Yeast LEU2. Repression of mRNA levels by leucine and primary structure of the gene product

It has been known that enzyme activity associated with the yeast LEU1 and LEU2 gene product (beta-isopropylmalate dehydrogenase) drops sharply when yeast is grown in the presence of leucine. RNA blot hybridizations with LEU2-specific probes establish that this is accompanied by a 5-fold repression in LEU2 mRNA levels. A similar repression was noted recently for LEU1 mRNA levels (Hsu, Y.-P., and Schimmel, P. (1984) J. Biol. Chem. 259, 3714-3719). Nuclease mapping of the 5'-end of the LEU2 mRNA shows a major start at approximately 16 nucleotides upstream of the AUG initiation codon. This initiation site in the gene is retained in an extensive LEU2 5'-noncoding region deletion which still expresses the LEU2 gene product (Erhart, E., and Hollenberg, C. P. (1983) J. Bacteriol. 156, 625-635). The primary structure of the LEU2 gene product was established from the nucleotide sequence of the gene-coding region and from fitting amino acid sequences of scattered internal peptides to the nucleotide sequence. The 364-amino acid protein has a 13-amino acid stretch which is highly homologous to the partially sequenced yeast LEU1 gene product (isopropylmalate isomerase). The homology occurs about 290 amino acids from the respective NH2 termini of the two proteins. The homology may represent residues which interact with beta-isopropylmalate, a common ligand for the enzymes.