Rapid evolutionary divergence of proteins in mammalian mitochondrial ribosomes

Abstract

Compared to cytoplasmic ribosomes (cytoribosomes), the mitochondrial ribosomes (mitoribosomes) of distantly related organisms are surprisingly diverse with respect to their physical and chemical properties [l] . Among more closely related organisms, such as the vertebrates, very little variation is seen in the overall structural properties of mitoribosomes. Nevertheless, in view of the general trend toward greater phylogenetic variation among mitoribosomes than among cytoribosomes, it seems likely that systematic differences among the mitoribosomes of these closely related species could be detected at a sufficiently detailed level of analysis, as for example in an electrophoretic analysis of the ribosomal proteins. Comparisons of the proteins of cytoribosomes by two-dimensional polyacrylamide gel electrophoresis have revealed that most or all of these proteins are electrophoretically indistinguishable in mammals, birds and reptiles [2-4]. Electrophoretic analyses of mitoribosomal proteins from rat [5,6] and bovine [7] liver have also been performed. However, different electrophoretic systems were used in these studies, so it is impossible to compare the electrophoretic properties of rat and bovine mitoribosomal proteins from these results. The present experiments permit a direct comparison, and a test of the proposition that mitochondrial ribosomes show a higher rate of evolutionary divergence than cytoplasmic ribosomes.