Genetic analysis of a 9 kDa phycocyanin-associated linker polypeptide

The structural basis for light harvesting in organisms producing phycobiliproteins

Cyanobacteria, red algae, and cryptophytes produce 2 classes of proteins for light harvesting: water-soluble phycobiliproteins (PBP) and membrane-intrinsic proteins that bind chlorophylls (Chls) and carotenoids. In cyanobacteria, red algae, and glaucophytes, phycobilisomes (PBS) are complexes of brightly colored PBP and linker (assembly) proteins. To date, 6 structural classes of PBS have been described: hemiellipsoidal, block-shaped, hemidiscoidal, bundle-shaped, paddle-shaped, and far-red-light bicylindrical. Two additional antenna complexes containing single types of PBP have also been described. Since 2017, structures have been reported for examples of all of these complexes except bundle-shaped PBS by cryogenic electron microscopy. PBS range in size from about 4.6 to 18 mDa and can include ∼900 polypeptides and bind >2000 chromophores. Cyanobacteria additionally produce membrane-associated proteins of the PsbC/CP43 superfamily of Chl a/b/d-binding proteins, including the iron-stress protein IsiA and other paralogous Chl-binding proteins (CBP) that can form antenna complexes with Photosystem I (PSI) and/or Photosystem II (PSII). Red and cryptophyte algae also produce CBP associated with PSI but which belong to the Chl a/b-binding protein superfamily and which are unrelated to the CBP of cyanobacteria. This review describes recent progress in structure determination for PBS and the Chl proteins of cyanobacteria, red algae, and cryptophytan algae.

A structure of the relict phycobilisome from a thylakoid-free cyanobacterium

Phycobilisomes (PBS) are antenna megacomplexes that transfer energy to photosystems II and I in thylakoids. PBS likely evolved from a basic, inefficient form into the predominant hemidiscoidal shape with radiating peripheral rods. However, it has been challenging to test this hypothesis because ancestral species are generally inaccessible. Here we use spectroscopy and cryo-electron microscopy to reveal a structure of a "paddle-shaped" PBS from a thylakoid-free cyanobacterium that likely retains ancestral traits. This PBS lacks rods and specialized ApcD and ApcF subunits, indicating relict characteristics. Other features include linkers connecting two chains of five phycocyanin hexamers (CpcN) and two core subdomains (ApcH), resulting in a paddle-shaped configuration. Energy transfer calculations demonstrate that chains are less efficient than rods. These features may nevertheless have increased light absorption by elongating PBS before multilayered thylakoids with hemidiscoidal PBS evolved. Our results provide insights into the evolution and diversification of light-harvesting strategies before the origin of thylakoids.

Attachment of Ferredoxin: NADP+ Oxidoreductase to Phycobilisomes Is Required for Photoheterotrophic Growth of the Cyanobacterium Synechococcus sp. PCC 7002

Two types of cyanobacterial phycobilisomes (PBS) are present: the hemidiscoidal PBS (CpcG-PBS) and the membrane-bound PBS (CpcL-PBS). Both types of PBS have ferredoxin:NADP⁺ oxidoreductase (FNR) attached to the termini of their rods through a CpcD domain. To date, the physiological significance of the attachment remains unknown. We constructed a mutant (dF338) which contains an FNR lacking the N-terminal CpcD domain in Synechococcus sp. PCC 7002. Isolated CpcG-PBS from dF338 did not contain FNR and the cell extracts of the mutant had a 35 kDa protein cross-reacting to anti-FNR antibodies. dF338 grows normally under photoautotrophic conditions, but little growth was observed under photoheterotrophic conditions. A cpcL (cpcG2) mutant grows extremely slowly under photoheterotrophic conditions while a cpcG (cpcG1) mutant, in which PBS rods could not attach to the cores of the CpcG-PBS, can grow photoheterotrophically, strongly suggesting that the attachment of FNR to CpcL-PBS is critical to photoheterotrophic growth. We show that electron transfer to the plastoquinone pool in dF338 and the cpcL mutant was impaired. We also provide evidence that trimeric photosystem I (PSI) and intact CpcL-PBS with a full-length FNR is critical to plastoquinone reduction. The presence of a NADPH-dehydrogenase (NDH)-CpcL-PBS-PSI trimer supercomplex and its roles are discussed.

Production of thermostable phycocyanin in a mesophilic cyanobacterium

Phycocyanin (PC) is a soluble phycobiliprotein found within the light-harvesting phycobilisome complex of cyanobacteria and red algae, and is considered a high-value product due to its brilliant blue colour and fluorescent properties. However, commercially available PC has a relatively low temperature stability. Thermophilic species produce more thermostable variants of PC, but are challenging and energetically expensive to cultivate. Here, we show that the PC operon from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 (cpcBACD) is functional in the mesophile Synechocystis sp. PCC 6803. Expression of cpcBACD in an 'Olive' mutant strain of Synechocystis lacking endogenous PC resulted in high yields of thermostable PC (112 ± 1 mg g-1 DW) comparable to that of endogenous PC in wild-type cells. Heterologous PC also improved the growth of the Olive mutant, which was further supported by evidence of a functional interaction with the endogenous allophycocyanin core of the phycobilisome complex. The thermostability properties of the heterologous PC were comparable to those of PC from T. elongatus, and could be purified from the Olive mutant using a low-cost heat treatment method. Finally, we developed a scalable model to calculate the energetic benefits of producing PC from T. elongatus in Synechocystis cultures. Our model showed that the higher yields and lower cultivation temperatures of Synechocystis resulted in a 3.5-fold increase in energy efficiency compared to T. elongatus, indicating that producing thermostable PC in non-native hosts is a cost-effective strategy for scaling to commercial production.

