title: A second DNA methyltransferase repair enzyme in Escherichia coli.
abstract: The Escherichia coli ada-alkB operon encodes a 39-kDa protein (Ada) that is a DNA-repair methyltransferase and a 27-kDa protein (AlkB) of unknown function. By DNA blot hybridization analysis we show that the alkylation-sensitive E. coli mutant BS23 [Sedgwick, B. & Lindahl, T. (1982) J. Mol. Biol. 154, 169-175] is a deletion mutant lacking the entire ada-alkB operon. Despite the absence of the ada gene and its product, the cells contain detectable levels of a DNA-repair methyltransferase activity. We conclude that the methyltransferase in BS23 cells is the product of a gene other than ada. A similar activity was detected in extracts of an ada-10::Tn10 insertion mutant of E. coli AB1157. This DNA methyltransferase has a molecular mass of about 19 kDa and transfers the methyl groups from O6-methylguanine and O4-methylthymine in DNA, but not those from methyl phosphotriester lesions. This enzyme was not induced by low doses of alkylating agent and is expressed at low levels in ada+ and a number of ada- E. coli strains.
authors.affiliation: Charles A. Dana Laboratory of Toxicology, Harvard School of Public Health, Boston, MA 02115.
authors.forename: G W
authors.initials: GW
authors.lastname: Rebeck
authors.forename: S
authors.initials: S
authors.lastname: Coons
authors.forename: P
authors.initials: P
authors.lastname: Carroll
authors.forename: L
authors.initials: L
authors.lastname: Samson
mesh_terms: D004260:DNA Repair; D004926:Escherichia coli / Q000201:enzymology*; D008780:Methyltransferases / Q000378:metabolism*; D008970:Molecular Weight; D009154:Mutation; D019853:O(6)-Methylguanine-DNA Methyltransferase; D009876:Operon
chemical_list: D008780:Methyltransferases; D019853:O(6)-Methylguanine-DNA Methyltransferase