A DIRECT ROLE FOR DNA-POLYMERASE-III IN ADAPTIVE REVERSION OF A FRAMESHIFT MUTATION IN ESCHERICHIA-COLI

The sequences of adaptive reversions of a lac frameshift mutation in E scherichia coli resemble DNA polymerase errors, and the adaptive rever sions decrease in strains with an antimutator DNA polymerase III (PolI II) allele. The latter finding could imply that DNA PolIII itself make s adaptive mutations. Alternatively, normal DNA PolIII errors could sa turate post-synthesis mismatch repair during adaptive mutation. If so, the antimutator strain would produce fewer adaptive mutations because it possesses greater capacity for mismatch repair which could correct errors made by a polymerase other than DNA PolIII. Mismatch repair ca pacity is limited specifically during adaptive mutation, necessitating a test of this indirect model. This indirect model is ruled out here by the observation that the antimutator PolIII allele decreases adapti ve mutation even in mismatch repair-defective cells. This supports a d irect role for DNA PolIII in recombination-dependent adaptive mutation .