Chlorophyll and carotenoid binding in a simple red algal light-harvesting complex crosses phylogenetic lines

Confronting FCP structure with ultrafast spectroscopy data: evidence for structural variations

Diatoms are a major group of algae, responsible for a quarter of the global primary production on our planet. Their adaptation to marine environments is ensured by their light-harvesting antenna - the fucoxanthin-chlorophyll protein (FCP) complex, which absorbs strongly in the blue-green spectral region. Although these essential proteins have been the subject of many studies, for a long time their comprehensive description was not possible in the absence of structural data. Last year, the 3D structures of several FCP complexes were revealed. The structure of an FCP dimer was resolved by crystallography for the pennate diatom Phaeodactylum tricornutum [W. Wang et al., Science, 2019, 363, 6427] and the structure of the PSII supercomplex from the centric diatom Chaetoceros gracilis, containing several FCPs, was obtained by electron microscopy [X. Pi et al., Science, 2019, 365, 6452; R. Nagao et al., Nat. Plants, 2019, 5, 890]. In this Perspective article, we evaluate how precisely these structures may account for previously published ultrafast spectroscopy results, describing the excitation energy transfer in the FCP from another centric diatom Cyclotella meneghiniana. Surprisingly, we find that the published FCP structures cannot explain several observations obtained from ultrafast spectroscopy. Using the available structures, and results from electron microscopy, we construct a trimer-based FCP model for Cyclotella meneghiniana, consistent with ultrafast experimental data. As a whole, our observations suggest that the structures from the proteins belonging to the FCP family display larger variations than the equivalent LHC proteins in plants, which may reflect species-specific adaptations or original strategies for adapting to rapidly changing marine environments.

Carotenoid profiling of five microalgae species from large-scale production

The carotenoid profiles of biomass from five eukaryotic microalgae, Porphyridium cruentum, Isochrysis galbana, Phaeodactylum tricornutum, Tetraselmis suecica and Nannochloropsis gaditana, produced at an industrial plant in outdoor photobioreactors, were investigated. Pigments were solvent-extracted after an ultrasonic pre-treatment and separated by HPLC-photodiode-array using a reversed-phase C18 column. Microalgae showed species-specific carotenoid profiles. Carotenoids were mostly in their free form, with a prevalence of xanthophylls over carotenes. Beta-carotene was the only carotenoid common to all species. The Rhodophyta P. cruentum exhibited the lowest total carotenoid content (167.2 mg 100 g^-1 dw) and the simplest profile, with (all-E)-zeaxanthin (94.2 mg 100 g^-1 dw, 56% of total carotenoids) and (all-E)-β- carotene (53.4 mg 100 g^-1 dw, 32% of total carotenoids) as the major carotenoids. The Haptophyta Isochrysis galbana and the Bacillariophyta Phaeodactylum tricornutum were the species with the highest total carotenoid content (1760 mg and 1022 mg 100 g^-1 dw, respectively). These species were characterized by similar carotenoid profiles, with (all-E)-fucoxanthin as the chief compound (1346 mg and 776.8 mg 100 g^-1 dw for I. galbana and P. tricornutum, respectively), accounting for about 76% of total carotenoids. The Chlorophyta Tetraselmis suecica was the species showing the greatest variety of carotenoids, with both α- carotene and β- carotene and their derivatives present. (All-E)-lutein (85.4 mg 100 g^-1 dw) and (all-E)-violaxanthin (81.8 mg 100 g^-1 dw) were the major pigments in this species. In the Ochrophyta Nannochloropsis gaditana, (all-E)-violaxanthin was the prevalent carotenoid (336.7 mg 100 g^-1 dw), followed by (all-E)-β-carotene (100.1 mg 100 g^-1 algal dw). The carotenoid content of the microalgal biomass studied compared favourably to that of major vegetable sources. Due to their characteristics, these microalgae, most of them currently finding their main application in aquaculture, may be also regarded as valuable sources of carotenoids to be used in the formulation of functional food and nutraceuticals.

