Mss51p, a putative translational activator of cytochrome c oxidase subunit-1 (COX1) mRNA, is required for synthesis of Cox1p in Saccharomyces cerevisiae

Mutants of Saccharomyces cerevisiae that lack a functional MSS51 gene are r espiratory deficient due to the absence of cytochrome c oxidase subunit 1 ( Cox1p). It has been previously suggested, but not formally proven, that Mss 51p is required for translational activation of COX1 mRNA, rather than bein g involved in a subsequent step in the synthesis of Cox1p or its assembly i nto cytochrome c oxidase. Pulse-chase labelling experiments now show that t he absence of detectable levels of Cox1p in mss51-null strains is indeed du e to the lack of synthesis of Cox1p, and is not caused by reduced stability of the protein. To gain more insight into the exact function of Mss51p, we determined the subcellular localization of the protein. We were able to sh ow that an epitope-tagged version of Mss51p (Mss51HA) complements the mutat ion and can be localized in mitochondrial where it is firmly associated wit h the mitochondrial inner membrane. In addition, we characterized the previ ously identified mutant allele mss51-3. Sequence analysis revealed the pres ence of a short open reading frame upstream of MSS51 resulting from the cre ation of an extra ATG startcodon.