Conservative sorting in a primitive plastid. The cyanelle of Cyanophora paradoxa

Higher plant chloroplasts possess at least four different pathways for protein translocation across and protein integration into the thylakoid membranes. It is of interest with respect to plastid evolution, which pathways have been retained as a relic from the cyanobacterial ancestor ('conservative sorting'), which ones have been kept but modified, and which ones were developed at the organelle stage, i.e. are eukaryotic achievements as (largely) the Toc and Tic translocons for envelope import of cytosolic precursor proteins. In the absence of data on cyanobacterial protein translocation, the cyanelles of the glaucocystophyte alga Cyanophora paradoxa for which in vitro systems for protein import and intraorganellar sorting were elaborated can serve as a model: the cyanelles are surrounded by a peptidoglycan wall, their thylakoids are covered with phycobilisomes and the composition of their oxygen-evolving complex is another feature shared with cyanobacteria. We demonstrate the operation of the Sec and Tat pathways in cyanelles and show for the first time in vitro protein import across cyanobacteria-like thylakoid membranes and protease protection of the mature protein.     

Protein translocation into and within cyanelles (Review)

The cyanelles of the glaucocystophyte alga Cyanophora paradoxa resemble endosymbiotic cyanobacteria in morphology, pigmentation and, especially, in the presence of a peptidoglycan wall situated between the inner and outer envelope membranes. However, it is now clear that cyanelles in fact are primitive plastids. Phylogenetic analyses of plastid, nuclear and mitochondrial genes support a single primary endosymbiotic event. In this scenario cyanelles and all other plastid types are derived from an ancestral photosynthetic organelle combining the high plastid gene content of the Porphyra purpurea rhodoplast and the peptidoglycan wall of glaucocystophyte cyanelles. This means that the import apparatus of all primary plastids should be homologous. Indeed, heterologous in vitro import can now be shown in both directions, provided a phenylalanine residue essential for cyanelle import is engineered into the N-terminal part of chloroplast transit peptides. The cyanelle and likely also the rhodoplast import apparatus can be envisaged as prototypes with a single receptor showing this requirement for N-terminal phenylalanine. In chloroplasts, multiple receptors with overlapping and less stringent specificities have evolved explaining the efficient heterologous import of native precursors from C. paradoxa. With respect to conservative sorting in cyanelles, both the Sec and Tat pathways could be demonstrated. Another cyanobacterial feature, the dual location of the Sec translocase in thylakoid and inner envelope membranes, is also unique to cyanelles. For the first time, protease protection of internalized lumenal proteins could be shown for cyanobacteria-like, phycobilisome-bearing thylakoid membranes after import into isolated cyanelles.     

Ultrastructure of Acaryochloris marina, an oxyphotobacterium containing mainly chlorophyll d

We present a detailed investigation of the ultrastructure of the chlorophyll a/d-containing unicellular oxyphotobacterium Acaryochloris marina, combining light and transmission electron microscopy and showing freeze fractures of this organism for the first time. The cells were 1.8-2.1 microm x 1.5-1.7 microm in size. The cell envelope consisted of a peptidoglycan layer of approximately 10 nm thickness combined with an outer membrane. Cell division was intermediate between the constrictive and the septum type. The nucleoplasm, which contained several carboxysomes, was surrounded by 7-11 concentrically arranged thylakoids, which were predominantly stacked, with the exception of distinct areas where phycobiliproteins were located. The thylakoids were perforated by channel-like structures connecting the central and peripheral portions of the cytoplasm and not yet observed in other organisms. In freeze fractures, the protoplasmic fracture faces of thylakoid membranes were densely covered with particles of inhomogenous size. The particle size histogram peaked at 10-11, 13 and 18 nm. The 18-nm particles are assumed to represent photosystem I trimers. The particles on exoplasmic fracture faces, proposed to represent photosystem II complexes, were significantly larger than the corresponding particles of cyanobacteria and clustered to form large aggregates. This kind of arrangement is unique among photosynthetic organisms.     

