Evolution of the mitochondrial protein synthetic machinery.

Comparative analysis of the components of the mitochondrial translational apparatus reveals a remarkable variability. For example the mitochondrial ribosomal rRNAs, display a three-fold difference in size in different organisms as a result of insertions or deletions, which affect specific areas of the rRNA molecule. This suggests that such areas are either not essential for mitoribosome function or that they can be replaced by proteins. Also mitochondrial tRNAs and mitoribosomal proteins are much less conserved than their cytoplasmic counterparts. Not only do the mitochondrial translational molecules vary in properties, also the location of the genes from which they are derived is not the same in all cases: mitochondrial tRNA genes which usually are found in the mtDNA, may have a nuclear location in protozoa and, conversely, only in fungi one finds a mitoribosomal protein gene in the organellar genome. The high rate of change of the components of the mitochondrial protein synthesizing machinery is accompanied by a number of unique features of the translation process: (i) the mitochondrial genetic code differs substantially from the standard code in a species-specific manner; (ii) special codon-anticodon recognition rules are followed; (iii) unusual mechanisms of translational initiation may exist. These observations suggest that the evolutionary pressures that have shaped the present day mitochondrial translational apparatus have been different in different organisms and also distinct from those acting on the cytoplasmic machinery. In spite of the interspecies variability, however, many features of the mitochondrial and bacterial protein synthetic apparatus show a clear resemblance, providing support for the hypothesis of a prokaryotic endosymbiont ancestry of mitochondria.