A NUCLEAR MUTANT OF SACCHAROMYCES-CEREVISIAE DEFICIENT IN MITOCHONDRIAL DNA REPLICATION AND POLYMERASE ACTIVITY

We have isolated a thermosensitive mutant which is transformed into a population of cells devoid of mitochondrial DNA (rho0 cells) at 35 °C and is deficient in mitochondrial (mt) DNA polymerase activity. A single recessive nuclear mutation (mip1) is responsible for rho0 phenotype and mtDNA polymerase deficiency in vitro. At 25 °C (or 30 °C) a dominant suppressor mutation (SUP) masks the deficiency in vivo. The meiotic segregants (mip1 sup) which do not harbor the suppressor have a rho0 phenotype both at 25 and 35 °C. They have no mtDNA polymerase activity, in contrast with MIP rho0 mutants of mitochondrial inheritance which do exhibit mtDNA polymerase activity. In the thermosensitive mutant (mip1 SUP), the replication of mtDNA observed in vivo at 30 °C is completely abolished at 35 °C. In the meiotic segregants (mip1 sup), no mtDNA replication takes place at 30 and 35 °C. The synthesis of nuclear DNA is not affected. DNA polymerases may have replicative and/or repair activity. There is no evidence that mip mutants are deficient in mtDNA repair. In contrast the MIP gene product is strictly required for the replication of mtDNA and for the expression of the mtDNA polymerase activity. This enzyme might be the replicase of mtDNA.