Regulation of branched-chain amino acid biosynthesis in Neurospora crassa: cloning and characterization of the leu-1 and ilv-3 genes.

The genes coding for the branched-chain amino acid biosynthetic enzymes comprise an integrated regulatory system. The expression of the several structural genes coding for enzymes of the isoleucine-valine and leucine pathways is controlled in parallel by the positive-acting regulatory gene, leu-3. The leu-1 and ilv-3 genes, coding for beta-isopropyl-malate dehydrogenase and aceto-hydroxyacid synthase, respectively, were cloned from a cosmid library. Restriction fragment length polymorphism analysis revealed that the two cloned fragments indeed mapped to the genomic locations of the leu-1 and ilv-3 genes, respectively. Northern blot analysis demonstrated that the leu-1 gene is transcribed to give an mRNA of approximately 1.5 kb, whereas the ilv-3 transcript size is 2.6 kb. The expression of both genes appears to be regulated at the transcriptional level. One leu-3 regulatory mutant was greatly deficient in both leu-1 and ilv-3 mRNAs, whereas another leu-3 allele showed an unusual antiparallel pattern of regulation.