Dual DNA binding and coactivator functions of Aspergillus nidulans†TamA, a Zn(II)2Cys6 transcription factor.

Transcription factors containing DNA binding domains generally regulate transcription by direct interaction with DNA. For most transcription factors, including the fungal Zn(II)2Cys6 zinc binuclear cluster transcription factors, the DNA binding motif is essential for function. However, Aspergillus nidulans TamA and the related Saccharomyces cerevisiae Dal81p protein contain Zn(II)2Cys6 motifs shown to be dispensable for function. TamA acts at several promoters as a coactivator of the global nitrogen GATA transcription factor AreA. We now show that TamA is the major transcriptional activator of gdhA, encoding the key nitrogen metabolism enzyme NADP-glutamate dehydrogenase. Moreover, activation of gdhA by TamA occurs primarily by a mechanism requiring the TamA DNA binding motif. We show that the TamA DNA binding motif is required for DNA binding of FLAG-epitope-tagged TamA to the gdhA promoter. We identify a conserved promoter element required for TamA activation, and show that TamA and AreA are reciprocally required for full binding at the gdhA promoter under conditions where AreA is inactive at most promoters but active at gdhA. Therefore TamA has dual functions as a DNA-binding transcription factor and a non-DNA-binding coactivator. Dual DNA-binding and coactivator functions provide an additional level of combinatorial control to mediate gene-specific expression.

Multiple nuclear localization signals mediate nuclear localization of the GATA transcription factor AreA.

The Aspergillus nidulans GATA transcription factor AreA activates transcription of nitrogen metabolic genes in response to nitrogen limitation and is known to accumulate in the nucleus during nitrogen starvation. Sequence analysis of AreA revealed multiple nuclear localization signals (NLSs), five putative classical NLSs conserved in fungal AreA orthologs but not in the Saccharomyces cerevisiae functional orthologs Gln3p and Gat1p, and one putative noncanonical RRX33RXR bipartite NLS within the DNA-binding domain. In order to identify the functional NLSs in AreA, we constructed areA mutants with mutations in individual putative NLSs or combinations of putative NLSs and strains expressing green fluorescent protein (GFP)-AreA NLS fusion genes. Deletion of all five classical NLSs individually or collectively did not affect utilization of nitrogen sources or AreA-dependent gene expression and did not prevent AreA nuclear localization. Mutation of the bipartite NLS conferred the inability to utilize alternative nitrogen sources and abolished AreA-dependent gene expression likely due to effects on DNA binding but did not prevent AreA nuclear localization. Mutation of all six NLSs simultaneously prevented AreA nuclear accumulation. The bipartite NLS alone strongly directed GFP to the nucleus, whereas the classical NLSs collaborated to direct GFP to the nucleus. Therefore, AreA contains multiple conserved NLSs, which show redundancy and together function to mediate nuclear import. The noncanonical bipartite NLS is conserved in GATA factors from Aspergillus, yeast, and mammals, indicating an ancient origin.

Reprogramming of carbon metabolism by the transcriptional activators AcuK and AcuM in Aspergillus nidulans.

The ability of fungi to use carbon sources metabolized via the TCA cycle requires gluconeogenesis. In Aspergillus nidulans the AcuK and AcuM transcription factors regulate the expression of the gluconeogenic genes acuF, encoding phosphoenolpyruvate carboxykinase, and acuG, encoding fructose-1,6-bisphosphatase. Expressed proteins containing the AcuK/AcuM N-terminal DNA-binding domains bind together in vitro to motifs containing repeats of CGG separated by seven bases (CCGN7CCG) and the functionality of these sequences was verified in vivo by acuF-lacZ reporter studies. Chromatin immunoprecipitation analysis showed inter-dependent DNA binding of the proteins to the promoters of gluconeogenic genes in vivo independent of the carbon source. Deletion of the mdhC gene encoding a cytoplasmic/peroxisomal malate dehydrogenase showed that this activity is not essential for gluconeogenesis and indicated that induction of AcuK/AcuM regulated genes might result from malate accumulation. Deletion of the gene for the alternative oxidase did not affect growth on gluconeogenic carbon sources; however, expression was absolutely dependent on AcuK and AcuM. Orthologues of AcuK and AcuM, are present in a wide range of fungal taxa and the CCGN7CCG motif is present in the 5' of many genes involved in gluconeogenesis indicating a fundamental role for these transcription factors in reprogramming fungal carbon metabolism.

