CHARACTERIZATION OF PHYCOCYANIN-DEFICIENT PHYCOBILISOMES FROM A PIGMENT MUTANT OF PORPHYRIDIUM SP. (RHODOPHYTA)

The structure and function of phycobilisomes in the rhodophyte Porphyridium sp. were investigated by comparing the properties of the wild type with a pigment mutant called C12. When grown under low light, cells of C12 were bright orange, while wild-type cells were deep red. The results obtained from a characterization of purified phycobilisomes of the mutant C12 led us to propose the existence in Porphyridium sp. phycobilisomes of two types of rods, some containing only phycoerythrin and others containing phycoerythrin bound to phycocyanin, which is in turn linked to the core by the linker L_RC. By studying the partitioning of phycobiliproteins between phycobilisomes and pools of free phycobiliproteins, we found that phycocyanin in the C12 mutant was only present in the pool of free proteins and that its specific linker, L_RC, was totally absent. Phycoerythrin was present in the free pool and in the purified phycobilisomes as well. One of the three specific phycoerythrin linkers γ was missing. In light of the fact that in the C12 mutant, the linker L_RC is absent and that there is no phycocyanin bound to the phycobilisomes, we propose that the rods in the mutant contain only phycoerythrin. These phycobilisomes are nevertheless functional and exhibit an efficient excitation transfer from phycoerythrin directly to allophycocyanin. Electron microscopy showed the purified phycobilisomes of C12 to be less dense than those of the wild type. This change was attributed to the disappearance of the rods containing the combination phycocyanin/phycoerythrin. Light still regulates phycobiliprotein synthesis in the mutant, as shown by the change in the color of the culture, which turned green-yellow when cells were shifted from low light to high light growth conditions. Light also regulates the structure of the phycobilisomes, which have fewer rods under high light growth conditions.     

Interactions of linker proteins with the phycobiliproteins in the phycobilisome substructures of <Emphasis Type="Italic">Gloeobacter violaceus</Emphasis>

Gloeobacter violaceus PCC 7421 is a unicellular oxygenic photosynthetic organism, which precedes the diversification of cyanobacteria in the phylogenetic tree. It is the only cyanobacterium that does not contain internal membranes. The unique structure of the rods of the phycobilisome (PBS), grouped as one bundle of six parallel rods, distinguishes G. violaceus from the other PBS-containing cyanobacteria. It has been proposed that unique multidomain rod-linkers are responsible for this peculiarly organized shape. However, the localization of the multidomain linkers Glr1262 and Glr2806 in the PBS-rods remains controversial (Koyama et al. 2006, FEBS Lett 580:3457–3461; Krogmann et al. 2007, Photosynth Res 93:27–43). To further increase our understanding of the structure of the G. violaceus PBS, the identification of the proteins present in fractions obtained from sucrose gradient centrifugation and from native electrophoresis of partially dissociated PBS was conducted. The identification of the proteins, after electrophoresis, was done by spectrophotometry and mass spectrometry. The results support the localization of the multidomain linkers as previously proposed by us. The Glr1262 (92 kDa) linker protein was found to be the rod-core linker L_RC ⁹², and Glr2806 (81 kDa), a special rod linker L_R ⁸¹ that joins six disks of hexameric PC. Consequently, we propose to designate glr1262 as gene cpcGm (encoding L_RC ⁹²) and glr2806 as gene cpcJm (encoding L_R ⁸¹). We also propose that the cpeC (glr1263) gene encoding L_R ^31.8 forms the interface that binds PC to PE.     

