Paper ID: 360
Duke University Medical Center (USA)
Mitotic recombination between homologous chromosomes leads to the uncovering of recessive alleles through loss of heterozygosity (LOH). Analyses of spontaneous LOH events has suggested that most are initiated by random double-strand breaks (DSBs). To examine this more directly, we used a defined double-strand break to initiate reciprocal LOH between diverged homologs of chromosome IV in Saccharomyces cerevisiae. All LOH events reflected the repair of two broken chromatids, one of which was repaired as a crossover and the other as a noncrossover. Associated gene conversion tracts resulting from the donor-directed repair of mismatches in the heteroduplex DNA formed during strand exchange were mapped using microarrays. Gene conversion tracts associated with individual crossover and noncrossover events were similar in size and position, with half of the tracts being unidirectional and mapping to only a one side of the initiating DSB. Among crossover events, this likely reflected gene conversion on one side of the break, with restoration-type repair occurring on the other side. For noncrossover events, an ectopic system was used to directly compare gene conversion tracts produced in a wild-type strain to unrepaired, heteroduplex DNA tracts that persist in the absence of the Mlh1 mismatch-repair protein. There was a strong bias for unidirectional tracts in the absence of Mlh1 that was lost in strains containing Mlh1, suggesting that MMR acts on heteroduplex DNA that is only transiently present in noncrossover intermediates. Although the molecular features of LOH events generally agreed with those predicted by current recombination models, there were unexpected complexities in associated gene conversion tracts.