DNA repair systems identified in mitotic cells of the yeast Saccharomyces cerevisiae are being examined for a) their protection of cells undergoing meiosis, and b) the role of the corresponding genes in normal meiosis. We have developed unique sucrose gradient techniques to examine repair after low doses of UV or ionizing radiation and to follow changes in meiotic DNA during meiosis. The RAD50, RAD52 and RAD57 genes are essential in the repair of DNA double-strand breaks in mitotic cells. They are also required for meiosis. Mutations in these genes abolish normal meiotic recombination; RAD50 acts early in meiosis. Rare single-strand interruptions (SSIs) were observed in rad52 and rad57 strains shortly after the beginning of meiotic DNA synthesis and these appear to be related to recombination. Gentle isolation techniques have allowed the characterization of SSIs as breaks in DNA; many have 3' OH and 5' PO4 termini. The SSIs do not appear to be randomly distributed, based on experiments involving probes for specific chromosomal regions, suggesting specific sites or regions involved in normal meiotic recombination. While rad52 and rad57 mutants are defective in meiotic recombination, recombinants can be recovered prior to commitment to reductional division. The frequency in rad52 is much lower than in Rad+ strains, but it is comparable in rad57 mutants. The recombinants differ qualitatively from those in Rad+. When meiosis is arrested and rad52 or rad57 cells are exposed to growth medium, recombinants are not recovered. This is due to the extended time necessary for recombinants to form, suggesting that rad52 and rad57 are blocked at an intermediate step. The recombination intermediates are resolved slowly and growth prior to resolution prevents the appearance of recombinants. Both single-strand and double-strand interruptions are found in rad57 mutants. Based on genetic and biochemical changes, the order of gene function appears to be RAD50, RAD52, and RAD57.
