Different base/base mismatches are corrected with different efficiencies by the methyl-directed DNA mismatch-repair system of E. coli.

The efficiency of methyl-directed DNA mismatch-repair of E. coli acting in vivo on heteroduplex genomes of phage M13 was found to be strongly dependent on the nature of the base/base mismatch to be corrected. Three efficiency classes were characterized:high (T/G, C/A and G/G); intermediate (A/A); and low (G/A, A/G, T/T, C/C, C/T and T/C). Methyl-directed DNA mismatch repair was lost completely for any type of mismatch in strains carrying either mutL or mutS mutations. Data obtained with a mutH mutant suggest that this locus is involved in methyl-dependent DNA strand discrimination. A functional correlation is suggested between the differential repair efficiencies and the frequencies of the corresponding replication errors.