Molecular morphology of cyanobacterial phycobilisomes

Phycobilisomes were isolated from several cyanobacteria following cell lysis with Triton X-100. They were purified by phosphate precipitation and hydrophobic-interaction chromatography. Their phycobiliprotein compositions were quantitatively determined by application of sets of simultaneous absorbance equations to gel chromatographic separations of the chromoproteins. Phycobilisomes purified from several cyanobacteria had characteristic elution times on agarose gel chromatography. Combining electron microscope observations of phycobilisome structure, phycobiliprotein composition, and agarose gel chromatography estimates of molecular weight permitted the calculation of many details of phycobilisome molecular structure. Complementary chromatic adaptation resulted in a change of phycobilisome composition and structure. The polypeptide compositions of phycobilisomes were examined by sodium dodecyl sulfate-agarose gel chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The phycobilisomes were composed of phycobilipeptides derived from the constituent phycobiliproteins. Higher molecular-weight phycobilipeptide aggregates were also observed. The dominant forces responsible for the maintenance of phycobilisome structure are concluded to be hydrophobic interactions.     

Differentiation between Phycobiliprotein and Colorless Linker Polypeptides by Fluorescence in the Presence of ZnSO(4)

Microcystis aeruginosa, a unicellular cyanobacterium, contains small phycobilisomes consisting of C-phycocyanin, allophycocyanin, and linker polypeptides. SDS-polyacrylamide gels of the phycobilisomes were examined for fluorescent bands before and after spraying with a solution of ZnSO₄, followed by Coomassie brilliant blue staining for protein. This procedure provides a rapid and sensitive method for detecting small amounts of phycobilin-containing polypeptides and distinguishing them from other tetrapyrrole-containing polypeptides and from `colorless' ones. Three polypeptide bands, in addition to the α and β phycobiliprotein subunits, have been detected under these conditions. An 85 kilodalton polypeptide was identified as a phycobiliprotein due to its enhanced fluorescence in the presence of ZnSO₄. The other polypeptides do not contain chromophores and are colorless. They are approximately 34.5 and 30 kilodaltons in size.     

Dynamic aspects of phycobilisome structure: Modulation of phycocyanin content of Synechococcus phycobilisomes

Phycobilisomes of the cyanobacterium Synechococcus 6301 contain the phycobiliproteins phycocyanin, allophycocyanin, and allophycocyanin B, and four major non pigmented polypeptides of 75, 33, 30, and 27 kdaltons. The molar ratio of phycocyanin to allophycocyanin in wild type phycobilisomes can be varied over about a two-fold range by alterations in culture conditions with parallel changes in the amounts of the 33 and 30 kdalton polypeptides whereas the levels of the 27 and 75 kdalton polypeptides do not vary. Two nitrosoguanidine-induced mutants, AN112 and AN135, produce abnormally small phycobilisomes, containing only 35 and 50% of the wild type level of phycocyanin. AN135 phycobilisomes contain less 33 kdalton polypeptide than wild type and the 30 kdalton polypeptide is only detected in phycobilisomes from cultures grown under conditions favoring high levels of phycocyanin. AN112 lacks both the 30 and 33 kdalton polypeptides and produces phycobilisomes of constant size and composition, independent of growth conditions. Both mutant phycobilisomes have wild type levels of 27 and 75 kdalton polypeptides relative to allophycocyanin and have normal energy transfer properties. These results indicate that modulation of phycobilisome size involves concurrent regulation of the levels of phycocyanin and of both the 30 and 33 kdalton polypeptides with no change in the composition of the allophycocyanin-containing core.Abbreviations LP cells cells grown under conditions favoring low p phycobiliprotein levels - HP cells cells grown under conditions favoring high phycobiliprotein levels - SDS sodium dodecylsulfate - EDTA ethylenediamine tetraacetic acid - NaK-PO₄ NaH₂PO₄ titrated with K₂HPO₄ to a given pH A preliminary report of some of this work was presented at the 81st Annual Meeting of the American Society for Microbiology, Dallas, Texas, March 1981     

