Structure, function and assembly of Photosystem II and its light-harvesting proteins

Assembly and Repair of Photosystem II in Chlamydomonas reinhardtii

Oxygenic photosynthetic organisms use Photosystem II (PSII) to oxidize water and reduce plastoquinone. Here, we review the mechanisms by which PSII is assembled and turned over in the model green alga Chlamydomonas reinhardtii. This species has been used to make key discoveries in PSII research due to its metabolic flexibility and amenability to genetic approaches. PSII subunits originate from both nuclear and chloroplastic gene products in Chlamydomonas. Nuclear-encoded PSII subunits are transported into the chloroplast and chloroplast-encoded PSII subunits are translated by a coordinated mechanism. Active PSII dimers are built from discrete reaction center complexes in a process facilitated by assembly factors. The phosphorylation of core subunits affects supercomplex formation and localization within the thylakoid network. Proteolysis primarily targets the D1 subunit, which when replaced, allows PSII to be reactivated and completes a repair cycle. While PSII has been extensively studied using Chlamydomonas as a model species, important questions remain about its assembly and repair which are presented here.

The role of the pigment-protein complex LHCBM1 in nonphotochemical quenching in Chlamydomonas reinhardtii

Nonphotochemical quenching (NPQ) is the process that protects photosynthetic organisms from photodamage by dissipating the energy absorbed in excess as heat. In the model green alga Chlamydomonas reinhardtii, NPQ is abolished in the knock-out mutants of the pigment-protein complexes LHCSR3 and LHCBM1. However, while LHCSR3 is a pH sensor and switches to a quenched conformation at low pH, the role of LHCBM1 in NPQ has not been elucidated yet. In this work, we combined biochemical and physiological measurements to study short-term high-light acclimation of npq5, the mutant lacking LHCBM1. In low light in the absence of this complex, the antenna size of PSII was smaller than in its presence; this effect was marginal in high light (HL), implying that a reduction of the antenna was not responsible for the low NPQ. The mutant expressed LHCSR3 at the wild-type level in HL, indicating that the absence of this complex is also not the reason. Finally, NPQ remained low in the mutant even when the pH was artificially lowered to values that can switch LHCSR3 to the quenched conformation. We concluded that both LHCSR3 and LHCBM1 are required for the induction of NPQ and that LHCBM1 is the interacting partner of LHCSR3. This interaction can either enhance the quenching capacity of LHCSR3 or connect this complex with the PSII supercomplex.

Reversible protein phosphorylation in higher plants: focus on state transitions

Reversible protein phosphorylation is one of the comprehensive mechanisms of cell metabolism regulation in eukaryotic organisms. The review describes the impact of the reversible protein phosphorylation on the regulation of growth and development as well as in adaptation pathways and signaling network in higher plant cells. The main part of the review is devoted to the role of the reversible phosphorylation of light-harvesting proteins of photosystem II and the state transition process in fine-tuning the photosynthetic activity of chloroplasts. A separate section of the review is dedicated to comparing the mechanisms and functional significance of state transitions in higher plants, algae, and cyanobacteria that allows the evolution aspects of state transitions meaning in various organisms to be discussed. Environmental factors affecting the state transitions are also considered. Additionally, we gain insight into the possible influence of STN7-dependent phosphorylation of the target proteins on the global network of reversible protein phosphorylation in plant cells as well as into the probable effect of the STN7 kinase inhibition on long-term acclimation pathways in higher plants.

