The Swi5-Sfr1 complex stimulates Rhp51/Rad51- and Dmc1-mediated DNA strand exchange in vitro.

Nucleoprotein filaments made up of Rad51 or Dmc1 recombinases, the core structures of recombination, engage in ATP-dependent DNA-strand exchange. The ability of recombinases to form filaments is enhanced by recombination factors termed 'mediators'. Here, we show that the Schizosaccharomyces pombe Swi5-Sfr1 complex, a conserved eukaryotic protein complex, at substoichiometric concentrations stimulates strand exchange mediated by Rhp51 (the S. pombe Rad51 homolog) and Dmc1 on long DNA substrates. Reactions mediated by both recombinases are completely dependent on Swi5-Sfr1, replication protein A (RPA) and ATP, although RPA inhibits the reaction when it is incubated with single-stranded DNA (ssDNA) before the recombinase. The Swi5-Sfr1 complex overcomes, at least partly, the inhibitory effect of RPA, representing a novel class of mediator. Notably, the Swi5-Sfr1 complex preferentially stimulates the ssDNA-dependent ATPase activity of Rhp51, and it increases the amounts of Dmc1 bound to ssDNA.