Isolation by genetic complementation of two differentially expressed genes for beta-isopropylmalate dehydrogenase...

We have constructed an Aspergillus niger cDNA library with a yeast expression vector. The library DNA complemented a leucine auxotroph of Saccharomyces cerevisiae (strain BWG1-7a) at a frequency of 4x10(-4). Plasmids rescued from the yeast prototrophs also complemented Escherichia coli (strain MC1066) deficient in leucine biosynthesis. Sequence determination of the rescued plasmids revealed two genes for beta-isopropylmalate dehydrogenase, which we called leu2A and leu2B. Genomic-blot analysis suggested that both leu2A and leu2B were derived from single-copy genes. Northern-blot hybridization showed that in nutrient-rich medium a leu2A transcript accumulated during germination and log-phase growth while the leu2B transcript appeared late in the growth phase. In minimal medium, only leu2A expression was greatly stimulated. We examined the codon preference of these two genes. Whereas leu2A shows a bias in codon usage typical of A. niger genes, leu2B does not. These results indicate the presence in A. niger of two highly divergent, differentially regulated, isozymes for beta-isopropylmalate dehydrogenase.