Mitochondrial gene expression in saccharomyces cerevisiae. I. Optimal conditions for protein synthesis in isolated mitochondria

Phenotypic suppression and nuclear accommodation of the mit- oxi1-V25 mutation in isolated yeast mitochondria

Phenotypic suppression by the antibiotic, paromomycin, of the mitochondrial oxi1- -V25 mutation, a mutation which arrests by premature ochre codon the synthesis of the cox II subunit, was studied in isolated yeast mitochondria competent in translation. This antibiotic is known to suppress the mutation in vivo (Dujardin et al. 1984) and allowed in vitro, at concentrations of 20-1100 micrograms per ml. the synthesis of the cox II subunit. This strongly suggests that phenotypic suppression of mit- mutations is due to the direct action of paromomycin on mitochondrial ribosomes. The effect of paromomycin bears a resemblance to the function of the omnipotent nuclear suppressor mutation R705. The nuclear suppression was expressed in isolated mitochondria; suppressor mutation influenced the structure of the mitoribosome. Therefore, it appears that mitoribosomes are indeed the common target in the phenotypical and genetic nuclear suppression of the oxi1-V25 mutation.

Protein synthesis in mitochondria from yeast strains carrying nam and mim suppressor genes

Yeast mitochondria isolated from two different wild type strains (gal+ and gal-), whether grown on galactose or glucose, synthesize all mitochondrial polypeptides with similar efficiencies and in proportions approximating those detected in vivo. Mitochondria isolated from mit- mutants synthesize in vitro a mutant pattern of mitochondrial proteins, indistinguishable from the in vivo products. The mutant pattern is restored to the wild type one in mitochondria isolated from pseudorevertant strains carrying an additional nuclear (nam3-1 and R705) or mitochondrial (mim3-1) informational suppressor gene. Suppression is expressed in isolated mitochondria without the obligatory presence of cytosol at the level of both respiratory control and specific polypeptide synthesis. Translation in isolated mitochondria is sensitive to paromomycin. The antibiotic differentiates between translation in mitochondria from wild type strains and that in nam-type gene carrying strains. This strongly suggests that nam-type mutations affect the mitoribosome, enhancing ambiguity of translation, thus allowing for the pseudoreversion of mit- phenotypes.

Mitochondrial DNA expression in Drosophila melanogaster: neosynthesized polypeptides in isolated mitochondria

The expression of mitochondrial genome of D. melanogaster in isolated mitochondria was followed by incorporation of 35S methionine in neosynthesized polypeptides. A high level of protein synthesis was obtained after optimization of all the incubation parameters. Two kinds of energy-generating systems were used: an endogenous system where an oxidizable substrate were added for ATP synthesis; an exogenous system with an energy-rich compound for ATP regeneration, the latter proved to be the most effective. The effect of the oxidative phosphorylation uncoupler (Clccp), and an ATPase inhibitor (oligomycine) allow us to postulate the role of the electrochemical potential in the expression of the mitochondrial genome. Electrophoresis and autoradiography of neosynthesized mitochondrial proteins exhibits 18 to 24 protein bands, ranging from 6.5 to 65 Kd; incubation of KC 0% drosophila cells with 35S methionine and cycloheximide gave similar results. Both our results and those published elsewhere suggest that the expression of mitochondrial genome in higher organisms could be more complex than simple translation of the 13 genes presents on these genomes.