Electron microscope and physical chemical characterization of C-phycocyanin from fresh extracts of two blue-green algae

Cyanobacterial phycobilisomes

Cyanobacterial phycobilisomes harvest light and cause energy migration usually toward photosystem II reaction centers. Energy transfer from phycobilisomes directly to photosystem I may occur under certain light conditions. The phycobilisomes are highly organized complexes of various biliproteins and linker polypeptides. Phycobilisomes are composed of rods and a core. The biliproteins have their bilins (chromophores) arranged to produce rapid and directional energy migration through the phycobilisomes and to chlorophyll a in the thylakoid membrane. The modulation of the energy levels of the four chemically different bilins by a variety of influences produces more efficient light harvesting and energy migration. Acclimation of cyanobacterial phycobilisomes to growth light by complementary chromatic adaptation is a complex process that changes the ratio of phycocyanin to phycoerythrin in rods of certain phycobilisomes to improve light harvesting in changing habitats. The linkers govern the assembly of the biliproteins into phycobilisomes, and, even if colorless, in certain cases they have been shown to improve the energy migration process. The Lcm polypeptide has several functions, including the linker function of determining the organization of the phycobilisome cores. Details of how linkers perform their tasks are still topics of interest. The transfer of excitation energy from bilin to bilin is considered, particularly for monomers and trimers of C-phycocyanin, phycoerythrocyanin, and allophycocyanin. Phycobilisomes are one of the ways cyanobacteria thrive in varying and sometimes extreme habitats. Various biliprotein properties perhaps not related to photosynthesis are considered: the photoreversibility of phycoviolobilin, biophysical studies, and biliproteins in evolution. Copyright 1998 Academic Press.

Biliprotein assemble in the disc-shaped phycobilisomes of Rhodella violacea. On the molecular composition of energy-transfering complexes (tripartite units) forming the periphery of the phycobilisome

Heterogeneous complexes with a molecular weight of about 790000 containing B-phycoerythrin (Bangiales phycoerythrin) and C-phycocyanin (Cyanophyceae phycocyanin) in a molar pigment ratio of 2:1 were isolated from purified, dissociated phycobilisomes. Electron microscopical investigations revealed structures of three discs aggregated face to face with an apparent distance of 1.5 nm between each disc. Two discs may represent phycoerythrin and one phycocyanin. The complexes are structurally identical with tripartite units of the phycobilisome periphery. Fluorescence data confirmed the integrity of isolated tripartite units. Excitation at 546 nm gives a fluorescence maximum at 644 nm, indicating intermolecular transfer of excitation energy from phycoerythrin to phycocyanin. Comparative subunit analyses and spectral data suggested that no allophycocyanin is present. Cross-linking experiments gave evidence for a polar arrangement of phycocyanin within the complex. This pigment itself is an aggregate of two smaller molecules each having a molecular weight of about 140000. Tripartite units contain all the phycoerythrin and phycocyanin of the phycobilisome. On this basis, a phycobilisome model is proposed which combines the aspects of biliprotein distribution, energy transfer and fine structure.

Some properties of allophycocyanin from a thermophilic blue-green alga

Allophycocyanin was purified from the extremely thermophilic blue-green alga Synechococcus lividus. It was shown to be more stable to thermal or urea denaturation than allophycocyanin from a mesophilic organisms. Its amino acid composition and spectroscopic response to pH were investigated. An analysis was made of the relatively low fluorescence polarization of allophycocyanin compared to that of a comparable sized aggregate of the biliprotein, C-phycocyanin. A rather speculative conclusion was reached that suggests that the lower polarization of allophycocyanin may be caused by orientations or positioning of the chromophores that are more favorable for intra-protein energy transfer.

Further evidence for a phycobilisome model from selective dissociation, fluorescence emission, immunoprecipitation, and electron microscopy

Phycobilisomes, isolated in 500 mM Sorensen's phosphate buffer pH 6.8 from the red alga, Porphyridium cruetum, were analyzed by selective dissociation at various phosphate concentrations. The results are consistent with a structural model consisting of an allophycocyanin core, surrounding by a hemispherical layer of R-phycocyanin, with phycoerythrin being on the periphery. Such a structure also allows maximum energy transfer. Intact phycobilisomes transfer excitation energy ultimately to a pigment with a fluorescence emission maximum at 675 nm. This pigment is presumed to be allophycocyanin in an aggreagated state. Uncoupling of energy transfer among the pigments, and physical release of the phycobiliproteins from the phycobilisome follow a parallel time-course; phycoerythrin is released first, followed by R-phycocyanin, and then allophycocyanin. In 55 mM phosphate buffer, the times at which 50% of each phycobiliprotein has dissociated are: phycoerythrin 40 min, R-phycocyanin 75 min, and allophycocyanin 140 min. The proposed arrangement of phycobiliproteins within phycobilisomes is also consistent with the results from precipitation reactions with monospecific antisera on intact and dissociated phycobilisomes. Anti-phycoertythrin reacts almost immediately with intact phycobilisomes, but reactivity with anti-R-phycocyanin and anti-allophycocyanin is considerably delayed, suggesting that the antigens are not accessible until a loosening of the phycobilsome structure occurs. Reaction wbilisomes, but is much more rapid in phycobilisomes of Nostoc sp. which contains 6-8 times more allophycocyanin. It is proposed that allophycocyanin is partially exposed on the base of isolated intact phycobilisomes of both algae, but that in P. cruentum there are too few accessible sites to permit a rapid formation of a precipitate with anti-allophyocyanin.

Comparison of the biliproteins from two strains of the thermophilic cyanophyte Synechococcus lividus

C-Phycocyanins from two thermophilic strains of Synechococcus lividus that grow within different temperature ranges have been shown to be unalike. The aggregation ability of these two C-phycocyanins in sedimentation-velocity experiments varied dramatically. Surprisingly, the aggregation properties of mesophilic C-phycocyanins were found to lie between those of the two thermophilic proteins. Under identical conditions at pH7.0, one thermophilic protein (Sy I) was composed of 17S and larger aggregates, whereas the other (Sy III) was an almost homogeneous 6S aggregate. Mesophilic C-phycocyanins have a mixture of 6S, 11S and less stable 17S aggregates under these conditions. Amino acid analysis, absorption spectra, immunochemistry and fluorescence polarization all indicated differences in the composition and properties of the thermophilic proteins, which suggest that they have different modes of adaptation to very high temperatures. Allophycocyanins from the two strains of S. lividus were also purified and studied, but unlike the C-phycocyanins no major differences were found between them. Allophycocyanin was homogeneous at pH6.0, with a sedimentation coefficient of 5.54S and mol.wt. 1.03x10(5), as determined by sedimentation-equilibrium measurements.