Phycobilisome Structure of Porphyridium cruentum: POLYPEPTIDE COMPOSITION

Purified phycobilisomes of Porphyridium cruentum were solubilized in sodium dodecyl sulfate and resolved by sodium dodecyl sulfate-acrylamide gel electrophoresis into nine colored and nine colorless polypeptides. The colored polypeptides accounted for about 84% of the total stainable protein, and the colorless polypeptides accounted for the remaining 16%. Five of the colored polypeptides ranging in molecular weight from 13,300 to 19,500 were identified as the α and β subunits of allophycocyanin, R-phycocyanin, and phycoerythrin. Three others (29,000-30,500) were orange and are probably related to the γ subunit of phycoerythrin. Another colored polypeptide had a molecular weight of 95,000 and the characteristics of long wavelength-emitting allophycocyanin. Sequential dissociation of phycobilisomes, and analysis of the polypeptides in each fraction, revealed the association of a 32,500 molecular weight colorless polypeptide with a phycoerythrin fraction. The remaining eight colorless polypeptides were in the core fraction of the phycobilisome, which also was enriched in allophycocyanin. In addition, the core fraction was enriched in a colored 95,000 dalton polypeptide. Inasmuch as a polypeptide with the same molecular weight is found in thylakoid membranes (free of phycobilisomes), it is suggested that this polypeptide is involved in anchoring phycobilisomes to thylakoid membranes.     

Noncovalent Intermolecular Forces in Phycobilisomes of Porphyridium cruentum

Using sensitized fluorescence as a measure of intactness of phycobilisomes isolated from Porphyridium cruentum, the effects of various environmental perturbations on phycobilisome integrity were investigated. The rate of phycobilisome dissociation in 0.75 ionic strength sodium salts proceeds in the order: SCN⁻ > NO₃⁻ > Cl⁻ > C₆H₅O₇³⁻ > SO₄²⁻ > PO₄³⁻, as predicted from the lyotropic series of anions and their effects on hydrophobic interactions in proteins. Similarly, increasing temperature (to 30 C) and pH values approaching the isoelectric points of the biliproteins stabilize phycobilisomes. Deuterium substitution at exchangeable sites on the phycobiliproteins decreases the rate of phycobilisome dissociation, while substitution at nonexchangeable sites increases rates of dissociation. It is concluded that hydrophobic intermolecular interactions are the most important forces in maintaining the phycobilisome structure. Dispersion forces also seem to contribute to phycobilisome stabilization. The adverse effects of electrostatic repulsion must not be ignored; however, it seems that the requirement of phycobilisomes of high salt concentrations is not simply countershielding of charges on the proteins.     

In vitro reassociation of phycobiliproteins and membranes to form functional membrane-bound phycobilisomes

A membrane-bound phycobilisome complex has been isolated from the cyanobacterium Fremyella diplosiphon grown in green light, thus containing phycoerythrin in addition to phycocyanin and allophycocyanin. The complex was dissociated by lowering the salt concentration. In the mixture obtained, no energy transfer from phycoerythrin to chlorophyll (Chl) a was observed. Reassociation of the phycobiliproteins and membrane mixture was carried out by a gradual increase of the salt concentration. The complex obtained after reassociation was characterized by polypeptide composition, absorbance and fluorescence emission spectra and electron microscopy. These analyses revealed similar composition and structure for the original and reconstituted membrane-bound phycobilisomes. Fluorescence emission spectra and measurements of Photosystem II activity demonstrated energy transfer from phycoerythrin to Chl a (Photosystem II) in the reconstituted complex. Reassociation of mixtures with varying phycoerythrin / Chl ratio showed that the phycobiliprotein concentration was critical in the reassociation process. Measurements of the amount of phycobilisomes reassociated with the photosynthetic membrane did not show saturation of binding when increasing the phycobiliprotein concentration. The ratio phycoerythrin / Chl a in the native complex was 7:1 (mg / mg). When the phycobiliprotein concentration was increased during the reassociation process, a ratio of 13–15 mg phycoerythrin / mg Chl a could be obtained. Under these conditions, only part of the phycobilisomes attached to the thylakoids was able to transfer energy to Photosystem II.     

