvate kinase and the pgaX-coding region was used to achieve high level production of exopolygalacturonase under conditions where no endopolygalacturonases were produced.

Analysis of mutations in the creA gene involved in carbon catabolite repression in Aspergillus nidulans

The molecular nature of a number of creA mutant alleles has been determined. Three alleles analysed are missense mutations in the DNA binding domain and predicted to reduce but not abolish binding. Of the other four alleles, two result from frameshifts: one has a nonsense mutilation and the other has an inversion. All four alleles result in truncations of the protein after the zinc finger domain, such that the protein no longer contains at least the carboxy terminal 145 amino acids, so identifying a region required for repression. Transcriptional analysis of creA indicates that the transcript is autoregulated and analysis using 5' rapid amplification of cDNA ends indicates that transcriptional start points exist in clusters over a region of 200 bp located up to 595 bp 5' of the translational start point. The two major clusters have potential CREA-binding sites (SYGGRG) at appropriate positions to allow autoregulation. Autoregulation leads to the creA transcript being most abundant in carbon catabolite nonrepressing conditions, and this, together with the phenotypes of the mutant alleles, has led to the suggestion that CREA has effects under conditions generally not considered as carbon catabolite repressing, as well as in carbon catabolite repressing conditions.

The glucose repressor gene cre1 of Trichoderma: isolation and expression of a full-length and a truncated mutant form

The cre1 genes of the filamentous fungi Trichoderma reesei and T. harzianum were isolated and characterized. The deduced CREI proteins are 46% identical to the product of the glucose repressor gene creA of Aspergillus nidulans, encoding a DNA-binding protein with zinc fingers of the C2H2 type. The cre1 promoters contain several sequence elements that are identical to the previously identified binding sites for A. nidulans CREA. Steady-state mRNA levels for cre1 of the T. reesei strain QM9414 varied depending on the carbon source, being low on glucose-containing media. These observations suggest that cre1 expression may be autoregulated. The T. reesei strain Rut-C30, a hyper-producer of cellulolytic enzymes, was found to express a truncated form of the cre1 gene (cre1-1) with an ORF corresponding to a protein of 95 amino acids with only one zinc finger. Unlike QM9414 the strain Rut-C30 produced cellulase mRNAs on glucose-containing medium and transformation of the full-length cre1 gene into this strain caused glucose repression of cbh1 expression, demonstrating that cre1 regulates cellulase expression.

Mutations affecting extracellular protease production in the filamentous fungus Aspergillus nidulans

The extracellular proteases of Aspergillus nidulans are known to be regulated by carbon, nitrogen and sulphur metabolite repression. In this study, a mutant with reduced levels of extracellular protease was isolated by screening for loss of halo production on milk plates. Genetic analysis of the mutant showed that it contains a single, recessive mutation, in a gene which we have designated xprE, located on chromosome VI. The xprE1 mutation affected the production of extracellular proteases in response to carbon, nitrogen and, to a lesser extent, sulphur limitation. Three reversion mutations, xprF1, xprF2 and xprG1, which suppress xprE1, were characterised. Both xprF and xprG map to chromosome VII but the two genes are unlinked. The xprF1, xprF2 and xprG1 mutants showed high levels of milk-clearing activity on medium containing milk as a carbon source but reduced growth on a number of nitrogen sources. Evidence is presented that the xprE1 and xprG1 mutations alter expression of more than one protease and affect levels of alkaline protease gene mRNA.

Endoglucanase II (EGII) of Penicillium janthinellum: cDNA sequence, heterologous expression and promotor analysis

The cDNA coding for the endoglucanase EGII of P. janthinellum was cloned and sequenced. The open reading frame comprises 1230 nucleotides and the deduced amino-acid sequence shows an overall homology of 63% with the T. reesei egl2. The cellulose-binding domain of EGII represents a typical member of the A family of cellulases. The egl2 gene is only induced by cellulose or cellobiose and not by sophorose. A promotor fragment including 1 kb was cloned and sequenced. Three major transcription startpoints were identified. Five motifs matching the binding site of the carbon-catabolite repressor CREA of A. nidulans were detected. Their potential implication in repression was analyzed by bandshift assays.