The phycocyanin-associated rod linker proteins of the phycobilisome of Gloeobacter violaceus PCC 7421 contain unusually located rod-capping domains

Gloeobacter violaceus PCC 7421 is a unique cyanobacterium that has no thylakoids and whose genome has been sequenced [Y. Nakamura, T. Kaneko, S. Sato, M. Mimuro, H. Miyashita, T. Tsuchiya, S. Sasamoto, A. Watanabe, K. Kawashima, Y. Kishida, C. Kiyokawa, M. Kohara, M. Matsumoto, A. Matsuno, N. Nakazaki, S. Shimpo, C. Takeuchi, M. Yamada, S. Tabata, Complete Genome Structure of Gloeobacter violaceus PCC 7421, a cyanobacterium that lacks thylakoids. DNA Research 10 (2003) 137-145]. Phycobilisomes of G. violaceus were isolated and analyzed by SDS-PAGE followed by N-terminal sequencing. Three rod-linker subunits (CpeC, CpeD and CpeE) were identified as predicted from the genome sequence. The cpcC1 and cpcC2 genes at order locus named (OLN) glr0950 and gll 3219 encoding phycocyanin-associated linker proteins from G. violaceus are 56 and 55 amino acids longer at the N-terminus than the open reading frame proposed in the genome. The two amino acid extensions showed a 66% identity to one another. Also, the N-terminal extensions of these sequences were similar to domains in both the rod-capping-linker protein CpcD2 and to the C-terminus domain of the phycoerythrin-associated linker protein CpeC. These domains are not only unusual in their N-terminal location, but are unusual in that they are more closely related in sequence similarity to the C-terminus domain of the phycoerythrin-associated linker, CpeC of G. violaceus, than to the C-terminus domain of phycocyanin-associated linker CpcC in other cyanobacteria. These linker proteins with unique special domains are indicators of the unusual structure of the phycobilisomes of G. violaceus.     

The phycocyanin-associated rod linker proteins of the phycobilisome of Gloeobacter violaceus PCC 7421 contain unusually located rod-capping domains

Gloeobacter violaceus PCC 7421 is a unique cyanobacterium that has no thylakoids and whose genome has been sequenced [Y. Nakamura, T. Kaneko, S. Sato, M. Mimuro, H. Miyashita, T. Tsuchiya, S. Sasamoto, A. Watanabe, K. Kawashima, Y. Kishida, C. Kiyokawa, M. Kohara, M. Matsumoto, A. Matsuno, N. Nakazaki, S. Shimpo, C. Takeuchi, M. Yamada, S. Tabata, Complete Genome Structure of Gloeobacter violaceus PCC 7421, a cyanobacterium that lacks thylakoids. DNA Research 10 (2003) 137-145]. Phycobilisomes of G. violaceus were isolated and analyzed by SDS-PAGE followed by N-terminal sequencing. Three rod-linker subunits (CpeC, CpeD and CpeE) were identified as predicted from the genome sequence. The cpcC1 and cpcC2 genes at order locus named (OLN) glr0950 and gll 3219 encoding phycocyanin-associated linker proteins from G. violaceus are 56 and 55 amino acids longer at the N-terminus than the open reading frame proposed in the genome. The two amino acid extensions showed a 66% identity to one another. Also, the N-terminal extensions of these sequences were similar to domains in both the rod-capping-linker protein CpcD2 and to the C-terminus domain of the phycoerythrin-associated linker protein CpeC. These domains are not only unusual in their N-terminal location, but are unusual in that they are more closely related in sequence similarity to the C-terminus domain of the phycoerythrin-associated linker, CpeC of G. violaceus, than to the C-terminus domain of phycocyanin-associated linker CpcC in other cyanobacteria. These linker proteins with unique special domains are indicators of the unusual structure of the phycobilisomes of G. violaceus.     