Regulation of the NADP-glutamate dehydrogenase gene gdhA in Aspergillus nidulans by the Zn(II)2Cys6 transcription factor LeuB.

NADP-dependent glutamate dehydrogenase (NADP-GDH) is a key enzyme in the assimilation of alternative nitrogen nutrient sources through ammonium in fungi. In Aspergillus nidulans, NADP-GDH is encoded by gdhA. Several transcription factors are known to regulate gdhA expression, including AreA, the major transcription activator of nitrogen metabolic genes, and TamA, a co-activator of AreA. TamA also interacts with LeuB, the regulator of leucine biosynthesis. We have investigated the effects of leucine biosynthesis on gdhA regulation, and found that leucine regulates the levels of NADP-GDH activity and gdhA expression. We show, using mutants with perturbed levels of α-isopropylmalate (α-IPM), that this leucine biosynthesis intermediate affects gdhA regulation. Leucine regulation of gdhA requires a functional LeuB with an intact Zn(II)2Cys6 DNA-binding domain. By analysing the prevalence of putative LeuB DNA-binding sites in promoters of gdhA orthologues we predict broad conservation of leucine regulation of NADP-GDH expression within ascomycetes except in the fusaria and fission yeasts. Using promoter mutations in gdhA-lacZ reporter genes we identified two sites of action for LeuB within the A. nidulans gdhA promoter. These two sites lack sequence identity, with one site conforming to the predicted LeuB DNA-binding site consensus motif, whereas the second site is a novel regulatory sequence element conserved in Aspergillus gdhA promoters. These data suggest that LeuB regulates NADP-GDH expression in response to leucine levels, which may act as an important sensor of nitrogen availability.

Role of carnitine acetyltransferases in acetyl coenzyme A metabolism in Aspergillus nidulans.

The flow of carbon metabolites between cellular compartments is an essential feature of fungal metabolism. During growth on ethanol, acetate, or fatty acids, acetyl units must enter the mitochondrion for metabolism via the tricarboxylic acid cycle, and acetyl coenzyme A (acetyl-CoA) in the cytoplasm is essential for the biosynthetic reactions and for protein acetylation. Acetyl-CoA is produced in the cytoplasm by acetyl-CoA synthetase during growth on acetate and ethanol while ß-oxidation of fatty acids generates acetyl-CoA in peroxisomes. The acetyl-carnitine shuttle in which acetyl-CoA is reversibly converted to acetyl-carnitine by carnitine acetyltransferase (CAT) enzymes is important for intracellular transport of acetyl units. In the filamentous ascomycete Aspergillus nidulans, a cytoplasmic CAT, encoded by facC, is essential for growth on sources of cytoplasmic acetyl-CoA while a second CAT, encoded by the acuJ gene, is essential for growth on fatty acids as well as acetate. We have shown that AcuJ contains an N-terminal mitochondrial targeting sequence and a C-terminal peroxisomal targeting sequence (PTS) and is localized to both peroxisomes and mitochondria, independent of the carbon source. Mislocalization of AcuJ to the cytoplasm does not result in loss of growth on acetate but prevents growth on fatty acids. Therefore, while mitochondrial AcuJ is essential for the transfer of acetyl units to mitochondria, peroxisomal localization is required only for transfer from peroxisomes to mitochondria. Peroxisomal AcuJ was not required for the import of acetyl-CoA into peroxisomes for conversion to malate by malate synthase (MLS), and export of acetyl-CoA from peroxisomes to the cytoplasm was found to be independent of FacC when MLS was mislocalized to the cytoplasm.

AoxA is a major peroxisomal long chain fatty acyl-CoA oxidase required for beta-oxidation in A. nidulans.

