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

Abstract

Mitochondrial DNA recombination was reduced in an yeast mutant lacking the NUC1 endo/exonuclease. Between linked markers in either the ω or cob region the frequency of recombination decreased nearly 50% compared to wild-type. Gene conversion frequencies in the var1 gene and in the ω region were also lower in the mutant strain. In particular, the gradient of gene conversion at ω was most affected by the absence of the NUC1 nuclease. In crosses between nuclease-deficient and wild-type strains, gene conversion frequencies at ω were reduced only when the ω₊ 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 ω is hypothesized to be due to the slow mixing of parental motochondrial membranes as they fuse in the zygote.