Factors influencing the phycobilisome rod composition of the cyanobacterium Synechococcus sp. PCC 7942: Effects of reduced phycocyanin content, lack of rod-linkers, and over-expression of the rod-terminating linker

Four novel mutants with altered phycobilisomes were constructed in the cyanobacterium Synechococcus 7942 to study factors influencing the rod length and composition. These mutants show (1) reduced phycocyanin content, (2) reduced phycocyanin content combined with loss of the 33 kDa linker, (3) loss of the 30 kDa rod-linker and (4) overexpression of the 9 kDa rod terminating linker. For these mutants we determined the 33 to 27 kDa and 30 to 27 kDa linker ratios in the isolated phycobilisomes and compared these ratios with those in the wild type. The 30 kDa linker can be incorporated into the rods in absence of the 33 kDa linker. The incorporation of the 30 kDa linker is lower in absence of the 33 kDa linker. When the 30 kDa linker is missing, an increase in the level of the 33 kDa linker is seen, indicating that there could be an excess of the 33 kDa linker in the cells. Our results also show that a reduction in the phycocyanin content causes a decrease in the rod length simultaneously with a reduction of the 30/27 linker ratio, without altering the 33/27 ratio. Reduced phycocyanin content and absence of the 33 kDa linker cause a dramatic reduction in the incorporation of the 30 kDa linker into the rods in the mutant B2SMIKM. Over-expression of the 9 kDa linker results in a decreased incorporation of both the 33 and 30 kDa linkers into the rods, the effect being more pronounced for the 30 kDa linker. This result indicates that the level of the 9 kDa linker relative to those of the 33 and the 30 kDa linkers may be an important determinant of the phycobilisome rod length.     

Structural organisation of phycobilisomes from Synechocystis sp. strain PCC6803 and their interaction with the membrane

In cyanobacteria, the harvesting of light energy for photosynthesis is mainly carried out by the phycobilisome — a giant, multi-subunit pigment-protein complex. This complex is composed of heterodimeric phycobiliproteins that are assembled with the aid of linker polypeptides such that light absorption and energy transfer to photosystem II are optimised. In this work we have studied, using single particle electron microscopy, the phycobilisome structure in mutants lacking either two or all three of the phycocyanin hexamers. The images presented give much greater detail than those previously published, and in the best two-dimensional projection maps a resolution of 13 Å was achieved. As well as giving a better overall picture of the assembly of phycobilisomes, these results reveal new details of the association of allophycocyanin trimers within the core. Insights are gained into the attachment of this core to the membrane surface, essential for efficient energy transfer to photosystem II. Comparison of projection maps of phycobilisomes with and without reconstituted ferredoxin:NADP oxidoreductase suggests a location for this enzyme within the complex at the rod-core interface.     

A Phycoerythrin-Allophycocyanin Complex from the Intact Phycobilisomes of the Marine Red Alga Polysiphonia Urceolata

A phycoerythrin-allophycocyanin (PE-AP) complex was obtained from intact phycobilisomes of the marine red alga Polysiphonia urceolata. Study of spectral properties and polypeptide components showed that the complex contains PE, phycocyanin, AP, and higher proportional linker proteins of the four groups present in intact phycobilisomes.     

Structure of the genes encoding the rod-core linker polypeptides of Mastigocladus laminosus phycobilisomes and functional aspects of the phycobiliprotein/linker-polypeptide interactions.

The 3' portion of the cpc operon in Mastigocladus laminosus encloses the genes 5'-cpcF-cpcG1-cpcG2-cpcG3 3'. The three cpcG genes encode different phycocyanin-associated rod-core linker polypeptides of the phycobilisomes with predicted 279, 247 and 254 amino acids in length. The gene products CpcG show a high similarity at their N-terminal domains (190 amino acids) and an overall identity of 47-53% to one another. Each of the three CpcG polypeptides is highly related to one of the four CpcG gene products of Anabaena sp. PCC 7120 (66-81% identity). It is suggested that these pairs of rod-core linker polypeptides mediate the same specific type of phycocyanin----allophycocyanin interaction in the similar phycobilisomes of M. laminosus and Anabaena sp. PCC 7120. The similarity of the CpcG1, CpcG2 and CpcG3 polypeptides to the single CpcG rod-core linker polypeptide of Synechococcus sp. PCC 7002 (36-41% identity) is lower. The rod-core linker polypeptides are more distantly related to the rod linker polypeptides associated with phycocyanin or phycoerythrin. However, six conserved domains were identified within the N-terminal 190 amino acids of these linker proteins, which bear similar amino acid sequences, including highly conserved basic amino acids. A similar amino acid sequence but with conserved acidic amino acids can be found in the beta subunits of phycocyanin, phycoerythrin and phycoerythrocyanin, which is protruding into the central cavity of the phycobiliprotein hexamers. It is suggested that these domains are sites of phycobiliprotein-hexamer/rod and rod-core linker interactions.     

