title: Role of the MetR regulatory system in vitamin B12-mediated repression of the Salmonella typhimurium metE gene.
abstract: The vitamin B12 (B12)-mediated repression of the metE gene in Escherichia coli and Salmonella typhimurium requires the B12-dependent transmethylase, the metH gene product. It has been proposed that the MetH-B12 holoenzyme complex is involved directly in the repression mechanism. Using Escherichia coli strains lysogenized with a lambda phage carrying a metE-lacZ gene fusion, we examined B12-mediated repression of the metE-lacZ gene fusion. Although B12 supplementation results in a 10-fold repression of metE-lacZ expression, homocysteine addition to the growth medium overrides the B12-mediated repression. In addition, B12-mediated repression of the metE-lacZ fusion is dependent on a functional MetR protein. When a metB mutant was transformed with a high-copy-number plasmid carrying the metE gene, which would be expected to reduce intracellular levels of homocysteine, metE-lacZ expression was reduced and B12 supplementation had no further effect. In a metJ mutant, B12 represses metE-lacZ expression less than twofold. When the metJ mutant was transformed with a high-copy-number plasmid carrying the metH gene, which would be expected to reduce intracellular levels of homocysteine, B12 repression of the metE-lacZ fusion was partially restored. The results indicate that B12-mediated repression of the metE gene is primarily a loss of MetR-mediated activation due to depletion of the coactivator homocysteine, rather than a direct repression by the MetH-B12 holoenzyme.
authors.affiliation: Department of Microbiology, University of Iowa, Iowa City 52242.
authors.forename: W F
authors.initials: WF
authors.lastname: Wu
authors.forename: M L
authors.initials: ML
authors.lastname: Urbanowski
authors.forename: G V
authors.initials: GV
authors.lastname: Stauffer
mesh_terms: D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase / Q000235:genetics; D001426:Bacterial Proteins / Q000235:genetics*; D010582:Bacteriophage lambda / Q000235:genetics; D004794:Enzyme Repression / Q000187:drug effects*; D029968:Escherichia coli Proteins; D008715:Methionine / Q000096:biosynthesis; D008780:Methyltransferases; D010957:Plasmids / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D012486:Salmonella typhimurium / Q000235:genetics*; D015534:Trans-Activators / Q000235:genetics*; D014805:Vitamin B 12 / Q000494:pharmacology*
chemical_list: D001426:Bacterial Proteins; D029968:Escherichia coli Proteins; C058169:MetR protein, Bacteria; D011993:Recombinant Fusion Proteins; D015534:Trans-Activators; D008715:Methionine; D008780:Methyltransferases; D013766:5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase; C058170:metE protein, E coli; D014805:Vitamin B 12