Functional interactions among yeast Rad51 recombinase, Rad52 mediator, andreplication protein A in DNA strand exchange

Rad51-catalyzed DNA strand exchange is greatly enhanced by the single-stran ded (ss) DNA binding factor RPA if the latter is introduced after Rad51 has already nucleated onto the initiating ssDNA substrate. Paradoxically, co-a ddition of RPA with Rad51 to the ssDNA to mimic the in vivo situation dimin ishes the level of strand exchange, revealing competition between RPA and R ad51 for binding sites on ssDNA Rad52 promotes strand exchange but only whe n there is a need for Rad51 to compete with RPA for loading onto ssDNA Rad5 2 is multimeric, binds ssDNA, and targets Rad51 to ssDNA. Maximal restorati on of pairing and strand exchange requires amounts of Rad52 substoichiometr ic to Rad51 and involves a stable, equimolar complex between Rad51 and Rad5 2, The Rad51-Rad52 complex efficiently utilizes a ssDNA template saturated with RPA for homologous pairing but does not appear to be more active than Rad51 when an RPA-free ssDNA template is used. Rad52 does not substitute fo r RPA in the pairing and strand exchange reaction nor does it lower the dep endence of the reaction on Rad51 or RPA.