SOS mutator DNA polymerase IV functions in adaptive mutation and not adaptive amplification

Adaptive point mutation and amplification are induced responses to environm ental stress, promoting genetic changes that can enhance survival. A specia lized adaptive mutation mechanism has been documented in one Escherichia co li assay, but its enzymatic basis remained unclear. We report that the SOS- inducible, error-prone DNA polymerase (pol) IV, encoded by dinB, is require d for adaptive point mutation in the E. coli lac operon. A nonpolar dins mu tation reduces adaptive mutation frequencies by 85% but does not affect ada ptive amplification, growth-dependent mutation, or survival after oxidative or UV damage. We show that pol IV, together with the major replicase, pol III, can account for all adaptive point mutations at lac. The results ident ify a role for pol IV in inducible genetic change.