Analysis of the role of the NUC1 endo/exonuclease in yeast mitochondrial DNA recombination

Mitochondrial DNA recombination was reduced in an yeast mutant lacking the NUC1 endo/exonuclease. Between linked markers in either the or cob region the frequency of recombination decreased nearly 50% compared to wild-type. Gene conversion frequencies in the var1 gene and in the region were also lower in the mutant strain. In particular, the gradient of gene conversion at was most affected by the absence of the NUC1 nuclease. In crosses between nuclease-deficient and wild-type strains, gene conversion frequencies at were reduced only when the ₊ allele was contributed to the zygote by the nuclease-deficient parent. We propose that the 5 exonuclease activity of the NUC1 nuclease functions during recombination to enlarge heteroduplex tracts following a double-strand break in DNA. In crosses between nuclease-deficient and wild-type strains, the anisotropy in gene conversion frequencies at is hypothesized to be due to the slow mixing of parental motochondrial membranes as they fuse in the zygote.