Allophycocyanins were purified from diverse cyanobacteria and one rhodophytan alga (Cyanidium caldarium). The native proteins are trimeric molecules with the structure (alpha beta)3. Representative native allophycocyanins and their alpha and beta subunits were characterized with respect to molecular weight, amino acid composition, isoelectric point, absorption and fluorescence spectra and immunological properties. All of the allophycocyanins studied were strikingly similar with respect to each of these properties. Renatured alpha and beta subunits of allophycocyanin were distinct immunologically from each other, and both cross-reacted with the antiserum to the native protein. Trimeric allophycocyanin was readily reconstituted from the purified alpha and beta subunits. Formation of hybrid allophycocyanins was demonstrated by direct isolation and characterization of the hybrid proteins and by immunological techniques. The results support the view that allophycocyanins are a highly conserved group of proteins.

Amino-terminal sequence determinations are re orted of the subunits of biliproteins of prokaryotic unicel ular and filamentous cyanobacteria and of eukaryotic unicellular red algae. The biliproteins examined, allophycocyanin, -phycocyanin, R-phycocyanin, b-phycoerythrin, and phycoerythrocyanin, vary with respect to the chemical nature and the number and distribution of the bilin chromophores between the two dissimilar subunits. The amino-terminal sequences fall into two classes, "a-type" and ",-type", with a high degree of homology within each class.In those biliproteins where the number of bilin chromophores on the two subunits is unequal, the subunit with the greater number of chromophores has the j#-type amino-acid sequence.Extensive homology also exists between a-and #-type sequences, strongly supporting the view that these arose by gene duplication to give rise to the ancestral a-and a-type genes early in the evolution of the biliproteins. The subsequent generation of the various classes of biliproteins appears to be the result of further gene duplication of the aand a-type genes, ultimately to give rise to families of polypeptide chains of similar sequence, but varying in the number of chromophore attachment sites and the structure of the chromophores. The phycobiliproteins are the major light-harvesting pigments of photosystem II of cyanobacteria and Rhodophyta.

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