Homologous pairing promoted by the human Rad52 protein.

The Rad52 protein, which is unique to eukaryotes, plays important roles in the Rad51-dependent and the Rad51-independent pathways of DNA recombination. In the present study, we have biochemically characterized the homologous pairing activity of the HsRad52 protein (Homo sapiens Rad52) and found that the presynaptic complex formation with ssDNA is essential in its catalysis of homologous pairing. We have identified an N-terminal fragment (amino acid residues 1-237, HsRad52(1-237)) that is defective in binding to the human Rad51 protein, which catalyzed homologous pairing as efficiently as the wild type HsRad52. Electron microscopic visualization revealed that HsRad52 and HsRad52(1-237) both formed nucleoprotein filaments with single-stranded DNA. These lines of evidence suggest the role of HsRad52 in the homologous pairing step of the Rad51-independent recombination pathway. Our results reveal the striking similarity between HsRad52 and the Escherichia coli RecT protein, which functions in a RecA-independent recombination pathway.