Photosystem II monomeric antenna CP26 plays a key role in nonphotochemical quenching in Chlamydomonas

Thermal dissipation of excess excitation energy, called nonphotochemical quenching (NPQ), is 1 of the main photoprotective mechanisms in oxygenic photosynthetic organisms. Here, we investigated the function of the monomeric photosystem II (PSII) antenna protein CP26 in photoprotection and light harvesting in Chlamydomonas reinhardtii, a model organism for green algae. We used CRISPR/Cas9 genome editing and complementation to generate cp26 knockout mutants (named k6#) that did not negatively affect CP29 accumulation, which differed from previous cp26 mutants, allowing us to compare mutants specifically deprived of CP26, CP29, or both. The absence of CP26 partially affected PSII activity, causing reduced growth at low or medium light but not at high irradiances. However, the main phenotype observed in k6# mutants was a more than 70% reduction of NPQ compared to the wild type (Wt). This phenotype was fully rescued by genetic complementation and complemented strains accumulating different levels of CP26, demonstrating that ∼50% of CP26 content, compared to the Wt, was sufficient to restore the NPQ capacity. Our findings demonstrate a pivotal role for CP26 in NPQ induction, while CP29 is crucial for PSII activity. The genetic engineering of these 2 proteins could be a promising strategy to regulate the photosynthetic efficiency of microalgae under different light regimes.

Cryo-EM structure of a plant photosystem II supercomplex with light-harvesting protein Lhcb8 and α-tocopherol

The heart of oxygenic photosynthesis is the water-splitting photosystem II (PSII), which forms supercomplexes with a variable amount of peripheral trimeric light-harvesting complexes (LHCII). Our knowledge of the structure of green plant PSII supercomplex is based on findings obtained from several representatives of green algae and flowering plants; however, data from a non-flowering plant are currently missing. Here we report a cryo-electron microscopy structure of PSII supercomplex from spruce, a representative of non-flowering land plants, at 2.8 Å resolution. Compared with flowering plants, PSII supercomplex in spruce contains an additional Ycf12 subunit, Lhcb4 protein is replaced by Lhcb8, and trimeric LHCII is present as a homotrimer of Lhcb1. Unexpectedly, we have found α-tocopherol (α-Toc)/α-tocopherolquinone (α-TQ) at the boundary between the LHCII trimer and the inner antenna CP43. The molecule of α-Toc/α-TQ is located close to chlorophyll a614 of one of the Lhcb1 proteins and its chromanol/quinone head is exposed to the thylakoid lumen. The position of α-Toc in PSII supercomplex makes it an ideal candidate for the sensor of excessive light, as α-Toc can be oxidized to α-TQ by high-light-induced singlet oxygen at low lumenal pH. The molecule of α-TQ appears to shift slightly into the PSII supercomplex, which could trigger important structure-functional modifications in PSII supercomplex. Inspection of the previously reported cryo-electron microscopy maps of PSII supercomplexes indicates that α-Toc/α-TQ can be present at the same site also in PSII supercomplexes from flowering plants, but its identification in the previous studies has been hindered by insufficient resolution.

Sustainable Production of Pigments from Cyanobacteria

Pigments are intensely coloured compounds used in many industries to colour other materials. The demand for naturally synthesised pigments is increasing and their production can be incorporated into circular bioeconomy approaches. Natural pigments are produced by bacteria, cyanobacteria, microalgae, macroalgae, plants and animals. There is a huge unexplored biodiversity of prokaryotic cyanobacteria which are microscopic phototrophic microorganisms that have the ability to capture solar energy and CO2 and use it to synthesise a diverse range of sugars, lipids, amino acids and biochemicals including pigments. This makes them attractive for the sustainable production of a wide range of high-value products including industrial chemicals, pharmaceuticals, nutraceuticals and animal-feed supplements. The advantages of cyanobacteria production platforms include comparatively high growth rates, their ability to use freshwater, seawater or brackish water and the ability to cultivate them on non-arable land. The pigments derived from cyanobacteria and microalgae include chlorophylls, carotenoids and phycobiliproteins that have useful properties for advanced technical and commercial products. Development and optimisation of strain-specific pigment-based cultivation strategies support the development of economically feasible pigment biorefinery scenarios with enhanced pigment yields, quality and price. Thus, this chapter discusses the origin, properties, strain selection, production techniques and market opportunities of cyanobacterial pigments.

