We have previously reported the isolation of an Escherichia coli K12 mutant that 
is extremely sensitive to mutagenesis by low doses of ethylating agents. We now 
show by Southern analysis that the mutation involves a gross deletion covering 
at least the ogt and fnr genes and that no O6-alkylguanine-DNA-alkyltransferase 
activity is present in cell-free 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::kanr disruption mutation was 
introduced into the parental ogt+ bacteria, and the ogt::kanr mutation was then 
eliminated by cotransduction of ogt+ with the closely linked Tetr marker 
(zcj::Tn10). The delta(ogt-fnr) deletion or ogt::kanr disruption mutants were 
highly sensitive to ethyl methanesulphonate-induced mutagenesis, as measured by 
the induction of forward mutations to L-arabinose resistance (Arar). 
Furthermore, the number of Arar mutants increased linearly with dose, unlike the 
case in ogt+ bacteria, which had a threshold dose below which no mutants 
accumulated. Differences 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 deletions 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.