Mitochondrial Ribosomes

Mitochondrial ribosomes have been called mitoribosomes to distinguish them from cytoplasmic ribosomes or cytoribosomes. Mitoribosomes occur in a wide variety of forms, the predominant form in multicellular organisms being the 55S ribosomes.

Subclasses. 50-60S mitoribosomes have been demonstrated in rate, rabbits, pigs, cows and hamsters. They also occur in chicken, sharks and locusts. The sedimentation coefficients of verbebrate mitoribosomes range from 54S to 61S. Since the average is closer to 55S than 60S, the ribosomes are called 55S ribosomes to distinguish them from the 70S bacterial ribosomes and the 80S cytoplasmic ribosomes. The 55S mitoribosome consists of a small 30S subunit and a large 40S subunit.
Size. Earlier reports indicated that the 55S ribosomes were significantly smaller than the 80S ribosomes.

RNA content. The small subunit of the mitoribsome contain 12-13S rRNA and the large subunit 16-17S rRNA. The RNA content of mitoribosomes is much less than that of cytoribosomes. They contain only about half the RNA of bacterial ribosomes. Mitoribosomal RNAs have a comparatively low G+C content (30-40%), unlike most ribosomal RNAs. Mitochondrial RNA also generally has less secondary structure, as is shown by its relatively lower content of methylated bases, in comparison to other ribosomal RNAs.

Ribosomal proteins. A distinguishing feature of animal mitoribosomes is that they are rich in proteins. Xenopus mitoribosomes were shown to have 84 different proteins, 40 in the lage subunit and 44 in the small subunit.

Protein synthesis. Mitochondria contain all the components to support protein synthesis. Estimates of the number of different proteins synthesized range from 1 or 2 to 10-15. The proteins synthesized are relatively low molecular weight precursor polypeptides.

Homology between mitoribosomes and bacterial ribosomes. To support the theory of the end symbiotic origin of mitochondria several homologies between the protein synthesis of mitochondria and bacteria have been cited. The diversity of mitoribosomal types is, however, so great that it is not possible to consider mitoribosomes homologous to bacterial ribosomes solely on the basis of size or S values.
 
A number of homologies, however, do exist between the two types of ribosomes, which are not found in cytoribosomes of higher cells. These include:

1) Suceptibility to antibiotics like chloraphenicol, erythromycin, tylosin, venanamycin, etc.

2) Similarities in physical properties like sedimentation coefficient, molecular weights and buoyant densities.

3) Requirements of relatively high levels of Mg++ for stability.

4) Dissociation of both ribosomal types by initiating factor IF – 3.

5) Involvement of fMet-tRNA in protein synthesis initiation.

6) Interchangeability of elongation factors EF-G and EF-T. The differences as well as similarities between mitoribosomes and bacterial ribosomes suggest that the different mitoribosomal types may have arisen from separate endosymbioses.