The RuvA, RuvB andRuvCproteins ofEscherichia coliact together to process Holliday junctions formed during recombination and DNA repair. RuvA has a well-de®ned DNA binding surface that is sculptured speci®cally to accommodate a Holliday junction and allow subsequent loading of RuvB and RuvC. A negatively charged pin pro-jecting from the centre limits binding of linear duplexDNA. The amino-acid sequences forming the pin are highly con-served.