On the linkage of exoplasmatic freeze-fracture particles to phycobilisomes

The thylakoids of the thermophilic cyanobacterium Mastigocladus laminosus were examined by freeze-fracture analysis. The expolasmatic (EF)-freeze-fracture particles are organized in rows, separated by 45 nm or more with a 12-nm center-tocenter spacing of neighboring particles. Phycobilisomes, associated to the outer thylakoid surfaces show a similar spacing pattern. Fractures exposing simultaneously phycobilisomes and EF-freeze-fracture particles on the same thylakoid show a direct alignment of both systems. Consequently the phycobilisomes are concluded to be associated peripherally on top of the EF-freeze-fracture particles in a 1:1 assembly pattern. The periodicity of the EF-freeze-fracture particles determines the arrangement of the phycobilisomes in the rows. The planar phycobilisome model of Mörschel et al. (1977) easily allows a successive arrangement of the phycobilisomes in a row, whereas with the staggered model developed by Bryant et al. (1979), only a cogged arrangement of neighboring phycobilisomes is possible.     

Protein translocation into and within cyanelles (review)

The cyanelles of the glaucocystophyte alga Cyanophora paradoxa resemble endosymbiotic cyanobacteria in morphology, pigmentation and, especially, in the presence of a peptidoglycan wall situated between the inner and outer envelope membranes. However, it is now clear that cyanelles in fact are primitive plastids. Phylogenetic analyses of plastid, nuclear and mitochondrial genes support a single primary endosymbiotic event. In this scenario cyanelles and all other plastid types are derived from an ancestral photosynthetic organelle combining the high plastid gene content of the Porphyra purpurea rhodoplast and the peptidoglycan wall of glaucocystophyte cyanelles. This means that the import apparatus of all primary plastids should be homologous. Indeed, heterologous in vitro import can now be shown in both directions, provided a phenylalanine residue essential for cyanelle import is engineered into the N-terminal part of chloroplast transit peptides. The cyanelle and likely also the rhodoplast import apparatus can be envisaged as prototypes with a single receptor showing this requirement for N-terminal phenylalanine. In chloroplasts, multiple receptors with overlapping and less stringent specificities have evolved explaining the efficient heterologous import of native precursors from C. paradoxa. With respect to conservative sorting in cyanelles, both the Sec and Tat pathways could be demonstrated. Another cyanobacterial feature, the dual location of the Sec translocase in thylakoid and inner envelope membranes, is also unique to cyanelles. For the first time, protease protection of internalized lumenal proteins could be shown for cyanobacteria-like, phycobilisome-bearing thylakoid membranes after import into isolated cyanelles.     

The Supramolecular Structure of Photosystem II — Phycobilisome-Complexes of Porphyridium cruentum

The structure and arrangement of phycobilisomes of the unicellular red alga Porphyridium cruentum is compared with the organization of the thylakoid freeze-fracture particles in order to determine the relationship between phycobilisomes and photosystem II. The hemi-ellipsoidal phycobilisomes, 20 nm thick, are predominantly organized into rows; their centre to centre periodicity is 30–40 nm, so that they are well separated by a gap of 10–20 nm. The phycobilisomes are cleaved by a central faint furrow, parallel to the long axis from top to base. The organization of the exoplasmic particles in rows is similar to the arrangement of the phycobilisomes so that a structural relationship between both systems, previously demonstrated in cyanobacteria, is evident. Within the rows, the 10 nm EF-particles are grouped in tetrameric complexes separated by distances similar to those observed for phycobilisomes. We propose that the tetrameric EF-particle complexes correspond to tetrameric photosystem II complexes which bind one hemi-ellipsoidal phycobilisome on the stroma exposed surface of the thylakoid. A hypothetical model of this photosystem II-phycobilisome complex is presented.     

