Double-strand-break repair recombination in Escherichia coli: physical evidence for a DNA replication mechanism in vivo

DNA double-strand-break repair (DSBR) is, in many organisms, accomplished b y homologous recombination. In Escherichia coli DSBR was thought to result from breakage and reunion of parental DNA molecules, assisted by known endo nucleases, the Holliday junction resolvases. Under special circumstances, f or example, SOS induction, recombination forks were proposed to initiate re plication. We provide physical evidence that this is a major alternative me chanism in which replication copies information from one chromosome to anot her generating recombinant chromosomes in normal cells in vivo. This altern ative mechanism can occur independently of known Holliday junction cleaving proteins, requires DNA polymerase III, and produces recombined DNA molecul es that carry newly replicated DNA. The replicational mechanism underlies a bout half the recombination of linear DNA in E. coli; the other half occurs by breakage and reunion, which we show requires resolvases, and is replica tion-independent. The data also indicate that accumulation of recombination intermediates promotes replication dramatically.