Resolution of holliday junctions by RuvABC prevents dimer formation in rep mutants and UV-irradiated cells.

In this work, we present evidence that indicates that RuvABC proteins resolve Holliday junctions in a way that prevents dimer formation in vivo. First, although arrested replication forks are rescued by recombinational repair in cells deficient for the Rep helicase, rep mutants do not require the XerCD proteins or the dif site for viability. This shows that the recombination events at arrested replication forks are generally not accompanied by the formation of chromosome dimers. Secondly, resolution of dimers into monomers is essential in the rep ruv strain because of an increased frequency of RecFOR recombination events in the chromosome of this mutant. This suggests that, in the absence of the Ruv proteins, chromosomal recombination leads to frequent dimerization. Thirdly, dif or xerC mutations increase the UV sensitivity of ruv-deficient cells 100-fold, whereas they do not confer UV sensitivity to ruv+ cells. This shows that recombinational repair of UV lesions is not accompanied by dimer formation provided that the RuvABC proteins are active. The requirement for dimer resolution in ruv strains is suppressed by the expression of the RusA Holliday junction resolvase; therefore, RusA also prevents dimer formation. We conclude that the inviability arising from a high frequency of dimer formation in rep or UV-irradiated cells is only observed in the absence of known enzymes that resolve Holliday junctions.