Isolation and characterization of an Escherichia coli strain with a high frequency of C-to-T mutations at 5-methylcytosines.

We used a genetic selection system to isolate a strain of Escherichia coli with a high frequency of C-to-T transition mutations at the second C of the sequence CCAGG. Cytosines in other sequences do not mutate to thymine at a high frequency in this strain, and the frequencies of other base substitution mutations are not increased to the same extent. The gene responsible for the mutator phenotype has been mapped to 43 min on the E. coli chromosome. Several lines of evidence indicate that this gene is distinct from the very short patch repair gene vsr.