EVIDENCE FOR INDEPENDENT MISMATCH REPAIR PROCESSING ON OPPOSITE SIDESOF A DOUBLE-STRAND BREAK IN SACCHAROMYCES-CEREVISIAE

Double-strand break (DSB) induced gene conversion in Saccharomyces cer evisiae during meiosis and MAT switching is mediated primarily by mism atch repair of heteroduplex, DNA (hDNA). We used nontandem ura3 duplic ations containing palindromic frameshift insertion mutations near an H O nuclease recognition site to test whether mismatch repair also media tes DSB-induced mitotic gene conversion at a non-MAT locus. Palindromi c insertions included in hDNA are expected to produce a stem-loop mism atch, escape reto produce a sectored (Ura(+/-)) colony. If conversion occurs by gap repair, the insertion should be remobed on both strands, and converted colonies will not be secotred. For both a 14-bp a 14-bp palindrome, and a 37-bp near-palindrome, similar to 75% of recombinan t colonies were sectored, indicating that most DSB-induced mitotic gen e conversion involves mismatch repair of hDNA. We also investigated mi smatch repair of well-repaired markers flanking an unrepaired palindro me. As seen in previous studies, these additional markers increased lo op repair (likely reflecting corepair). Among sectored products, few h ad additional segregating markers, indicating that the lack of repair at one marker is not associated with inefficient repair at nearby mark ers. Clear evidence was obtained for low levels of short tract mismatc h repair. As seen with full gene conversions, donor alleles in sectore d products were not altered. Markers on the same side of the DSB as th e palindrome were involved in hDNA less often among sectored products than non-sectored products, but markers on the opposite side of the DB showed similar hDNA involvement among both product classes. These res ults can be explained in terms of corepair, and they suggest that mism atch repair on opposite sides of a DSB involves distinct repair tracts .