Mutagenic DNA repair in Escherichia coli: conditions for error-free filling of daughter strand gaps.

Two situations have been observed in which daughter strand gaps in DNA synthesized after exposure of excision-deficient Escherichia coli to ultraviolet light are filled but in which no mutations are formed as judged by loss of photoreversibility: (i) during the first 20 min of growth after u.v. irradiation, and (ii) when repair is allowed to occur in buffer. We suggest as an explanation that the majority of daughter strand gap-filling is error free and that mutations arise through a minor error-prone repair pathway which is inoperative under these conditions.