EFFECT OF OGT EXPRESSION ON MUTATION-INDUCTION BY METHYLMETHANESULPHONATE, ETHYLMETHANESULFONATE AND PROPYLMETHANESULPHONATE IN ESCHERICHIA-COLI K12 STRAINS

We have previously reported the isolation of an Escherichia coli K12 m utant that is extremely sensitive to mutagenesis by low doses of ethyl ating agents. We now show by Southern analysis that the mutation invol ves a gross deletion covering at least the ogt and fnr genes and that no O6-alkylguanine-DNA-alkyltransferase activity is present in cell-fr ee extracts of an ada=Tn10 derivative of these bacteria. Confirmation that sensitisation to ethylation-induced mutagenesis was attributable to ogt and not to any other loci covered by the deletion was obtained by constructing derivatives. Thus an ogt=kan(r) disruption mutation wa s introduced into the parental ogt+ bacteria, and the ogt=kan(r) mutat ion was then eliminated by cotransduction of ogt+ with the closely lin ked Tet(r) marker (zcj=Tn 10). The DELTA(ogt-fnr) deletion or ogt=kan( r) disruption mutants were highly sensitive to ethyl methanesulphonate -induced mutagenesis, as measured by the induction of forward mutation s to L-arabinose resistance (Ara(r)). Furthermore, the number of Ara(r ) mutants increased linearly with dose, unlike the case in ogt+ bacter ia, which had a threshold dose below which no mutants accumulated. Dif ferences in mutability were even greater with propyl methanesulphonate . Overproduction of the ogt alkyltransferase from a multicopy plasmid reduced ethylmethanesulphonate-induced mutagenesis in the ogt- mutant strains and also methylmethanesulphonate mutagenesis in ada- bacteria. A sample of AB1157 obtained from the E. coli K12 genetic stock centre also had a deletion covering the ogt and fnr genes. Since such deleti ons greatly influence the mutagenic responses to alkylating agents, a survey of the presence of the ogt gene in the E. coli K12 strain being used is advisable.