S-adenosylmethionine synthetase in methionine regulatory mutants of Salmonella typhimurium.

Abstract

In wild-type bacteria, S-adenosylmethionine (SAM) synthetase activity was repressed by growth in methionine. MetJ regulatory mutants had elevated activities which did not show this repression. Two metK mutants with normal regulation of the methionine biosynthetic enzymes had elevated Km's (methionine) for SAM synthetase while five metK mutants with constitutive methionine enzymes showed no measurable SAM synthetase activity. One mutant, metK X 721, similar in phenotype to these five had a wild-type level of SAM synthetase in conditions where SAM decarboxylase activity was blocked. By using an F′-factor carrying the metK region of the genome, this mutant was shown to complement six other metK mutants. These results indicate that SAM or a derivative of it, rather than methionine itself, is the co-repressor of the methionine system, regulatory abnormalities resulting from the absence or reduction of the amount of SAM formed by the SAM synthetase reaction. As SAM is essential for bacteria it is likely that there is some alternative biosynthetic route for SAM.