Structure and expression of two aminoacyl-tRNA synthetase genes from Saccharomyces cerevisiae

We have isolated the structural genes for methionyl-tRNA synthetase (L-methionine:tRNAMet ligase, EC 6.1.1.10) and isoleucyl-tRNA synthetase (L-isoleucine:tRNAIle ligase, EC 6.1.1.5) from the yeast Saccharomyces cerevisiae by transformation. Both genes exist in a single copy in the yeast genome and show no homology to either yeast mitochondrial or Escherichia coli DNA as judged by Southern analysis. The region corresponding to the DNA segment altered in mes1 or ils1-1 mutants was defined by transformation of synthetase mutants with subclones of the respective gene. Although the isolated fragment containing the methionyl-tRNA synthetase gene complements the mes1 mutation in yeast, it does not complement the analogous metG mutation in E. coli. A single transcript of about 3100 nucleotides codes for the isoleucyl-tRNA synthetase. This transcript is about 2-fold elevated in strains starved for isoleucine, histidine, or arginine as compared to wild type. It is regulated by the general control of amino acid biosynthesis, which affects several biosynthetic pathways. Two polyadenylated transcripts are homologous to the methionyl-tRNA synthetase gene. The major MES1 transcript is not affected by the general control or starvation for several different amino acids, including methionine.