Proteomic Insights into Phycobilisome Degradation, A Selective and Tightly Controlled Process in The Fast-Growing Cyanobacterium Synechococcus elongatus UTEX 2973

Phycobilisomes (PBSs) are large (3-5 megadalton) pigment-protein complexes in cyanobacteria that associate with thylakoid membranes and harvest light primarily for photosystem II. PBSs consist of highly ordered assemblies of pigmented phycobiliproteins (PBPs) and linker proteins that can account for up to half of the soluble protein in cells. Cyanobacteria adjust to changing environmental conditions by modulating PBS size and number. In response to nutrient depletion such as nitrogen (N) deprivation, PBSs are degraded in an extensive, tightly controlled, and reversible process. In Synechococcus elongatus UTEX 2973, a fast-growing cyanobacterium with a doubling time of two hours, the process of PBS degradation is very rapid, with 80% of PBSs per cell degraded in six hours under optimal light and CO₂ conditions. Proteomic analysis during PBS degradation and re-synthesis revealed multiple proteoforms of PBPs with partially degraded phycocyanobilin (PCB) pigments. NblA, a small proteolysis adaptor essential for PBS degradation, was characterized and validated with targeted mass spectrometry. NblA levels rose from essentially 0 to 25,000 copies per cell within 30 min of N depletion, and correlated with the rate of decrease in phycocyanin (PC). Implications of this correlation on the overall mechanism of PBS degradation during N deprivation are discussed.

The amazing phycobilisome

Cyanobacteria and red-algae share a common light-harvesting complex which is different than all other complexes that serve as photosynthetic antennas - the Phycobilisome (PBS). The PBS is found attached to the stromal side of thylakoid membranes, filling up most of the gap between individual thylakoids. The PBS self assembles from similar homologous protein units that are soluble and contain conserved cysteine residues that covalently bind the light absorbing chromophores, linear tetra-pyrroles. Using similar construction principles, the PBS can be as large as 16.8 MDa (68×45×39nm), as small as 1.2 MDa (24 × 11.5 × 11.5 nm), and in some unique cases smaller still. The PBS can absorb light between 450 nm to 650 nm and in some cases beyond 700 nm, depending on the species, its composition and assembly. In this review, we will present new observations and structures that expand our understanding of the distinctive properties that make the PBS an amazing light harvesting system. At the end we will suggest why the PBS, for all of its excellent properties, was discarded by photosynthetic organisms that arose later in evolution such as green algae and higher plants.

Roles of CpcF and CpcG1 in Peroxiredoxin-Mediated Oxidative Stress Responses and Cellular Fitness in the Cyanobacterium Synechocystis sp. PCC 6803

As a component of the photosynthetic apparatus in cyanobacteria, the phycobilisome (PBS) plays an important role in harvesting and transferring light energy to the core photosynthetic reaction centers. The size, composition (phycobiliprotein and chromophore), and assembly of PBSs can be dynamic to cope with tuning photosynthesis and associated cellular fitness in variable light environments. Here, we explore the role of PBS-related stress responses by analyzing deletion mutants of cpcF or cpcG1 genes in Synechocystis sp. PCC 6803. The cpcF gene encodes a lyase that links the phycocyanobilin (PCB) chromophore to the alpha subunit of phycocyanin (PC), a central phycobiliprotein (PBP) in PBSs. Deletion of cpcF (i.e., ΔcpcF strain) resulted in slow growth, reduced greening, elevated reactive oxygen species (ROS) levels, together with an elevated accumulation of a stress-related Peroxiredoxin protein (Sll1621). Additionally, ΔcpcF exhibited reduced sensitivity to a photosynthesis-related stress inducer, methyl viologen (MV), which disrupts electron transfer. The cpcG1 gene encodes a linker protein that serves to connect PC to the core PBP allophycocyanin. A deletion mutant of cpcG1 (i.e.,ΔcpcG1) exhibited delayed growth, a defect in pigmentation, reduced accumulation of ROS, and insensitivity to MV treatment. By comparison, ΔcpcF and ΔcpcG1 exhibited similarity in growth, pigmentation, and stress responses; yet, these strains showed distinct phenotypes for ROS accumulation, sensitivity to MV and Sll1621 accumulation. Our data emphasize an importance of the regulation of PBS structure in ROS-mediated stress responses that impact successful growth and development in cyanobacteria.

Chlorosis as a Developmental Program in Cyanobacteria: The Proteomic Fundament for Survival and Awakening

Cyanobacteria that do not fix atmospheric nitrogen gas survive prolonged periods of nitrogen starvation in a chlorotic, dormant state where cell growth and metabolism are arrested. Upon nutrient availability, these dormant cells return to vegetative growth within 2-3 days. This resuscitation process is highly orchestrated and relies on the stepwise reinstallation and activation of essential cellular structures and functions. We have been investigating the transition to chlorosis and the return to vegetative growth as a simple model of a cellular developmental process and a fundamental survival strategy in biology. In the present study, we used quantitative proteomics and phosphoproteomics to describe the proteomic landscape of a dormant cyanobacterium and its dynamics during the transition to vegetative growth. We identified intriguing alterations in the set of ribosomal proteins, in RuBisCO components, in the abundance of central regulators and predicted metabolic enzymes. We found O-phosphorylation as an abundant protein modification in the chlorotic state, specifically of metabolic enzymes and proteins involved in photosynthesis. Nondegraded phycobiliproteins were hyperphosphorylated in the chlorotic state. We provide evidence that hyperphosphorylation of the terminal rod linker CpcD increases the lifespan of phycobiliproteins during chlorosis.

The proteolysis adaptor, NblA, binds to the N-terminus of β-phycocyanin: Implications for the mechanism of phycobilisome degradation

Phycobilisome (PBS) complexes are massive light-harvesting apparati in cyanobacteria that capture and funnel light energy to the photosystem. PBS complexes are dynamically degraded during nutrient deprivation, which causes severe chlorosis, and resynthesized during nutrient repletion. PBS degradation occurs rapidly after nutrient step down, and is specifically triggered by non-bleaching protein A (NblA), a small proteolysis adaptor that facilitates interactions between a Clp chaperone and phycobiliproteins. Little is known about the mode of action of NblA during PBS degradation. In this study, we used chemical cross-linking coupled with LC-MS/MS to investigate the interactions between NblA and phycobiliproteins. An isotopically coded BS³ cross-linker captured a protein interaction between NblA and β-phycocyanin (PC). LC-MS/MS analysis identified the amino acid residues participating in the binding reaction, and demonstrated that K⁵² in NblA is cross-linked to T² in β-PC. These results were modeled onto the existing crystal structures of NblA and PC by protein docking simulations. Our data indicate that the C-terminus of NblA fits in an open groove of β-PC, a region located inside the central hollow cavity of a PC rod. NblA may mediate PBS degradation by disrupting the structural integrity of the PC rod from within the rod. In addition, M¹-K⁴⁴ and M¹-K⁵² cross-links between the N-terminus of NblA and the C-terminus of NblA are consistent with the NblA crystal structure, confirming that the purified NblA is structurally and biologically relevant. These findings provide direct evidence that NblA physically interacts with β-PC.

