Genetic evidence that different functional domains of the PET54 gene product facilitate expression of the mitochondrial genes COX1 and COX3 in Saccharomyces cerevisiae.

Expression of the yeast mitochondrial genes COX1 and COX3, which encode subunits I and III of cytochrome oxidase, respectively, is controlled by a common nuclear-encoded trans-acting factor. This protein, encoded by the PET54 gene, controls expression of COX1 at the level of RNA splicing and COX3 at the level of mRNA translation. While the steps of COX1 and COX3 gene expression affected by the PET54 gene product are different, it is possible that the PET54 protein is monofunctional and affects expression of each gene by a single mechanism, such as modulation of RNA secondary structure. The goal of this study was to address whether the PET54 protein is monofunctional or multifunctional with respect to its role in COX1 and COX3 gene expression. Ten insertion mutations, which each resulted in the in-frame addition of four amino acids within the PET54 polypeptide, were generated, and the resulting mutants were characterized for respiration phenotype and mitochondrial gene expression. Five of the ten mutants were respiration deficient. Two of these five mutants were defective in expression of COX3 but not in expression of COX1, while two other mutants had the opposite phenotype (primarily defective in expression of COX1). The fifth mutant was equally defective in expression of both genes. These results demonstrate that the two functions of PET54 are genetically separable and support the idea that the PET54 protein is multifunctional.