SUPPRESSION OF A DEFECT IN THE 5' UNTRANSLATED LEADER OF MITOCHONDRIAL COX3 MESSENGER-RNA BY A MUTATION AFFECTING AN MESSENGER RNA-SPECIFICTRANSLATIONAL ACTIVATOR PROTEIN

Translation of the Saccharomyces cerevisiae mitochondrial COX3 mRNA, e ncoding subunit III of cytochrome c oxidase, specifically requires the action of the nuclear gene products PET54, PET122, and PET494 at a si te encoded in the 612-base 5' untranslated leader. To identify more pr ecisely the site of action of the translational activators, we constru cted two large deletions of the COX3 mRNA 5' untranslated leader. Both deletions blocked translation without affecting mRNA stability. Howev er, one of the large deletions was able to revert to partial function by a small secondary deletion within the remaining 5' leader sequences . Translation of the resulting mutant (cox3-15) mRNA was still depende nt on the nuclear-encoded specific activators but was cold sensitive. We selected revertants of this mitochondrial mutant at low temperature to identify genes encoding proteins that might interact with the COX3 mRNA 5' leader. One such revertant carried a missense mutation in the PET122 gene that was a strong and dominant suppressor of the cold-sen sitive defect in the mRNA, indicating that the PET122 protein interact s functionally (possibly directly) with the COX3 mRNA 5' leader. The c ox3-15 mutation was not suppressed by overproduction of the wild-type PET122 protein but was very weakly suppressed by overproduction of PET 494 and slightly better suppressed by co-overproduction of PET494 and PET122.