Structure and mutation of a gene encoding a Mr 33000 phycocyanin-associated linker polypeptide

The gene encoding a phycocyanin-associated linker polypeptide of M_r 33000 from the cyanobacterium Synechococcus sp. PCC 7002 was found to be located adjacent and 3 to the genes encoding the and subunits of phycocyanin. The identity of this gene, designated cpcC, was proven by matching the amino-terminal sequence of the authentic polypeptide with that predicted by the nucleotide sequence. A cpcC mutant strain of this cyanobacterium was constructed. The effect of the mutation was to prevent assembly of half the total phycocyanin into phycobilisomes. By electron microscopy, phycobilisomes from this mutant were shown to contain rod substructures composed of a single disc of hexameric phycocyanin, as opposed to two discs in the wild type. It was concluded that the M_r 33000 linker polypeptide is required for attachment of the core-distal phycocyanin hexamer to the core-proximal one. Using absorption spectra of the wild type, CpcC^–, and phycocyanin-less phycobilisomes, the in situ absorbances expected for specific phycocyanin-linker complexes were calculated. These data confirm earlier findings on isolated complexes regarding the influence of linkers on the spectroscopic properties of phycocyanin.Abbreviations PC phycocyanin - PEC phycoerythrocyanin - AP allophycocyanin - SDS-PAGE polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate. Linker polypeptides are abbreviated according to Glazer (1985). L _infX ^supY refers to a linker having a mass Y, located at a position X in the phycobilisome, where X can be R (rod), RC (rod or core), C (core) or CM (core to membrane). When necessary, the abbreviation for a linker is appended with that of its associated phycobiliprotein. Thus, L _infR ^sup34.5PEC is a rod linker of M_r 34 500 that is associated with phycoerythrocyanin     

Evidence for a Transient Association of New Proteins with the Spirulina maxima Phycobilisome in Relation to Light Intensity

Environmental parameters are known to affect phycobilisomes. Variations of their structure and relative composition in phycobiliproteins have been observed. We studied the effect of irradiance variations on the phycobilisome structure in the cyanobacterium Spirulina maxima and discovered the appearance of new polypeptides associated with the phycobilisomes under an increased light intensity. In high light, the six rods of phycocyanin associated with the central core of allophycocyanin contained only one to two phycocyanin hexamers instead of the two to three they contained in low light. The concomitant disappearance of a 33-kD linker polypeptide was observed. Moreover, in high light three polypeptides of 29, 30, and 47 kD, clearly unrelated to linkers, were found to be associated with the phycobilisome fraction: protein labeling showed that a specific association of these polypeptides was induced by high light. One polypeptide, at least, would play the role of a chaperone protein. Not only the synthesis of these proteins, which appeared slightly increased in high light, but also their association with phycobilisome structure are light intensity dependent.     

