The PET122 protein is one of three Saccharomyces cerevisiae nuclear gene products required specifically to activate translation of the mitochondrially coded COX3 mRNA. We have previously observed that mutations which remove the carboxy-terminal region of PET122 block translation of the COX3 mRNA but can be suppressed by unlinked nuclear mutations in several genes, two of which have been shown to code for proteins of the small subunit of mitochondrial ribosomes. Here we describe and map two more new genes identified as allele-specific suppressors that compensate for carboxy-terminal truncation of PET122. One of these genes, MRP17, is essential for the expression of all mitochondrial genes and encodes a protein of M(r) 17343. The MRP17 protein is a component of the small ribosomal subunit in mitochondria, as demonstrated by the fact that a missense mutation, mrp17-1, predicted to cause a charge change indeed alters the charge of a mitochondrial ribosomal protein of the expected size. In addition, mrp17-1, in combination with some mutations affecting another mitochondrial ribosomal protein, caused a synthetic defective phenotype. These findings are consistent with a model in which PET122 functionally interacts with the ribosomal small subunit. The second new suppressor gene described here, PET127, encodes a protein too large (M(r) 95900) to be a ribosomal protein and appears to operate by a different mechanism. PET127 is not absolutely required for mitochondrial gene expression and allele-specific suppression of pet122 mutations results from the loss of PET127 function: a pet127 deletion exhibited the same recessive suppressor activity as the original suppressor mutation. These findings suggest the possibility that PET127 could be a novel component of the mitochondrial translation system with a role in promoting accuracy of translational initiation.