Molecular mechanisms in alkylation mutagenesis. Induced reversion of bacteriophage T4rII AP72 by ethyl methanesulphonate in relation to extent and mode of ethylation of purines in bacteriophage deoxyribonucleic acid.

Survival and reversion to T4r+ of bacteriophage T4rII AP72 after treatment with ethyl methanesulphonate at 37 degrees or 45 degrees C were studied in relation to the extent and mode of alkylation of purines in DNA of ethylated bacteriophage. A single-burst technique was used for reversion assay. Survival was lower at 45 degrees C than at 37 degrees C at a given extent of ethylation of bacteriophage DNA, confirming that events subsequent to ethylation, probably depurinations, are the main cause of decreased survival. Reversion was positively correlated (approximately linearly except at low extents at 37 degrees C) with ethylation of bacteriophage DNA, showing that ethylation itself causes mutation. Following the concept that reversion results from G-C leads to A-T transition at a single site (Krieg, 1963a,b) and the suggestion that O6-alkylation of guanine generates the miscoding base (Loveless, 1969), it was calculated that about one-third of induced O6-ethylguanines at this site would miscode to induce mutation.