Contribution of ogt-encoded alkyltransferase to resistance to chloroethylnitrosoureas in nucleotide excision repair-deficient Escherichia coli

We investigated the relative contribution of the two Escherichia coli DNA alkyltransferases (ATases) to the increased sensitivity of ATase-deficient bacteria to the mutagenic and lethal effects of chloroethylnitrosoureas (CNU). The ogt-encoded protein was the principal determinant in resistance to the mutagenic effects of CNU in E. coli, Thus, only when the ogt gene was inactivated was sensitivity to mutagenesis greatly increased; the contribution of inactivation of the ada gene was relatively minor, Furthermore, induction of the adaptive response provided essentially no protection against CNU mutagenesis in either an ogt(+) or ogt(-) background, Finally, overexpression of the ogt gene into ogt(-)ada(-) double mutants provided the greatest protection against CNU; introduction of the full-length or truncated ada gene was protective, but to a much lesser extent, Mammalian ATases were not as protective against mutation induction by CNU as Ogt, even though they were apparently expressed at higher level, In order of effectiveness the ATases ranked Ogt > human > truncated Ada = Ada > rat. This order was not observed in the protection against killing by 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea, where truncated Ada = human > Ogt > rat = Ada, Higher mutation frequency and toxicity were observed in uvr(-) mutants, suggesting that one or more of the potentially mutagenic and/or toxic lesions are also substrates for the excision repair proteins.