Excision-repair in mutants ofEscherichia coli deficient in DNA polymerase I and/or its associated 5′→3′ exonuclease

Abstract

The ultraviolet (UV) sensitivity ofEscherichia coli mutants deficient in the 5′→3′ exonuclease activity of DNA polymerase I is intermediate between that ofpol ⁺ strains and mutants which are deficient in the polymerizing activity of pol I (polA1). LikepolA1 mutants, the 5′-exonuclease deficient mutants exhibit increased UV-induced DNA degradation and increased repair synthesis compared to apol ⁺ strain, although the increase is not as great as inpolA1 or in the conditionally lethal mutant BT4113ts deficient inboth polymerase I activities. When dimer excision was measured at UV doses low enough to avoid interference from extensive DNA degradation, all three classes of polymerase I deficient mutants were found to remove dimers efficiently from their DNA. We conclude that enzymes alternative to polymerase I can operate in both the excision and resynthesis steps of excision repair and that substitution for either of the polymerase I functions results in longer patches of repair. A model is proposed detailing the possible events in the alternative pathways.