Selected Methods of Extracting Carotenoids, Characterization, and Health Concerns: A Review

Carotenoids are the most powerful nutrients (medicine) on earth due to their potent antioxidant properties. The ability of these tetraterpenoids in obviating human chronic ailments like cancer, cardiovascular disease, osteoporosis, and diabetes has drawn public attention toward these novel compounds. Conventionally, carotenoids have been extracted from plant materials and agro-industrial byproduct using different solvents, but these procedures result in contaminating the target compound (carotenoids) with extraction solvents. Furthermore, some utilized solvents are not safe and hence are harmful to the environment. This has attracted criticism from consumers, ecologists, environmentalists, and public health workers. However, there is clear consumer preference for carotenoids from natural origin without traces of extracting solvent. Therefore, this review seeks to discuss methods for higher recovery of pure carotenoids without contamination from a solvent. Methods such as enzyme-based extraction, supercritical fluid extraction, microwave-assisted extraction, Soxhlet extraction, ultrasonic extraction, and postextraction treatment (saponification) are discussed. Merits and demerits of these methods along with health concerns during intake of carotenoids were also considered.

Transcriptome analysis of the typical freshwater rhodophytes Sheathia arcuata grown under different light intensities

The Rhodophyta Sheathia arcuata is exclusively distributed in freshwater, constituting an important component in freshwater flora. This study presents the first transcriptome profiling of freshwater Rhodophyta taxa. A total of 161,483 assembled transcripts were identified, annotated and classified into different biological categories and pathways based on BLAST against diverse databases. Different gene expression patterns were caused principally by different irradiances considering the similar water conditions of the sampling site when the specimens were collected. Comparison results of gene expression levels under different irradiances revealed that photosynthesis-related pathways significantly up-regulated under the weak light. Molecular responses for improved photosynthetic activity include the transcripts corresponding to antenna proteins (LHCA1 and LHCA4), photosynthetic apparatus proteins (PSBU, PETB, PETC, PETH and beta and gamma subunits of ATPase) and metabolic enzymes in the carbon fixation. Along with photosynthesis, other metabolic activities were also regulated to optimize the growing and development of S. arcuata under appropriate sunlight. Protein-protein interactive networks revealed the most responsive up-expressed transcripts were ribosomal proteins. The de-novo transcriptome assembly of S. arcuata provides a foundation for further investigation on the molecular mechanism of photosynthesis and environmental adaption for freshwater Rhodophyta.

Algal light sensing and photoacclimation in aquatic environments

Anoxygenic photosynthetic prokaryotes arose in ancient oceans ~3.5 billion years ago. The evolution of oxygenic photosynthesis by cyanobacteria followed soon after, enabling eukaryogenesis and the evolution of complex life. The Archaeplastida lineage dates back ~1.5 billion years to the domestication of a cyanobacterium. Eukaryotic algae have subsequently radiated throughout oceanic/freshwater/terrestrial environments, adopting distinctive morphological and developmental strategies for adaptation to diverse light environments. Descendants of the ancestral photosynthetic alga remain challenged by a typical diurnally fluctuating light supply ranging from ~0 to ~2000 μE m^-2  s^-1 . Such extreme changes in light intensity and variations in light quality have driven the evolution of novel photoreceptors, light-harvesting complexes and photoprotective mechanisms in photosynthetic eukaryotes. This minireview focuses on algal light sensors, highlighting the unexpected roles for linear tetrapyrroles (bilins) in the maintenance of functional chloroplasts in chlorophytes, sister species to streptophyte algae and land plants.

Evolution and function of light harvesting proteins

Photosynthetic eukaryotes exhibit very different light-harvesting proteins, but all contain membrane-intrinsic light-harvesting complexes (Lhcs), either as additional or sole antennae. Lhcs non-covalently bind chlorophyll a and in most cases another Chl, as well as very different carotenoids, depending on the taxon. The proteins fall into two major groups: The well-defined Lhca/b group of proteins binds typically Chl b and lutein, and the group is present in the 'green lineage'. The other group consists of Lhcr/Lhcf, Lhcz and Lhcx/LhcSR proteins. The former are found in the so-called Chromalveolates, where they mostly bind Chl c and carotenoids very efficient in excitation energy transfer, and in their red algae ancestors. Lhcx/LhcSR are present in most Chromalveolates and in some members of the green lineage as well. Lhcs function in light harvesting, but also in photoprotection, and they influence the organisation of the thylakoid membrane. The different functions of the Lhc subfamilies are discussed in the light of their evolution.

