Regulation of methionine biosynthesis in Escherichia coli: mapping of the metJ locus and properties of a metJ plus-metJ minus diploid.

MetJ(-) mutants of Escherichia coli have elevated nonrepressible levels of the enzymes of methionine biosynthesis and S-adenosylmethionine synthetase (ATP:L-methionine S-adenosyltransferase, EC 2.5.1.6). In E. coli, as in Salmonella typhimurium, the metJ locus is close to metB (95% cotransduction of metB and metJ markers), but in E. coli the order is reversed, with metJ mapping clockwise to metB. A stable merodiploid, heterozygous for metJ, is subject to repression by methionine. Thus, metJ functions via a diffusible product. MetJ could either be a regulatory locus or could code for an enzyme required for the synthesis of a methionine metabolite that functions in the control system.