| Abstract: | We have isolated a thermosensitive mutant which is transformed into a population of cells devoid of mitochondrial DNA (rho 0 cells) at 35 degrees C and is deficient in mitochondrial (mt) DNA polymerase activity. A single recessive nuclear mutation (mip1) is responsible for rho 0 phenotype and mtDNA polymerase deficiency in vitro. At 25 degrees C (or 30 degrees C) a dominant suppressor mutation (SUP) masks the deficiency in vivo. The meiotic segregants (mip1 sup) which do not harbor the suppressor have a rho 0 phenotype both at 25 and 35 degrees C. They have no mtDNA polymerase activity, in contrast with MIP rho 0 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 degrees C is completely abolished at 35 degrees C. In the meiotic segregants (mip1 sup), no mtDNA replication takes place at 30 and 35 degrees 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. |
