Homologous recombinational repair of double-strand breaks in yeast is enhanced by MAT heterozygosity through yKU-dependent and -independent mechanisms

DNA double-strand breaks (DSBs) are repaired by homologous recombination (H R) and nonhomologous end-joining (NHEJ). NHEJ in yeast chromosomes has been observed only when HR is blocked, as in rad52 mutants or in the absence of a homologous repair template. We detected yKu70p-dependent imprecise NHEJ at a frequency of similar to0.1% in HR-competent Rad(+) haploid cells. Inte restingly, yku70 mutation increased DSB-induced HR between direct repeats b y 1.3-fold in a haploid strain and by 1.5-fold in a AMT homozygous (a/a) di ploid, but yku70 had no effect on HR in a MAT heterozygous (a/alpha) diploi d. yku70 might increase HR because it eliminates the competing precise NHEJ (religation) pathway and/or because yKu70p interferes directly or indirect ly with HR. Despite the yku70-dependent increase in a/a cells, HR remained 2-fold lower than in a/alpha cells. Cell survival was also lower in a/a cel ls and correlated with the reduction in HR. These results indicate that MAT heterozygosity enhances DSB-induced HR by yKu-dependent and -independent m echanisms, with the latter mechanism promoting cell survival. Surprisingly, yku70 strains survived a DSB slightly better than wild type. We propose th at this reflects enhanced HR, not by elimination of precise NHEJ since this pathway produces viable products, but by elimination of yKu-dependent inte rference of HR.