Mitochondrial DNA repairs double-strand breaks in yeast chromosomes

The endosymbiotic theory for the origin of eukaryotic cells' proposes that genetic information can be transferred from mitochondria to the nucleus of a cell, and genes that are probably of mitochondrial origin have been found in nuclear chromosomes(2). Occasionally, short or rearranged sequences hom ologous to mitochondrial DNA are seen in the chromosomes of different organ isms including yeast, plants and humans(3). Here we report a mechanism by w hich fragments of mitochondrial DNA, in single or tandem array, are transfe rred to yeast chromosomes under natural conditions during the repair of dou ble-strand breaks in haploid mitotic cells. These repair insertions origina te from noncontiguous regions of the mitochondrial genome. Our analysis of the Saccharomyces cerevisiae mitochondrial genome(4) indicates that the yea st nuclear genome does indeed contain several short sequences of mitochondr ial origin which are similar in size and composition to those that repair d ouble-strand breaks. These sequences are located predominantly in non-codin g regions of the chromosomes, frequently in the vicinity of retrotransposon long terminal repeats, and appear as recent integration events. Thus, colo nization of the yeast genome by mitochondrial DNA is an ongoing process.