Plastoquinone pool redox state and control of state transitions in Chlamydomonas reinhardtii in darkness and under illumination

Movement of LHCII between two photosystems has been assumed to be similarly controlled by the redox state of the plastoquinone pool (PQ-pool) in plants and green algae. Here we show that the redox state of the PQ-pool of Chlamydomonas reinhardtii can be determined with HPLC and use this method to compare the light state in C. reinhardtii with the PQ-pool redox state in a number of conditions. The PQ-pool was at least moderately reduced under illumination with all tested types of visible light and oxidation was achieved only with aerobic dark treatment or with far-red light. Although dark incubations and white light forms with spectral distribution favoring one photosystem affected the redox state of PQ-pool differently, they induced similar Stt7-dependent state transitions. Thus, under illumination the dynamics of the PQ-pool and its connection with light state appears more complicated in C. reinhardtii than in plants. We suggest this to stem from the larger number of LHC-units and from less different absorption profiles of the photosystems in C. reinhardtii than in plants. The data demonstrate that the two different control mechanisms required to fulfill the dual function of state transitions in C. reinhardtii in photoprotection and in balancing light utilization are activated via different means.

High light-induced changes in thylakoid supercomplexes organization from cyclic electron transport mutants of Chlamydomonas reinhardtii

The localization of carotenoids and macromolecular organization of thylakoid supercomplexes have not been reported yet in Chlamydomonas reinhardtii WT and cyclic electron transport mutants (pgrl1 and pgr5) under high light. Here, the various pigments, protein composition, and pigment-protein interactions were analyzed from the cells, thylakoids, and sucrose density gradient (SDG) fractions. Also, the supercomplexes of thylakoids were separated from BN-PAGE and SDG. The abundance of light-harvesting complex (LHC) II trimer complexes and pigment-pigment interaction were changed slightly under high light, shown by circular dichroism. However, a drastic change was seen in photosystem (PS)I-LHCI complexes than PSII complexes, especially in pgrl1 and pgr5. The lutein and β-carotene increased under high light in LHCII trimers compared to other supercomplexes, indicating that these pigments protected the LHCII trimers against high light. However, the presence of xanthophylls, lutein, and β-carotene was less in PSI-LHCI, indicating that pigment-protein complexes altered in high light. Even the real-time PCR data shows that the pgr5 mutant does not accumulate zeaxanthin dependent genes under high light, which shows that violaxanthin is not converting into zeaxanthin under high light. Also, the protein data confirms that the LHCSR3 expression is absent in pgr5, however it is presented in LHCII trimer in WT and pgrl1. Interestingly, some of the core proteins were aggregated in pgr5, which led to change in photosynthesis efficiency in high light.

Study on high-CO2 tolerant Dunaliella salina and its mechanism via transcriptomic analysis

Microalgae has been regarded as a promising method for reducing CO2 emission. High CO2 concentration generally inhibits algal growth, and previous studies have mostly focused on breeding freshwater algae with high CO2 tolerance. In this study, one marine algal strain Dunaliella salina (D. salina) was grown under 0.03%-30 % CO2 and 3% NaCl conditions, and was evaluated to determine its potential for CO2 assimilation. The results showed that D. salina could tolerate 30% CO2 , and its maximum biomass concentration could reach 1.13 g·L-1 after 8 days incubation, which was 1.85 times higher than that of incubation in air (0.03%). The phenomenon of high-CO2 tolerance in D. salina culture was discussed basing on transcriptome analysis. The results showed that D. salina was subjected to oxidative stress under 30% CO2 conditions, and the majority genes involving in antioxidant system, such as SOD, CAT, and APX genes were up-regulated to scavenge ROS. In addition, most of the key enzyme genes related to photosynthesis, carbon fixation and metabolism were up-regulated, which are consistent with the higher physiological and biochemical values for D. salina incubation under 30% CO2 .

