Role of RAD51 in sister-chromatid exchanges in mammalian cells

To measure the impact of the RAD51 pathway on Sister-Chromatid Exchanges (SCE), we used hamster cells expressing either the wild-type MmRAD51, which stimulates, or the dominant negative SMRAD51, which inhibits, gene conversion without affecting cell viability of untreated as well as gamma-rays irradiated cells. We show that MmRAD51 did not affect the rate of spontaneous SCE while it strongly stimulated spontaneous recombination between tandem repeats. No spontaneous recombinant was detected when expressing SMRAD51 while spontaneous SCE were only slightly diminished. After treatment by an alkylating agent (MNU), MmRAD51 stimulated MNU-induced recombination whereas no recombinant was detected when expressing SMRAD51. MNU induced SCE in all cell lines, even in the SMRAD51 expressing lines, but the induction of SCE was slightly more efficient in lines expressing MmRAD51 and less efficient in lines expressing SMRAD51. Thus, in mammalian cells, the RAD51-dependent gene conversion pathway drastically affects recombination between intrachromosomal tandem repeats, whereas it only partially participates in SCE formation, measured at a chromosomal level. These results show that RAD51-gene conversion can participate in induced SCE but that alternative pathways should exist.