Mechanism of repression of methionine biosynthesis in Escherichia coli. II. The effect of metJ mutations on the free amino acid pool

Summary Ethionine-resistant mutants (metJ mutants) were isolated and characterized as constitutive in the biosynthesis of methionine. Such mutations resulted in marked differences or alterations in the free amino acid pool. In some strains the levels of threonine and histidine were elevated by as much as 13 and 22 times that of the wild type level. The possibility that structural modifications of methionyl-tRNA were giving rise to constitutive methionine biosynthesis and the apparent aberrations in the free amino acid pool, was in large part ruled out by a comparison of the mobilities of wild type and mutant methionyl-tRNA on benzoylated DEAE-cellulose columns. The results obtained are consistent with the view that the product of the metJ locus is a repressor protein which is directly involved in the repression of the methionine genes.