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The Rad51 protein, which catalyzes homologous-pairing and strand-exchange reactions, is an essential enzyme for homologous recombinational repair (HRR) and meiotic homologous recombination in eukaryotes. In humans, the conventional Rad51 (HsRad51) protein has a Lys residue at position 313; however, the HsRad51-Q313 protein, in which the Lys313 residue is replaced by Gln, was reported as an isoform, probably corresponding to a polymorphic variant. In this study, we purified the HsRad51-K313 and HsRad51-Q313 isoforms and analyzed their biochemical activities in vitro. Compared to the conventional HsRad51-K313 protein, the HsRad51-Q313 protein exhibited significantly enhanced strand-exchange activity under conditions with Ca2+, although the difference was not observed without Ca2+. A double-stranded DNA (dsDNA) unwinding assay revealed that the HsRad51-Q313 protein clearly showed enhanced DNA unwinding activity, probably due to its enhanced filament-formation ability. Mutational analyses of the HsRad51-Lys313 residue revealed that positively charged residues (Lys and Arg), but not negatively charged, polar and hydrophobic residues (Glu, Gln and Met, respectively), at position 313 reduced the strand-exchange and DNA unwinding abilities of the HsRad51 protein. These results suggest that the electrostatic environment around position 313 is important for the regulation of the HsRad51 recombinase activity. [ABSTRACT FROM AUTHOR]