Distribution of isoforms of ferredoxin-NADP+ reductase (FNR) in cyanobacteria in two growth conditions

Ferredoxin-NADP⁺ reductase (FNR) transfers reducing equivalents between ferredoxin and NADP(H) in the photosynthetic electron transport chains of chloroplasts and cyanobacteria. In most cyanobacteria, FNR is coded by a single petH gene. The structure of FNR in photosynthetic organisms can be constituted by FAD-binding and NADPH-binding domains (FNR-2D), or by these and an additional N-terminal domain (FNR-3D). In this article, biochemical evidence is provided supporting the induction of FNR-2D by iron or combined nitrogen deficiency in the cyanobacteria Synechocystis PCC 6803 and Anabaena variabilis ATCC 29413. In cell extracts of these cyanobacteria, most of FNR was associated to phycobilisomes (PBS) or phycocyanin (PC), and the rest was found as free enzyme. Free FNR activity increased in both cyanobacteria under iron stress and during diazotrophic conditions in A. variabilis. Characterization of FNR from both cyanobacteria showed that the PBS-associated enzyme was FNR-3D and the free enzyme was mostly a FNR-2D isoform. Predominant isoforms in heterocysts of A. variabilis were FNR-2D; where its N-terminal sequence lacked an initial (formyl)methionine. This means that FNR-3D is targeted to thylakoid membrane, and anchored to PBS, and FNR-2D is found as a soluble protein in the cytoplasm, when iron or fixed nitrogen deficiencies prevail in the environment. Moreover, given that Synechocystis and Anabaena variabilis are dissimilar in genotype, phenotype and ecology, the presence of these two-domain proteins in these species suggests that the mechanism of FNR induction is common among cyanobacteria regardless of their habitat and morphotype.

The phycobilisomes: an early requisite for efficient photosynthesis in cyanobacteria

Cyanobacteria trap light energy by arrays of pigment molecules termed “phycobilisomes (PBSs)”, organized proximal to "reaction centers" at which chlorophyll perform the energy transduction steps with highest quantum efficiency. PBSs, composed of sequential assembly of various chromophorylated phycobiliproteins (PBPs), as well as nonchromophoric, basic and hydrophobic polypeptides called linkers. Atomic resolution structure of PBP is a heterodimer of two structurally related polypeptides but distinct specialised polypeptides- a and ß, made up of seven alpha-helices each which played a crucial step in evolution of PBPs. PBPs carry out various light dependent responses such as complementary chromatic adaptation. The aim of this review is to summarize and discuss the recent progress in this field and to highlight the new and the questions that remain unresolved.

Reconstruction and comparison of the metabolic potential of cyanobacteria Cyanothece sp. ATCC 51142 and Synechocystis sp. PCC 6803

Cyanobacteria are an important group of photoautotrophic organisms that can synthesize valuable bio-products by harnessing solar energy. They are endowed with high photosynthetic efficiencies and diverse metabolic capabilities that confer the ability to convert solar energy into a variety of biofuels and their precursors. However, less well studied are the similarities and differences in metabolism of different species of cyanobacteria as they pertain to their suitability as microbial production chassis. Here we assemble, update and compare genome-scale models (iCyt773 and iSyn731) for two phylogenetically related cyanobacterial species, namely Cyanothece sp. ATCC 51142 and Synechocystis sp. PCC 6803. All reactions are elementally and charge balanced and localized into four different intracellular compartments (i.e., periplasm, cytosol, carboxysome and thylakoid lumen) and biomass descriptions are derived based on experimental measurements. Newly added reactions absent in earlier models (266 and 322, respectively) span most metabolic pathways with an emphasis on lipid biosynthesis. All thermodynamically infeasible loops are identified and eliminated from both models. Comparisons of model predictions against gene essentiality data reveal a specificity of 0.94 (94/100) and a sensitivity of 1 (19/19) for the Synechocystis iSyn731 model. The diurnal rhythm of Cyanothece 51142 metabolism is modeled by constructing separate (light/dark) biomass equations and introducing regulatory restrictions over light and dark phases. Specific metabolic pathway differences between the two cyanobacteria alluding to different bio-production potentials are reflected in both models.

A proteomic approach to the analysis of the components of the phycobilisomes from two cyanobacteria with complementary chromatic adaptation: Fremyella diplosiphon UTEX B590 and Tolypothrix PCC 7601

Tolypothrix PCC 7601 and Fremyella diplosiphon UTEX B590 can produce two alternative phycobilisome (PBS) rods. PE-PBSs with one phycocyanin (PC) disk and multiple phycoerythrin (PE) disks are found in cells grown under green light (GL). PC-PBSs with only PC disks are obtained from cells grown under red light (RL). In this manuscript, we show the localization of the linker proteins and ferredoxin-NADP(+) oxidoreductase (FNR) in the PC-PBS and of PE-PBS rods using visible spectroscopy and mass spectrometry. PE-PBSs with different [PE]/[PC] ratios and PC-PBSs with different [PC]/[AP] (AP, allophycocyanin) ratios were isolated. CpeC was the primary rod linker protein found in the PBSs with a [PE]/[PC] ratio of 1.1, which indicates that this is the rod linker at the interphase PC-PE. CpeC and CpeD were identified in the PBSs with a [PE]/[PC] ratio of 1.6, which indicates that CpcD is the linker between the first and the second PE hexamers. Finally, CpeC, CpeD, and CpeE were found in the PBSs with a [PE]/[PC] ratio of 2.9, indicating the position of CpeE between the second and third PE moieties. CpcI2 was identified in the two PC-PBSs obtained from cells grown under RL, which indicates that CpcI2 is the linker between the first and second PC hexamers. CpcH2 was identified only in the PC-PBSs from Tolypothrix with a high [PC]/[AP] ratio of 1.92, which indicates that CpcH2 is the linker between the second and third PC hexamers. The PC-PBSs contained the rod cap protein L(R)(10) (CpcD), but this protein was absent in the PE-PBSs. PE-PBSs (lacking L(R)(10)) incorporated exogenous rFNR in a stoichiometry of up to five FNRs per PBS. A maximum of two FNRs per PBS were found in PC-PBSs (with L(R)(10)). These observations support the hypothesis that FNR binds at the distal ends of the PBS rods in the vacant site of CpcD L(R)(10). Finally, the molecular mass of the core membrane linker (L(CM)) was determined to be 102 kDa from a mass spectrometry analysis.

