POORLY REPAIRED MISMATCHES IN HETERODUPLEX DNA ARE HYPER-RECOMBINAGENIC IN SACCHAROMYCES-CEREVISIAE

In yeast meiotic recombination, alleles used as genetic markers fall i nto two classes as regards their fate when incorporated into heterodup lex DNA. Normal alleles are those that form heteroduplexes that are ne arly always recognized and corrected by the mismatch repair system ope rating in meiosis. High PMS (postmeiotic segregation) alleles form het eroduplexes that are inefficiently mismatch repaired. We report that p lacing any of several high PMS alleles very close to normal alleles ca uses hyperrecombination between these markers. We propose that this hy perrecombination is caused by the high PMS allele blocking a mismatch repair tract initiated from the normal allele, thus preventing corepai r of the two alleles, which would prevent formation of recombinants. T he results of three point crosses involving two PMS alleles and a norm al allele suggest that high PMS alleles placed between two alleles tha t are normally corepaired block that corepair.