Genetic studies have indicated that the facB gene of Aspergillus nidulans is a major regulatory gene involved in acetamide and acetate utilisation. Sequencing of the facB gene revealed that it encodes a protein that contains an N-terminal GAL4-like Zn(II)2Cys6 (or C6 zinc) binuclear cluster for DNA binding, leucine zipper-like heptad repeat motifs and central and C-terminal acidic alpha-helical regions, consistent with a function as a DNA-binding transcriptional activator. The Zn(II)2Cys6 cluster shows strong similarity with those of the Saccharomyces cerevisiae carbon metabolism regulatory proteins CAT8 and SIP4. A significant level of similarity with CAT8 is found throughout the length of the protein, suggesting at least partial functional homology. The facB genes of Aspergillus oryzae and Aspergillus niger were also sequenced and found to be highly conserved. Deletion of the facB gene confirmed that it is required for growth on acetate as a sole carbon source. Functional dissection using deletion and fusion constructs and in vitro mutagenesis indicated that the Zn(II)2Cys6 cluster and the C-terminal end of the protein are required for function.

Expression of many microbial genes required for the utilisation of less favoured carbon sources is carbon catabolite repressed in the presence of a preferred carbon source such as D-glucose. In Aspergillus nidulans, creC mutants show derepression in the presence of D-glucose of some, but not all, systems normally subject to carbon catabolite repression. These mutants also fail to grow on some carbon sources, and show minor morphological impairment and altered sensitivity to toxic compounds including molybdate and acriflavin. The pleiotropic nature of the phenotype suggests a role for the creC gene product in the carbon regulatory cascade. The creC gene was cloned and found to encode a protein which contains five WD40 motifs. The sequence changes in three mutant alleles were found to lead to production of truncated proteins which lack one or more of the WD40 repeats. The similarity of the phenotypes conferred by these alleles implies that these alleles represent loss of function alleles. Deletion analysis also showed that at least the most C-terminal WD40 motif is required for function. The CreC protein is highly conserved relative to the Schizosaccharomyces pombe protein Yde3--whose function is unknown--and human and mouse DMR-N9, which may be associated with myotonic dystrophy.

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