Filamentous fungi can use a variety of fatty acids (FA) as sole carbon and energy sources. Aspergillus nidulans has been shown to possess both peroxisomal and mitochondrial beta-oxidation pathways. In these studies, the major peroxisomal long chain fatty acyl coenzyme A oxidase AoxA was identified. AoxA was shown to be localised to peroxisomes and deletion of the aoxA gene leads to reduced growth on long chain FA, but not on short chain FA. AoxA is predicted to be part of the same peroxisomal beta-oxidation pathway as the bifunctional protein FoxA. In addition, an aoxA(p)lacZ reporter gene construct is induced by short and long chain FA and the induction is dependent on the transcriptional regulators FarA, FarB and ScfA with FarA being required for the induction by short chain as well as long chain FA and FarB and ScfA being required for induction of aoxA by short chain FA. It is proposed that there are additional peroxisomal beta-oxidation pathways in A. nidulans, which include fatty acyl-CoA dehydrogenases with a partially overlapping substrate range and include a pathway for short chain FA.

Aspergillus nidulans contains six possible fatty acyl-CoA synthetases with FaaB being the major synthetase for fatty acid degradation.

Aspergillus nidulans can use a variety of fatty acids as sole carbon and energy sources via its peroxisomal and mitochondrial beta-oxidation pathways. Prior to channelling the fatty acids into beta-oxidation, they need to be activated to their acyl-CoA derivates. Analysis of the genome sequence identified a number of possible fatty acyl-CoA synthetases (FatA, FatB, FatC, FatD, FaaA and FaaB). FaaB was found to be the major long-chain synthetase for fatty acid degradation. FaaB was shown to localise to the peroxisomes, and the corresponding gene was induced in the presence of short and long chain fatty acids. Deletion of the faaB gene leads to a reduced/abolished growth on a variety of fatty acids. However, at least one additional fatty acyl-CoA synthetase with a preference for short chain fatty acids and a potential mitochondrial candidate (AN4659.3) has been identified via genome analysis.

Deletion and overexpression of the Aspergillus nidulans GATA factor AreB reveals unexpected pleiotropy.

The Aspergillus nidulans transcription factor AreA is a key regulator of nitrogen metabolic gene expression. AreA contains a C-terminal GATA zinc finger DNA-binding domain and activates expression of genes necessary for nitrogen acquisition. Previous studies identified AreB as a potential negative regulator of nitrogen catabolism showing similarity with Penicillium chrysogenum NreB and Neurospora crassa ASD4. The areB gene encodes multiple products containing an N-terminal GATA zinc finger and a leucine zipper motif. We deleted the areB gene and now show that AreB negatively regulates AreA-dependent nitrogen catabolic gene expression under nitrogen-limiting or nitrogen-starvation conditions. AreB also acts pleiotropically, with functions in growth, conidial germination and asexual development, though not in sexual development. AreB overexpression results in severe growth inhibition, aberrant cell morphology and reduced AreA-dependent gene expression. Deletion of either the DNA-binding domain or the leucine zipper domain results in loss of both nitrogen and developmental phenotypes.

Genetic analysis of the role of peroxisomes in the utilization of acetate and fatty acids in Aspergillus nidulans.

Peroxisomes are organelles containing a diverse array of enzymes. In fungi they are important for carbon source utilization, pathogenesis, development, and secondary metabolism. We have studied Aspergillus nidulans peroxin (pex) mutants isolated by virtue of their inability to grow on butyrate or by the inactivation of specific pex genes. While all pex mutants are able to form colonies, those unable to import PTS1 proteins are partially defective in asexual and sexual development. The pex mutants are able to grow on acetate but are affected in growth on fatty acids, indicating a requirement for the peroxisomal localization of beta-oxidation enzymes. However, mislocalization of malate synthase does not prevent growth on either fatty acids or acetate, showing that the glyoxylate cycle does not require peroxisomal localization. Proliferation of peroxisomes is dependent on fatty acids, but not on acetate, and on PexK (Pex11), expression of which is activated by the FarA transcription factor. Proliferation was greatly reduced in a farADelta strain. A mutation affecting a mitochodrial ketoacyl-CoA thiolase and disruption of a mitochondrial hydroxy-acyl-CoA dehydrogenase gene prevented growth on short-chain but not long-chain fatty acids. Together with previous results, this is consistent with growth on even-numbered short-chain fatty acids requiring a mitochondrial as well as a peroxisomal beta-oxidation pathway. The mitochondrial pathway is not required for growth on valerate or for long-chain fatty acid utilization.

Sumoylation in Aspergillus nidulans: sumO inactivation, overexpression and live-cell imaging.

