Biliproteins of cyanobacteria and Rhodophyta: Homologous family of photosynthetic accessory pigments (phycobiliproteins/amino-terminal sequences/evolution/structure-function relationships)

Amino-terminal sequence determinations are reported of the subunits of biliproteins of prokaryotic unicellular and filamentous cyanobacteria and of eukaryotic unicellular red algae. The biliproteins examined, allophyco- cyanin, C-phycocyanin, R-phycocyanin, b?-phycoerythrin, and phycoerythrocyanin, vary with respect to the chemical na- ture and the number and distribution of the bilin chromo- phores between the two dissimilar subunits. The amino-ter- minal sequences fall into two classes, "a-type" and "dl-type", with a high degree of homology within each class. In those biliproteins where the number of bilin chromo- phores on the two subunits is unequal, the subunit with the greater number of chromophores has the p-type amino-acid sequence. Extensive homology also exists between a- and A-type se- quences, strongly supporting the view that these arose by gene duplication to give rise to the ancestral a- and d-type genes early in the evolution of the biliproteins. The subse- quent generation of the various classes of biliproteins ap- pears to be the result of further gene duplication of the a- and d-type genes, ultimately to give rise to families of poly- peptide chains of similar sequence, but varying in the num- ber of chromophore attachment sites and the structure of the