Functional assembly in vitro of phycobilisomes with isolated photosystem II particles of eukaryotic chloroplasts

Phycobiliproteins obtained by dissociation of phycobilisomes were reassociated in vitro with intact thylakoids or isolated photosystems I and II preparations obtained from cyanophytes (prokaryotes) or green algae (eukaryotes) to form bound phycobilisome complexes. Energy transfer from Fremyella diplosiphon phycobiliproteins to chlorophyll a of reaction centers I and II was measured in: complexes containing intact thylakoids of the cyanophytes F. diplosiphon or Anacystis nidulans and the eukaryotic algae Euglena gracilis and mutants of Chlamydomonas reinhardtii; complexes containing isolated photosystem II particles of A. nidulans or C. reinhardtii; and complexes containing reaction center I of F. diplosiphon or C. reinhardtii. Energy transfer from phycoerythrin to chlorophyll a of photosystem II could be demonstrated in complexes containing phycobilisomes bound to cyanophyte thylakoids or isolated photosystem II particles of A. nidulans or C. reinhardtii. Bound phycobilisomes did not transfer energy to photosystem II within green algae thylakoids containing altered forms of light-harvesting chlorophyll a/b-protein complex (LHC) II antenna, reduced amounts of LHC II, or chlorophyll b, or chlorophyll b-less mutants, nor to chlorophyll a of photosystem I of intact thylakoids or isolated reaction centers. We conclude that phycobilisomes can form a specific and functional association with photosystem II particles of both cyanophytes and eukaryotic thylakoids. This interaction appears to be hindered by the presence of LHC II antenna in the eukaryotic thylakoids.     

Phycobiliproteins and phycobilisomes: the early observations

The purpose of this minireview is to highlight the early observations that led to the discovery of the physico-chemical properties of the phycobiliproteins, their structure and function, and to their architectural organization in supramolecular complexes, the phycobilisomes. Generally attached on the stromal surface of the thylakoid membranes in both prokaryotic (cyanobacteria) and eukaryotic cells (cyanelles, red algae and cryptomonads), these complexes represent the most abundant soluble proteins and the major light-harvesting antennae for photosynthesis. This review mainly focuses on the years prior to the development of the molecular biology of cyanobacteria that flourished in the 1980s. We refer the reader to the comprehensive and excellent review by Sidler (1994) for more recent discoveries and more detailed literature on this topic. [-pt] `It would be difficult to find another series of colouring matters of greater beauty or with such remarkable and instructive chemical and physical peculiarities'. H. Sorby, 1877. This revised version was published online in August 2006 with corrections to the Cover Date.     

Changes in thylakoid structure associated with the differentiation of heterocysts in the cyanobacterium, Anabaena cylindrica

The thylakoids of vegetative cells of the filamentous cyanobacterium, Anabaena cylindrica, are capable of oxygen-evolving photosynthesis and contain both Photosystems I and II (PSI and PSII). The heterocysts, cells specialized for nitrogen fixation, do not produce oxygen and lack Photosystem II activity, the major accessory pigments, and perhaps the chlorophyll a associated with PSII. Freeze-fracture replicas of vegetative cells and of heterocysts reveal differences in the structure of the thylakoids. A histogram of particle sizes on the expolasmic fracture face (E-face, EF) of vegetative cell thylakoids has two major peaks, at 75 and 100 Å. The corresponding histogram for heterocyst thylakoids lacks the 100 Å size class, but has a very large peak at about 55 Å with a shoulder at 75 Å. Histograms of protoplasmic fracture face (P-face, PF) particle diameters show single broad peaks, the mean diameter being 71 Å for vegetative cells and 64 Å for heterocysts. The thylakoids of both cell types have about 5600 particles/μm² on the P-face. On the E-face, the density drops from 939 particles/μm² on vegetative cell thylakoids to 715 particles/μm² on heterocyst thylakoids. The data suggest that the 100 Å E-face particle of vegetative cell thylakoids is a PSII complex. The 55 Å EF particle of heterocysts may be part of the nitrogenase complex or a remnant of the PSII complex. The role of the 75 Å EF particle is unknown. Other functions localized on cyanobacterial thylakoids, such as respiration and hydrogenase activity, must be considered when interpreting the structure of these complex thylakoids.     