Characterization of the activities of the CpeY, CpeZ, and CpeS bilin lyases in phycoerythrin biosynthesis in Fremyella diplosiphon strain UTEX 481

When grown in green light, Fremyella diplosiphon strain UTEX 481 produces the red-colored protein phycoerythrin (PE) to maximize photosynthetic light harvesting. PE is composed of two subunits, CpeA and CpeB, which carry two and three phycoerythrobilin (PEB) chromophores, respectively, that are attached to specific Cys residues via thioether linkages. Specific bilin lyases are hypothesized to catalyze each PEB ligation. Using a heterologous, coexpression system in Escherichia coli, the PEB ligation activities of putative lyase subunits CpeY, CpeZ, and CpeS were tested on the CpeA and CpeB subunits from F. diplosiphon. Purified His(6)-tagged CpeA, obtained by coexpressing cpeA, cpeYZ, and the genes for PEB synthesis, had absorbance and fluorescence emission maxima at 566 and 574 nm, respectively. CpeY alone, but not CpeZ, could ligate PEB to CpeA, but the yield of CpeA-PEB was lower than achieved with CpeY and CpeZ together. Studies with site-specific variants of CpeA(C82S and C139S), together with mass spectrometric analysis of trypsin-digested CpeA-PEB, revealed that CpeY/CpeZ attached PEB at Cys(82) of CpeA. The CpeS bilin lyase ligated PEB at both Cys(82) and Cys(139) of CpeA but very inefficiently; the yield of PEB ligated at Cys(82) was much lower than observed with CpeY or CpeY/CpeZ. However, CpeS efficiently attached PEB to Cys(80) of CpeB but neither CpeY, CpeZ, nor CpeY/CpeZ could ligate PEB to CpeB.

Interactions of linker proteins with the phycobiliproteins in the phycobilisome substructures of Gloeobacter violaceus

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.

Complementation of a phycocyanin-bilin lyase from Synechocystis sp. PCC 6803 with a nucleomorph-encoded open reading frame from the cryptophyte Guillardia theta

BACKGROUND

Cryptophytes are highly compartmentalized organisms, expressing a secondary minimized eukaryotic genome in the nucleomorph and its surrounding remnant cytoplasm, in addition to the cell nucleus, the mitochondrion and the plastid. Because the members of the nucleomorph-encoded proteome may contribute to essential cellular pathways, elucidating nucleomorph-encoded functions is of utmost interest. Unfortunately, cryptophytes are inaccessible for genetic transformations thus far. Therefore the functions of nucleomorph-encoded proteins must be elucidated indirectly by application of methods in genetically accessible organisms.

RESULTS

Orf222, one of the uncharacterized nucleomorph-specific open reading frames of the cryptophyte Guillardia theta, shows homology to slr1649 of Synechocystis sp. PCC 6803. Recently a further homolog from Synechococcus sp. PCC 7002 was characterized to encode a phycocyanin-beta155-bilin lyase. Here we show by insertion mutagenesis that the Synechocystis sp. PCC 6803 slr1649-encoded protein also acts as a bilin lyase, and additionally contributes to linker attachment and/or stability of phycobilisomes. Finally, our results indicate that the phycocyanin-beta155-bilin lyase of Synechocystis sp. PCC 6803 can be complemented in vivo by the nucleomorph-encoded open reading frame orf222.

CONCLUSION

Our data show that the loss of phycocyanin-lyase function causes pleiotropic effects in Synechocystis sp. PCC 6803 and indicate that after separating from a common ancestor protein, the phycoerythrin lyase from Guillardia theta has retained its capacity to couple a bilin group to other phycobiliproteins. This is a further, unexpected example of the universality of phycobiliprotein lyases.

CpcM posttranslationally methylates asparagine-71/72 of phycobiliprotein beta subunits in Synechococcus sp. strain PCC 7002 and Synechocystis sp. strain PCC 6803

Cyanobacteria produce phycobilisomes, which are macromolecular light-harvesting complexes mostly assembled from phycobiliproteins. Phycobiliprotein beta subunits contain a highly conserved gamma-N-methylasparagine residue, which results from the posttranslational modification of Asn71/72. Through comparative genomic analyses, we identified a gene, denoted cpcM, that (i) encodes a protein with sequence similarity to other S-adenosylmethionine-dependent methyltransferases, (ii) is found in all sequenced cyanobacterial genomes, and (iii) often occurs near genes encoding phycobiliproteins in cyanobacterial genomes. The cpcM genes of Synechococcus sp. strain PCC 7002 and Synechocystis sp. strain PCC 6803 were insertionally inactivated. Mass spectrometric analyses of phycobiliproteins isolated from the mutants confirmed that the CpcB, ApcB, and ApcF were 14 Da lighter than their wild-type counterparts. Trypsin digestion and mass analyses of phycobiliproteins isolated from the mutants showed that tryptic peptides from phycocyanin that included Asn72 were also 14 Da lighter than the equivalent peptides from wild-type strains. Thus, CpcM is the methyltransferase that modifies the amide nitrogen of Asn71/72 of CpcB, ApcB, and ApcF. When cells were grown at low light intensity, the cpcM mutants were phenotypically similar to the wild-type strains. However, the mutants were sensitive to high-light stress, and the cpcM mutant of Synechocystis sp. strain PCC 6803 was unable to grow at moderately high light intensities. Fluorescence emission measurements showed that the ability to perform state transitions was impaired in the cpcM mutants and suggested that energy transfer from phycobiliproteins to the photosystems was also less efficient. The possible functions of asparagine N methylation of phycobiliproteins are discussed.

