The pentatricopeptide repeat (PPR) proteins are a large family of proteins characterized by tandem repeats of a degenerate 35-amino-acid motif.PPR proteins play critical roles in all aspects of organellar RNA metabolism.Here we show that Ppr10, a newly identified fission yeast Schizosaccharomyces pombe PPR protein, associates with a novel PPR motif-containing protein that we called Ppr11.Both are located on the matrix side of the inner mitochondrial membrane.Deletion of either gene impaired mitochondrial respiration.However, deletion of ppr10 or ppr11 did not dramatically affect mitochondrial RNA levels.Analysis of mitochondrial protein synthesis revealed that deletion of either ppr10 or ppr11 severely impaired protein synthesis of mitochondrial-encoded cytochrome c oxidase (COX) subunits.Western blot analysis confirmed the impairment in mitochondrial translation in both deletion mutants.These findings demonstrate that both Ppr10 and Ppr11 may function together as specific activators for translation ofmitochondrial COX proteins.