Changes in thylakoid structure associated with the differentiation of heterocysts in the cyanobacterium, Anabaena cylindrica.

The thylakoids of vegetative cells of the filamentous cyanobacterium, Anabaena cylindrica, are capable of oxygen-evolving photosynthesis and contain both Photosystems I and II (PSI and PSII). The heterocysts, cells specialized for nitrogen fixation, do not produce oxygen and lack Photosystem II activity, the major accessory pigments, and perhaps the chlorophyll a associated with PSII. Freeze-fracture replicas of vegetative cells and of heterocysts reveal differences in the structure of the thylakoids. A histogram of particle sizes on the exoplasmic fracture face (E-face, EF) of vegetative cell thylakoids has two major peaks, at 75 and 100 A. The corresponding histogram for heterocyst thylakoids lacks the 100 A size class, but has a very large peak at about 55 A with a shoulder at 75 A. Histograms of protoplasmic fracture face (P-face, PF) particle diameters show single broad peaks, the mean diameter being 71 A for vegetative cells and 64 A for heterocysts. The thylakoids of both cell types have about 5600 particles/micrometers2 on the P-face. On the E-face, the density drops from 939 particles/micrometers2 on vegetative cell thylakoids to 715 particles/micrometers2 on heterocyst thylakoids. The data suggest that the 100 A E-face particle of vegetative cell thylakoids is a PSII complex. The 55 A EF particle of heterocysts may be part of the nitrogenase complex or a remnant of the PSII complex. The role of the 75 A EF particle is unknown. Other functions localized on cyanobacterial thylakoids, such as respiration and hydrogenase activity, must be considered when interpreting the structure of these complex thylakoids.     

A carboxysomal carbon-concentrating mechanism in the cyanelles of the 'coelacanth' of the algal world, Cyanophora paradoxa?

Cyanelles are the peculiar plastids of glaucocystophyte algae that retained a peptidoglycan wall from the ancestral cyanobacterial endosymbiont. All cyanobacteria and most algae possess an inorganic carbon-concentrating mechanism (CCM) that involves a microcompartment--carboxysomes in prokaryotes and pyrenoids in eukaryotes--harboring the bulk of cellular (plastidic) Rubisco. In the case of the living fossil, Cyanophora paradoxa, the existence of a CCM was a matter of debate. Microarray data revealing 142 CO(2)-responsive genes (induced or repressed through a shift from high to low CO(2) conditions), gas exchange measurements and measurements of photosynthetic affinity provided strong support for a CCM. We favor a recent hypothesis that glaucocystophyte cyanelles as the closest cousins to cyanobacteria among plastids contain 'eukaryotic carboxysomes': bicarbonate enrichment within cyanelles should be considerably higher than in chloroplasts with their pyrenoid-based CCM. Thus, the stress-bearing function of the peptidoglycan layer, the other unique heritage, would be indispensable. An isolation method for cyanelle 'carboxysomes' was developed and the protein components other than Rubisco analyzed by MS. Rubisco activase was identified and corroborated by western blotting. The well-established cyanelle in vitro import system allows to use them as 'honorary cyanobacteria': assembly processes of supramolecular structures as phycobilisomes and carboxysomes thus can be studied after import of nucleus-encoded precursor proteins and subsequent fractionation. Even minor components can easily be tracked and a surprisingly dynamic view is obtained. Labeled pre-activase was imported into isolated cyanelles and 30% of the mature protein was found to be incorporated into the carboxysome fraction. A final decision between carboxysome or pyrenoid must await the identification of cyanelle carbonic anhydrase and, especially, the demonstration of shell proteins.     

