cDNA and deduced amino acid sequences of cytochrome c fromChlamydomonas reinhardtii: Unexpected functional and phylogenetic implications

Summary We have isolated complementary DNA (cDNA) clones for apocytochrome c from the green algaChlamydomonas reinhardtii and shown that they are encoded by a single nuclear gene termedcyc.Cyc mRNA levels are found to depend primarily on the presence of acetate as a reduced carbon source in the culture medium. The deduced amino acid sequence shows that, apart from the probable removal of the initiating methionine,C. reinhardtii apocytochrome c is syntheszed in its mature form. Its structure is generally similar to that of cytochromes c from higher plants. Several punctual deviations from the general pattern of cytochrome c sequences that is found in other organisms have interesting structural and functional implications. These include, in particular, valines 19 and 39, asparagine 78, and alanine 83. A phylogenetic tree was constructed by the matrix method from cytochrome c data for a representative range of species. The results suggest thatC. reinhardtii diverged from higher plants approximately 700–750 million years ago; they also are not easy to reconcile with the current attribution ofChlamydomonas reinhardtii andEnteromorpha intestinalis to a unique phylum, because these two species probably diverged from one another at about the same time as they diverged from the line leading to higher plants.