A Polypeptide with Similarity to Phycocyanin α-Subunit Phycocyanobilin Lyase Involved in Degradation of Phycobilisomes

To optimize the utilization of photosynthate and avoid damage that can result from the absorption of excess excitation energy, photosynthetic organisms must rapidly modify the synthesis and activities of components of the photosynthetic apparatus in response to environmental cues. During nutrient-limited growth, cyanobacteria degrade their light-harvesting complex, the phycobilisome, and dramatically reduce the rate of photosynthetic electron transport. In this report, we describe the isolation and characterization of a cyanobacterial mutant that does not degrade its phycobilisomes during either sulfur or nitrogen limitation and exhibits an increased ratio of phycocyanin to chlorophyll during nutrient-replete growth. The mutant phenotype was complemented by a gene encoding a polypeptide with similarities to polypeptides that catalyze covalent bond formation between linear tetrapyrrole chromophores and subunits of apophycobiliproteins. The complementing gene, designated nblB, is expressed at approximately the same level in cells grown in nutrient-replete medium and medium devoid of either sulfur or nitrogen. These results suggest that the NblB polypeptide may be a constitutive part of the machinery that coordinates phycobilisome degradation with environmental conditions.     

Integration of Apo-α-Phycocyanin into Phycobilisomes and Its Association with FNRL in the Absence of the Phycocyanin α-Subunit Lyase (CpcF) in Synechocystis sp. PCC 6803

Phycocyanin is an important component of the phycobilisome, which is the principal light-harvesting complex in cyanobacteria. The covalent attachment of the phycocyanobilin chromophore to phycocyanin is catalyzed by the enzyme phycocyanin lyase. The photosynthetic properties and phycobilisome assembly state were characterized in wild type and two mutants which lack holo-α-phycocyanin. Insertional inactivation of the phycocyanin α-subunit lyase (ΔcpcF mutant) prevents the ligation of phycocyanobilin to α-phycocyanin (CpcA), while disruption of the cpcB/A/C2/C1 operon in the CK mutant prevents synthesis of both apo-α-phycocyanin (apo-CpcA) and apo-β-phycocyanin (apo-CpcB). Both mutants exhibited similar light saturation curves under white actinic light illumination conditions, indicating the phycobilisomes in the ΔcpcF mutant are not fully functional in excitation energy transfer. Under red actinic light illumination, wild type and both phycocyanin mutant strains exhibited similar light saturation characteristics. This indicates that all three strains contain functional allophycocyanin cores associated with their phycobilisomes. Analysis of the phycobilisome content of these strains indicated that, as expected, wild type exhibited normal phycobilisome assembly and the CK mutant assembled only the allophycocyanin core. However, the ΔcpcF mutant assembled phycobilisomes which, while much larger than the allophycocyanin core observed in the CK mutant, were significantly smaller than phycobilisomes observed in wild type. Interestingly, the phycobilisomes from the ΔcpcF mutant contained holo-CpcB and apo-CpcA. Additionally, we found that the large form of FNR (FNR_L) accumulated to normal levels in wild type and the ΔcpcF mutant. In the CK mutant, however, significantly less FNR_L accumulated. FNR_L has been reported to associate with the phycocyanin rods in phycobilisomes via its N-terminal domain, which shares sequence homology with a phycocyanin linker polypeptide. We suggest that the assembly of apo-CpcA in the phycobilisomes of ΔcpcF can stabilize FNR_L and modulate its function. These phycobilisomes, however, inefficiently transfer excitation energy to Photosystem II.     