Effect of Light Quality on Phycobilisome Components of the Cyanobacterium Spirulina platensis

Phycobilisomes from the nonchromatic adapting cyanobacterium Spirulina platensis are composed of a central core containing allophycocyanin and rods with phycocyanin and linker polypeptides in a regular array. Room temperature absorption spectra of phycobilisomes from this organism indicated the presence of phycocyanin and allophycocyanin. However, low temperature absorption spectra showed the association of a phycobiliviolin type of chromophore within phycobilisomes. This chromophore had an absorption maximum at 590 nanometers when phycobilisomes were suspended in 0.75 molar K-phosphate buffer (pH 7.0). Purified phycocyanin from this cyanobacterium was found to consist of three subparticles and the phycobiliviolin type of chromophore was associated with the lowest density subparticle. Circular dichroism spectra of phycocyanin subparticles also indicated the association of this chromophore with the lowest density subparticle. Absorption spectral analysis of α and β subunits of phycocyanin showed that phycobiliviolin type of chromophore was attached to the α subunit, but not the β subunit. Effect of light quality showed that green light enhanced the synthesis of this chromophore as analyzed from the room temperature absorption spectra of phycocyanin subparticles and subunits, while red or white light did not have any effect. Low temperature absorption spectra of phycobilisomes isolated from green, red, and white light conditions also indicated the enhancement of phycobiliviolin type of chromophore under green light.     

Changes in polypeptide composition of Synechocystis sp. strain 6308 phycobilisomes induced by nitrogen starvation.

Phycobilisomes isolated from actively growing Synechocystis sp. strain 6308 (ATCC 27150) consist of 12 polypeptides ranging in molecular mass from 11.5 to 95 kilodaltons. The phycobilisome anchor and linker polypeptides are glycosylated. Nitrogen starvation causes the progressive loss of phycocyanin and allophycocyanin subunits with molecular masses between 16 and 20 kilodaltons and of two linker polypeptides with molecular masses of 27 and 33 kilodaltons. Nitrogen starvation also leads to enrichment of four additional polypeptides with molecular masses of 46, 53, 57, and 61 kilodaltons and a transient enrichment of 35- and 41-kilodalton polypeptides in isolated phycobilisomes. The 57-kilodalton additional polypeptide was identified by immunoblotting as the large subunit of ribulosebisphosphate carboxylase/oxygenase. Proteins with the same molecular weights as the additional polypeptides were also coisolated with the 12 phycobilisome polypeptides in the supernatant of nitrogen-replete Synechocystis thylakoid membranes extracted in high-ionic-strength buffer and washed with deionized water. These observations suggest that the additional polypeptides in phycobilisomes from nitrogen-starved cells may be soluble or loosely bound membrane proteins which associate with phycobilisomes. The composition and degree of association of phycobilisomes with soluble and adjacent membrane polypeptides appear to be highly dynamic and specifically regulated by nitrogen availability. Possible mechanisms for variation in the strength of association between phycobilisomes and other polypeptides are suggested.     

Characterization, structure and function of linker polypeptides in phycobilisomes of cyanobacteria and red algae: An overview

Cyanobacteria and red algae have intricate light-harvesting systems comprised of phycobilisomes that are attached to the outer side of the thylakoid membrane. The phycobilisomes absorb light in the wavelength range of 500-650 nm and transfer energy to the chlorophyll for photosynthesis. Phycobilisomes, which biochemically consist of phycobiliproteins and linker polypeptides, are particularly wonderful subjects for the detailed analysis of structure and function due to their spectral properties and their various components affected by growth conditions. The linker polypeptides are believed to mediate both the assembly of phycobiliproteins into the highly ordered arrays in the phycobilisomes and the interactions between the phycobilisomes and the thylakoid membrane. Functionally, they have been reported to improve energy migration by regulating the spectral characteristics of colored phycobiliproteins. In this review, the progress regarding linker polypeptides research, including separation approaches, structures and interactions with phycobiliproteins, as well as their functions in the phycobilisomes, is presented. In addition, some problems with previous work on linkers are also discussed.     

