Methyl-directed repair of frameshift heteroduplexes in cell extracts from Escherichia coli

The methyl-directed DNA repair efficiency of a series of M13mp9 frameshift heteroduplexes containing 1, 2, or 3 unpaired bases was determined by using an in vitro DNA mismatch repair assay. Repair of hemimethylated frameshift heteroduplexes in vitro was directed to the unmethylated strand; was dependent on MutH, MutL, and MutS; and was equally efficient on base insertions and deletions. However, fully methylated frameshift heteroduplexes were resistant to repair, while totally unmethylated substrates were repaired with no strand bias. Hemimethylated 1-, 2-, or 3-base insertion and deletion heteroduplexes were repaired by the methyl-directed mismatch repair pathway as efficiently as the G · T mismatch. These results are consistent with earlier in vivo studies and demonstrate the involvement of methyl-directed DNA repair in the efficient prevention of frameshift mutations.