Role of DNA polymerases in excision repair in Escherichia coli.

The excision repair process following ultraviolet irradiation has been fractionated into its individual steps in toluene-treated cells. Incision can be examined in vitro independently of other reactions by omission of the deoxyribonucleoside triphosphates which prohibits repair synthesis and causes incisions to accumulate. Incision requires ATP, continues from 10 to 15 min at 37”, and is specific for the excision repair pathway in toluene-treated cells. The excision of pyrimidine dimers in strains containing DNA polymerase I is rapid when all components are present and results in 30 to 40% excision in the first 5 min. When the deoxyribonucleoside triphosphates are omitted, the excision rate, but not extent, is much reduced. This pattern of excision is comparable to that observed in intact cells deficient in DNA polymerase I. Neither DNA polymerase II nor III appears to influence the rate of dimer removal. The requirement for repair synthesis in excision repair has been evaluated by the addition of the deoxyribonucleoside triphosphates subsequent to incision accumulation, thus allowing repair synthesis and ligation to return the DNA to its original size. The reformation of the DNA to high molecular weight is rapid and nearly complete by 2 min in cells containing DNA polymerase I. The reformation is slower and less complete in the absence of DNA polymerase I. This slower reformation is apparently catalyzed by DNA polymerase III as synthesis is observed in a mutant lacking both DNA polymerase I and II.