THE NAM9-1 SUPPRESSOR MUTATION IN A NUCLEAR GENE ENCODING RIBOSOMAL MITOCHONDRIAL PROTEIN OF SACCHAROMYCES-CEREVISIAE

The nuclear gene NAM9 from Saccharomyces cerevisiae (Sc) codes for a p rotein which, on the basis of sequence homology, was previously postul ated 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 confir med 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 subs titution changing Ser(82) to Leu within the part of NAM9 corresponding to the S4 region involved in interaction with the 16S rRNA. This indi cates that the mechanism of NAM9-1 suppression could be analogous to t he suppression due to ram (ribosomal ambiguity) mutations in the Ec st ructural 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.