Transcriptomic Analyses of Grapevine Leafroll-Associated Virus 3 Infection in Leaves and Berries of 'Cabernet Franc'

Grapevine leafroll-associated virus 3 (GLRaV-3) is one of the most important viruses affecting global grape and wine production. GLRaV-3 is the chief agent associated with grapevine leafroll disease (GLRD), the most prevalent and economically destructive grapevine viral disease complex. Response of grapevine to GLRaV-3 infection at the gene expression level is poorly characterized, limiting the understanding of GLRaV-3 pathogenesis and viral-associated symptom development. In this research, we used RNA-Seq to profile the changes in global gene expression of Cabernet franc, a premium red wine grape, analyzing leaf and berry tissues at three key different developmental stages. We have identified 1457 differentially expressed genes (DEGs) in leaves and 1181 DEGs in berries. The expression profiles of a subset of DEGs were validated through RT-qPCR, including those involved in photosynthesis (VvPSBP1), carbohydrate partitioning (VvSUT2, VvHT5, VvGBSS1, and VvSUS), flavonoid biosynthesis (VvUFGT, VvLAR1, and VvFLS), defense response (VvPR-10.3, and VvPR-10.7), and mitochondrial activities (ETFB, TIM13, and NDUFA1). GLRaV-3 infection altered source-sink relationship between leaves and berries. Photosynthesis and photosynthate assimilation were inhibited in mature leaves while increased in young berries. The expression of genes involved in anthocyanin biosynthesis increased in GLRaV-3-infected leaves, correlating with interveinal tissue reddening, a hallmark of GLRD symptoms. Notably, we identified changes in gene expression that suggest a compromised sugar export and increased sugar retrieval in GLRaV-3-infected leaves. Genes associated with mitochondria were down-regulated in both leaves and berries of Cabernet franc infected with GLRaV-3. Results of the present study suggest that GLRaV-3 infection may disrupt mitochondrial function in grapevine leaves, leading to repressed sugar export and accumulation of sugar in mature leaf tissues. The excessive sugar accumulation in GLRaV-3-infected leaves may trigger downstream GLRD symptom development and negatively impact berry quality. We propose a working model to account for the molecular events underlying the pathogenesis of GLRaV-3 and symptom development.

Phospholipase D activation is required for 1-aminocyclopropane 1-carboxylic acid signaling during sexual reproduction in the marine red alga Neopyropia yezoensis (Rhodophyta)

BACKGROUND

1-aminocyclopropane 1-carboxylic acid (ACC) is the immediate precursor of the plant hormone ethylene. However, recent studies have suggested that ACC also acts as a signaling molecule to regulate development and growth independently from ethylene biosynthesis. In red algae, ACC stimulates the switch from a vegetative to a sexual reproductive phase. However, despite evidence that ACC signaling in plants and algae is widespread, the mechanistic basis of the ACC signaling pathway remains unknown.

RESULTS

We demonstrate that exogenous ACC increased the activity of phospholipase D (PLD) and induced the accumulation of PLD transcripts in the marine red alga Neopyropia yezoensis. The product of PLD, the lipid second messenger phosphatidic acid (PA), also increased in response to ACC. Furthermore, the pharmacological inhibition of PLD by 1-butanol blocked ACC-induced spermatangia and carpospore production, but the inactive isomer t-butanol did not. In addition, 1-butanol prevented ACC-induced growth inhibition and inhibited transcript accumulation of genes upregulated by ACC, including extracellular matrix (ECM)-related genes, and alleviated the transcriptional decrease of genes downregulated by ACC, including photosynthesis-related genes.

CONCLUSIONS

These results indicate that PLD is a positive regulator of sexual cell differentiation and a negative regulator of growth. This study demonstrates that PLD and its product, PA, are components of ACC signaling during sexual reproduction in N. yezoensis.

