Error-prone DNA repair consists of replicative filling-in of DNA gaps carrying lesions. We have reconstituted E. coli SOS error-prone repair using purified DNA polymerase III holoenzyme, SSB, RecA, UmuD', a UmuC fusion protein, and a gap lesion plasmid. In the absence of UmuDC, or without SOS induction, replication skips over the lesion, forming mostly one-nucleotide deletions. These cause translational frameshifts that usually inactivate genes. UmuD' and UmuC, in the presence of RecA and SSB, stimulate translesion replication and change its mutagenic specificity such that deletions are prevented and base substitutions are increased. This results in mutagenic but nondetrimental gap repair and provides an effective mechanism for generating genetic variation in bacteria adapting to environmental stress.