Light Intensity Adaptation and Phycobilisome Composition of Microcystis aeruginosa

Phycobilisomes isolated from Microcystis aeruginosa grown to midlog at high light (270 microeinsteins per square meter per second) or at low light intensities (40 microeinsteins per square meter per second) were found to be identical. Electron micrographs established that they have a triangular central core apparently consisting of three allophycocyanin trimers surrounded by six rods, each composed of two hexameric phycocyanin molecules. The apparent mass of a phycobilisome obtained by gel filtration is 2.96 × 10⁶ daltons. The molar ratio of the phycobiliproteins per phycobilisome is 12 phycocyanin hexamers:9 allophycocyanin trimers. The electron microscopic observations combined with the phycobilisome apparent mass and the phycobiliprotein stoichiometry data indicate that M. aeruginosa phycobilisomes are composed of a triangular central core of three stacks of three allophycocyanin trimers and six rods each containing two phycocyanin hexamers. Adaptation of M. aeruginosa to high light intensity results in a decrease in the number of phycobilisomes per cell with no alteration in phycobilisome composition or structure.     

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 cruentum, were analyzed by selective dissociation at various phosphate concentrations. The results are consistent with a structural model consisting of an allophycocyanin core, surrounded 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 aggregated 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-phycoerythrin 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 phycobilisome structure occurs. Reaction with anti-allophycocyanin is very slow in P. cruentum phycobilisomes, 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.Phycobilisome dissociation is inversely proportional to phosphate concentration (500 mM to 2 mM), and is essentially unaffected by protein concentration in the range used (30–200 μg/ml). Phycobiliprotein release occurs in the same order (phycoerythrin > R-phycocyanin > allophycocyanin) in the pH range 5.4–8.0.     

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     

Distinct roles of CpcG1-phycobilisome and CpcG2-phycobilisome in state transitions in a cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803

State transitions in cyanobacteria regulate the relative energy transfer from phycobilisome to photosystem I and II. Although it has been shown that phycobilisome mobility is essential for phycobilisome-dependent state transitions, the biochemical mechanism is not known. Previously we reported that two distinct forms of phycobilisome are assembled with different CpcG copies, which have been referred to as “rod-core linker,” in a cyanobacterium Synechocystis sp. PCC 6803. CpcG2-phycobilisome is devoid of a typical central core, while CpcG1-phycobilisome is equivalent to the conventional phycobilisome supercomplex. Here, we demonstrated that the cpcG1 disruptant has a severe specific defect in the phycobilisome-dependent state transition. However, fluorescence recovery after photobleaching measurements showed no obvious difference in phycobilisome mobility between the wild type and the cpcG1 disruptant. This suggests that both CpcG1 and CpcG2 phycobilisomes have an unstable interaction with the reaction centres. However, only CpcG1 phycobilisomes are involved in state transitions. This suggests that state transitions require the phycobilisome core.     

The proteolysis adaptor,NblA, initiates protein pigment degradation by interacting with the cyanobacterial light‐harvesting complexes

Degradation of the cyanobacterial protein pigment complexes, the phycobilisomes, is a central acclimation response that controls light energy capture. The small protein, NblA, is essential for proteolysis of these large complexes, which may reach a molecular mass of up to 4 MDa. Interactions of NblA in vitro supported the suggestion that NblA is a proteolysis adaptor that labels the pigment proteins for degradation. The mode of operation of NblA in situ, however, remained unresolved. Particularly, it was unclear whether NblA interacts with phycobilisome proteins while part of the large complex, or alternatively interaction with NblA, necessitates dissociation of pigment subunits from the assembly. Fluorescence intensity profiles demonstrated the preferential presence of NblA::GFP (green fluorescent protein) at the photosynthetic membranes, indicating co‐localization with phycobilisomes. Furthermore, fluorescence lifetime imaging microscopy provided in situ evidence for interaction of NblA with phycobilisome protein pigments. Additionally, we demonstrated the role of NblA in vivo as a proteolysis tag based on the rapid degradation of the fusion protein NblA::GFP compared with free GFP. Taken together, these observations demonstrated in vivo the role of NblA as a proteolysis adaptor. Additionally, the interaction of NblA with phycobilisomes indicates that the dissociation of protein pigment subunits from the large complex is not a prerequisite for interaction with this adaptor and, furthermore, implicates NblA in the disassembly of the protein pigment complex. Thus, we suggest that, in the case of proteolysis of the phycobilisome, the adaptor serves a dual function: undermining the complex stability and designating the dissociated pigments for degradation.     

