A sequence-specific endonuclease, Endo.SceI, can efficiently induce gene conversion in yeast mitochondria lacking a major exonuclease.

Endo.SceI most likely initiates homologous recombination of yeast mitochondrial genome through sequence-specific double-strand scission of DNA. According to the double-strand break-repair model for the mechanism of homologous recombination, DNA ends created by sequence-specific endonucleases have to be processed by exonucleases. The major mitochondrial exonuclease (encoded by NUC1) has been shown to greatly affect the length of conversion tracts at the 21S rRNA locus when site-specific gene conversion is induced by omega endonuclease. In order to examine the role of the NUC1 nuclease in the Endo.SceI-induced recombination, recombination frequencies were measured after crossing of parental strains either in the presence or absence of NUC1 nuclease activity. The frequency of gene conversion in the oli2 locus induced by Endo.SceI was not reduced by disruption of the NUC1 gene. This result strongly implicates the presence of multiple exonucleases for the processing of the DNA ends created by sequence-specific endonucleases.