Mismatch-stimulated killing.

DNA duplexes with or without mismatches and with or without adenine-methylated GATC sequences were prepared from separated strands of bacteriophage lambda DNA and used to transfect Escherichia coli. Unmethylated heteroduplexes containing one or more repairable mismatches transfect cells with a functioning mismatch repair system less efficiently than they transfect cells deficient in mismatch repair. No difference is observed when the duplexes contain no mismatch or a poorly repaired mismatch or when the heteroduplexes are fully or hemimethylated. These results and the phenotypes of E. coli dam mutants suggest that the E. coli mismatch repair system may introduce double-strand breaks in unmethylated DNA at or near repairable mismatches.