Recombination‐dependent mtDNA partitioning: in vivo role of Mhr1p to promote pairing of homologous DNA

Yeast mhr1‐1 was isolated as a defective mutation in mitochondrial DNA (mtDNA) recombination. About half of mhr1‐1 cells lose mtDNA during growth at a higher temperature. Here, we show that mhr1‐1 exhibits a defect in the partitioning of nascent mtDNA into buds and is a base‐substitution mutation in MHR1 encoding a mitochondrial matrix protein. We found that the Mhr1 protein (Mhr1p) has activity to pair single‐stranded DNA and homologous double‐stranded DNA to form heteroduplex joints in vitro , and that mhr1‐1 causes the loss of this activity, indicating its role in homologous mtDNA recombination. While the majority of the mtDNA in the mother cells consists of head‐to‐tail concatemers, more than half of the mtDNA in the buds exists as genome‐sized monomers. The mhr1‐1 δ cce1 double mutant cells do not maintain any mtDNA, indicating the strict dependence of mtDNA maintenance on recombination functions. These results suggest a mechanism for mtDNA inheritance similar to that operating in the replication and packaging of phage DNA.