Recombination between divergent sequences leads to cell death in a mismatch-repair-independent manner

Homologous recombination is an important DNA repair mechanism in vegetative cells. During the repair of double-strand breaks, genetic information is transferred between the interacting DNA sequences, thus creating a gene-conversion event. Gene conversion of a functional member of a gene family, which uses an inactive member (such as a pseudogene) as a template, might have deleterious consequences. It is therefore important for the cell to prevent recombination between divergent sequences. We have studied the repair of a double-strand break by recombination in a haploid yeast strain carrying 99% identical alleles located on different chromosomes. The fate of the broken chromosome was followed in the whole cell population without imposing selective constraints. Our results show that all the cells were able to repair the broken chromosome by gene conversion. During the repair, the cells arrest in the cell cycle with a "dumbbell" configuration characteristic of G2/M-arrested cells. Surprisingly, although all the cells repaired the broken chromosome, 60% of them were unable to resume growth and to form colonies after the repair was completed. The low level of cell recovery was due to the 1% divergence between the alleles, but was not dependent on the function of the mismatch-repair system. Cell death, however, could be prevented by the presence of an alternative source of perfect homology located on a different chromosome.