PRIMARY STRUCTURE AND EVOLUTION OF CYTOCHROME C

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Abstract
The amino acid sequences of the cytochromes c from horse, man, pig, rabbit, chicken, tuna, and baker''s yeast show extensive identities. It is concluded that these proteins are all homologous structures in the evolutionary sense. Some residues and sequences in the peptide chains are invariant, while others are highly variable. Evolution exhibits a considerable degree of conservatism with regard to the remarkable clusters of basic and of hydrophobic residues in these proteins. The cytochromes c from relatively closely related species show few differences, while those from phylogenetically distant species are more widely dissimilar. Particular residues in certain positions appear to be characteristic of certain species or groups of closely related species. Considering only the number of variant residues, the proteins from individual species of one class, within relatively narrow limits, are probably all equally differentf rom those of another. The results of such comparisons are consistentwith the common-ly accepted over-all scheme of evolution. The extent of variation of the pri-mary structure of the various cytochromes c may give rough approximations of the time elapsed since the lines of evolution leading to any two species diverged. The factors introducing errors in such estimates are discussed. Cytochrome c is a protein which has accumulated mutations at a relatively slow rate. The confrontation of completely determined ribonucleic acid "codes" for amino acid residues in proteins with the amino acid sequence of a large enough set of homologous cytochromes c can be expected to yield an estimate of the primary structure of the primordial cytochrome c along one or more lines of evolution, as well as predictions of the particular residues occurring in certain positions in intermediate, extant, or extinct species.

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