MEIOTIC RECOMBINATION INITIATED BY A DOUBLE-STRAND BREAK IN RAD50-DELTA YEAST-CELLS OTHERWISE UNABLE TO INITIATE MEIOTIC RECOMBINATION

Meiotic recombination in Saccharomyces cerevisiae is initiated by doub le-strand breaks (DSBs). We have developed a system to compare the pro perties of meiotic DSBs with those created by the site-specific HO end onuclease. HO endonuclease was expressed under the control of the meio tic-specific SP013 promoter, creating a DSB at a single site on one of yeast's 16 chromosomes. In Rad(+) strains the times of appearance of the HO-induced DSBs and of subsequent recombinants are coincident with those induced by normal meiotic DSBs. Physical monitoring of DNA show ed that SP013::HO induced gene conversions both in Rad(+) and in rad50 Delta cells that cannot initiate normal meiotic DSBs. We find that th e RAD50 gene is important, but not essential, for recombination even a fter a DSB has been created in a meiotic cell. In rad50 Delta cells, s ome DSBs are not repaired until a broken chromosome has been packaged into a spore and is subsequently germinated. This suggests that a brok en chromosome does not signal an arrest of progression through meiosis . The recombination defect in rad50 Delta diploids is not, however, me iotic specific, as mitotic rad50 diploids, experiencing an HO-induced DSB, exhibit similar departures from wild-type recombination.