Spectral properties of a divinyl chlorophyll a harboring mutant of Synechocystis sp. PCC6803

A divinyl chlorophyll (DV-Chl) a harboring mutant of Synechocystis sp. PCC 6803, in which chlorophyll species is replaced from monovinyl(normal)-Chl a to DV-Chl a, was characterized. The efficiency of light utilization for photosynthesis was decreased in the mutant. Absorption spectra at 77 K and their fourth derivative analyses revealed that peaks of each chlorophyll forms were blue-shifted by 1-2 nm, suggesting lowered stability of chlorophylls at their binding sites. This was also true both in PSI and PSII complexes. On the other hand, fluorescence emission spectra measured at 77 K were not different between wild type and the mutant. This indicates that the mode of interaction between chlorophyll and its binding pockets responsible for emitting fluorescence at 77 K is not altered in the mutant. P700 difference spectra of thylakoid membranes and PSI complexes showed that the spectrum in Soret region was red-shifted by 7 nm in the mutant. This is a characteristic feature of DV-Chl a. Microenvironments of iron-sulfur center of a terminal electron acceptor of PSI complex, P430, were practically the same as that of wild type.

Benefits of an inclusive US education system

Presented is a historical perspective of one scientist's journey from war-torn Europe to the opportunities presented by a flexible US educational system. It celebrates the opening of the science establishment that began in the 1950s and its fostering of basic research, and recognizes individuals who were instrumental in guiding the author's education as well as those with whom she later participated in collaborative algal plant research. The initial discovery and later elucidation of phycobilisome structure are elaborated, including the structural connection with photosystem II. Furthermore, she summarizes some of her laboratory's results on carotenoids and its exploration of the isoprenoid pathway in cyanobacteria. Finally, she comments on the gender gap and how her generation benefited when opportunities for women scientists were enlarged.

A structural phylogenetic map for chloroplast photosynthesis

Chloroplasts are cytoplasmic organelles and the sites of photosynthesis in eukaryotic cells. Advances in structural biology and comparative genomics allow us to identify individual components of the photosynthetic apparatus precisely with respect to the subcellular location of their genes. Here we present outline maps of four energy-transducing thylakoid membranes. The maps for land plants and red and green algae distinguish protein subunits encoded in the nucleus from those encoded in the chloroplast. We find no defining structural feature that is common to all chloroplast gene products. Instead, conserved patterns of gene location are consistent with photosynthetic redox chemistry exerting gene regulatory control over its own rate-limiting steps. Chloroplast DNA carries genes whose expression is placed under this control.

Evolution of light-harvesting complex proteins from Chl c-containing algae

Background

Light harvesting complex (LHC) proteins function in photosynthesis by binding chlorophyll (Chl) and carotenoid molecules that absorb light and transfer the energy to the reaction center Chl of the photosystem. Most research has focused on LHCs of plants and chlorophytes that bind Chl a and b and extensive work on these proteins has uncovered a diversity of biochemical functions, expression patterns and amino acid sequences. We focus here on a less-studied family of LHCs that typically bind Chl a and c, and that are widely distributed in Chl c-containing and other algae. Previous phylogenetic analyses of these proteins suggested that individual algal lineages possess proteins from one or two subfamilies, and that most subfamilies are characteristic of a particular algal lineage, but genome-scale datasets had revealed that some species have multiple different forms of the gene. Such observations also suggested that there might have been an important influence of endosymbiosis in the evolution of LHCs.

Results

We reconstruct a phylogeny of LHCs from Chl c-containing algae and related lineages using data from recent sequencing projects to give ~10-fold larger taxon sampling than previous studies. The phylogeny indicates that individual taxa possess proteins from multiple LHC subfamilies and that several LHC subfamilies are found in distantly related algal lineages. This phylogenetic pattern implies functional differentiation of the gene families, a hypothesis that is consistent with data on gene expression, carotenoid binding and physical associations with other LHCs. In all probability LHCs have undergone a complex history of evolution of function, gene transfer, and lineage-specific diversification.

Conclusion

The analysis provides a strikingly different picture of LHC diversity than previous analyses of LHC evolution. Individual algal lineages possess proteins from multiple LHC subfamilies. Evolutionary relationships showed support for the hypothesized origin of Chl c plastids. This work also allows recent experimental findings about molecular function to be understood in a broader phylogenetic context.

