Two different Swi5-containing protein complexes are involved in mating-type switching and recombination repair in fission yeast

Homologous recombination is an important biological process that occurs in all organisms and facilitates genome rearrangements and repair of DNA double-strand breaks. Eukaryotic Rad51 proteins (Rad51 sp or Rhp51 in fission yeast) are functional and structural homologs of bacterial RecA protein, an evolutionarily conserved protein that plays a key role in homologous pairing and strand exchange between homologous DNA molecules in vitro . Here we show that the fission yeast swi5 + gene, which was originally identified as a gene required for normal mating-type switching, encodes a protein conserved among eukaryotes and is involved in a previously uncharacterized Rhp51 (Rad51 sp )-dependent recombination repair pathway that does not require the Rhp55/57 (Rad55/57 sp ) function. Protein interactions with both Swi5 and Rhp51 were found to be mediated by a domain common to Swi2 and Sfr1 ( S wi f ive-dependent r ecombination repair protein 1 , a previously uncharacterized protein with sequence similarity to the C-terminal part of Swi2). Genetic epistasis analyses suggest that the Swi5–Sfr1–Rhp51 interactions function specifically in DNA recombination repair, whereas the Swi5–Swi2–Rhp51 interactions may function, together with chromodomain protein Swi6 (HP1 homolog), in mating-type switching.