Interaction of human Rad51 recombination protein with single-stranded DNA binding protein, RPA

Replication protein A (RPA), a heterotrimeric single-stranded DNA binding p rotein, is required for recombination, and stimulates homologous pairing an d DNA strand exchange promoted in vitro by human recombination protein HsRa d51. Co-immunoprecipitation revealed that purified RPA interacts physically with HsRad51, as well as with HsDmc1,the homolog that is expressed specifi cally in meiosis. The interaction with HsRad51 was mediated by the 70 kDa s ubunit of RPA, and according to experiments with deletion mutants, this int eraction required amino acid residues 169-326, In exponentially growing mam malian cells, 22% of nuclei showed foci of RPA protein and 1-2% showed foci of Rad51. After gamma-irradiation, the percentage of cells with RPA foci i ncreased to similar to 50%, and those with Rad51 foci to 30%. All of the ce lls with foci of Rad51 had foci of RPA, and in those cells the two proteins co-localized in a high fraction of foci. The interactions of human RPA wit h Rad51, replication proteins and DNA are suited to the linking of recombin ation to replication.