Biogenesis of phycobiliproteins: I. cpcS-I and cpcU mutants of the cyanobacterium Synechococcus sp. PCC 7002 define a heterodimeric phyococyanobilin lyase specific for beta-phycocyanin and allophycocyanin subunits

Phycobilin lyases covalently attach phycobilin chromophores to apo-phycobiliproteins (PBPs). Genome analyses of the unicellular, marine cyanobacterium Synechococcus sp. PCC 7002 identified three genes, denoted cpcS-I, cpcU, and cpcV, that were possible candidates to encode phycocyanobilin (PCB) lyases. Single and double mutant strains for cpcS-I and cpcU exhibited slower growth rates, reduced PBP levels, and impaired assembly of phycobilisomes, but a cpcV mutant had no discernable phenotype. A cpcS-I cpcU cpcT triple mutant was nearly devoid of PBP. SDS-PAGE and mass spectrometry demonstrated that the cpcS-I and cpcU mutants produced an altered form of the phycocyanin (PC) beta subunit, which had a mass approximately 588 Da smaller than the wild-type protein. Some free PCB (mass = 588 Da) was tentatively detected in the phycobilisome fraction purified from the mutants. The modified PC from the cpcS-I, cpcU, and cpcS-I cpcU mutant strains was purified, and biochemical analyses showed that Cys-153 of CpcB carried a PCB chromophore but Cys-82 did not. These results show that both CpcS-I and CpcU are required for covalent attachment of PCB to Cys-82 of the PC beta subunit in this cyanobacterium. Suggesting that CpcS-I and CpcU are also required for attachment of PCB to allophycocyanin subunits in vivo, allophycocyanin levels were significantly reduced in all but the CpcV-less strain. These conclusions have been validated by in vitro experiments described in the accompanying report (Saunée, N. A., Williams, S. R., Bryant, D. A., and Schluchter, W. M. (2008) J. Biol. Chem. 283, 7513-7522). We conclude that the maturation of PBP in vivo depends on three PCB lyases: CpcE-CpcF, CpcS-I-CpcU, and CpcT.

Biogenesis of phycobiliproteins: II. CpcS-I and CpcU comprise the heterodimeric bilin lyase that attaches phycocyanobilin to CYS-82 OF beta-phycocyanin and CYS-81 of allophycocyanin subunits in Synechococcus sp. PCC 7002

The Synechococcus sp. PCC 7002 genome encodes three genes, denoted cpcS-I, cpcU, cpcV, with sequence similarity to cpeS. CpcS-I copurified with His(6)-tagged (HT) CpcU as a heterodimer, CpcSU. When CpcSU was assayed for bilin lyase activity in vitro with phycocyanobilin (PCB) and apophycocyanin, the reaction product had an absorbance maximum of 622 nm and was highly fluorescent (lambda(max) = 643 nm). In control reactions with PCB and apophycocyanin, the products had absorption maxima at 635 nm and very low fluorescence yields, indicating they contained the more oxidized mesobiliverdin (Arciero, D. M., Bryant, D. A., and Glazer, A. N. (1988) J. Biol. Chem. 263, 18343-18349). Tryptic peptide mapping showed that the CpcSU-dependent reaction product had one major PCB-containing peptide that contained the PCB binding site Cys-82. The CpcSU lyase was also tested with recombinant apoHT-allophycocyanin (aporHT-AP) and PCB in vitro. AporHT-AP formed an ApcA/ApcB heterodimer with an apparent mass of approximately 27 kDa. When aporHT-AP was incubated with PCB and CpcSU, the product had an absorbance maximum of 614 nm and a fluorescence emission maximum at 636 nm, the expected maxima for monomeric holo-AP. When no enzyme or CpcS-I or CpcU was added alone, the products had absorbance maxima between 645 and 647 nm and were not fluorescent. When these reaction products were analyzed by gel electrophoresis and zinc-enhanced fluorescence emission, only the reaction products from CpcSU had PCB attached to both AP subunits. Therefore, CpcSU is the bilin lyase-responsible for attachment of PCB to Cys-82 of CpcB and Cys-81 of ApcA and ApcB.

Subcellular localization of ferredoxin-NADP(+) oxidoreductase in phycobilisome retaining oxygenic photosysnthetic organisms

Ferredoxin-NADP(+) oxidoreductase (FNR) catalyzing the terminal step of the linear photosynthetic electron transport was purified from the cyanobacterium Spirulina platensis and the red alga Cyanidium caldarium. FNR of Spirulina consisted of three domains (CpcD-like domain, FAD-binding domain, and NADP(+)-binding domain) with a molecular mass of 46 kDa and was localized in either phycobilisomes or thylakoid membranes. The membrane-bound FNR with 46 kDa was solublized by NaCl and the solublized FNR had an apparent molecular mass of 90 kDa. FNR of Cyanidium consisted of two domains (FAD-binding domain and NADP(+)-binding domain) with a molecular mass of 33 kDa. In Cyanidium, FNR was found on thylakoid membranes, but there was no FNR on phycobilisomes. The membrane-bound FNR of Cyanidium was not solublized by NaCl, suggesting the enzyme is tightly bound in the membrane. Although both cyanobacteria and red algae are photoautotrophic organisms bearing phycobilisomes as light harvesting complexes, FNR localization and membrane-binding characteristics were different. These results suggest that FNR binding to phycobilisomes is not characteristic for all phycobilisome retaining oxygenic photosynthetic organisms, and that the rhodoplast of red algae had possibly originated from a cyanobacterium ancestor, whose FNR lacked the CpcD-like domain.