Gel mobility shift scanning of the acetate-inducible promoters from Neurospora crassa reveals a common co-inducible DNA-binding protein

The promoter regions of four acetate-inducible genes of Neurospora crassa, acu-3, acu-5, acu-8 and acu-9, have been sequenced. Using a scanning gel mobility shift assay particular DNA regions in each promoter have been shown specifically to bind partially purified protein extracted from acetate-induced mycelia. The protein-binding regions so defined have common sequence motifs, elements of which are similar to those required for acetate induction in aspergillus nidulans.

The cloning and sequencing of the alcB gene, coding for alcohol dehydrogenase II, in Aspergillus nidulans

Alcohol dehydrogenase II (ADH II, structural gene alcB) was purified from a strain H1035, biA1; alcE1; alc500 alcD1, which produces 100-times more ADH II activity than the alcAalcR deletion strain (alc500). Antibodies were raised against this ADH, and were used to screen a cDNA library in lambda gt11. We have isolated the gene for an ADH which is over-expressed in H1035, and which we believe to be the alcB gene: cDNA and genomic clones were sequenced. The sequence contains three introns and encodes a protein of 367 amino acids. This protein shows a clear level of identity to a range of alcohol dehydrogenases, but is no more closely related to the ADH I and ADH III previously described in A. nidulans than to the ADHs of S. pombe and S. cerevisiae. The significance of consensus sequences found in the 5' region of the gene is discussed in relation to the regulation of the gene.

Carbon catabolite repression inAspergillus nidulans: a review

We describe the experimental methodology that led to the discovery of the creA gene in Aspergillus nidulans. This gene codes for a transcriptional repressor mediating carbon catabolite repression in many pathways in this organism. We compare both the mode and the mechanism of action in two pathways subject to CreA-mediated repression. The genes comprising the ethanol regulon are subject to carbon catabolite repression independently of the nitrogen source, while the genes involved in proline utilization are repressed by glucose only when a repressing nitrogen source is also present. In the ethanol regulon, CreA drastically represses the expression of the positive regulatory gene alcR, thus preventing the expression of the structural genes. Direct repression of the structural genes is also existant. In the proline utilization pathway, repression operates directly at the level of the structural genes. In the ethanol regulon, CreA prevents the self-induction of alcR and the induction of the structural genes by competing with the binding of the AlcR protein. In proline gene cluster, CreA does not interfere with induction mediated by PrnA but with the activity of an unknown and more general transcription factor. Key words: carbon catabolite repression, ascomycetes, Zn fingers.

Nitrogen metabolite repression inAspergillus nidulans: an historical perspective

The paper of Arst and Cove (Mol. Gen. Genet. 126: 111 – 141, 1973) on "Nitrogen metabolite repression in Aspergillus nidulans" has influenced studies and perceptions of gene regulation in filamentous fungi during the past 21 years. Here I attempt to appraise the contributions of that paper and assess its role in further developments. Nitrogen metabolite repression, carbon catabolite repression, pathway-specific and integrated induction, as-acting regulatory mutations, a useful class of growth inhibitors, and a homologous Neurospora crassa gene are all discussed. Key words: Aspergillus nidulans, carbon catabolite repression, nitrogen metabolite repression.

Cloning, structural organization and regulation of expression of the Penicillium chrysogenum paf gene encoding an abundantly secreted protein with antifungal activity

An abundantly secreted, highly basic 12-kDa protein (PAF) was purified from the culture medium of Penicillium chrysogenum (Pc). Based on the N-terminal amino acid (aa) sequence of the protein, an oligodeoxyribonucleotide probe was derived and used for amplification of the encoding cDNA by PCR. This cDNA fragment encodes a Cys-rich preproprotein of 92 aa which appears to be processed to a mature product of 55 aa. The deduced aa sequence of the preproprotein reveals 42.6% identity to an antifungal protein (AFP) of Aspergillus giganteus. Agar diffusion tests confirmed that the Pc protein exhibits antifungal activity. In order to investigate the promoter region and the structural organization of the paf gene, a genomic 6-kb fragment was isolated and partially sequenced. Comparison of the nucleotide sequence of the genomic fragment and the cDNA clone revealed the presence of a coding region of 279 bp which is interrupted by two introns of 76 and 68 bp in length. In the promoter region, a typical TATA box, a motif resembling the fungal carbon catabolite repression element, as well as several putative GATA factor binding motifs, were found. Northern blot analysis indicated that the regulation of paf expression occurs at the level of mRNA transcription and is under control of carbon catabolite and nitrogen metabolite repression regulatory circuits.