Evidence for the existence of one antenna-associated, lipid-dissolved and two protein-bound pools of diadinoxanthin cycle pigments in diatoms

We studied the localization of diadinoxanthin cycle pigments in the diatoms Cyclotella meneghiniana and Phaeodactylum tricornutum. Isolation of pigment protein complexes revealed that the majority of high-light-synthesized diadinoxanthin and diatoxanthin is associated with the fucoxanthin chlorophyll protein (FCP) complexes. The characterization of intact cells, thylakoid membranes, and pigment protein complexes by absorption and low-temperature fluorescence spectroscopy showed that the FCPs contain certain amounts of protein-bound diadinoxanthin cycle pigments, which are not significantly different in high-light and low-light cultures. The largest part of high-light-formed diadinoxanthin cycle pigments, however, is not bound to antenna apoproteins but located in a lipid shield around the FCPs, which is copurified with the complexes. This lipid shield is primarily composed of the thylakoid membrane lipid monogalactosyldiacylglycerol. We also show that the photosystem I (PSI) fraction contains a tightly connected FCP complex that is enriched in protein-bound diadinoxanthin cycle pigments. The peripheral FCP and the FCP associated with PSI are composed of different apoproteins. Tandem mass spectrometry analysis revealed that the peripheral FCP is composed mainly of the light-harvesting complex protein Lhcf and also significant amounts of Lhcr. The PSI fraction, on the other hand, shows an enrichment of Lhcr proteins, which are thus responsible for the diadinoxanthin cycle pigment binding. The existence of lipid-dissolved and protein-bound diadinoxanthin cycle pigments in the peripheral antenna and in PSI is discussed with respect to different specific functions of the xanthophylls.

Removal and reconstitution of the carotenoid antenna of xanthorhodopsin

Salinixanthin, a C₄₀-carotenoid acyl glycoside, serves as a light-harvesting antenna in the retinal-based proton pump xanthorhodopsin of Salinibacter ruber. In the crystallographic structure of this protein, the conjugated chain of salinixanthin is located at the protein–lipid boundary and interacts with residues of helices E and F. Its ring, with a 4-keto group, is rotated relative to the plane of the π-system of the carotenoid polyene chain and immobilized in a binding site near the β-ionone retinal ring. We show here that the carotenoid can be removed by oxidation with ammonium persulfate, with little effect on the other chromophore, retinal. The characteristic CD bands attributed to bound salinixanthin are now absent. The kinetics of the photocycle is only slightly perturbed, showing a 1.5-fold decrease in the overall turnover rate. The carotenoid-free protein can be reconstituted with salinixanthin extracted from the cell membrane of S. ruber. Reconstitution is accompanied by restoration of the characteristic vibronic structure of the absorption spectrum of the antenna carotenoid, its chirality, and the excited-state energy transfer to the retinal. Minor modification of salinixanthin, by reducing the carbonyl C=O double bond in the ring to a C-OH, suppresses its binding to the protein and eliminates the antenna function. This indicates that the presence of the 4-keto group is critical for carotenoid binding and efficient energy transfer.

The supramolecular architecture, function, and regulation of thylakoid membranes in red algae: an overview

Red algae are a group of eukaryotic photosynthetic organisms. Phycobilisomes (PBSs), which are composed of various types of phycobiliproteins and linker polypeptides, are the main light-harvesting antennae in red algae, as in cyanobacteria. Two morphological types of PBSs, hemispherical- and hemidiscoidal-shaped, are found in different red algae species. PBSs harvest solar energy and efficiently transfer it to photosystem II (PS II) and finally to photosystem I (PS I). The PS I of red algae uses light-harvesting complex of PS I (LHC I) as a light-harvesting antennae, which is phylogenetically related to the LHC I found in higher plants. PBSs, PS II, and PS I are all distributed throughout the entire thylakoid membrane, a pattern that is different from the one found in higher plants. Photosynthesis processes, especially those of the light reactions, are carried out by the supramolecular complexes located in/on the thylakoid membranes. Here, the supramolecular architecture, function and regulation of thylakoid membranes in red algal are reviewed.

Interactions between the photosystem II subunit PsbS and xanthophylls studied in vivo and in vitro

The photosystem II subunit PsbS is essential for excess energy dissipation (qE); however, both lutein and zeaxanthin are needed for its full activation. Based on previous work, two models can be proposed in which PsbS is either 1) the gene product where the quenching activity is located or 2) a proton-sensing trigger that activates the quencher molecules. The first hypothesis requires xanthophyll binding to two PsbS-binding sites, each activated by the protonation of a dicyclohexylcarbodiimide-binding lumen-exposed glutamic acid residue. To assess the existence and properties of these xanthophyll-binding sites, PsbS point mutants on each of the two Glu residues PsbS E122Q and PsbS E226Q were crossed with the npq1/npq4 and lut2/npq4 mutants lacking zeaxanthin and lutein, respectively. Double mutants E122Q/npq1 and E226Q/npq1 had no qE, whereas E122Q/lut2 and E226Q/lut2 showed a strong qE reduction with respect to both lut2 and single glutamate mutants. These findings exclude a specific interaction between lutein or zeaxanthin and a dicyclohexylcarbodiimide-binding site and suggest that the dependence of nonphotochemical quenching on xanthophyll composition is not due to pigment binding to PsbS. To verify, in vitro, the capacity of xanthophylls to bind PsbS, we have produced recombinant PsbS refolded with purified pigments and shown that Raman signals, previously attributed to PsbS-zeaxanthin interactions, are in fact due to xanthophyll aggregation. We conclude that the xanthophyll dependence of qE is not due to PsbS but to other pigment-binding proteins, probably of the Lhcb type.