The immunologically conserved phycobilisome-thylakoid linker polypeptide

We have isolated phycobilisomes from two classes of red algae, several subdivisions of the cyanobacteria, and the cyanelles of Cyanophora paradoxa. In addition to the major light harvesting biliproteins, these phycobilisomes also contain several other polypeptides, the largest of which ranges from 75 to 120 kilodaltons in the different species surveyed. This protein, previously isolated and characterized from three species, was shown to be the final emitter of excitation energy in phycobilisomes and is also thought to be involved in the attachment of the phycobilisomes to the thylakoid membrane. We have obtained polyclonal antibodies to the 95 kilodalton polypeptide isolated from phycobilisomes of the cyanobacterium, Nostoc sp. This protein shares no common antigenic determinants with either the α or β subunits of allophycocyanin, or any of the other biliproteins, as determined by the sensitive Western immunoblotting technique. However, this antiserum cross-reacts with the highest molecular weight polypeptide of all the rhodophytan and cyanobacterial phycobilisomes tested. That these proteins are immunologically related, but are unrelated to other biliproteins, is reminiscent of previous immunological studies of biliproteins which showed that while the three major spectroscopically distinct classes of biliproteins (phycoerythrin, phycocyanin, and allophycocyanin) shared no common antigenic determinants, there was a strong antigenic determinant to specific biliprotein classes which crossed taxonomic divisions.     

Phycobilisome structure and function

Phycobilisomes are aggregates of light-harvesting proteins attached to the stroma side of the thylakoid membranes of the cyanobacteria (blue-green algae) and red algae. The water-soluble phycobiliproteins, of which there are three major groups, tetrapyrrole chromophores covalently bound to apoprotein. Several additional protiens are found within the phycobilisome and serve to link the phycobiliproteins to each other in an ordered fashion and also to attach the phycobilisome to the thylakoid membrane. Excitation energy absorbed by phycoerythrin is transferred through phycocyanin to allophycocyanin with an efficiency approximating 100%. This pathway of excitation energy transfer, directly confirmed by time-resolved spectroscopic measurements, has been incorporated into models describing the ultrastructure of the phycobilisome. The model for the most typical type of phycobilisome describes an allophycocyanin-containing core composed of three cylinders arranged so that their longitudinal axes are parallel and their ends form a triangle. Attached to this core are six rod structures which contain phycocyanin proximal to the core and phycoerythrin distal to the core. The axes of these rods are perpendicular to the longitudinal axis of the core. This arrangement ensures a very efficient transfer of energy. The association of phycoerythrin and phycocyanin within the rods and the attachment of the rods to the core and the core to the thylakoid require the presence of several linker polypeptides. It is recently possible to assemble functionally and structurally intact phycobilisomes in vitro from separated components as well as to reassociate phycobilisomes with stripped thylakoids. Understanding of the biosynthesis and in vivo assembly of phycobilisomes will be greatly aided by the current advances in molecular genetics, as exemplified by recent identification of several genes encoding phycobilisome components.Combined ultrastructural, biochemical and biophysical approaches to the study of cyanobacterial and red algal cells and isolated phycobilisome-thylakoid fractions are leading to a clearer understanding of the phycobilisome-thylakoid structural interactions, energy transfer to the reaction centers and regulation of excitation energy distribution. However, compared to our current knowledge concerning the structural and functional organization of the isolated phycobilisome, this research area is relatively unexplored.     

