BACKGROUND: The RAD51 gene of Saccharomyces cerevisiae is homologous to the Escherichia coli recA gene and plays a key role in genetic recombination and DNA double-strand break repair. To construct an improved experimental system of homologous recombination in higher eukaryotes, we have chosen the South African clawed frog, Xenopus laevis, whose egg extracts might be useful for the in vitro studies. We identified and characterized a Xenopus homologue of RAD51 gene, the XRAD51.1. RESULTS: Recombinant XRad51.1 was expressed in E. coli. The purified XRad51.1 protein showed ssDNA-dependent ATPase activity and promoted the DNA strand exchange reaction between two 55-mer oligonucleotides. The binding stoichiometry of XRad51.1 to ssDNA was determined by fluorescence of poly(d epsilonA), a chemically modified poly(dA), and was found to be about six bases/XRad51.1 monomer in a nucleoprotein filament, a similar value to E. coli RecA protein. The kinetics of the fluorescence change of poly(d epsilonA) after XRad51.1 binding in the presence of ATP was significantly different from that observed with RecA protein. The affinity of XRad51.1 to ssDNA in the presence of ATP was higher than that of RecA protein, and the dissociation of the XRad51.1-ssDNA complex was slower than the RecA-ssDNA complex. CONCLUSIONS: Purified recombinant XRad51.1 protein promoted the strand exchange between short DNA molecules. While the binding stoichiometry of XRad51.1 protein to ssDNA was identical to that of the RecA protein, XRad51.1 has a significantly higher affinity and binding stability to ssDNA than did the RecA protein in the presence of ATP.

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