Amino Acid Sequences of Cytochrome C and Plastocyanins in Phylogenetic Studies of Higher Plants

Abstract

Boulter, Donald (Dept. of Botany, University of Durham, Durham, England) 1974. Amino Acid Sequences of Cytochrome c and Plastocyanins in Phylogenetic Studies of Higher Plants. Syst. Zool. 22:549-553.-The yields of cytochrome c from different types of plant organs is given and the use of cytochrome c sequence data for phylogenetic studies is briefly discussed. Sequences of cytochrome c of nineteen angiosperm species and a gymnosperm have been used to construct a molecular tree. This tree is discussed in relationship to existing phylogenetic schemes. The preliminary results of investigations on plastocyanin are mentioned. During the last two decades, chemical data on plants have accumulated at an ever increasing rate. Much of this data is now being collected in a way which is useful to systematists and is being critically evaluated by them. Macromolecular data on plants are still sparse however, and consequently there has not been very much debate as to their relevance for systematics. The present paper briefly presents the methods used by the Durham group and their latest results; these are discussed in relationship to the major phylogenetic schemes. Species and gene phylogenies are constructed by using homologous proteins, i.e., those specified by genes with a common ancestor. However, in assembling a set of protein sequences to be compared, it is necessary first of all to establish that they are a homologous set; similarities between sequences may also arise during evolution, either because of convergence of unrelated proteins, due to similar residues in the sequences having a common function, or because of chance. For the plant cytochromes discussed in the present paper, homology has been established, (1) by the use of the semi-rigorous methods devised by Fitch and his co-workers (Fitch and Margoliash, 1967a; Fitch and Markowitz, 1970), and (2) by constructing a model of plant cytochromes c using the a-carbon x-ray diffraction data of horse heart and bonito ferricytochrome c obtained by Dickerson et al. (1971), in conjunction with the sequences of fourteen plant cytochromes. This model indicates that the three-dimensional structure of plant cytochromes c is essentially the same as that of animal cytochromes c. Since about thirty residues are essential for function, the only reason the other seventy or so residues should occupy the same three-dimensional space in the different plant and animal molecules, is by virtue of their common ancestry. The proof of this suggestion must await the accurate determination of the three-dimensional structure of plant cytochrome c by x-ray diffraction methods. Klein and Cronquist (1967) have classified c-type cytochromes into three groups: firstly, those with low redox potentials, which function in anaerobic energy-yielding reactions as reductases; secondly, those with higher redox potentials which function in photo-reductive processes, such as photosynthesis; and thirdly, those which function in aerobic energy-yielding reactions. These types can usually be distinguished one from another by their a-absorption spectra, molecular weights and redox potentials, although in the procaryotes (blue-green algae and bacteria) the relationship of different c-type cytochromes is not always clear, and probably will not be so until more sequences have been determined. Cytochromes from two different groups cannot be used to construct species phylogenies, thus, the cytochrome sequences dis-