Homologous DNA pairing by human recombination factors Rad51 and Rad54

Human Rad51 (hRad51) and Rad54 proteins are key members of the RAD52 group required for homologous recombination. We show an ability of hRad54 to promote transient separation of the strands in duplex DNA via its ATP hydrolysis-driven DNA supercoiling function. The ATPase, DNA supercoiling, and DNA strand opening activities of hRad54 are greatly stimulated through an interaction with hRad51. Importantly, we demonstrate that hRad51 and hRad54 functionally cooperate in the homologous DNA pairing reaction that forms recombination DNA intermediates. Our results should provide a biochemical model for dissecting the role of hRad51 and hRad54 in recombination reactions in human cells.