Theoretical study on the thermodynamic properties of chlorophyll d-peptides coordinating ligand

The chlorophyll d containing cyanobacterium, Acaryochloris marina has provided a model system for the study of chlorophyll replacement in the function of oxygenic photosynthesis. Chlorophyll d replaces most functions of chlorophyll a in Acaryochloris marina. It not only functions as the major light-harvesting pigment, but also acts as an electron transfer cofactor in the primary charge separation reaction in the two photosystems. The Mg-chlorophyll d-peptide coordinating interaction between the amino acid residues and chlorophylls using the latest semi-empirical PM5 method were examined. It is suggested that chlorophyll d possesses similar coordination ligand properties to chlorophyll a, but chlorophyll b possesses different ligand properties. Compared with other studies involving theoretical correlation and our prior experiments, this study suggests that the chlorophyll a-bound proteins will bind chlorophyll d without difficulty when chlorophyll d is available.

Chimeric Plastid Proteome in the Florida “Red Tide” Dinoflagellate Karenia brevis

Current understanding of the plastid proteome comes almost exclusively from studies of plants and red algae. The proteome in these taxa has a relatively simple origin via integration of proteins from a single cyanobacterial primary endosymbiont and the host. However, the most successful algae in marine environments are the chlorophyll c-containing chromalveolates such as diatoms and dinoflagellates that contain a plastid of red algal origin derived via secondary or tertiary endosymbiosis. Virtually nothing is known about the plastid proteome in these taxa. We analyzed expressed sequence tag data from the toxic "Florida red tide" dinoflagellate Karenia brevis that has undergone a tertiary plastid endosymbiosis. Comparative analyses identified 30 nuclear-encoded plastid-targeted proteins in this chromalveolate that originated via endosymbiotic or horizontal gene transfer (HGT) from multiple different sources. We identify a fundamental divide between plant/red algal and chromalveolate plastid proteomes that reflects a history of mixotrophy in the latter group resulting in a highly chimeric proteome. Loss of phagocytosis in the "red" and "green" clades effectively froze their proteomes, whereas chromalveolate lineages retain the ability to engulf prey allowing them to continually recruit new, potentially adaptive genes through subsequent endosymbioses and HGT. One of these genes is an electron transfer protein (plastocyanin) of green algal origin in K. brevis that likely allows this species to thrive under conditions of iron depletion.

Acclimation of chlorophyll a and carotenoid levels to different irradiances in four freshwater cyanobacteria

This study investigated carotenoid and chlorophyll a (Chl-a) contents under two different growth irradiances in four freshwater cyanobacterial strains. We found an increased weight ratio of zeaxanthin to Chl-a after exposure to high irradiances over several days. Two out of four strains showed higher zeaxanthin amounts on a biomass basis as well. It appears that cyanobacteria enhance their carotenoid pool in response to high light conditions, as increased production of other carotenoids with photoprotective abilities has also been observed under high irradiance levels. Cyanobacteria do not possess the violaxanthin cycle, which enables a rapid reversible conversion from violaxanthin into zeaxanthin and functioning as a quencher of excessive energy, and elevated zeaxanthin concentrations could therefore be seen as an adaptive strategy against excess light energy. Some differences in the acclimation pattern were revealed between different cyanobacteria. Anabaena torulosa contained higher amounts of every carotenoid, while Nostoc sp. mainly increased zeaxanthin, and myxoxanthophyll. Anabaenopsis elenkinii produced exceptionally high amounts of myxoxanthophyll and beta-carotene under higher irradiances. Anabaena cylindrica generally showed less variation of carotenoids under different irradiances.

