Mitochondrial ribosomal RNA genes of yeast: their mutations and a common nuclear suppressor

Due to the absence of repetition of the rRNA genes in S. cerevisiae mitochondria, isolation of ribosomal mutants at the level of the rRNA genes is relatively easy in this system. We describe here a novel thermosensitive mutation, ts1297, localized by rho- deletion mapping in (or very close to) the sequence corresponding to the small ribosomal RNA (15S) gene. Defective mutations of the small rRNA have not been reported so far. In the mutant, the amount of 15S rRNA and of the small ribosomal subunit, 37S, is reduced. The quantity of the large ribosomal RNA (21S), directly extracted from mitochondria, appears normal. However, the large ribosomal subunit, 50S, seems to be fragile and could be recovered only in the presence of Ca2+ in place of Mg2+. The 50S particles seem to be completely degraded under normal conditions of extraction with Mg2+. The thermosensitive phenotype of the ts1297 mutant is suppressed by a nuclear mutation SU101. The SU101 mutation had been originally isolated as a suppressor of another mitochondrial mutation, ts902, which is located within the 21S rRNA gene. These results suggest that the mitochondrial mutations ts1297 and ts902 are both involved in the interaction of the large and small ribosomal subunits.