Sumoylation, the reversible covalent attachment of small ubiquitin-like modifier (SUMO) peptides has emerged as an important regulator of target protein function. In Saccharomyces cerevisiae, but not in Schizosaccharyomes pombe, deletion of the gene encoding SUMO peptides is lethal. We have characterized the SUMO-encoding gene, sumO, in the filamentous fungus Aspergillus nidulans. The sumO gene was deleted in a diploid and sumODelta haploids were recovered. The mutant was viable but exhibited impaired growth, reduced conidiation and self-sterility. Overexpression of epitope-tagged SumO peptides revealed multiple sumoylation targets in A. nidulans and SumO overexpression resulted in greatly increased levels of protein sumoylation without obvious phenotypic consequences. Using five-piece fusion PCR, we generated a gfp-sumO fusion gene expressed from the sumO promoter for live-cell imaging of GFP-SumO and GFP-SumO-conjugated proteins. Localization of GFP-SumO is dynamic, accumulating in punctate spots within the nucleus during interphase, lost at the onset of mitosis and re-accumulating during telophase.

Transcriptional control of gluconeogenesis in Aspergillus nidulans.

Aspergillus nidulans can utilize carbon sources that result in the production of TCA cycle intermediates, thereby requiring gluconeogenesis. We have cloned the acuG gene encoding fructose-1,6 bisphosphatase and found that expression of this gene is regulated by carbon catabolite repression as well as by induction by a TCA cycle intermediate similar to the induction of the previously studied acuF gene encoding phosphoenolpyruvate carboxykinase. The acuN356 mutation results in loss of growth on gluconeogenic carbon sources. Cloning of acuN has shown that it encodes enolase, an enzyme involved in both glycolysis and gluconeogenesis. The acuN356 mutation is a translocation with a breakpoint in the 5' untranslated region resulting in loss of expression in response to gluconeogenic but not glycolytic carbon sources. Mutations in the acuK and acuM genes affect growth on carbon sources requiring gluconeogenesis and result in loss of induction of the acuF, acuN, and acuG genes by sources of TCA cycle intermediates. Isolation and sequencing of these genes has shown that they encode proteins with similar but distinct Zn(2) Cys(6) DNA-binding domains, suggesting a direct role in transcriptional control of gluconeogenic genes. These genes are conserved in other filamentous ascomycetes, indicating their significance for the regulation of carbon source utilization.

Cloning and sequence characterization of a non-reducing polyketide synthase gene from the lichen Xanthoparmelia semiviridis.

Lichens produce a diverse array of secondary metabolites that have shown various biological activities. Of particular interest are the coupled phenolics that originate from polyketide pathways, such as depsides, depsidones and usnic acids, which are produced almost solely by lichens. Based on the presumed catalytic domains required for the synthesis of the key intermediates beta-orsellinic acid and methylphloroacetophenone, two pairs of degenerate primers were designed to target specifically the beta-ketoacylsynthase (KS) and C-methyltransferase (CMeT) domains of fungal non-reducing polyketide synthase (NR-PKS) genes with CMeT domains. These primers were used to explore the genome of the lichen Xanthoparmelia semiviridis, which produces beta-orcinol depsidones and usnic acid. One of the two KS domains amplified from genomic DNA of field-collected X. semiviridis was used as a probe to recover the candidate PKS gene. A 13 kb fragment containing an intact putative PKS gene (xsepks1) of 6555 bp was recovered from a partial genomic library. The inferred amino acid sequence indicated that xsepks1 encodes a protein of 2164 amino acids and contains KS, acyltransferase (AT), acyl carrier protein (ACP) and CMeT domains as expected. This demonstrated a successful strategy for targeting non-reducing PKS genes with CMeT domains. Integration of the 5' fragment of xsepks1, including the native promoter, into Aspergillus nidulans by cotransformation resulted in the transcription of the 5'xsepks1 and the splicing of a 63 bp intron, suggesting that A. nidulans could be a suitable heterologous host for xsepks1 expression.

Genetic manipulation of Aspergillus nidulans: meiotic progeny for genetic analysis and strain construction.