Two family G xylanase genes from Chaetomium gracile and their expression in Aspergillus nidulans

With oligonucleotides based on the amino-terminal and internal amino-acid sequences of a xylanase, two xylanase genes, cgxA and cgxB, were isolated and sequenced from Chaetomium gracile wild and mutant strains. Each gene isolated from both strains was essentially the same as far as nucleotide sequences were compared. The mature CgXA and CgXB xylanases comprise 189 and 211 amino acids, respectively, and share 68.5% homology. The CgXA was found to be the major enzyme in the mutant strain. Comparison of these amino-acid sequences with xylanase sequences from other origins showed that they have a high degree of identity to the family G xylanases. The cgxA and cgxB genes were introduced into Aspergillus nidulans and found to be expressed with their own promoters.

Cloning, characterization and overproduction of nuclease S1 gene (nucS) from Aspergillus oryzae

The nuclease S1 gene (nucS) from Aspergillus oryzae was isolated using a polymerase-chain-reaction-amplified DNA fragment as a probe, and a 2.6-kb SalI-EcoRI fragment containing the nucS gene was sequenced. It was deduced that the nucS gene had two short introns, 49 and 50 nucleotides in length. The nucS gene had an open-reading frame of 963 base pairs and coded for a protein of 287 amino acid residues, comprising the signal peptide of 20 amino acids and a mature protein of 267 amino acids. The deduced amino acid sequence agreed well with the published amino acid sequence except for one substitution. Southern hybridization analysis showed that the nucS gene existed as a single copy in the A. oryzae chromosome. When the structural gene of nucS was fused with the promoter of the glaA gene and introduced into A. oryzae, the yield of secreted nuclease S1 increased about 100-fold compared with the recipient strain.

Cre1, the carbon catabolite repressor protein from Trichoderma reesei

In order to investigate the mechanism of carbon catabolite repression in the industrially important fungus Trichoderma reesei, degenerated PCR-primers were designed to amplify a 0.7-bp fragment of the cre1 gene, which was used to clone the entire gene. It encodes a 402-amino acid protein with a calculated M(r) of 43.6 kDa. Its aa-sequence shows 55.6% and 54.7% overall similarity to the corresponding genes of Aspergillus nidulans and A. niger, respectively. Similarity was restricted to the aa-region containing the C2H2 zinc finger and several aa-regions rich in proline and basic amino acids, which may be involved in the interaction with other proteins. Another aa-region rich in the SPXX-motif that has been considered analogous to a region of yeast RGR1p, was instead identified as a domain occurring in several eucaryotic transcription factors. The presence of the cre1 translation product was demonstrated with polyclonal antibodies against Cre1, which identified a protein of 43 (+/- 2) kDa in cell-free extracts from T. reesei. A Cre1 protein fragment from the two zinc fingers to the region similar to the aa-sequence of eucaryotic transcription factors, was expressed in Escherichia coli as a fusion protein with glutathione S-transferase. EMSA and in vitro footprinting revealed binding of the fusion protein to the sequence 5'-GCGGAG-3', which matches well with the A. nidulans consensus sequence for CreA binding (5'-SYGGRG-3'). Cell-free extracts of T. reesei formed different complexes with DNA-fragments carrying this binding sites, and the presence of Cre1 and additional proteins in these complexes was demonstrated. We conclude that T. reesei Cre1 is the functional homologue of Aspergillus CreA and that it binds to its target sequence probably as a protein complex.

Regulation of the expression of three housekeeping genes encoding subunits of the Neurospora crassa vacuolar ATPase

The vacuolar ATPase is a complex enzyme and is encoded by at least nine genes, which appear to be scattered throughout the genome. We have examined the vma-1 vma-2, and vma-3 genes, which encode subunits present in multiple copies within the Neurospora crassa vacuolar ATPase. We wished to see if the expression of these genes is coordinately regulated and if these genes contain similar promoter elements. A region was sequenced of approximately 1 kb located upstream of the protein coding region for each gene. Several sequence elements were found in similar positions in each of the three genes. Each of the genes had several strong transcription initiation sites, clustered within 13-60 bp and located 112-193 bp upstream of the translation start site. The size and abundance of the RNA transcripts was also determined: the amount of RNA transcribed from each gene was roughly proportional to the numbers of each subunit present in the enzyme. A series of plasmids was constructed containing parts of the putative promoter region fused to beta-galactosidase. Analysis of these plasmids indicated that the essential region of the vma promoters lies within 370 bp of the protein coding region. Overall, the vma genes appear to have similar characteristics to "housekeeping" genes described in other organisms.

