Translation of mitochondrial mRNAs in Saccharomyces cerevisiae depends on mRNA-specific translational activators that recognize the 5' untranslated leaders (5'-UTLs) of their target mRNAs. We have identified mutations in two new nuclear genes that suppress translation defects due to certain alterations in the 5'-UTLs of both the COX2 and COX3 mRNAs, indicating a general function in translational activation. One gene, MRP21, encodes a protein with a domain related to the bacterial ribosomal protein S21 and to unidentified proteins of several animals. The other gene, MRP51, encodes a novel protein whose only known homolog is encoded by an unidentified gene in S. kluyveri. Deletion of either MRP21 or MRP51 completely blocked mitochondrial gene expression. Submitochondrial fractionation showed that both Mrp21p and Mrp51p cosediment with the mitochondrial ribosomal small subunit. The suppressor mutations are missense substitutions, and those affecting Mrp21p alter the region homologous to E. coli S21, which is known to interact with mRNAs. Interactions of the suppressor mutations with leaky mitochondrial initiation codon mutations strongly suggest that the suppressors do not generally increase translational efficiency, since some alleles that strongly suppress 5'-UTL mutations fail to suppress initiation codon mutations. We propose that mitochondrial ribosomes themselves recognize a common feature of mRNA 5'-UTLs which, in conjunction with mRNA-specific translational activation, is required for organellar translation initiation.