Substrate specificity of RusA resolvase reveals the DNA structures targeted by RuvAB and RecG in vivo.

RusA endonuclease cleaves Holliday junctions by introducing paired strand incisions 5' to CC dinucleotides. Coordinated catalysis is achieved when both subunits of the homodimer interact simultaneously with cleavage sites located symmetrically. This requirement confers Holliday junction specificity. Uncoupled catalysis occurs when binding interactions are disturbed. Genetic studies indicate that uncoupling occurs rarely in vivo, and DNA cleavage is therefore restricted to Holliday junctions. We exploited the specificity of RusA to identify the DNA substrates targeted by the RuvAB and RecG branch-migration proteins in vivo. We present evidence that replication restart in UV-irradiated cells relies on the processing of stalled replication forks by RecG helicase and of Holliday junctions by the RuvABC resolvasome, and that RuvAB alone may not promote repair.