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 omega or cob region the frequency of recombination decreased nearly 50% com pared to wild-type. Gene conversion frequencies in the var1 gene and i n the omega region were also lower in the mutant strain. In particular , the gradient of gene conversion at omega was most affected by the ab sence of the NUC1 nuclease. In crosses between nuclease-deficient and wild-type strains, gene conversion frequencies at omega were reduced o nly when the omega(+) allele was contributed to the zygote by the nucl ease-deficient parent. We propose that the 5' exonuclease activity of the NUC1 nuclease functions during recombination to enlarge heterodupl ex tracts following a double-strand break in DNA. In crosses between n uclease-deficient and wild-type strains, the anisotropy in gene conver sion frequencies at omega is hypothesized to be due to the slow mixing of parental mitochondrial membranes as they fuse in the zygote.