Structural and compositional analyses of the phycobilisomes of Synechococcus sp. PCC 7002. Analyses of the wild-type strain and a phycocyanin-less mutant constructed by interposon mutagenesis

The phycobilisomes and phycobiliproteins of Synechococcus sp. PCC 7002 wild-type strain PR6000 have been isolated and characterized. The hemidiscoidal phycobilisomes of strain PR6000 are composed of eleven different polypeptides: phycocyanin and subunits; allophycocyanin and subunits; subunit of allophycocyanin B; the allophycocyanin -subunit-like polypeptide of M_r 18 000; the linker phycobiliprotein of M_r 99 000; and non-chromophore-carrying linker polypeptides of M_r 33 000, 29 000, 9000, and 8000. Several of these polypeptides were purified to homogeneity and their amino acid compositions and amino-terminal amino acid sequences were determined. Analyses of the phycobiliproteins of Synechococcus sp. PCC 7002 were greatly facilitated by comparative studies performed with a mutant strain, PR6008, constructed to be devoid of the phycocyanin and subunits by recombinant DNA techniques and transformation of strain PR6000. The absence of phycocyanin did not greatly affect the allophycocyanin content of the mutant strain but caused the doubling time to increase 2–7-fold depending upon the light intensity at which the cells were grown. Although intact phycobilisome cores could not be isolated from this mutant, it is probable that functionally intact cores do exist in vivo.Abbreviations used SDS-PAGE polyacrylamide gel electrophoresis in the presence of sodium dodecylsulfate - 2D-PAGE two-dimensional gel electrophoresis in which the first dimension consisted of isoelectric focusing in the presence of 8.0 M urea in the pH range 4–6 and the second dimension consisted of electrophoresis in the presence of sodium dodecylsulfate. The nomenclature employed for the phycobiliprotein subunits and linker polypeptides is that defined by Glazer (1985)     

Role of the Colorless Polypeptides in Phycobilisome Assembly in Nostoc sp

We have identified the function of the `extra' polypeptides involved in phycobilisome assembly in Nostoc sp. These phycobilisomes, as those of other cyanobacteria, are composed of an allophycocyanin core, phycoerythrin- and phycocyanin-containing rods, and five additional polypeptides of 95, 34.5, 34, 32, and 29 kilodaltons. The 95 kilodalton polypeptide anchors the phycobilisome to the thylakoid membrane (Rusckowski, Zilinskas 1982 Plant Physiol 70: 1055-1059); the 29 kilodalton polypeptide attaches the phycoerythrin- and phycocyanin-containing rods to the allophycocyanin core (Glick, Zilinskas 1982 Plant Physiol 69: 991-997). Two populations of rods can exist simultaneously or separately in phycobilisomes, depending upon illumination conditions. In white light, only one type of rod with phycoerythrin and phycocyanin in a 2:1 molar ratio is synthesized. Associated with this rod are the 29, 32, and 34 kilodalton colorless polypeptides; the 32 kilodalton polypeptide links the two phycoerythrin hexamers, and the 34 kilodalton polypeptide attaches a phycoerythrin hexamer to a phycocyanin hexamer. The second rod, containing predominantly phycocyanin, and the 34.5 and 29 kilodalton polypeptides, is synthesized by redlight-adapted cells; the 34.5 kilodalton polypeptide links two phycocyanin hexamers. These assignments are based on isolation of rods, dissociation of these rods into their component biliproteins, and analysis of colorless polypeptide composition, followed by investigation of complexes formed or not formed upon their recombination.     

