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.

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.