Fission yeast Rad54 prevents intergenerational buildup of Rad51 aggregates in proliferating cells

Homologous recombination is central to the maintenance of genome stability. Using fission yeast, we found that mutation of the rad54 gene leads to robust Rad51 accumulation in vegetatively growing cells. By developing a protocol to track Rad51 in live yeast cells, we traced the origin and fate of Rad51 aggregates formed in rad54 mutants. Our observations strongly suggest that DNA breaks arising in late S phase act as the primary initiators of Rad51 accumulation. Rad51 initially appears as foci during late S phase, which continue to enlarge throughout the G2 phase. These Rad51 accumulations frequently persist into M phase and are distributed along with chromosomes into daughter cells. The inherited Rad51 mass in daughter cells continues to grow, forming robust Rad51 aggregates that are often associated with cell cycle arrest. Thus, the primary role of Rad54 in vegetative fission yeast cells is to facilitate the repair of DNA breaks arising in late S phase. The intergenerational accumulation of Rad51 aggregates in rad54 mutants reveals a novel mechanism through which defective homologous recombination drives genome instability.