The Loroxanthin Cycle: A New Type of Xanthophyll Cycle in Green Algae (Chlorophyta)

Xanthophyll cycles (XC) have proven to be major contributors to photoacclimation for many organisms. This work describes a light-driven XC operating in the chlorophyte Chlamydomonas reinhardtii and involving the xanthophylls Lutein (L) and Loroxanthin (Lo). Pigments were quantified during a switch from high to low light (LL) and at different time points from cells grown in Day/Night cycle. Trimeric LHCII was purified from cells acclimated to high or LL and their pigment content and spectroscopic properties were characterized. The Lo/(L + Lo) ratio in the cells varies by a factor of 10 between cells grown in low or high light (HL) leading to a change in the Lo/(L + Lo) ratio in trimeric LHCII from .5 in low light to .07 in HL. Trimeric LhcbMs binding Loroxanthin have 5 ± 1% higher excitation energy (EE) transfer (EET) from carotenoid to Chlorophyll as well as higher thermo- and photostability than trimeric LhcbMs that only bind Lutein. The Loroxanthin cycle operates on long time scales (hours to days) and likely evolved as a shade adaptation. It has many similarities with the Lutein-epoxide - Lutein cycle (LLx) of plants.

Transcriptional response of a green alga (Raphidocelis subcapitata) exposed to triclosan: photosynthetic systems and DNA repair

Recent studies show that triclosan (TCS) exposure causes reduction in pigments, suppression of photosynthesis, and induction of oxidative stress at the physiological level, resulting in morphological alteration and growth inhibition in algae including Raphidocelis subcapitata (R. subcapitata, a freshwater model green alga). However, the underlying molecular mechanisms remain to be elucidated, especially at environmentally relevant concentrations. The present study uncovered the transcriptional profiles and molecular mechanisms of TCS toxicity in R. subcapitata using next-generation sequencing. The algal growth was drastically inhibited following a 7-day exposure at both 75 and 100 μg/L TCS, but not at 5 μg/L (environmentally realistic level). The transcriptomic analysis shows that molecular signaling pathways including porphyrin and chlorophyll metabolism, photosynthesis - antenna proteins, and photosynthesis were suppressed in all three TCS treatments, and the perturbations of these signaling pathways were exacerbated with increased TCS exposure concentrations. Additionally, signaling of replication-coupled DNA repair was only activated in 100 μg/L TCS treatment. These results indicate that photosynthesis systems were sensitive targets of TCS toxicity in R. subcapitata, which is distinct from the inhibition of lipid synthesis by TCS in bacteria. This study provides novel knowledge on molecular mechanisms of TCS toxicity in R. subcapitata.

Comprehensive RNAseq analysis for identification of genes expressed under heat stress in lentil

Lentils are highly sensitive to abrupt increases in temperature during the mid to late reproductive stages, leading to severe biomass and seed yield reduction. Therefore, we carried out an RNAseq analysis between IG4258 (heat tolerant) and IG3973 (heat sensitive) lentil genotypes at the reproductive stage under both normal and heat stress conditions in the field. It resulted in 209,549 assembled transcripts and among these 161,809 transcripts had coding regions, of which 94,437 transcripts were annotated. The differential gene expression analysis showed upregulation of 678 transcripts and downregulation of 680 transcripts between the tolerant and sensitive genotypes at the early reproductive stage. While 76 transcripts were upregulated and 47 transcripts were downregulated at the late reproductive stage under heat stress conditions. The validation of 12 up-or downregulated transcripts through RT-PCR corresponded well with the expression analysis data of RNAseq, with a correlation of R²  = 0.89. Among these transcripts, the DN364_c1_g1_i9 and DN2218_c0_g1_i5 transcripts encoded enzymes involved in the tryptophan pathway, indicating that tryptophan biosynthesis plays a role under heat stress in lentil. Moreover, KEGG pathways enrichment analysis identified transcripts associated with genes encoding proteins/regulating factors related to different metabolic pathways including signal transduction, fatty acid biosynthesis, rRNA processing, ribosome biogenesis, gibberellin (GA) biosynthesis, and riboflavin biosynthesis. This analysis also identified 6852 genic-SSRs leading to the development of 4968 SSR primers that are potential genomic resources for molecular mapping of heat-tolerant genes in lentil.

