NUCLEAR MUTATIONS IN SACCHAROMYCES-CEREVISIAE THAT AFFECT THE ESCAPE OF DNA FROM MITOCHONDRIA TO THE NUCLEUS

We have inserted a yeast nuclear DNA fragment bearing the TRP1 gene an d its associated origin of DNA replication, ARS1, into the functional mitochondrial chromosome of a strain carrying a chromosomal trp1 delet ion. TRPI was not phenotypically expressed within the organelle. Howev er, this Trp- strain readily gave rise to respiratory competent Trp+ c lones that contained the TPP1/ARS1 fragment, associated with portions of mitochondrial DNA (mtDNA), replicating in their nuclei. Thus the Tr p+ clones arose as a result of DNA escaping from mitochondria and migr ating to the nucleus. We have isolated 21 nuclear mutants in which the rate of mtDNA escape is increased by screening for increased rates of papillation to Trp+. All 21 mutations were recessive and fell into si x complementation groups, termed YME1-YME6. In addition to increasing the rate of mtDNA escape, yme1 mutations also caused a heat-sensitive respiratory deficient phenotype at 37-degrees and a cold-sensitive gro wth defect on complete glucose medium at 14-degrees. While the other y me mutations had no detectable growth phenotypes, synergistic interact ions were observed in two double mutant combinations: a yme1, yme2 dou ble mutant failed to respire at 30-degrees and a yme4, yme6 double mut ant failed to respire at all temperatures tested. None of the respirat ory defects were caused by loss of functional mtDNA. These findings su ggest that yme1, yme2, yme4 and yme6 mutations alter mitochondrial fun ctions and thereby lead to an increased rate of DNA escape from the or ganelle.