The multicellular microbial eukaryote Aspergillus nidulans is an excellent model for the study of a wide array of biological processes. Studies in this system contribute significantly to understanding fundamental biological principles and are relevant for biotechnology and industrial applications, as well as human, animal and plant fungal pathogenesis. A. nidulans is easily manipulated using classical and molecular genetics. Here, we describe the storage and handling of A. nidulans and procedures for genetic crossing, progeny analysis and growth testing. These procedures are used for Mendelian analysis of segregation of alleles to show whether a mutant phenotype segregates as a single gene and independent assortment of genes to determine the linkage relationship between genes. Meiotic crossing is used for construction of multiple mutant strains for genetic analysis. Genetic crossing and analysis of progeny can be undertaken in 2-3 weeks and growth testing takes 2-3 days.

Genetic manipulation of Aspergillus nidulans: heterokaryons and diploids for dominance, complementation and haploidization analyses.

The haploid microbial eukaryote Aspergillus nidulans is a powerful genetic system, which allows analysis of a broad range of biological phenomena. In addition to conventional analysis of meiotic progeny in a single generation, parasexual analysis affords a rapid and convenient method for genetic analysis. We describe the construction of A. nidulans heterokaryons and diploids for use in genetic analysis to determine dominance and conduct complementation tests. We also describe the rapid mapping of mutations to chromosomes by haploidization of diploids carrying marked chromosomes. Balanced heterokaryons may be established within 10 days and diploids may be constructed in 2-3 weeks. Dominance tests and complementation tests using balanced heterokaryons or diploids may be completed in 2-3 days. Haploidization analysis of heterozygous diploids can be achieved within 10 days. These protocols should be adaptable for use in related Aspergilli and Penicillia, which lack a known meiotic cycle.

Transcriptional control of nmrA by the bZIP transcription factor MeaB reveals a new level of nitrogen regulation in Aspergillus nidulans.

Fungi can use a diverse range of nitrogen sources. Some nitrogen sources sustain a rapid growth rate and are used in preference to less readily metabolized nitrogen sources. The mechanisms involved in this control of nitrogen utilization have been studied in the model filamentous ascomycete, Aspergillus nidulans. The GATA transcription factor AreA is necessary for the expression of nitrogen-catabolic permeases and enzymes. AreA activity is controlled by multiple mechanisms including regulated areA transcript levels and regulated AreA nuclear export. During nitrogen sufficiency, AreA activation is also prevented by the co-repressor NmrA. We have investigated nitrogen signalling to NmrA. NmrA overexpression prevents AreA function irrespective of the nitrogen status. The mRNA levels of areA and nmrA are inversely regulated, suggesting that the relative levels of AreA and NmrA are critical in determining AreA activation. The bZIP transcription factor MeaB was found to activate nmrA expression and a conserved element, TTGCACCAT, bound by MeaB in vitro is present in the promoters of NmrA homologues in other filamentous ascomycetes. Expression of meaB was not strongly regulated suggesting that transcriptional activation by MeaB is modulated by the nitrogen status. This work highlights a new level of complexity in the regulation of nitrogen catabolism.

Differential expression of Aspergillus nidulans ammonium permease genes is regulated by GATA transcription factor AreA.

The movement of ammonium across biological membranes is mediated in both prokaryotes and eukaryotes by ammonium transport proteins (AMT/MEP) that constitute a family of related sequences. We have previously identified two ammonium permeases in Aspergillus nidulans, encoded by the meaA and mepA genes. Here we show that meaA is expressed in the presence of ammonium, consistent with the function of MeaA as the main ammonium transporter required for optimal growth on ammonium as a nitrogen source. In contrast, mepA, which encodes a high-affinity ammonium permease, is expressed only under nitrogen-limiting or starvation conditions. We have identified two additional AMT/MEP-like genes in A. nidulans, namely, mepB, which encodes a second high-affinity ammonium transporter expressed only in response to complete nitrogen starvation, and mepC, which is expressed at low levels under all nitrogen conditions. The MepC gene product is more divergent than the other A. nidulans AMT/MEP proteins and is not thought to significantly contribute to ammonium uptake under normal conditions. Remarkably, the expression of each AMT/MEP gene under all nitrogen conditions is regulated by the global nitrogen regulatory GATA factor AreA. Therefore, AreA is also active under nitrogen-sufficient conditions, along with its established role as a transcriptional activator in response to nitrogen limitation.

Regulatory genes controlling fatty acid catabolism and peroxisomal functions in the filamentous fungus Aspergillus nidulans.