Reconstitution of the peridinin-chlorophyll a protein (PCP): evidence for functional flexibility in chlorophyll binding

The coding regions for the N-domain, and full length peridinin-chlorophyll a apoprotein (full length PCP), were expressed in Escherichia coli. The apoproteins formed inclusion bodies from which the peptides could be released by hot buffer. Both the above constructs were reconstituted by addition of a total pigment extract from native PCP. After purification by ion exchange chromatography, the absorbance, fluorescence excitation and CD spectra resembled those of the native PCP. Energy transfer from peridinin to Chl a was restored and a specific fluorescence activity calculated which was approximately 86% of that of native PCP. Size exclusion analysis and CD spectra showed that the N-domain PCP dimerized on reconstitution. Chl a could be replaced by Chl b, 3-acetyl Chl a, Chl d and Bchl using the N-domain apo protein. The specific fluorescence activity was the same for constructs with Chl a, 3-acetyl Chl a, and Chl d but significantly reduced for those made with Chl b. Reconstitutions with mixtures of chlorophylls were also made with eg Chl b and Chl d and energy transfer from the higher energy Qy band to the lower was demonstrated.

Chlamydomonas reinhardtii in the landscape of pigments

This review focuses on the biosynthesis of pigments in the unicellular alga Chlamydomonas reinhardtii and their physiological and regulatory functions in the context of information gathered from studies of other photosynthetic organisms. C. reinhardtii is serving as an important model organism for studies of photosynthesis and the pigments associated with the photosynthetic apparatus. Despite extensive information pertaining to the biosynthetic pathways critical for making chlorophylls and carotenoids, we are just beginning to understand the control of these pathways, the coordination between pigment and apoprotein synthesis, and the interactions between the activities of these pathways and those for other important cellular metabolites branching from these pathways. Other exciting areas relating to pigment function are also emerging: the role of intermediates of pigment biosynthesis as messengers that coordinate metabolism in the chloroplast with nuclear gene activity, and the identification of photoreceptors and their participation in critical cellular processes including phototaxis, gametogenesis, and the biogenesis of the photosynthetic machinery. These areas of research have become especially attractive for intensive development with the application of potent molecular and genomic tools currently being applied to studies of C. reinhardtii.

A core catalytic domain of the TyrA protein family: arogenate dehydrogenase from Synechocystis

The TyrA protein family includes prephenate dehydrogenases, cyclohexadienyl dehydrogenases and TyrA(a)s (arogenate dehydrogenases). tyrA(a) from Synechocystis sp. PCC 6803, encoding a 30 kDa TyrA(a) protein, was cloned into an overexpression vector in Escherichia coli. TyrA(a) was then purified to apparent homogeneity and characterized. This protein is a model structure for a catalytic core domain in the TyrA superfamily, uncomplicated by allosteric or fused domains. Competitive inhibitors acting at the catalytic core of TyrA proteins are analogues of any accepted cyclohexadienyl substrate. The homodimeric enzyme was specific for L-arogenate (K(m)=331 microM) and NADP+ (K(m)=38 microM), being unable to substitute prephenate or NAD+ respectively. L-Tyrosine was a potent inhibitor of the enzyme (K(i)=70 microM). NADPH had no detectable ability to inhibit the reaction. Although the mechanism is probably steady-state random order, properties of 2',5'-ADP as an inhibitor suggest a high preference for L-arogenate binding first. Comparative enzymology established that both of the arogenate-pathway enzymes, prephenate aminotransferase and TyrA(a), were present in many diverse cyanobacteria and in a variety of eukaryotic red and green algae.

Isolation of a Novel Carotenoid-rich Protein in Cyanophora paradoxa that is Immunologically Related to the Light-harvesting Complexes of Photosynthetic Eukaryotes

Novel eukaryotic chlorophyll-carotenoid proteins have evolved at least twice following the origin of the plastid and include the widely distributed integral membrane light-harvesting complexes (LHCs) and the dinoflagellate-specific soluble peridinin-chlorophyll proteins. In the glaucophytes, homologs of these proteins are reportedly absent. We have identified a novel carotenoid-rich protein (CRP) in the glaucophyte Cyanophora paradoxa that is 28 kDa and immunologically related to the family of LHCs. CRP is associated with the thylakoid membrane, though it can be removed by stringent washes, suggesting that there are probably significant structural differences between CRP and the LHCs. CRP co-localizes with a zeaxanthin-rich thylakoid membrane fraction that also contains beta-carotene, chlorophyll and an unidentified carotenoid. Despite this, we found no evidence for carotenoid-chlorophyll energy transfer in the isolated complex, suggesting that light harvesting may not be a primary function. The presence of CRP in C. paradoxa is evidence for the evolution of a novel pigment-binding protein in the glaucophytes.

