Ribosome in Baker's Yeast Mitochondria are Heterogeneous

Ribosomes are macromolecular machines of conveyor type, which move along the mRNA from triplet to triplet and polymerize cognate amino acids. They are regarded as uniform entities with constant molecular composition bearing no regulatory capacity. However, their ability to interact with multiple proteins involved in translation suggests existence of specialized ribosomes dedicated to biosynthesis of particular proteins. This can be easily imagined in yeast mitochondria, whose genomes encode eight polypeptides, and where protein-specific translation is already represented by translational activators - a group of proteins each regulating a single mRNA translation. Despite this, the subject remains poorly investigated. We report an exploratory approach to search for distinct populations of ribosomes in the yeast mitochondria. Affinity tags were added to mitochondrial ribosomal proteins, ribosomes were isolated by immunoprecipitation and their protein and RNA content was characterized. It was shown that the mitochondrial ribosomes isolated from yeast strains bearing affinity tags on different ribosomal proteins recover different sets of components. The differences were rather quantitative than qualitative, because almost full sets of mitochondrial ribosomal proteins were identified in each sample, but the ratios demonstrated variance ranging from 10 to less than 0.05 molecules per ribosome. In addition, we explored the power of translational activators as a bait to recover ribosomes translating specific mRNAs in yeast mitochondria.