Phycobilisomes linker family in cyanobacterial genomes: divergence and evolution

Cyanobacteria are the oldest life form making important contributions to global CO2 fixation on the Earth. Phycobilisomes (PBSs) are the major light harvesting systems of most cyanobacteria species. Recent availability of the whole genome database of cyanobacteria provides us a global and further view on the complex structural PBSs. A PBSs linker family is crucial in structure and function of major light-harvesting PBSs complexes. Linker polypeptides are considered to have the same ancestor with other phycobiliproteins (PBPs), and might have been diverged and evolved under particularly selective forces together. In this paper, a total of 192 putative linkers including 167 putative PBSs-associated linker genes and 25 Ferredoxin-NADP oxidoreductase (FNR) genes were detected through whole genome analysis of all 25 cyanobacterial genomes (20 finished and 5 in draft state). We compared the PBSs linker family of cyanobacteria in terms of gene structure, chromosome location, conservation domain, and polymorphic variants, and discussed the features and functions of the PBSs linker family. Most of PBSs-associated linkers in PBSs linker family are assembled into gene clusters with PBPs. A phylogenetic analysis based on protein data demonstrates a possibility of six classes of the linker family in cyanobacteria. Emergence, divergence, and disappearance of PBSs linkers among cyanobacterial species were due to speciation, gene duplication, gene transfer, or gene loss, and acclimation to various environmental selective pressures especially light.

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) alpha and beta 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.

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.

A suppressor mutation in the alpha-phycocyanin gene in the light/glucose-sensitive phenotype of the psbK-disruptant of the cyanobacterium Synechocystis sp. PCC 6803

psbK encodes a small transmembrane component of PSII. Here we report that the psbK-disruptant of Synechocystis sp. PCC 6803 cannot survive under photomixotrophic conditions of light and glucose after transient growth, while the wild type is able to grow. A spontaneous yellow-green mutant that recovered the sustained growth under the same conditions was isolated from the psbK-disruptant. Instead of recovery, the mutant largely lost photoautotrophic growth. By phenotype complementation, the mutation was identified in cpcA as a sequence replacement with a close downstream segment, generating an inverted repeat of 23 bp. The mutant phenotype was characterized by (i) the complete loss of alpha- and beta-phycocyanin; (ii) increased accumulation of PSII; and (iii) greatly reduced transcripts harboring cpcA in abundance and in size. The inverted repeat generated in cpcA probably led to the early termination of transcription. A possible mechanism for such a mutation is discussed.

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.

Elucidation of the molecular structures of components of the phycobilisome: reconstructing a giant

The molecular architectures of photosynthetic complexes are rapidly becoming available through the power of X-ray crystallography. These complexes are comprised of antenna complexes, which absorb and transfer energy into photochemical reaction centers. Most reaction centers, found in both oxygenic and non-oxygenic species, are connected to transmembrane chlorophyll containing antennas, and the crystal structures of these antennas contain information on the structure of the entire complex as well as clear indications on their modes of functional association. In cyanobacteria and red alga, most of the Photosystem II associated light harvesting is performed by an enormous (3-7 MDa) membrane attached complex called the phycobilisome (PBS). While the crystal structures of many isolated components of different PBSs have been determined, the structure of the entire complex as well as its manner of association with Photosystem II can only be suggested. In this review, the structural information obtained on the isolated components will be described. The structural information obtained from the components provides the basis for the modeled reconstruction of this giant complex.

CpeR is an activator required for expression of the phycoerythrin operon (cpeBA) in the cyanobacterium Fremyella diplosiphon and is encoded in the phycoerythrin linker-polypeptide operon (cpeCDESTR)

In the cyanobacteria, phycobilisomes are assembled from (alphabeta)(6) hexamers of the coloured phycobiliproteins, allophycocyanin, phycocyanin and phycoerythrin (PE). The precise architecture of the phycobilisome is determined by the various colourless linker proteins that bind to the biliprotein hexamers. Genes for beta and alpha subunits of PE make up one operon (cpeBA), whereas genes for PE-associated linker polypeptides are in a second operon. In the chromatically adapting cyanobacterium Fremyella diplosiphon green light is required for the transcription of both cpeBA and the operon encoding the PE-associated linkers (cpeCDE). From the genome of F. diplosiphon we have identified an open reading frame, cpeR, which, when expressed from a shuttle plasmid, is capable of suppressing various mutations that cause a decrease in PE synthesis. The introduction of a shuttle plasmid bearing cpeR+ into wild-type F. diplosiphon caused PE expression in red light. Fremyella diplosiphon cpeR-, created by in vitro mutagenesis and in vivo homologous recombination, is fully PE and, in this strain, cpeCDE is transcribed normally whereas the transcript from cpeBA is undetectable. Polymerase chain reaction (PCR) amplification of cDNA showed that cpeR is transcribed as part of the cpeCDE operon on an extended transcript. As CpeR is an activator required for expression of the cpeBA operon, we propose that at the onset of green light the operons cpeCDESTR and cpeBA are expressed in series as a genetic cascade.

The complete purification and characterization of three forms of ferredoxin-NADP(+) oxidoreductase from a thermophilic cyanobacterium Synechococcus elongatus

The petH gene, encoding ferredoxin-NADP(+) oxidoreductase (FNR), was isolated from a thermophilic cyanobacterium, Synechococcus elongatus (the same strain as Thermosynechococcus elongatus). The petH gene of S. elongatus was a single copy gene, and the N-terminal region of PetH showed a sequence similarity to the CpcD-phycobilisome linker polypeptide. The amino acid sequence of the catalytic domains of PetH was markedly similar to those from mesophilic cyanobacterial PetH and higher plant FNR. The enzymatically active FNR protein was purified to homogeneity from S. elongatus as three forms corresponding to the 45-kDa form retaining the CpcD-like domain, the 34-kDa form lacking the CpcD-like domain, and the 78-kDa complex with phycocyanin. The FNR in the 78-kDa complex was tolerant to proteolytic cleavage. However, the dissociation of phycocyanin from the 78-kDa complex induced to specific proteolysis between the CpcD-like domain and the FAD-binding domain to give rise to the 34-kDa form of FNR. The enzymatic activity of the 45-kDa form was thermotolerant, but the 45-kDa form readily aggregated under the storage at -30 degrees C. These results suggest that the association with phycocyanin via CpcD-like domain gives remarkable stability to S. elongatus FNR.