Constant Phycobilisome Size in Chromatically Adapted Cells of the Cyanobacterium Tolypothrix tenuis, and Variation in Nostoc sp

Phycobilisomes of Tolypothrix tenuis, a cyanobacterium capable of complete chromatic adaptation, were studied from cells grown in red and green light, and in darkness. The phycobilisome size remained constant irrespective of the light quality. The hemidiscoidal phycobilisomes had an average diameter of about 52 nanometers and height of about 33 nanometers, by negative staining. The thickness was equivalent to a phycocyanin molecule (about 10 nanometers). The molar ratio of allophycocyanin, relative to other phycobiliproteins always remained at about 1:3. Phycobilisomes from red light grown cells and cells grown heterotrophically in darkness were indistinguishable in their pigment composition, polypeptide pattern, and size. Eight polypeptides were resolved in the phycobilin region (17.5 to 23.5 kilodaltons) by isoelectric focusing followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Half of these were invariable, while others were variable in green and red light. It is inferred that phycoerythrin synthesis in green light resulted in a one for one substitution of phycocyanin, thus retaining a constant phycobilisome size. Tolypothrix appears to be one of the best examples of phycobiliprotein regulation with wavelength. By contrast, in Nostoc sp., the decrease in phycoerythrin in red light cells was accompanied by a decrease in phycobilisome size but not a regulated substitution.     

Dynamic aspects of phycobilisome structure

In exponentially growing cells of Synechococcus sp. 6301, over 95% of the phycobiliproteins are located in phycobilisomes, and the remainder is present in the form of low molecular weight aggregates. In addition to the subunits of the phycobiliproteins (C-phycocyanin, allophycocyanin, allophycocyanin B), the phycobilisomes of this unicellular cyanobacterium contain five non-pigmented polypeptides. During the initial phase of starvation (24 h after removal of combined nitrogen from the growth medium), the phycobiliproteins in the low molecular weight fraction largely disappeared. Phycocyanin was lost more rapidly from this fraction than allophycocyanin. Simultaneous changes in the phycobilisome were (1) a decrease in sedimentation coefficient, (2) a decrease in phycocyanin: allophycocyanin ratio, (3) a shift in the fluorescence emission maximum from 673 to 676 nm, and (4) a selective complete loss of a 30,000 dalton non-pigmented polypeptide. Upon extensive nitrogen starvation (72 h), the intracellular level of phycocyanin decreased by over 30-fold. These results indicate that in the early stage of nitrogen starvation, the free phycobiliproteins of the cell are degraded, as well as a significant proportion of the phycocyanin from the periphery of the phycobilisome. However, the structures partially depleted of phycocyanin still function efficiently in energy transfer. On extended starvation, total degradation of residual phycobilisomes takes place, possibly in conjunction with the detachment of these structures from the thylakoids.None of the effects of the absence of combined nitrogen were seen when cells were starved in the presence of chloramphenicol, or in a methionine auxotroph starved for methionine.Abbreviations Used NaK-PO₄ NaH₂PO₄ titrated with K₂HPO₄ to a given pH - SDS sodium dodecyl sulfate - Tris Tris(hydroxymethyl)aminomethane     

Diversity and evolution of phycobilisomes in marine <Emphasis Type="Italic">Synechococcus</Emphasis>spp.: a comparative genomics study

Background  

Marine Synechococcus owe their specific vivid color (ranging from blue-green to orange) to their large extrinsic antenna complexes called phycobilisomes, comprising a central allophycocyanin core and rods of variable phycobiliprotein composition. Three major pigment types can be defined depending on the major phycobiliprotein found in the rods (phycocyanin, phycoerythrin I or phycoerythrin II). Among strains containing both phycoerythrins I and II, four subtypes can be distinguished based on the ratio of the two chromophores bound to these phycobiliproteins. Genomes of eleven marine Synechococcus strains recently became available with one to four strains per pigment type or subtype, allowing an unprecedented comparative genomics study of genes involved in phycobilisome metabolism.     

