Homology Requirements and Competition between Gene Conversion and Break-Induced Replication during Double-Strand Break Repair

Saccharomyces cerevisiae mating-type switching is initiated by a double-strand break (DSB) at MATa, leaving one cut end perfectly homologous to the HML alpha donor, while the second end must be processed to remove a non-homologous tail before completing repair by gene conversion (GC). When homology at the matched end is <= 150 bp, efficient repair depends on the recombination enhancer, which tethers HML alpha near the DSB. Thus, homology shorter than an apparent minimum efficient processing segment can be rescued by tethering the donor near the break. When homology at the second end is <= 150 bp, second-end capture becomes inefficient and repair shifts from GC to break-induced replication (BIR). But when pol32 or pif1 mutants block BIR, GC increases 3-fold, indicating that the steps blocked by these mutations are reversible. With short second-end homology, absence of the RecQ helicase Sgs1 promotes gene conversion, whereas deletion of the FANCM-related Mph1 helicase promotes BIR.