REDUCED DOSAGE OF GENES ENCODING RIBOSOMAL-PROTEIN S18 SUPPRESSES A MITOCHONDRIAL INITIATION CODON MUTATION IN SACCHAROMYCES-CEREVISIAE

A yeast mitochondrial translation initiation codon mutation affecting the gene for cytochrome oxidase subunit III (COX3) was partially suppr essed by a spontaneous nuclear mutation. The suppressor mutation also caused cold-sensitive fermentative growth on glucose medium. Suppressi on and cold sensitivity resulted from inactivation of the gene product of RPS18A, one of mio unlinked genes that code the essential cytoplas mic small subunit ribosomal protein termed S18 in yeast. The two S18 g enes differ only by 21 silent substitutions in their exons; both are i nterrupted by a single intron after the 15th codon. Yeast S18 is homol ogous to the human S11 (70% identical) and the Escherichia coli S17 (3 5% identical) ribosomal proteins. This highly conserved family of ribo somal proteins has been implicated in maintenance of translational acc uracy and is essential for assembly of the small ribosomal subunit. Ch aracterization of the original rps18a-1 missense mutant and rps18a Del ta and rps18b Delta null mutants revealed that levels of suppression, cold sensitivity and paromomycin sensitivity all varied directly with a limitation of small ribosomal subunits. The rps18a-1 mutant was most affected, followed by rps18a Delta then rps18b Delta. Mitochondrial m utations that decreased COX3 expression without altering the initiatio n codon were not suppressed. This allele specificity implicates mitoch ondrial translation in the mechanism of suppression. We could not dete ct an epitope-tagged variant of S18 in mitochondria. Thus, it appears that suppression of the mitochondrial translation initiation defect is caused indirectly by reduced levels of cytoplasmic small ribosomal su bunits, leading to changes in either cytoplasmic translational accurac y or the relative levels of cytoplasmic translation products.