Plant and Algal PSII-LHCII Supercomplexes: Structure, Evolution and Energy Transfer

Photosynthesis is the process conducted by plants and algae to capture photons and store their energy in chemical forms. The light-harvesting, excitation transfer, charge separation and electron transfer in photosystem II (PSII) are the critical initial reactions of photosynthesis and thereby largely determine its overall efficiency. In this review, we outline the rapidly accumulating knowledge about the architectures and assemblies of plant and green algal PSII-light harvesting complex II (LHCII) supercomplexes, with a particular focus on new insights provided by the recent high-resolution cryo-electron microscopy map of the supercomplexes from a green alga Chlamydomonas reinhardtii. We make pair-wise comparative analyses between the supercomplexes from plants and green algae to gain insights about the evolution of the PSII-LHCII supercomplexes involving the peripheral small PSII subunits that might have been acquired during the evolution and about the energy transfer pathways that define their light-harvesting and photoprotective properties.

Structural basis of LhcbM5-mediated state transitions in green algae

In green algae and plants, state transitions serve as a short-term light-acclimation process in the regulation of the light-harvesting capacity of photosystems I and II (PSI and PSII, respectively). During the process, a portion of light-harvesting complex II (LHCII) is phosphorylated, dissociated from PSII and binds with PSI to form the supercomplex PSI-LHCI-LHCII. Here, we report high-resolution structures of PSI-LHCI-LHCII from Chlamydomonas reinhardtii, revealing the mechanism of assembly between the PSI-LHCI complex and two phosphorylated LHCII trimers containing all four types of LhcbM protein. Two specific LhcbM isoforms, namely LhcbM1 and LhcbM5, directly interact with the PSI core through their phosphorylated amino terminal regions. Furthermore, biochemical and functional studies on mutant strains lacking either LhcbM1 or LhcbM5 indicate that only LhcbM5 is indispensable in supercomplex formation. The results unravel the specific interactions and potential excitation energy transfer routes between green algal PSI and two phosphorylated LHCIIs.

Structural characterization of the photosystems in the green alga Chlorella sorokiniana

The supramolecular organization of the photosystem supercomplexes in the green alga Chlorella sorokiniana belonging to Trebouxiophyceae are essentially the same as those of Chlamydomonas reinhardtii belonging to Chlorophyceae. The photosynthetic conversion of light energy into chemical energy is performed by photosystems II and I (PSII and PSI) embedded within the thylakoid membranes. In plants and green algae, PSII and PSI comprise the core complex and light-harvesting complexes (LHCII and LHCI), forming PSII-LHCII and PSI-LHCI supercomplexes, respectively. The structural information about photosystem supercomplexes of green algae has been limited to chlorophytic algae. Here, to obtain an insight into the evolution of Chlorophyta, we determined the supramolecular organization of the PSII-LHCII and PSI-LHCI supercomplexes from the freshwater green alga Chlorella sorokiniana, which belongs to Trebouxiophyceae. The obtained results showed that the supramolecular organizations of the photosystem supercomplexes in C. sorokiniana were essentially the same as those of the model green alga C. reinhardtii, which belongs to Chlorophyceae, namely PSII-LHCII supercomplex formed the C₂S₂M₂L₂ configuration and PSI-LHCI supercomplex was associated with 10 LHCI subunits.

Photoprotective Capabilities of Light-Harvesting Complex II Trimers in the Green Alga Chlamydomonas reinhardtii

Major light-harvesting complex (LHCII) trimers in plants induce the thermal dissipation of absorbed excitation energy against photooxidative damage under excess light conditions. LHCII trimers in green algae have been thought to be incapable of energy dissipation without additional quencher proteins, although LHCIIs in plants and green algae are homologous. In this study, we investigated the energy-dissipative capabilities of four distinct types of LHCII trimers isolated from the model green alga Chlamydomonas reinhardtii using spectroscopic analysis. Our results revealed that the LHCII trimers possessing LHCII type II (LHCBM5) and LHCII type IV (LHCBM1) had efficient energy-dissipative capabilities, whereas LHCII type I (LHCBM3/4/6/8/9) and type III (LHCBM2/7) did not. On the basis of the amino acid sequences of LHCBM5 and LHCBM1 compared with the other LHCBMs, we propose that positively charged extra N-terminal amino acid residues mediate the interactions between LHCII trimers to form energy-dissipative states.