THE γ SUBUNITS OF PHYCOERYTHRIN FROM A RED ALGA: POSITION IN PHYCOBILISOMES AND SEQUENCE CHARACTERIZATION

Aglaothamnion neglectum Feldman-Mazoyer has two γ subunits, γ³¹ and γ³³, that are associated with phycoerythrin in the light-harvesting phycobilisomes. We demonstrate that these subunits are spatially separated within the phycobilisome, with the γ³¹ subunit present at the distal end of phycobilisome rods and the γ³³ subunit present on the proximal end. These subunits are thought to link phycoerythrin hexamers together in the rod substructure, serving a role analogous to that of linker polypeptides of cyanobacteria (although unlike the cyanobacterial linker polypeptides they are chromophorylated). The sequencing of tryptic polypeptides of the γ subunits enabled us to prepare oligonucleotides encoding different regions of γ³¹. These oligonucleotides were used as primers to generate a probe for isolating a γ³¹ cDNA clone. Characterization of the cDNA clone predicts a polypeptide of 280 amino acids with a 42 amino acid presequence that is characteristic of a transit peptide, the peptide that targets proteins to chloroplasts of vascular plants. The γ³¹ subunit has 50% similarity to the previously characterized γ³³ subunit but has no identifiable similarity to functionally related polypeptides present in cyanobacterial phycobilisomes or to any other polypeptides in the databases. A repeat of 95 amino acids is present in the red algal γ subunit sequences, suggesting that these proteins were generated by a gene duplication followed by fusion of the duplicate sequences.     

Growth and Chromatic Adaptation of Nostoc sp. Strain MAC and the Pigment Mutant R-MAC

A spontaneous, stable, pigmentation mutant of Nostoc sp. strain MAC was isolated. Under various growth conditions, this mutant, R-MAC, had similar phycoerythrin contents (relative to allophycocyanin) but significantly lower phycocyanin contents (relative to allophycocyanin) than the parent strain. In saturating white light, the mutant grew more slowly than the parent strain. In nonsaturating red light, MAC grew with a shorter generation time than the mutant; however, R-MAC grew more quickly in nonsaturating green light.

When the parental and mutant strains were grown in green light, the phycoerythrin contents, relative to allophycocyanin, were significantly higher than the phycoerythrin contents of cells grown in red light. For both strains, the relative phycocyanin contents were only slightly higher for cells grown in red light than for cells grown in green light. These changes characterize both MAC and R-MAC as belonging to chromatic adaptation group II: phycoerythrin synthesis alone photocontrolled.

A comparative analysis of the phycobilisomes, isolated from cultures of MAC and R-MAC grown in both red and green light, was performed by polyacrylamide gel electrophoresis in the presence of 8.0 molar urea or sodium dodecyl sulfate. Consistent with the assignment of MAC and R-MAC to chromatic adaptation group II, no evidence for the synthesis of red light-inducible phycocyanin subunits was found in either strain. Phycobilisomes isolated from MAC and R-MAC contained linker polypeptides with relative molecular masses of 95, 34.5, 34, 32, and 29 kilodaltons. When grown in red light, phycobilisomes of the mutant R-MAC appeared to contain a slightly higher amount of the 32-kilodalton linker polypeptide than did the phycobilisomes isolated from the parental strain under the same conditions. The 34.5-kilodalton linker polypeptide was totally absent from phycobilisomes isolated from cells of either MAC or R-MAC grown in green light.

     

Acclimation Processes in the Light-Harvesting System of the Cyanobacterium Anacystis nidulans following a Light Shift from White to Red Light

Cyanobacteria acclimate to changes in light by adjusting the amounts of different cellular compounds, for example the light-harvesting macromolecular complex. Described are the acclimatization responses in the light-harvesting system of the cyanobacterium Anacystis nidulans following a shift from high intensity, white light to low intensity, red light.

