The nuclear gene NAM9 from Saccharomyces cerevisiae (Sc) codes for a protein which, on the basis of sequence homology, was previously postulated to be a mitochondrial (mt) equivalent of the Escherichia coli (Ec) S4 ribosomal protein (r-protein) [Boguta et al., Mol. Cell. Biol. 12 (1992) 402-412]. The mt-r character of the NAM9 product is now confirmed by cross-reaction with the antisera for the Sc mt r-proteins. The NAM9-1 mutation, characterized previously as the nuclear suppressor of some ochre mt mit - mutants, is found to be a single nucleotide substitution changing Ser 8 2 to Leu within the part of NAM9 corresponding to the S4 region involved in interaction with the 16S rRNA. This indicates that the mechanism of NAM9-1 suppression could be analogous to the suppression due to ram (ribosomal ambiguity) mutations in the Ec structural gene encoding r-protein S4. The NAM9-1 mutation leads also to defect in respiratory growth in the background of the wild-type mit + genome.