A genetic link between an mRNA-specific translational activator and the translation system in yeast mitochondria

Translation of the Saccharomyces cerevisiae mitochondrial mRNA encoding cytochrome c oxidase subunit III (coxIII) specifically requires the products of at least three nuclear genes, PET122, PET494 and PET54. pet122 mutations that remove 24-67 amino acid residues from the carboxyterminus of the gene product were found to be suppressed by unlinked nuclear mutations. These unlinked suppressors fail to suppress both a pet122 missense mutation and a complete pet122 deletion. One of the suppressor mutations causes a heat-sensitive nonrespiratory growth phenotype in an otherwise wild-type strain and reduces translation of all mitochondrial gene products in cells grown at high temperature. This suppressor maps to a newly identified gene on chromosome XV termed PET123. The sequence of a DNA fragment carrying PET123 contains one major open reading frame encoding a basic protein of 318 amino acids. Inactivation of the chromosomal copy of PET123 by interruption of this open reading frame causes cells to become rho- (sustain large deletions in their mtDNA). This phenotype is characteristic for null alleles of genes whose products are essential for general mitochondrial protein synthesis. Thus our data strongly suggest that the PET123 protein is a component of the mitochondrial translation apparatus that interacts directly with the coxIII-mRNA-specific translational activator PET122.