Saccharomyces cerevisiae RAD51 gene is required for genetic recombination and recombinational repair of DNA strand breaks. Rad51 protein has a DNA-dependent ATPase activity, and it catalyzes ATP-dependent pairing and strand exchange between homologous DNA molecules. We show here that the rad51 Arg-191 protein, which is devoid of ATPase activity, mediates the pairing and strand exchange reaction upon binding ATP. In addition, the wild type Rad51 protein can catalyze pairing and strand exchange in the presence of the nonhydrolyzable ATP analogues adenylyl-imidodiphosphate and adenosine 5'-O-thiotriphosphate. Thus, homologous pairing and the unidirectional transfer of greater than 5 kilobases of DNA can occur efficiently without the need for nucleotide hydrolysis. Consistent with the results from the biochemical analyses, expression of the rad51 Arg-191 protein in a rad51 null mutant confers normal cellular resistance to the DNA damaging agent methylmethane sulfonate, suggesting that nucleotide binding by Rad51 is sufficient for biological function.