In vivo hydrolysis of S-adenosylmethionine induces the met regulon of Escherichia coli

Abstract

Regulation of methionine biosynthesis inEscherichia coliinvolves a complex of the MetJ aporepressor protein andS-adenosylmethionine (SAM) repressing expression of most genes in themetregulon. To test the role of SAM in the regulation ofmetgenes directly, SAM pools were depleted by thein vivoexpression of the cloned plasmid vector-based coliphage T3 SAM hydrolase (SAMase) gene. Cultures within vivoSAMase activity were assayed for expression of themetA,B,C,E,F,H,J,KandRgenes in cells grown in methionine-rich complete media as well as in defined media with and withoutl-methionine.In vivoSAMase activity dramatically induced expression between 11- and nearly 1000-fold depending on the gene assayed for all butmetJandmetH, and these genes were induced over twofold.metJ : : Tn5(aporepressor defective) andmetK : : Tn5(SAM synthetase impaired; produces <5 % of wild-type SAM) strains containingin vivoSAMase activity produced even highermetgene activity than that seen in comparably prepared cells with wild-type genes for all butmetJin a MetJ-deficient background. The SAMase-mediated hyperinduction ofmetHin wild-type cells and of themetgenes assayed inmetJ : : Tn5andmetK : : Tn5cells provokes questions about how other elements such as the MetR activator protein or factors beyond themetregulon itself might be involved in the regulation of genes responsible for methionine biosynthesis.