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

Abstract

Regulation of methionine biosynthesis in Escherichia coli involves a complex of the MetJ aporepressor protein and S-adenosylmethionine (SAM) repressing expression of most genes in the met regulon. To test the role of SAM in the regulation of met genes directly, SAM pools were depleted by the in vivo expression of the cloned plasmid vector-based coliphage T3 SAM hydrolase (SAMase) gene. Cultures with in vivo SAMase activity were assayed for expression of the metA, B, C, E, F, H, J, K and R genes in cells grown in methionine-rich complete media as well as in defined media with and without L-methionine. In vivo SAMase activity dramatically induced expression between 11-and nearly 1000-fold depending on the gene assayed for all but metJ and metH, and these genes were induced over twofold. metJ : : Tn5 (aporepressor defective) and metK : : Tn5 (SAM synthetase impaired; produces <5 % of wild-type SAM) strains containing in vivo SAMase activity produced even higher met gene activity than that seen in comparably prepared cells with wild-type genes for all but metJ in a MetJ-deficient background. The SAMasemediated hyperinduction of metH in wild-type cells and of the met genes assayed in metJ : : Tn5 and metK : : Tn5 cells provokes questions about how other elements such as the MetR activator protein or factors beyond the met regulon itself might be involved in the regulation of genes responsible for methionine biosynthesis.

Cell division, one-carbon metabolism and methionine synthesis in a metK-deficient Escherichia coli mutant, and a role for MmuM

“…It is clear that in minimal medium, addition of SAM is insufficient, and only one other metabolite, methionine, is required by a SAM synthase deficient strain. This is consistent with the fact that expression from all the promoters of the methionine biosynthetic genes is repressed by SAM and MetJ (Lawrence et al, 1968;LaMonte & Hughes, 2006) so that an adequate supply of methionine cannot be synthesized in the presence of SAM.…”

Mechanism of methionine synthase overexpression in chaperonin-depleted Escherichia coli

“…These results indicated that MetJ regulates the levels of both MetE and MetR, and suggested that a shortage of SAM through MetK dysfunction in GroE-depleted cells increases the levels of the activator MetR. A similar overexpression of MetR has been observed when SAM is depleted through the strong SAM hydrolase of a phage (LaMonte & Hughes, 2006). Since the metE promoter is regulated by MetR and UuMetK suppresses overexpression of the metE promoter lacking MetJ-binding regions, it is plausible that the increased level of MetR contributes to the overexpression of MetE in GroE-depleted cells.…”

“…Reporter assays and suppression by UuMetK suggested that the increased expression of MetR in GroE-depleted cells contributes substantially to the overexpression of MetE, as has been suggested elsewhere (LaMonte & Hughes, 2006). Accumulation of the MetR co-activator homocysteine caused by MetF dysfunction contributed little to the overexpression of MetE (Table 2).…”

Growth Deficiencies of Neisseria meningitidis pfs and luxS Mutants Are Not Due to Inactivation of Quorum Sensing

“…SAM is another likely AMC metabolite that is known to influence numerous phenotypic traits (19), as it has a number of fates within the cell (15,16,27 Thus, it is apparent that affecting levels of AMC metabolites may result in more global deregulated metabolism, which in turn could contribute to the growth defects observed. Accordingly, the current metabolite analysis did not identify a definitive mechanism that explains the changes observed.…”