Mitochondrial gene expression in saccharomyces cerevisiae. II. Fidelity of translation in isolated mitochondria from wild type and respiratory-deficient mutant cells.

The fidelity of mitochondrial translation has been examined in isolated yeast mitochondria incubated in an optimized protein-synthesizing medium (McKee, E. E., and Poyton, R. O., (1984) J. Biol. Chem. 259, 9320-9331). These studies have revealed: that isolated mitochondria synthesize bona fide mitochondrial gene products which are identical in kind and relative amounts to those synthesized in vivo; that mitochondria isolated from both mitochondrial mit- mutants and nuclear Pet mutants, which retain the capacity for mitochondrial protein synthesis, produce a mutant pattern of mitochondrial gene products which is similar to that produced in vivo; and that isolated mitochondria synthesize up to 7% of their protein mass in vitro at a rate of about one polypeptide bond/polypeptide chain/s. These studies also reveal that isolated wild type yeast mitochondria are competent in all steps in mitochondrial translation, including initiation. Using pactamycin as a specific inhibitor of translational initiation we have demonstrated that polypeptide chain initiation continues throughout a 60-min incubation period. By using this in vitro system to calculate the stoichiometry of synthesis of the major proteins coded by yeast mitochondrial DNA we have found that the var1 polypeptide is synthesized at a level which is significantly lower than all other mitochondrial gene products and that cytochrome c oxidase subunits I, II, and III and ATPase subunit 9 are synthesized in nearly equimolar amounts. These results suggest that the synthesis of these four gene products is controlled coordinately.