The Main Structural and Functional Characteristics of Photosystem-II-Enriched Membranes Isolated from Wild Type and cia3 Mutant Chlamydomonas reinhardtii

Photosystem II (PSII)-enriched membranes retain the original PSII architecture in contrast to PSII cores or PSII supercomplexes, which are usually isolated from Chlamydomonas reinhardtii. Here, we present data that fully characterize the structural and functional properties of PSII complexes in isolated PSII-enriched membranes from C. reinhardtii. The preparations were isolated from wild-type (WT) and CAH3-deficient mutant cia3 as the influence of CAH3 on the PSII function was previously proposed. Based on the equal chlorophyll content, the PSII-enriched membranes from WT and cia3 have the same amount of reaction centers (RCs), cytochrome b559, subunits of the water-oxidizing complex, Mn ions, and carotenes. They differ in the ratio of other carotenoids, the parts of low/intermediate redox forms of cytochrome b559, and the composition of outer light-harvesting complexes. The preparations had 40% more chlorophyll molecules per RC compared to higher plants. Functionally, PSII-enriched membranes from WT and cia3 show the same photosynthetic activity at optimal pH 6.5. However, the preparations from cia3 contained more closed RCs even at pH 6.5 and showed more pronounced suppression of PSII photosynthetic activity at shift pH up to 7.0, established in the lumen of dark-adapted cells. Nevertheless, the PSII photosynthetic capacities remained the same.

Structural insight into light harvesting for photosystem II in green algae

Green algae and plants rely on light-harvesting complex II (LHCII) to collect photon energy for oxygenic photosynthesis. In Chlamydomonas reinhardtii, LHCII molecules associate with photosystem II (PSII) to form various supercomplexes, including the C₂S₂M₂L₂ type, which is the largest PSII-LHCII supercomplex in algae and plants that is presently known. Here, we report high-resolution cryo-electron microscopy (cryo-EM) maps and structural models of the C₂S₂M₂L₂ and C₂S₂ supercomplexes from C. reinhardtii. The C₂S₂ supercomplex contains an LhcbM1-LhcbM2/7-LhcbM3 heterotrimer in the strongly associated LHCII, and the LhcbM1 subunit assembles with CP43 through two interfacial galactolipid molecules. The loosely and moderately associated LHCII trimers interact closely with the minor antenna complex CP29 to form an intricate subcomplex bound to CP47 in the C₂S₂M₂L₂ supercomplex. A notable direct pathway is established for energy transfer from the loosely associated LHCII to the PSII reaction centre, as well as several indirect routes. Structure-based computational analysis on the excitation energy transfer within the two supercomplexes provides detailed mechanistic insights into the light-harvesting process in green algae.

Four distinct trimeric forms of light-harvesting complex II isolated from the green alga Chlamydomonas reinhardtii

Light-harvesting complex II (LHCII) absorbs light energy and transfers it primarily to photosystem II in green algae and land plants. Although the trimeric structure of LHCII is conserved between the two lineages, its subunit composition and function are believed to differ significantly. In this study, we purified four LHCII trimers from the green alga Chlamydomonas reinhardtii and analyzed their biochemical properties. We used several preparation methods to obtain four distinct fractions (fractions 1-4), each of which contained an LHCII trimer with different contents of Type I, III, and IV proteins. The pigment compositions of the LHCIIs in the four fractions were similar. The absorption and fluorescence spectra were also similar, although the peak positions differed slightly. These results indicate that this green alga contains four types of LHCII trimer with different biochemical and spectroscopic features. Based on these findings, we discuss the function and structural organization of green algal LHCII antennae.

