THE RuvA and RuvB proteins of Escherichia coli, which are induced in response to DNA damage, are important in the formation of heteroduplex DNA during genetic recombination and related recombinational repair processes(1). In vitro studies show that RuvA binds Holliday junctions(2-5) and acts as a specificity factor that targets the RuvB ATPase, a hexameric ring protein(6,7), to the junction. Together, RuvA and RuvB promote branch migration, an ATP-dependent reaction that increases the length of the heteroduplex DNA(3,8-10). Electron microscopic visualization of RuvAB now provides a new insight into the mechanism of this process. We observe the formation of a tripartite protein complex in which RuvA binds the crossover and is sandwiched between two hexameric rings of RuvB. The Holliday junction within this complex adopts a square-planar structure. We propose a molecular model for branch migration, a unique feature of which is the role played by the two oppositely oriented RuvB ring motors.