Cellular Defense Mechanisms Against Alkylation of DNA
Several repair enzymes are induced in Escherichia coli as a consequence of exposure to methylating agents such as N-methyl-N’- nitro-N-nitrosoguanidine. These include at least two different glycosylase functions, which act specifically on alkylated DNA by catalyzing the release of 3-methyladenine and 7-methy1guanine in free form. The apparently most important repair function induced during this adaptive response to alkylating agents, however, reduces the mutagenic activity of these agents by allowing improved repair of O6- methylguanine (Cairns, 1980; Schendel and Robins, 1978). This reaction can be faithfully duplicated in vitro employing cell-free extracts from adapted E. coli. The active factor has been shown to be an induced protein of molecular weight about 17,000, which catalyzes the transfer of a methyl group from the O6-position of an alkylated DNA guanine residue to one of its own cysteine residues. The protein acts in an analogous fashion on ethylated DNA by removing the ethyl group from O6-ethylguanine. The reaction is associated with suicide inactivation of the protein, since it appears to lack the capacity to release the blocking alkyl group. There is no detectable change in molecular weight of the protein accompanying methylation, so the inactivation probably depends on the modification of a reactive sulfhydryl group. S-Methylcysteine has been identified in the methylated protein by amino acid analysis and by conversion of this residue to its sulfone derivative by hydrogen peroxide treatment. S-Methylcysteine has not been found previously in enzymatically methylated proteins. Moreover, the reaction represents the only known case of protein methylation that is not dependent on S- adenosylmethionine as methyl donor. E. coli mutants unable to respond to alkylating agents by adaptation (isolated by P. Jeggo) also lack the methyltransferase, while mutants that express the adaptive response in a constitutive fashion (isolated by B. Sedgwick) also produce the methyltransferase constitutively. Thus, the DNA methyltransferase acting on O6-methylguanine appears to be primarily responsible for the adaptive response to the mutagenic effect of simple alkylating agents.
KeywordsAlkylating Agent Imidazole Ring Methyl Donor Micrococcus Luteus Methyl Transferase
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