Saccharomyces cerevisiae Rad51 protein is the paradigm for eukaryotic ATP-dependent DNA strand exchange proteins. To explain some of the unique characteristics of DNA strand exchange promoted by Rad51 protein, when compared with its prokaryotic homologue the Escherichia coli RecA protein, we analyzed the DNA binding properties of the Rad51 protein. Rad51 protein binds both single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) in an ATP- and Mg2+-dependent manner, over a wide range of pH, with an apparent binding stoichiometry of approximately 1 protein monomer per 4 (+/-1) nucleotides or base pairs, respectively. Only dATP and adenosine 5'-gamma-(thiotriphosphate) (ATPgammaS) can substitute for ATP, but binding in the presence of ATPgammaS requires more than a 5-fold stoichiometric excess of protein. Without nucleotide cofactor, Rad51 protein binds both ssDNA and dsDNA but only at pH values lower than 6.8; in this case, the apparent binding stoichiometry covers the range of 1 protein monomer per 6-9 nucleotides or base pairs. Therefore, Rad51 protein displays two distinct modes of DNA binding. These binding modes are not inter-convertible; however, their initial selection is governed by ATP binding. On the basis of these DNA binding properties, we conclude that the main reason for the low efficiency of the DNA strand exchange promoted by Rad51 protein in vitro is its enhanced dsDNA-binding ability, which inhibits both the presynaptic and synaptic phases of the DNA strand exchange reaction as follows: during presynapsis, Rad51 protein interacts with and stabilizes secondary structures in ssDNA thereby inhibiting formation of a contiguous nucleoprotein filament; during synapsis, Rad51 protein inactivates the homologous dsDNA partner by directly binding to it.