Cloning and characterization of the yeast methionyl-tRNA synthetase mutation mes1

The chromosomal mes 1 mutation appears to elevate the Km of methionine for yeast methionyl-tRNA synthetase. The mutation was cloned on a multicopy plasmid by gap repair of a plasmid bearing the wild type MES1 gene for a fragment corresponding to the mes 1 mutation. DNA sequencing established that the mutation consists of a single conversion of guanine into adenine which results in the replacement of a glycine by an aspartic acid at position 502. This causes the enzyme to be labile and inactive in vitro and to show a requirement for high concentrations of methionine in vivo. The mutation is in the COOH-terminal domain of the mononucleotide binding fold of the yeast enzyme and suggests participation of this region in the binding of the amino acid residue.