PHOTOREGULATION OF PHYCOBILISOME STRUCTURE DURING COMPLEMENTARY CHROMATIC ADAPTATION IN THE MARINE CYANOPHYTE PHORMIDIUM SP. C861

Changes in the molecular structure of phycobilisomes during complementary chromatic adaptation were studied in the marine cyanophyte Phormidium sp. C86. This strain forms phycoerythrin (PE)-less phycobilisomes under red light but synthesizes PE-rich phycobilisomes under green light. Analysis of phycobiliprotein composition and electron microscopic examination of phycobilisomes in ultra-thin sections of cells and of isolated phycobilisomes were performed for cells acclimated to red and green light, respectively. The structure of phycobilisomes formed under red light conditions was typically hemidiscoidal. Phycobilisomes in cells acclimated to green light were twice as large in size as those in cells acclimated to red light. This increase in phycobilisome size was a result of the increase in the molar ratio of antenna pigment (PE and phycocyanin) to allophycocyanin, from 3.5 to 11.3. Pigment composition and fine structure of phycobilisomes formed under green light were similar to those of “nonhemidiscoidal” phycobilisomes reported in Phormidium persicinum. These results suggest that changes occur not only in the molecular species of peripheral rods but also in the structure of rods and probably of cores in relation to their connection with rods during chromatic adaptation of Phormidium sp. C86.     

The structure of cyanobacterial phycobilisomes: a model

Phycobilisomes, supramolecular complexes of water-soluble accessory pigments, serve as the major light-harvesting antennae in cyanobacteria and red algae. Regular arrays of these organelles are found on the surface of the thylakoid membranes of these organisms. In the present study, the hemi-discoidal phycobilisomes of several species of cyanobacteria were examined in thin sections of cells and by negative staining after isolation and fixation. Their fundamental structures were found to be the same. Isolated phycobilisomes possessed a triangular core assembled from three stacks of disc-shaped subunits. Each stack contained two discs which were 12 nm in diameter and 6–7 nm thick. Each of these discs was probably subdivided into halves 3–3.5 nm thick. Radiating from each of two sides of the triangular core were three rods 12 nm in diameter. Each rod consisted of stacks of 2 to 6 disc-shaped subunits 6 nm thick. These discs were subdivided into halves 3 nm thick.The average number of discs of 6 nm thickness forming the peripheral rods varied among the strains studied. For certain chromatically adapting strains, the average rod length was dependent upon the wavelength of light to which cells were exposed during growth. Analyses of phycobilisomes by spectroscopic techniques, polyacrylamide gel electrophoresis, and electron microscopy were compared. These analyses suggested that the triangular core was composed of allophycocyanin and that the peripheral rods contained phycocyanin and phycoerythrin (when present). A detailed model of the hemi-discoidal phycobilisome is proposed. This model can account for many aspects of phycobiliprotein assembly and energy transfer.Abbreviations PBS phycobilisome(s) - PBP phycobiliprotein(s) - AP allophycocyanin - PC phycocyanin - PE phycoerythrin - PEC phycoerythrocyanin - AP-B allophycocyanin B - C- cyanobacterial - R- rhodophytan - B- Bangiophycean - SDS sodium dodecyl sulfate - LPP Lyngbya-Plectonema-Phormidium group - Na-KPO₄ buffers NaH₂PO₄ titrated with a solution of KH₂PO₄ of equivalent molarity to a given pH