Analyses of ribosomal RNA sequences from glaucocystophyte cyanelles provide new insights into the evolutionary relationships of plastids

Glaucocystophyte algae (sensu Kies, Berl. Deutsch. Bot. Ges. 92, 1979) contain plastids (cyanelles) that retain the peptidoglycan wall of the putative cyanobacterial endosymbiont; this and other ultrastructural characters (e.g., unstacked thylakoids, phycobilisomes) have suggested that cyanelles are primitive plastids that may represent undeveloped associations between heterotrophic host cells (i.e., glaucocystophytes) and cyanobacteria. To test the monophyly of glaucocystophyte cyanelles and to determine their evolutionary relationship to other plastids, complete 16S ribosomal RNA sequences were determined for Cyanophora paradoxa, Glaucocystis nostochinearum, Glaucosphaera vacuolata, and Gloeochaete wittrockiana. Plastid rRNAs were analyzed with the maximum-likelihood, maximumparsimony, and neighbor joining methods. The phylogenetic analyses show that the cyanelles of C. paradoxa, G. nostochinearum, and G. wittrockiana form a distinct evolutionary lineage; these cyanelles presumably share a monophyletic origin. The rDNA sequence of G. vacuolata was positioned within the nongreen plastid lineage. This result is consistent with analyses of nuclear-encoded rRNAs that identify G. vacuolata as a rhodophyte and support its removal from the Glaucocystophyta. Results of a global search with the maximumlikelihood method suggest that cyanelles are the first divergence among all plastids; this result is consistent with a single loss of the peptidoglycan wall in plastids after the divergence of the cyanelles. User-defined tree analyses with the maximum-likelihood method indicate, however, that the position of the cyanelles is not stable within the rRNA phylogenies. Both maximumparsimony and neighbor-joining analyses showed a close evolutionary relationship between cyanelles and nongreen plastids; these phylogenetic methods were sensitive to inclusion/exclusion of the G. wittrockiana cyanelle sequence. Base compositional bias within the G. wittrockiana 16S rRNA may explain this result. Taken together the phylogenetic analyses are interpreted as supporting a near-simultaneous radiation of cyanelles and green and nongreen plastids; these organelles are all rooted within the cyanobacteria.Correspondence to: D. Bhattacharya     

Ultrastructure of cell wall and photosynthetic apparatus of the phycobilisome-less Synechocystis sp. strain BO 8402 and phycobilisome-containing derivative strain BO 9201

The ultrastructures of two closely related strains of a novel diazotrophic cyanobacterium, Synechocystis sp. BO 8402 and BO 9201, were examined using ultrathin sections and freeze-fracture electron microscopy. Cells of both strains were surrounded by an unusual thick peptidoglycan layer. Substructures in the layer indicated the presence of microplasmodesmata aligned perpendicular to the free cell surface and in the septum of dividing cells. Synechocystis sp. strain BO 8402 contained lobed, electronopaque, highly fluorescent inclusion bodies consisting of phycocyanin-linker complexes. The thylakoids lacked phycobilisomes and accommodated, in addition to randomly distributed exoplasmic freeze-fracture particles, patches of two-dimensionally ordered arrays of dimeric photosystem II particles in the exoplasmic fracture face. Determination of photosystem I and photosystem II suggested an increase of photosystem II in strain BO 8402. Strain BO 9201 performed phycobilisome-supported photosynthesis and showed rows of dimeric photosystem II particles in the exoplasmic fracture face. Corresponding particle-free grooves in the protoplasmic fracture face were lined by a class of large particles tentatively assigned as trimers of photosystem I. The different lateral organization of protein complexes in the thylakoid membranes and the fine structure of the cell wall are discussed with respect to absorption cross-section of photosynthesis and nitrogen fixation.Abbreviations EF Exoplasmic freeze-fracture face - P 700 Reaction centre chlorophyll of photosystem I - PF Protoplasmic freeze-fracture face - PS I Photosystem I - PS II Photosystem II     

Evidence for a peptidoglycan envelope in the cyanelles of Glaucocystis nostochinearum Itzigsohn

