Branch migration during homologous recombination: assembly of a RuvAB-Holliday junction complex in vitro.

The RuvA and RuvB proteins of E. coli promote the branch migration or movement of Holliday junctions during genetic recombination and DNA repair. Using small synthetic Holliday junctions in which the crossover point is confined near one end of the DNA molecule, we show that RuvAB-mediated branch migration occurs with a defined polarity. The assembly of RuvA and RuvB on the Holliday junction has been investigated by sedimentation analysis and by DNase I footprinting. We find that RuvA protein binds and protects all four strands of DNA at the crossover point, whereas RuvB protein binds the DNA asymmetrically. The polarity of branch migration is defined by the asymmetric assembly of the RuvAB branch migration complex relative to the junction and is consistent with a model in which RuvAB drives branch migration by passing the DNA through the hexameric rings of RuvB.