The phycobiliproteins of the blue-green algae Synechococcus sp. and Aphanocapsa sp. were characterized with respect to homogeneity, isoelectric point, and subunit composition. Each of the biliproteins consisted of two different noncovalently associated subunits, with molecular weights of about 20,000 and 16,000 for phycocyanin, 17,500 and 15,500 for allophycocyanin, and 22,000 and 20,000 for phycoerythrin. Covalently bound chromophore was associated with each subunit.The phycobiliproteins-phycoerythrin, phycocyanin, and allophycocyanin-function as accessory photosynthetic pigments both in the prokaryotic Cyanophyta and in the eukaryotic Rhodophyta and Cryptophyta (1), a biological distribution that makes them valuable chemical markers for the evolutionist. As a preliminary to more detailed chemical and immunological studies, we have ascertained the subunit structure of all three types of phycobiliproteins isolated from blue-green algae.When freshly extracted from the cell under nondenaturing conditions, phycocyanin and phycoerythrin exist predominantly as dissociable aggregates of high molecular weight: reported values range from 220,000 to 340,000 (1-5) for phycoerythrin and from 180,000 to 360,000 for phycocyanin (6-9). The lowest molecular weight components observed under nondenaturing conditions (assumed to be monomers) are reported to have molecular weights of about 40,000 (10, 11) and 23,000 (11) for phycoerythrin, and about 30,000 for phycocyanin (6,9,12). Kao and Berns (12) have reported the molecular weight of the fundamental subunit of cyanophytan phycocyanins in denaturing solvents to be 30,000. Vaughan (4) has shown that the phycoerythrin of the red alga, Ceramium rubrum, consists of two subunits of molecular weight about 17,000, each carrying chromophore(s). No information is available on the subunit structure of allophycocyanin.In the present study, the phycobiliproteins of two unicellular blue-green algae, with DNAs differing by about 20 mol% in G + C content, have been characterized with respect to homogeneity, isoelectric point, and subunit composition.
MATERIALS AND METHODS Preparation of phycobiliproteinsThe source, culture conditions, and properties of the strains used here have been described elsewhere (13). Unless otherwise specified, all operations were performed in sodium acetate buffers that contained 1 mM j3-mercaptoethanol at pH 5.5.Washed Synechococcus sp. (strain 6301) cells were suspended in buffer (1 g wet weight per 10 ml) and broken in a French pressure cell at 20,000 lb/in2 (1330 atm). The suspension was centrifuged at 20,000 X g for 20 min. The supernatant was decanted, and the pellet was washed exhaustively with buffer.The pooled supernatants were brought to 60% saturation with (NH4)2SO4 and left at 4°C for 18 hr. The resulting precipitate from 2 g of cells was dissolved in the minimum volume of buffer, dialyzed against 0.05 M buffer, and applied to a 100 X 2.5 cm column of Sephadex G-200. The fractions with the highest A622/A.a0 (phycocyanin) and A652/An2o (allophyco...