Mitochondrial assembly in respiration-deficient mutants of Saccharomyces cerevisiae. IV. Effects of nuclear amber suppressors on the accumulation of a mitochondrially made subunit of cytochrome c oxidase.

Earlier studies from this laboratory have shown that cytochrome c oxidase from bakers' yeast contains seven subunits, three of which are made in the mitochondrion (Mason, T. L., and Schatz, G. (1973) J. Biol. Chem. 248, 1355). Moreover, a cytochrome c oxidase-less yeast mutant (pet 494-1) was isolated which lacked one of the mitochondrially made subunits (Ebner, E., Mason, T. L., and Schatz, G. (1973) J. Biol. Chem. 248, 5369). Surprisingly, the mutated gene was localized in the nucleus. The results presented here demonstrate that this mutant phenotype can be suppressed by nuclear amber suppressors which affect translation on cytoplasmic ribosomes. This fact was established by two methods, (a) By constructing pet 494-1 strains possessing various amber and ochre markers, isolating respiring revertants from these strains, and demonstrating co-reversion of the amber (but not of the ochre) markers. (b) By coupling the pet 494-1 allele with the well characterized amber suppressor gene SUP 4-3. These data show that suppressor genes located on nuclear chromosomes may control the accumulation of a mitochondrially synthesized polypeptide. The present results also allow some tentative conclusions about the mechanism of the pet 494 mutation. Because it is highly unlikely that the cytoplasmic and the mitochondrial translation system share a common suppressor, the pet 494 locus probably does not code for the missing mitochondrially made subunit, but for a cytoplasmically made protein. This as yet unidentified protein seems to control the synthesis or the integration of the mitochondrially made subunit. Nuclear suppressor genes may thus be useful tools for studying the role of cytoplasmic protein synthesis in mitochondrial formation.