Unique origin and lateral transfer of prokaryotic chlorophyll-b and chlorophyll-d light-harvesting systems

pcb genes, encoding proteins binding light-harvesting chlorophylls, were cloned and sequenced from the Chl d-containing cyanobacterium, Acaryochloris marina, and the Chl b-containing cyanobacterium, Prochloron didemni. Both organisms contained two tandem pcb genes. Peptide fingerprinting confirmed the expression of one of the A. marina pcb genes. Phylogenetic tree reconstruction using distance-matrix and maximum-likelihood methods indicated a single origin of the pcb gene family, whether occurring in Chl b-containing or Chl d-containing organisms. This may indicate widespread lateral transfer of the Pcb protein-based light-harvesting system.

Structure and function of the water-soluble carotenoid-binding proteins of cyanobacteria

The orange carotenoid protein (OCP) and its derivative, the red carotenoid protein (RCP), appear to play important photoprotective roles in cyanobacteria. Structural and functional characterization is gradually elucidating the specific details of how carotenoid-protein interactions, including the role of six methionine residues oriented toward the pigment, contribute to the spectral and functional properties of these proteins.

Endosymbiosis and the design of eukaryotic electron transport

The bioenergetic organelles of eukaryotic cells, mitochondria and chloroplasts, are derived from endosymbiotic bacteria. Their electron transport chains (ETCs) resemble those of free-living bacteria, but were tailored for energy transformation within the host cell. Parallel evolutionary processes in mitochondria and chloroplasts include reductive as well as expansive events: On one hand, bacterial complexes were lost in eukaryotes with a concomitant loss of metabolic flexibility. On the other hand, new subunits have been added to the remaining bacterial complexes, new complexes have been introduced, and elaborate folding patterns of the thylakoid and mitochondrial inner membranes have emerged. Some bacterial pathways were reinvented independently by eukaryotes, such as parallel routes for quinol oxidation or the use of various anaerobic electron acceptors. Multicellular organization and ontogenetic cycles in eukaryotes gave rise to further modifications of the bioenergetic organelles. Besides mitochondria and chloroplasts, eukaryotes have ETCs in other membranes, such as the plasma membrane (PM) redox system, or the cytochrome P450 (CYP) system. These systems have fewer complexes and simpler branching patterns than those in energy-transforming organelles, and they are often adapted to non-bioenergetic functions such as detoxification or cellular defense.

Reversed-phase HPLC determination of chlorophyll a' and naphthoquinones in photosystem I of red algae: existence of two menaquinone-4 molecules in photosystem I of Cyanidium caldarium

Chlorophyll (Chl) a', the C13(2)-epimer of Chl a, is one of the two Chl molecules constituting the primary electron donor (P700) of photosystem (PS) I of a thermophilic cyanobacterium Synechococcus elongatus. To examine whether PS I of other oxygenic photosynthetic organisms in general contain one Chl a' molecule in P700, the pigment composition of thylakoid membranes and PS I preparations isolated from red algae Porphyridium purpureum and Cyanidium caldarium was examined by reversed-phase HPLC with particular attention to Chl a' and phylloquinone (PhQ), the secondary electron acceptor of PS I. The two red algae contained one Chl a' molecule at the core part of PS I. In PS I of C. caldarium, two menaquinone-4 (MQ-4) molecules were detected in place of PhQ used by higher plants and cyanobacteria. The 1:2:1 stoichiometry among Chl a', PhQ (MQ-4) and P700 in PS I of the red algae indicates that one Chl a' molecule universally exists in PS I of oxygenic photosynthetic organisms, and two MQ-4 molecules are associated with PS I of C. caldarium.

Changes in the LHCI aggregation state during iron repletion in the unicellular red alga Rhodella violacea

Red algae are well suited to study the effects of iron deficiency on light-harvesting complex for photosystem I (LHCI), since they are totally devoid of light-harvesting complex for photosystem II (LHCII). Iron starvation results in a reduction of the pigment content, an increase of the fluorescence yield and a new emission band at 705 nm in the 77 K fluorescence emission spectra. These changes reflect the accumulation of uncoupled, aggregated LHCI in iron-depleted cells. Reconnection of LHCI to de novo synthesized reaction center I (RCI) is the first event, which takes place after iron addition. The changes in the aggregation state of LHCI are likely to occur also in brown and green algae.

