The Saccharomyces cerevisiae nuclear gene MRP-S9 was identified as part of the European effort in sequencing chromosome II. MRP-S9 encodes for a hydrophilic and basic protein of 278 amino acids with a molecular mass of 32 kDa. The C-terminal part (aa 153–278) of the MRP-S9 protein exhibits significant sequence similarity to members of the eubacterial and chloroplast S9 ribosomal-protein family. Cells disrupted in the chromosomal copy of MRP-S9 were unable to respire and displayed a characteristic phenotype of mutants with defects in mitochondrial protein synthesis as indicated by a loss of cytochrome c oxidase activity. Additionally, no activities of the gluconeogenetic enzymes, fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase, could be observed under conditions of glucose de-repression. The respiration-deficient phenotype could not be restored by transformation of the disruption strain with a wild-type copy of MRP-S9, indicating that MRP-S9 disruption led to rho- or rhoo cells. Sequence similarities of MRP-S9 to other members of the ribosomal S9-protein family and the phenotype of disrupted cells are consistent with an essential role of MRP-S9 is assembly and/or function of the 30s subunit of yeast mitochondrial ribosomes.