Roles of transcription and repair in alkylation mutagenesis

Mutations occurring in Escherichia coli cells exposed to alkylating agents have been analyzed using an assay for forward mutations in the E. coli rpsL gene cloned on a high copy number plasmid. N-Methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced mutations were recovered from wild-type and O6-methylguanine methyltransferase-deficient mutant (ada- ogt-) cells and their sequence alterations determined. We found that the mutations recovered from the wild-type strain were predominantly G:C to A:T transitions located at several hot spots in the rpsL sequence. A vast majority of the mutations were found at guanine residues preceded by thymine on the transcribed strand of the target gene. Although the methyltransferase mutant showed hypersensitivity to the alkylating reagent in terms of mutagenic effect and cell killing effects, the class and site distributions of the rpsL- mutations recovered from MNNG-treated ada- ogt- cells were similar to those observed with MNNG-treated wild-type cells. Therefore, the site preference of MNNG-induced rpsL- mutations seems to be due not to the specificity of methyl-transferring repair enzymes but probably to the distribution of the mutagenic lesions (O6-methylguanine) in the target sequence. Mutations induced by methyl methanesulfonate, an SN2 alkylating agent, showed similar class and site distributions in the rpsL system. The site preference of MNNG-induced mutations was significantly changed when the level of transcription of the rpsL gene was decreased to 120-fold lower than that promoted by the authentic rpsL promoter. Under these conditions, 78% of mutations were induced at the central guanine of 5'-GG(A or C)-3' and 2/3 of them were on the non-transcribed strand of the rpsL gene. These results suggested that the site preference of MNNG-induced mutations is determined by at least three factors: (i) a flanking-base effect on the chemical reactivity of a guanine residue, (ii) transcribed strand-specific repair, probably by the UvrABC system, and (iii) the effects of transcription of the target gene on the alkylation of DNA and the strand-specific repair.