The catabolism of fatty acids is important in the lifestyle of many fungi, including plant and animal pathogens. This has been investigated in Aspergillus nidulans, which can grow on acetate and fatty acids as sources of carbon, resulting in the production of acetyl coenzyme A (CoA). Acetyl-CoA is metabolized via the glyoxalate bypass, located in peroxisomes, enabling gluconeogenesis. Acetate induction of enzymes specific for acetate utilization as well as glyoxalate bypass enzymes is via the Zn2-Cys6 binuclear cluster activator FacB. However, enzymes of the glyoxalate bypass as well as fatty acid beta-oxidation and peroxisomal proteins are also inducible by fatty acids. We have isolated mutants that cannot grow on fatty acids. Two of the corresponding genes, farA and farB, encode two highly conserved families of related Zn2-Cys6 binuclear proteins present in filamentous ascomycetes, including plant pathogens. A single ortholog is found in the yeasts Candida albicans, Debaryomyces hansenii, and Yarrowia lipolytica, but not in the Ashbya, Kluyveromyces, Saccharomyces lineage. Northern blot analysis has shown that deletion of the farA gene eliminates induction of a number of genes by both short- and long-chain fatty acids, while deletion of the farB gene eliminates short-chain induction. An identical core 6-bp in vitro binding site for each protein has been identified in genes encoding glyoxalate bypass, beta-oxidation, and peroxisomal functions. This sequence is overrepresented in the 5' region of genes predicted to be fatty acid induced in other filamentous ascomycetes, C. albicans, D. hansenii, and Y. lipolytica, but not in the corresponding genes in Saccharomyces cerevisiae.

Amino acid catabolism by an areA-regulated gene encoding an L-amino acid oxidase with broad substrate specificity in Aspergillus nidulans.

The filamentous fungus Aspergillus nidulans can use a wide range of compounds as nitrogen sources. The synthesis of the various catabolic enzymes needed to breakdown these nitrogen sources is regulated by the areA gene, which encodes a GATA transcription factor required to activate gene expression under nitrogen-limiting conditions. The areA102 mutation results in pleiotropic effects on nitrogen source utilization, including better growth on certain amino acids as nitrogen sources. Mutations in the sarA gene were previously isolated as suppressors of the strong growth of an areA102 strain on l-histidine as a sole nitrogen source. We cloned the sarA gene by complementation of a sarA mutant and showed that it encodes an l-amino acid oxidase enzyme with broad substrate specificity. Elevated expression of this enzyme activity in an areA102 background accounts for the strong growth of these strains on amino acids that are substrates for this enzyme. Loss of function sarA mutations, which abolish the l-amino acid oxidase activity, reverse the areA102 phenotype. Growth tests with areA102 and sarA mutants show that this enzyme is the primary route of catabolism for some amino acids, while other amino acids are metabolized through alternative pathways that yield either ammonium or glutamate for growth.

Nuclear accumulation of the GATA factor AreA in response to complete nitrogen starvation by regulation of nuclear export.

Both the availability and the quality of nutrients affect cellular functions by controlling gene activity. AreA, a member of the GATA family of transcription factors, globally activates expression of genes involved in nitrogen source utilization in Aspergillus nidulans. The quality of the nitrogen source determines the level and activation capacity of AreA through controls at the level of areA mRNA stability and by interaction of AreA with the corepressor NmrA. The availability of potential nitrogen sources also affects the activation capacity of AreA. We show that the complete absence of a nitrogen source results in an enhanced level of AreA-dependent gene expression and that this response is independent of mechanisms regulating AreA activity in response to nitrogen source quality. During nitrogen starvation AreA accumulates in the nucleus, but the presence of a potential nitrogen source or carbon starvation prevents this accumulation. Furthermore, accumulated AreA is rapidly lost from the nuclei of nitrogen-starved cells when a nitrogen source is supplied or when a carbon source is absent, and this accompanies arrest of the AreA-dependent nitrogen starvation response on regulated gene expression. By the generation of a leptomycin B-sensitive mutant, we have been able to show that nuclear exit occurs via the CrmA exportin. We conclude that sensing mechanisms discriminate between starvation and the presence of potential nutrients that can signal to the AreA transcription factor. Nitrogen source availability, but not quality, affects nuclear accumulation by regulating nuclear exit of AreA, providing a rapid response to changes in the supply of nutrients.