Glaucocystis nostochinearum is a eukaryotic organism with chloroplasts that have usually been assumed to be cyanelles — i.e., endosymbiotic cyanobacteria. Previous attempts by others to support this assumption by demonstrating the presence of a limiting peptidoglycan envelope have been unsuccessful.In the present study disruption of intact Glaucocystis cells with a glass tissue homogenizer permitted the isolation of the uniquely-shaped cyanelles. That these cyanelles were lunited by a peptidoglycan-containing envelope was concluted from the following evidence: (1) stability of isolated cyanelles in distilled water as determined by the preservation of their intactness and peculiar asymmetrical shape; (2) lysozyme sensitivity as demonstrated by lysis of isolated cyanelles when treated with low concentrations of lysozyme; (3) inhibition of the lysozyme-mediated lysis by N-acetyl-glucosamine-2, a known competitive inhibitor of lysozyme, (4) visualization of a thin, electron dense layer between the two limiting membranes around the cyanelle, and (5) isolation and identification of the peptidoglycan-specific amino acid, diaminopimelic acid, from the cyanelles.     

Fluorescence Properties Indicate that Photosystem II Reaction Centers and Light-Harvesting Complex Are Modified by Low Temperature Growth in Winter Rye

Thylakoids isolated from winter rye (Secale cereale L. cv Puma) grown at 20°C (nonhardened rye, RNH) or 5°C (cold-hardened rye, RH) were characterized using chlorophyll (Chl) fluorescence. Low temperature fluorescence emission spectra of RH thylakoids contained emission bands at 680 and 695 nanometers not present in RNH thylakoids which were interpreted as changes in the association of light-harvesting Chl a/b proteins and photosystem II (PSII) reaction centers. RH thylakoids also exhibited a decrease in the emission ratio of 742/685 nanometers relative to RNH thylakoids.

Room temperature fluorescence induction revealed that a larger proportion of Chl in RH thylakoids was inactive in transferring energy to PSII reaction centers when compared with RNH thylakoids. Fluorescence induction kinetics at 20°C indicated that RNH and RH thylakoids contained the same proportions of fast (α) and slow (β) components of the biphasic induction curve. In RH thylakoids, however, the rate constant for α components increased and the rate constant for β components decreased relative to RNH thylakoids. Thus, energy was transferred more quickly within a PSII reaction center complex in RH thylakoids. In addition, PSII reaction centers in RH thylakoids were less connected, thus reducing energy transfers between reaction center complexes. We concluded that both PSII reaction centers and light-harvesting Chl a/b proteins had been modified during development of rye chloroplasts at 5°C.

     

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.     

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.     

Effect of Light Quality on the Composition, Function, and Structure of Photosynthetic Thylakoid Membranes of Asplenium australasicum (Sm.) Hook

The effect of light quality on the composition, function and structure of the thylakoid membranes, as well as on the photosynthetic rates of intact fronds from Asplenium australasicum, a shade plant, grown in blue, white, or red light of equal intensity (50 microeinsteins per square meter per second) was investigated. When compared with those isolated from plants grown in white and blue light, thylakoids from plants grown in red light have higher chlorophyll a/chlorophyll b ratios and lower amounts of light-harvesting chlorophyll a/b-protein complexes than those grown in blue light. On a chlorophyll basis, there were higher levels of PSII reaction centers, cytochrome f and coupling factor activity in thylakoids from red light-grown ferns, but lower levels of PSI reaction centers and plastoquinone. The red light-grown ferns had a higher PSII/PSI reaction center ratio of 4.1 compared to 2.1 in blue light-grown ferns, and a larger apparent PSI unit size and a lower PSII unit size. The CO₂ assimilation rates in fronds from red light-grown ferns were lower on a unit area or fresh weight basis, but higher on a chlorophyll basis, reflecting the higher levels of electron carriers and electron transport in the thylakoids.

The structure of thylakoids isolated from plants grown under the three light treatments was similar, with no significant differences in the number of thylakoids per granal stack or the ratio of appressed membrane length/nonappressed membrane length. The large freeze-fracture particles had the same size in the red-, blue-, and white-grown ferns, but there were some differences in their density. Light quality is an important factor in the regulation of the composition and function of thylakoid membranes, but the effects depend upon the plant species.