Induced mutagenesis in dam- mutants of Escherichia coli: a role for 6-methyladenine residues in mutation avoidance.

E. coli strains carrying the dam-3 and dam-4 mutations resulting in reduced levels of 6-methyladenine in the DNA have been found to be more sensitive to base analogue mutagenesis than dam+ strains. Mutagenesis by EMS was also found to be enhanced in dam- strains. Dam- mutants however were not found to be hypermutable by UV light. It is concluded that the dam- strains are deficient in the correct repair of mispairing lesions. The data are consistent with the hypothesis that 6-methyladenine residues in the DNA are involved in strand discrimination during mismatch correction.