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; it appears that RAD50 acts at an early step 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 mutants is much lower than in RAD+ strains; the frequency is somewhat lower in rad57 mutants. The recombinants are also qualitatively different from those in RAD+. When meiosis is arrested and rad52 or rad57 are exposed to growth medium, recombinants are not recovered. This is due to the extended time necessary for recombinants to form, and suggests that rad52 and rad57 are blocked at an intermediate step in recombination. The recombination intermediates are resolved slowly, and growth prior to resolution prevents the appearance of recombinants.
