Replication-dependent sister chromatid recombination in rad1 mutants of Saccharomyces cerevisiae

Homolog recombination and unequal sister chromatid recombination were monitored in rad1-1/rad1-1 diploid yeast cells deficient for excision repair, and in control cells, RAD1/rad1-1, after exposure to UV irradiation. In a rad1-1/rad1-1 diploid, UV irradiation stimulated much more sister chromatid recombination relative to homolog recombination when cells were irradiated in the G₁ or the G₂ phases of the cell cycle than was observed in RAD1/rad1-1 cells. Since sister chromatids are not present during G₁, this result suggested that unexcised lesions can stimulate sister chromatid recombination events during or subsequent to DNA replication. The results of mating rescue experiments suggest that unexcised UV dimers do not stimulate sister chromatid recombination during the G₂ phase, but only when they are present during DNA replication. We propose that there are two types of sister chromatid recombination in yeast. In the first type, unexcised UV dimers and other bulky lesions induce sister chromatid recombination during DNA replication as a mechanism to bypass lesions obstructing the passage of DNA polymerase, and this type is analogous to the type of sister chromatid exchange commonly observed cytologically in mammalian cells. In the second type, strand scissions created by X-irradiation or the excision of damaged bases create recombinogenic sites that result in sister chromatid recombination directly in G₂. Further support for the existence of two types of sister chromatid recombination is the fact that events induced in rad1- 1/rad1-1 were due almost entirely to gene conversion, whereas those in RAD1/rad1-1 cells were due to a mixture of gene conversion and reciprocal recombination.