The phycocyanin and chlorophyll content and the relative amount of the two linker peptides (33 and 30 kilodaltons) in the phycobilisome were studied. Both the phycocyanin and chlorophyll content per cell increased after the shift, although the phycocyanin increased relatively more. The increase in phycocyanin was biphasic in nature, a fast initial phase and a slower second phase, while the chlorophyll increase was completed in one phase. The phycocyanin and chlorophyll responses to red light were immediate and were completed within 30 and 80 hours for chlorophyll and phycocyanin, respectively. An immediate response was also seen for the two phycobilisome linker peptides. The amount of both of them increased after the shift, although the 33 kilodalton linker peptide increased faster than the 30 kilodalton linker peptide. The increase of the content of the two linker peptides stopped when the phycocyanin increase shifted from the first to the second phase. We believe that the first phase of phycocyanin increase was due mainly to an increase in the phycobilisome size while the second phase was caused only by an increase in the amount of phycobilisomes. The termination of chlorophyll accumulation, which indicates that no further reaction center chlorophyll antennae were formed, occurred parallel to the onset of the second phase of phycocyanin accumulation.

     

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.     

Chromatic adaptation and the events involved in phycobilisome biosynthesis

Abstract. The major light-harvesting complex in cyanobacteria and red algae is the phycobilisome, a macromolecular complex that is attached to the surface of the photosynthetic membranes. The phycobilisome is composed of a number of different chromophoric polypeptides called phycobiliproteins and nonchromophoric polypeptides called linker proteins. Several environmental parameters modulate the synthesis, assembly and degradation of phycobilisome components. In many cyanobacteria, the composition of the phycobilisome can change to accommodate the prevalent wavelengths of light in the environment. This phenomenon is called complementary chromatic adaptation. Organisms that exhibit complementary chromatic adaptation must perceive the wavelengths of light in the environment and transduce the light signals into a sequence of biochemical events that result in altering the activities of genes encoding specific phycobiliprotein and linker polypeptides. Other environmental parameters such as light intensity and nutrient status can also have marked effects on both the number and composition of the phycobilisomes. The major concern of this article is the molecular events involved in chromatic adaptation. Most of the information concerning this process has been gained from studies involving the filamentous cyanobacterium Fremyella diplosiphon . However, also briefly considered are some of the complexities involved in phycobilisome biosynthesis and degradation; they include post-translational modification of phycobilisome polypeptides, the coordinate expression of chromophore and apobiliprotein, the specific degradation of phycobilisomes when cyanobacteria are deprived of macronutrients such as nitrogen, sulphur and phosphorus, and the assembly of the individual phycobilisome components into substructures of the light harvesting complex.     

The presence of multidomain linkers determines the bundle-shape structure of the phycobilisome of the cyanobacterium Gloeobacter violaceus PCC 7421

The complete genome sequence of Gloeobacter violaceus [Nakamura et al. (2003a, b) DNA Res 10:37–45, 181–201] allows us to understand better the structure of the phycobilisomes (PBS) of this cyanobacterium. Genomic analysis revealed peculiarities in these PBS: the presence of genes for two multidomain linker proteins, a core membrane linker with four repetitive sequences (REP domains), the absence of rod core linkers, two sets of phycocyanin (PC) α and β subunits, two copies of a rod PC associated linker (CpcC), and two rod cap associated linkers (CpcD). Also, there is one ferredoxin–NADP⁺ oxidoreductase with only two domains. The PBS proteins were investigated by gel electrophoresis, amino acid sequencing and peptide mass fingerprinting (PMF). The two unique multidomain linkers contain three REP domains with high similarity and these were found to be in tandem and were separated by dissimilar Arms. One of these, with a mass of 81 kDa, is found in heavy PBS fragments rich in PC. We propose that it links six PC hexamers in two parallel rows in the rods. The other unique linker has a mass of 91 kDa and is easily released from the heavy fragments of PBS. We propose that this links the rods to the core. The presence of these multidomain linkers could explain the bundle shaped rods of the PBS. The presence of 4 REP domains in the core membrane linker protein (129 kDa) was established by PMF. This core linker may hold together 16 AP trimers of the pentacylindrical core, or alternatively, a tetracylindrical core of the PBS of G. violaceus.