Spectroscopic study of the light-harvesting protein C-phycocyanin associated with colorless linker peptides

C-Phycocyanin (PC) trimers associated with linker polypeptides were isolated from the phycobilisome (PBS) rods of Synechococcus sp. PCC 7002. LXY refers to a linker polypeptide (L) having an apparent mass of Y kDa, located at position X in the phycobilisome where X can be R (rod), C (core) or RC (rod-core junction). Measurements of the absorption, fluorescence and excitation anisotropy of PC trimer, PC.LR32.3 and PC.LRC28.5 complexes document the spectroscopic modulation of each linker polypeptide on the PC chromophores. The difference spectra between the PC trimer and the PC-linker complexes show that although the effect induced by the linker polypeptides is qualitatively similar in behavior, the extent of the modulation is greater in PC.LRC28.5. Measurements taken at 77 K show that a red-wavelength component of the PC trimer absorption-fluorescence spectra is the target of the linker's influence and that this component is altered to a greater extent by LRC28.5. In addition the 77 K absorbance of the PC trimer resolves band features that are consistent with an excitonic coupling interaction between neighboring alpha 84 and beta 84 chromophores. These band features are also evident in the absorbance of PC.LR32.3 but are absent in PC.LRC28.5 indicating that LRC28.5 may be perturbing the coupling interaction established in the PC trimer alpha 84-beta 84 chromophore pairs. Structurally, the linker polypeptide should disrupt the C3 symmetry in the central cavity of the associated phycobiliprotein and this asymmetric interaction should serve to guide the transfer of excitation energy along PBS rods toward the core elements.

The blue-colored linker polypeptide L55 is a fusion protein of phycobiliproteins in the cyanobacterium synechocystis sp. strain BO 8402

The cyanobacterium Synechocystis sp. strain BO 8402, isolated from Lake Constance, lacks phycobilisomes but instead forms inclusion bodies containing remnants of phycobiliproteins. The inclusion bodies are surrounded by a proteinaceous capsule and contain alpha-phycocyanin and beta-phycocyanin, the rod linker polypeptide L35RPC and a novel blue-colored protein L55 with an apparent molecular mass of 55 kDa. An antibody raised against beta-phycocyanin showed a strong cross-reaction with L55. Mass spectrometry analysis of proteolytic peptides from L55 revealed mass identity to proteolytic peptides derived from L35RPC and beta-phycocyanin. However, analysis of the genome of strain BO 8402 revealed only one cpcBACE operon, encoding the apoproteins of beta-phycocyanin and alpha-phycocyanin, L35RPC and a subunit of the phycocyanin alpha subunit phycocyanobilin lyase, respectively. The gene structure, sequence and transcription of these genes were identical to that of a revertant strain, Synechocystis sp. strain BO 9201, which formed phycobilisomes and did not express L55. Based on these observations, we concluded that L55 did not derive from a particular gene or from a special form of mRNA-processing. We propose that L55 is formed by post-translational fusion of L35RPC and beta-phycocyanin. Cross-linking may stabilize the formation of the large paracrystalline phycocyanin aggregates unique to Synechocystis sp. strain BO 8402.

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.

Reconstitution, characterisation and mass analysis of the pentacylindrical allophycocyanin core complex from the cyanobacterium Anabaena sp. PCC 7120

The phycobilisome (PBS) of Anabaena sp. PCC 7120 was allowed to dissociate into its constituents and the resulting allophycocyanin (AP) fraction was purified. Its reconstitution yielded a complex which according to negative stain electron microscopy and spectral analysis was identical to the native pentacylindrical PBS core domain. Each cylinder of the central tricylindric unit was comprised of four AP (alphabeta)3 disks. Mass analysis using the scanning transmission electron microscope (STEM) showed the presence of 16 AP trimers in the intact reconstitute, which had a total mass of 1966(+/-66) kDa. Composition analysis indicated an AP trimer distribution of (AP-II):(AP-LCM):(AP-B):(AP-I)=6:2:2:6, i.e. an addition of two AP-I and two AP-II complexes compared to a tricylindrical PBS core domain. Therefore, we suggest that each supplementary half-core cylinder found in pentacylindrical AP core domains is comprised of one AP-I and one AP-II trimer, in agreement with the current model. The structural significance of the 127 kDa core membrane linker polypeptide was further investigated by subjecting the AP core reconstitute to mild chymotryptic degradation. After isolation, the digested complex exhibited a tricylindrical appearance while STEM mass analysis confirmed the presence of only 12 AP complexes. Polypeptide analysis by SDS-PAGE and Edman degradation related the half-cylinder loss to cleavage of the Rep4 domain of the core membrane linker polypeptide. On the basis of these data, a general model for the assembly of the three hemidiscoidal PBS types known to date is discussed.

Interaction of positively charged amino acid residues of recombinant, cyanobacterial ferredoxin:NADP+ reductase with ferredoxin probed by site directed mutagenesis

The petH genes encoding ferredoxin:NADP+ reductase (FNR) from two Anabaena species (PCC 7119 and ATCC 29413) were cloned and overexpressed in E. coli. Several positively charged residues (Arg, Lys) have been implicated to be involved in ferredoxin binding and electron transfer by cross-linking, chemical modification and protection experiments, and crystallographic studies. The following substitutions were introduced by site-directed mutagenesis: R153Q, K209Q, K212Q, R214Q, K275N, K430Q and K431Q in Anabaena 29413 FNR, and R153E, K209E, K212E, R214E, K275E, R401E, K427E, and K431E in Anabaena 7119 FNR. Comparison of the diaphorase activities, the specific rates of ferredoxin dependent NADP(+)-photoreduction and cytochrome c reduction catalyzed by FNR showed that all these amino acid residues were required for efficient electron transfer between FNR and ferredoxin. Replacement of any one of these basic residues produced a much more pronounced effect on the cytochrome c reductase activity, where FNR, reduced by NADPH, acted as electron donor, than in the reduction of NADP+ by photosystem I via FNR. A mutation involving the replacement of positive charge by a neutral amide produced in all cases a smaller inhibitory effect on the activity than a charge reversal mutation. In addition, it has been found that R214 was necessary for stable integration of the non covalently bound FAD-cofactor.

