STRUCTURE OF A MULTISUBUNIT COMPLEX THAT PROMOTES DNA BRANCH MIGRATION

THE RuvA and RuvB proteins of Escherichia coli, which are induced in r esponse to DNA damage, are important in the formation of heteroduplex DNA during genetic recombination and related recombinational repair pr ocesses(1). In vitro studies show that RuvA binds Holliday junctions(2 -5) and acts as a specificity factor that targets the RuvB ATPase, a h exameric ring protein(6,7), to the junction. Together, RuvA and RuvB p romote branch migration, an ATP-dependent reaction that increases the length of the heteroduplex DNA(3,8-10). Electron microscopic visualiza tion of RuvAB now provides a new insight into the mechanism of this pr ocess. We observe the formation of a tripartite protein complex in whi ch RuvA binds the crossover and is sandwiched between two hexameric ri ngs 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 ori ented RuvB ring motors.