A common mechanism for repair of O6-methylguanine and O6-ethylguanine in DNA

The mutagenic effects of several ethylating and methylating agents were assessed in Encherichia coli strains that are defective in the adaptive response to alkylating agents. These mutants were either deficient in the response or expressed it constitutively. When expressed, the repair pathway removed the major mutagenic lesion produced by either methylating or ethylating agents. This lesion was almost certainly O⁶-alkylguanine produced by alkylation of DNA, and the mechanism for its removal was characterized in vitro. E. coli cells expressing the adaptive response contain relatively large amounts of a protein that transfers the methyl group from O⁶-methylguanine to one of its own cysteine residues (Olsson & Lindahl, 1980). This methyltransferase was shown to act in an analogous fashion on O⁶-ethylguanine. Incubation of ethylated DNA with purified transferase led to disappearance of the O⁶-ethylguanine residues, and S-ethylcysteine was simultaneously generated in the protein. The greater sensitivity of E. coli wild-type to ethylating than methylating agents may be explained by a slower repair of O⁶-ethylguanine than O⁶-methylguanine and also a weaker ability of ethylating agents to induce the adaptive response.