Isolation, characterization and electron microscopy analysis of a hemidiscoidal phycobilisome type from the cyanobacterium Anabaena sp. PCC 7120

In this work we present the characterization of a hemidiscoidal phycobilisome type of the heterocyst-forming cyanobacterium Anabaena sp. PCC 7120. The phycobilisome of this organism contains allophycocyanin, phycocyanin and phycoerythrocyanin, similar to the closely related thermophilic cyanobacterium Mastigocladus laminosus. Intact phycobilisomes exhibit an absorption maximum at 619 nm and two fluorescence maxima at 664 nm and 680 nm, corroborating the presence of a complete energy pathyway along the antenna. Upon dissociation, the phycobiliproteins were released from the phycobilisome. One phycoerythrocyanin, one phycocyanin and three allophycocyanin complexes were isolated by ion-exchange chromatography and characterized by absorption and fluorescence spectroscopy and by SDS/PAGE. The amino-terminal sequences of the polypeptides belonging to the phycoerythrocyanin and phycocyanin families were identical with the derived sequences of their corresponding genes. Partial amino-terminal sequences of the polypeptides belonging to the allophycocyanin family are presented here. Our results show that the phycobiliproteins and linker polypeptides from Anabaena sp. PCC 7120 are similar to the phycobilisome components characterized in other cyanobacteria. The phycobilisome of Anabaena sp. PCC 7120 was extensively analyzed by electron microscopy. It differs from the common hemidiscoidal tricylindrical, six-rod phycobilisome type by a core domain consisting of five core cylinders surrounded by up to eight rods radiating in a hemidiscoidal manner. One rod is linked to each basal core cylinder, whereas the remaining core cylinders bind two rods each. On the basis of the data presented in this work, a revised model for the hemidiscoidal pentacylindrical phycobilisome of Anabaena sp. PCC 7120, M. laminosus and Anabaena variabilis is proposed. This model accounts more accurately for the 'grape' pattern typically exhibited by these phycobilisomes in electron micrographs.

Selective disruption of energy flow from phycobilisomes to Photosystem I

Efficient production of ATP and NADPH by the 'light' reactions of oxygen-evolving photosynthesis demands continuous adjustment of transfer of absorbed light energy from antenna complexes to Photosystem I (PS I) and II (PS II) reaction center complexes in response to changes in light quality. Treatment of intact cyanobacterial cells with N-ethylmaleimide appears to disrupt energy transfer from phycobilisomes to Photosystem I (PS I). Energy transfer from phycobilisomes to Photosystem II (PS II) is unperturbed. Spectroscopic analysis indicates that the individual complexes (phycobilisomes, PS II, PS I) remain functionally intact under these conditions. The results are consistent with the presence of connections between phycobiliproteins and both PS II and PS I, but they do not support the existence of direct contacts between the two photosystems.

Cloning of the phycobilisome rod linker genes from the cyanobacterium Synechococcus sp. PCC 6301 and their inactivation in Synechococcus sp. PCC 7942

The phycobilisome rod linker genes in the two closely related cyanobacteria Synechococcus sp. PCC 6301 and Synechococcus sp. PCC 7942 were studied. Southern blot analysis showed that the genetic organization of the phycobilisome rod operon is very similar in the two strains. The phycocyanin gene pair is duplicated and separated by a region of about 2.5 kb. The intervening region between the duplicated phycocyanin gene pair was cloned from Synechococcus sp. PCC 6301 and sequenced. Analysis of this DNA sequence revealed the presence of three open reading frames corresponding to 273, 289 and 81 amino acids, respectively. Insertion of a kanamycin resistance cassette into these open reading frames indicated that they corresponded to the genes encoding the 30, 33 and 9 kDa rod linkers, respectively, as judged by the loss of specific linkers from the phycobilisomes of the insertional mutants. Amino acid compositions of the 30 and 33 kDa linkers derived from the DNA sequence were found to deviate from those of purified 33 and 30 kDa linkers in the amounts of glutamic acid/glutamine residues. On the basis of similarity of the amino acid sequence of the rod linkers between Synechococcus sp. PCC 6301 and Calothrix sp. PCC 7601 we name the genes encoding the 30, 33 and 9 kDa linkers cpcH, cpcI and cpcD, respectively. The three linker genes were found to be co-transcribed on an mRNA of 3700 nucleotides. However, we also detected a smaller species of mRNA, of 3400 nucleotides, which would encode only the cpcH and cpcI genes.(ABSTRACT TRUNCATED AT 250 WORDS)

Core mutations of Synechococcus sp. PCC 7002 phycobilisomes: a spectroscopic study

Three cyanobacterial strains harboring mutations affecting phycobilisome (PBS) cores were studied using steady state absorption and fluorescence and time-resolved fluorescence. The apcF mutant, missing beta 18, and the apcDF mutant, missing both alpha APB and beta 18, showed only small spectroscopic differences from the wild-type strain; their PBS emission was blue shifted by 10 nm, whereas their absorption spectra and time-resolved fluorescence kinetics were virtually unchanged. The third mutant studied was the apcE/C186S mutant in which the chromophore-binding cysteine-186 in the LCM99 polypeptide has been substituted with serine. The apcE/C186S mutant contained a modified chromophore which significantly changed the spectroscopic properties of the PBS complex. The apcE/C186S PBS absorbed more than the wild-type strain at 705 nm, and the emission spectrum gave two peaks at 660 nm and 715 nm. The time-resolved kinetics of the apcE/C186S mutant PBS were also significantly altered from those of the wild-type strain.

Structure and mutation of a gene encoding a Mr 33,000 phycocyanin-associated linker polypeptide

The gene encoding a phycocyanin-associated linker polypeptide of Mr 33,000 from the cyanobacterium Synechococcus sp. PCC 7002 was found to be located adjacent and 3' to the genes encoding the alpha and beta 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 Mr 33,000 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.

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 alpha and beta subunits; allophycocyanin alpha and beta subunits; alpha subunit of allophycocyanin B; the allophycocyanin beta-subunit-like polypeptide of Mr 18,000; the linker phycobiliprotein of Mr 99,000; and non-chromophore-carrying linker polypeptides of Mr 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 alpha and beta 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.