Role of the Saccharomyces cerevisiae Rad51 Paralogs in Sister Chromatid Recombination

Abstract

Rad51 requires a number of other proteins, including the Rad51 paralogs, for efficient recombination in vivo. Current evidence suggests that the yeast Rad51 paralogs, Rad55 and Rad57, are important in formation or stabilization of the Rad51 nucleoprotein filament. To gain further insights into the function of the Rad51 paralogs, reporters were designed to measure spontaneous or double-strand break (DSB)-induced sister or nonsister recombination. Spontaneous sister chromatid recombination (SCR) was reduced 6000-fold in the rad57 mutant, significantly more than in the rad51 mutant. Although the DSBinduced recombination defect of rad57 was suppressed by overexpression of Rad51, elevated temperature, or expression of both mating-type alleles, the rad57 defect in spontaneous SCR was not strongly suppressed by these same factors. In addition, the UV sensitivity of the rad57 mutant was not strongly suppressed by MAT heterozygosity, even though Rad51 foci were restored under these conditions. This lack of suppression suggests that Rad55 and Rad57 have different roles in the recombinational repair of stalled replication forks compared with DSB repair. Furthermore, these data suggest that most spontaneous SCR initiates from single-stranded gaps formed at stalled replication forks rather than DSBs.

XRCC2 and XRCC3 Regulate the Balance between Short- and Long-Tract Gene Conversions between Sister Chromatids

“…These results suggest that each of these Rad51 paralogs has a nonredundant function in HR. Interestingly, although overexpression of S. cerevisiae RAD51 reverses a defect in DSB-induced HR in rad55 and rad57 mutants, RAD51 overexpression was not able to suppress the spontaneous SCR defect in rad57 mutants, suggesting a specialized role for Rad51 paralogs in the repair of daughter strand gaps that arise as a result of stalled replication forks (43). In this regard, mammalian XRCC3 has been shown to be required for replication fork progression in cells treated with DNA replication inhibitors (23).…”

Mitotic gene conversion events induced in G1-synchronized yeast cells by gamma rays are similar to spontaneous conversion events

“…Replication of nicked DNA molecules can give rise to a recombinogenic DSB (22,30). In addition, nicked or gapped DNA molecules can stimulate conversion directly, without being converted to a DSB (31)(32)(33).…”

Suppression of the Double-Strand-Break-Repair Defect of the Saccharomyces cerevisiae rad57 Mutant

“…Mutation of RAD51, RAD55, or RAD57 confers sensitivity of ionizing radiation (IR) and defects in mitotic and meiotic recombination, indicating that their functions are not redundant (Symington 2002). rad51 mutants generally exhibit more severe defects than rad55 or rad57 mutants in DSB-induced recombination assays; however, rad55 and rad57 mutants are more defective than rad51 in some assays that measure spontaneous recombination between repeated sequences (Rattray and Symington 1995;Mozlin et al 2008).The molecular details of homologous recombination are largely based on genetic, physical, and cytological studies of DSB repair (DSBR) and on biochemical characterization of purified proteins (Paques and Haber 1999;Sugawara et al 2003;Lisby et al 2004;San Filippo et al 2008). The single-stranded DNA (ssDNA)-binding protein, replication protein A (RPA), initially binds ssDNA that forms by nucleolytic processing of DNA ends at DSBs.…”