Production of Trichoderma reesei cellulases on glucose-containing media

The filamentous fungus Trichoderma reesei was shown to secrete active cellobiohydrolase I and the endoglucanase I catalytic core domain into the culture medium when the fungus was grown on glucose-containing medium. The expression of the proteins was driven by the promoters of the elongation factor 1 alpha, tef1, and the unidentified gene for cDNA1. The cDNA1 promoter gave the best yields. The highest amounts of cellobiohydrolase I and the endoglucanase I core, being 50 to 100 mg/liter, accounted for more than half of the total protein secreted by the fungus. The levels obtained with the tef1 promoter were 20 to 50 times lower.

Cloning and characterization of an ATPase gene from Pneumocystis carinii which closely resembles fungal H+ ATPases

A gene encoding a P-type cation translocating ATPase was cloned from a genomic library of rat-derived Pneumocystis carinii. The nucleotide sequence of the gene contains a 2781 base-pair open reading frame that is predicted to encode a 101,401 dalton protein composed of 927 amino acids. The P. carinii ATPase protein (pcal) is 69-75% identical when compared with eight proton pumps from six fungal species. The Pneumocystis ATPase is less than 34% identical to ATPase proteins from protozoans, vertebrates or the Ca++ ATPases of yeast. The P. carinii ATPase contains 115 of 121 residues previously identified as characteristic of H+ ATPases. Alignment of the Pneumocystis and fungal proton pumps reveals five homologous domains specific for fungal H+ ATPases.

Nuclear DNA-binding proteins which recognize the intergenic control region of penicillin biosynthetic genes

The biosynthesis of penicillin, a secondary metabolite produced by Penicillium chrysogenum, is subject to sophisticated genetic and metabolic regulation. The structural genes, pcbC and pcbAB, which encode two of the penicillin biosynthetic enzymes are separated by a 1.16-kb intergenic region and transcribed divergently from one another. To identify and characterize nuclear proteins which interact with the pcbAB-pcbC intergenic promoter region, crude and partially purified nuclear extracts were used in mobility shift and DNA footprinting assays. Multiple DNA-binding proteins appear to bind to different regions of this DNA segment. An abundant nuclear protein, nuclear factor A (NF-A), binds at a single site in the intergenic promoter region and recognizes an 8-bp sequence, GCCAAGCC. Penicillin production is sensitive to nitrogen catabolite repression. The global-acting nitrogen regulatory protein NIT2 of Neurospora crassa binds strongly to the intergenic promoter region of the pcbAB and pcbC genes at a single site that contains two closely spaced GATA sequences.

Overexpression of two penicillin structural genes in Aspergillus nidulans

We have placed two different penicillin structural genes from Aspergillus nidulans, ipnA (encoding isopenicillin N synthetase, IPNS) and acyA (encoding acyl-CoA:6-aminopenicillanic acid acyltransferase, AAT), under the control of the strong alcA promoter [alcA(p)]. Single copies of these transcriptional fusions were targeted to the same chromosomal location and conditions have been worked out which simultaneously allow induction of the alcA(p) and support penicillin biosynthesis. Transcriptional induction of the chimeric genes alcA(p)::ipnA or alcA(p)::acyA(cdna) in the relevant recombinant strains results in 10-fold higher levels of the ipnA or acyA transcripts than those resulting from transcription of the corresponding endogenous genes. This increase causes a 40-fold rise in IPNS activity or a 8-fold rise in AAT activity. Despite this rise in enzyme levels, forced expression of the ipnA gene results in only a modest increase in levels of exported penicillin, whereas forced expression of the acyA gene reduces penicillin production, showing that neither of these enzymes is rate-limiting for penicillin biosynthesis in A. nidulans. A genomic version of the alcA(p)::acyA fusion in which the acyA gene is interrupted by three small introns, is inducible by threonine to a lesser extent (as determined by both acyA mRNA levels and AAT enzyme levels) than the corresponding cDNA version, suggesting that processing of the introns present in the primary transcript may limit acyA expression.