Cells are continually exposed to endogenous and exogenous elements that induce DNA double-strand breaks (DSB)s, which, if not properly removed, can cause death or gross chromosome aberrations. Homologous recombination (HR) is a major pathway for the elimination of DSBs. There is compelling evidence that homology-directed DNA repair is needed for cancer avoidance in humans. Genetic analyses in yeast have led to the identification of the RAD52 epistasis group of genes needed for HR. Subsequent cloning, biochemical, and genetic studies have shown remarkable conservation of the structure and function of the RAD52 group genes and proteins during eukaryotic evolution. Biochemical analyses of the RAD51-encoded product, a key member of the RAD52 group, have uncovered in it a homologous DNA pairing and strand exchange activity that can serve to link recombining chromosomes. The recombinase activity of Rad51 is subject to multiple layers of control. This research project will utilize a variety of molecular tools to delineate the homologous DNA pairing and strand exchange reaction mediated by human Rad51 protein and define the role of various human recombination factors in regulating the hRad51 recombinase activity. Specifically, we will (1) examine how ATP binding and hydrolysis modulate the affinity of hRad51 for its DNA substrates and hRad51 presynaptic filament dynamics, and (2) define the biochemical properties of the hRad54 and Rad54B proteins, dissect the multifaceted role of these factors in HR, and identify and characterize complexes that contain these factors. The results should shed some light on the mechanistic underpinnings of the homologous recombination machinery in human cells and will have important implications for cancer etiology and prevention.