The crystal structure of a cyanobacterial water-soluble carotenoid binding protein

Carotenoids undergo a wide range of photochemical reactions in animal, plant, and microbial systems. In photosynthetic organisms, in addition to light harvesting, they perform an essential role in protecting against light-induced damage by quenching singlet oxygen, superoxide anion radicals, or triplet-state chlorophyll. We have determined the crystal structure of a water-soluble orange carotenoid protein (OCP) isolated from the cyanobacterium Arthrospira maxima at a resolution of 2.1 A. OCP forms a homodimer with one carotenoid molecule per monomer. The carotenoid binding site is lined by a striking number of methionine residues. The structure reveals several possible ways in which the protein environment influences the spectral properties of the pigment and provides insight into how the OCP carries out its putative functions in photoprotection.

Assembly of the photosynthetic apparatus in embryos from Fucus serratus L

The assembly of the photosynthetic apparatus was studied during the first six days of development of Fucus serratus L. embryos. HPLC analysis revealed that oospheres and zygotes contain the same photosynthetic pigments (i.e., chlorophyll a, chlorophyll c, fucoxanthin, violaxanthin, and beta-carotene) as fully developed thalli. Total pigment amount increased after fertilization, mainly due to an active synthesis of Chl a and fucoxanthin. Spectral modifications revealing the progressive integration of Chl a and Chl c in the photosynthetic units are described. In particular, a distinct emission at 705 nm, reflecting the accumulation of LHC I, was clearly detected. The emission bands at 705 nm and 725 nm were characterized by 77 K excitation fluorescence measurements. Their spectra differed by the presence of a large band at approximately 550 nm due to fucoxanthin in the excitation spectrum of F705 nm. Room temperature variable fluorescence was first observed 30 h after fertilization indicating a functional Photosystem II electron transfer at this developmental stage.

Chlorophyll b inhibits the formation of photosystem I trimer in Synechocystis sp. PCC6803

Chlorophyllide a oxygenase (CAO) catalyzes two-step oxygenation reactions and converts chlorophyllide a to chlorophyllide b. When CAO was introduced into the Synechocystis sp. PCC6803 genome, chlorophyll b was synthesized and incorporated into P700-chlorophyll a-protein complexes. Curve analysis of photosystem I particles showed that Ca687 was decreased with a concomitant increase in Cb652 suggesting that chlorophyll b was incorporated into Ca687-binding sites. When the level of chlorophyll b was high, Ca704, which is known as red chlorophyll and photosystem I trimers were decreased. Formation of photosystem I trimers is discussed in relation to red chlorophyll and chlorophyll b accumulation.

Chlorophyll b can serve as the major pigment in functional photosystem II complexes of cyanobacteria

An Arabidopsis thaliana chlorophyll(ide) a oxygenase gene (cao), which is responsible for chlorophyll b synthesis from chlorophyll a, was introduced and expressed in a photosystem I-less strain of the cyanobacterium Synechocystis sp. PCC 6803. In this strain, most chlorophyll is associated with the photosystem II complex. In line with observations by Satoh et al. [Satoh, S., Ikeuchi, M., Mimuro, M. & Tanaka, A. (2001) J. Biol. Chem. 276, 4293-4297], chlorophyll b was made but accounted for less than 10% of total chlorophyll. However, when lhcb encoding light-harvesting complex (LHC)II from pea was present in the same strain (lhcb(+)/cao(+)), chlorophyll b accumulated in the cell to levels exceeding those of chlorophyll a, although LHCII did not accumulate. In the lhcb(+)/cao(+) strain, the total amount of chlorophyll, the number of chlorophylls per photosystem II center, and the oxygen-evolving activity on a per-chlorophyll basis were similar to those in the photosystem I-less strain. Furthermore, the chlorophyll a/b ratio of photosystem II core particles (retaining CP47 and CP43) and of whole cells of the lhcb(+)/cao(+) strain was essentially identical, and PS II activity could be obtained efficiently by chlorophyll b excitation. These data indicate that chlorophyll b functionally substitutes for chlorophyll a in photosystem II. Therefore, the availability of chlorophylls, rather than their binding specificity, may determine which chlorophyll is incorporated at many positions of photosystem II. We propose that the transient presence of a LHCII/chlorophyll(ide) a oxygenase complex in the lhcb(+)/cao(+) strain leads to a high abundance of available chlorophyll b that is subsequently incorporated into photosystem II complexes. The apparent LHCII requirement for high chlorophyll(ide) a oxygenase activity may be instrumental to limit the occurrence of chlorophyll b in plants to LHC proteins.