Transient generation of displaced single-stranded DNA during nick translation

Abstract We show that displaced single-stranded overhangs are transiently generated and destroyed during nick translation by E. coli DNA polymerase I. Evidence that hyper- rec mutants have an increased frequency of such overhang structures is discussed. The transient generation of overhangs may be significant for general recombination. The 5′ → 3′ exonuclease activity of polymerase I specifically hydrolyzes such overhangs to yield a nick. Overhangs are generated by polymerization, but after every polymerization step, either polymerase or exonuclease can act— 55% of the time, polymerization occurred first. At this frequency overhangs of ⩾12 nucleotides are generated every 1300 nucleotides polymerized. We suggest that many DNA strand discontinuities are displaced single-strand overhangs, rather than gaps or simple nicks.