title: Repair of alkylated DNA in Escherichia coli. Methyl group transfer from O6-methylguanine to a protein cysteine residue.
abstract: O6-Methylguanine residues disappear from alkylated DNA by an inducible repair process in Escherichia coli. The reaction can be studied in a cell-free system, using DNA treated with a radioactive methylating agent as substrate. The disappearance of labeled O6-methylguanine from DNA is not accompanied by release of radioactive material in an acid-soluble form. Instead, the methyl group of O6-methylguanine appears to be transferred enzymatically to a protein cysteine residue. Radioactively labeled S-methylcysteine has been identified in protein hydrolysates after incubation of the alkylated DNA with a partly purified E. coli methyltransferase activity. The radioactive amino acid residue shows properties identical with those of S-methylcysteine by automatic amino acid analysis and paper chromatography in several solvent systems. Moreover, oxidation of the compound with hydrogen peroxide yields a product which co-chromatographs with S-methylcysteine sulfone.
mesh_terms: D000478:Alkylation; D001426:Bacterial Proteins / Q000378:metabolism; D003545:Cysteine / Q000031:analogs & derivatives / Q000032:analysis / Q000378:metabolism; D004260:DNA Repair; D004269:DNA, Bacterial / Q000378:metabolism*; D004926:Escherichia coli / Q000378:metabolism*; D006147:Guanine / Q000031:analogs & derivatives; D008745:Methylation; D008780:Methyltransferases / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D006147:Guanine; C008449:O-(6)-methylguanine; C008425:S-methylcysteine; D008780:Methyltransferases; D003545:Cysteine