Observations leading to the discovery of mitochondrial ribosomes are recounted. Mammalian mitochondrial ribosomes, the first to be discovered (O’Brien and Kalf, 1967 a,b), are 55S ribosomes, with a sedimentation rate much slower than that of 70S bacterial or 80S eukaryotic cytoplasmic ribosomes. This result was not widely received because of the prevailing notion that mitochondrial ribosomes would resemble bacterial 70S ribosomes, reminiscent of their ancestral prokaryotic origins. After rigorous purification procedures to eliminate contaminating microsomal ribosomes (O’Brien and Kalf, 1967a) and further studies to confirm this finding (O’Brien, 1971) the 55S mitochondrial ribosome emerges as a ribosome with properties remarkably different from bacterial and eukaryotic, cytoplasmic ribosomes (O’Brien, 2003).
Despite their lower sedimentation coefficient, the 55S mitoribosomes are physically larger than bacterial 70S ribosomes. Their mass is greater and so also are their physical dimensions (Hamilton and O’Brien (1974). While larger than bacterial ribosomes, they actually contain smaller RNAs (Andersen, et al., 1981). Their increased size is due to the presence of several extra proteins which are unique to mitochondrial ribosomes (O’Brien, 2003). All of the MRPs are candidates for mitochondrial disease (Sylvester, et al., 2004).