Structure of a C2S2M2N2-type PSII-LHCII supercomplex from the green alga Chlamydomonas reinhardtii

Photosystem II (PSII) in the thylakoid membranes of plants, algae, and cyanobacteria catalyzes light-induced oxidation of water by which light energy is converted to chemical energy and molecular oxygen is produced. In higher plants and most eukaryotic algae, the PSII core is surrounded by variable numbers of light-harvesting antenna complex II (LHCII), forming a PSII-LHCII supercomplex. In order to harvest energy efficiently at low-light-intensity conditions under water, a complete PSII-LHCII supercomplex (C₂S₂M₂N₂) of the green alga Chlamydomonas reinhardtii (Cr) contains more antenna subunits and pigments than the dominant PSII-LHCII supercomplex (C₂S₂M₂) of plants. The detailed structure and energy transfer pathway of the Cr-PSII-LHCII remain unknown. Here we report a cryoelectron microscopy structure of a complete, C₂S₂M₂N₂-type PSII-LHCII supercomplex from C. reinhardtii at 3.37-Å resolution. The results show that the Cr-C₂S₂M₂N₂ supercomplex is organized as a dimer, with 3 LHCII trimers, 1 CP26, and 1 CP29 peripheral antenna subunits surrounding each PSII core. The N-LHCII trimer partially occupies the position of CP24, which is present in the higher-plant PSII-LHCII but absent in the green alga. The M trimer is rotated relative to the corresponding M trimer in plant PSII-LHCII. In addition, some unique features were found in the green algal PSII core. The arrangement of a huge number of pigments allowed us to deduce possible energy transfer pathways from the peripheral antennae to the PSII core.

Structural determination of the large photosystem II-light-harvesting complex II supercomplex of Chlamydomonas reinhardtii using nonionic amphipol

In photosynthetic organisms, photosystem II (PSII) is a large membrane protein complex, consisting of a pair of core complexes surrounded by an array of variable numbers of light-harvesting complex (LHC) II proteins. Previously reported structures of the PSII-LHCII supercomplex of the green alga Chlamydomonas reinhardtii exhibit significant structural heterogeneity, but recently improved purification methods employing ionic amphipol A8-35 have enhanced supercomplex stability, providing opportunities for determining a more intact structure. Herein, we present a 5.8 Å cryo-EM map of the C. reinhardtii PSII-LHCII supercomplex containing six LHCII trimers (C₂S₂M₂L₂). Utilizing a newly developed nonionic amphipol-based purification and stabilizing method, we purified the largest photosynthetic supercomplex to the highest percentage of the intact configuration reported to date. We found that the interprotein distances within the light-harvesting complex array in the green algal photosystem are larger than those previously observed in higher plants, indicating that the potential route of energy transfer in the PSII-LHCII supercomplex in green algae may be altered. Interestingly, we also observed an asymmetric PSII-LHCII supercomplex structure comprising C₂S₂M₁L₁ in the same sample. Moreover, we found a new density adjacent to the PSII core complex, attributable to a single-transmembrane helix. It was previously unreported in the cryo-EM maps of PSII-LHCII supercomplexes from land plants.

Non-photochemical quenching-dependent acclimation and thylakoid organization of Chlamydomonas reinhardtii to high light stress

Light is essential for all photosynthetic organisms while an excess of it can lead to damage mainly the photosystems of the thylakoid membrane. In this study, we have grown Chlamydomonas reinhardtii cells in different intensities of high light to understand the photosynthetic process with reference to thylakoid membrane organization during its acclimation process. We observed, the cells acclimatized to long-term response to high light intensities of 500 and 1000 µmol m^-2 s^-1 with faster growth and more biomass production when compared to cells at 50 µmol m^-2 s^-1 light intensity. The ratio of Chl a/b was marginally decreased from the mid-log phase of growth at the high light intensity. Increased level of zeaxanthin and LHCSR3 expression was also found which is known to play a key role in non-photochemical quenching (NPQ) mechanism for photoprotection. Changes in photosynthetic parameters were observed such as increased levels of NPQ, marginal change in electron transport rate, and many other changes which demonstrate that cells were acclimatized to high light which is an adaptive mechanism. Surprisingly, PSII core protein contents have marginally reduced when compared to peripherally arranged LHCII in high light-grown cells. Further, we also observed alterations in stromal subunits of PSI and low levels of PsaG, probably due to disruption of PSI assembly and also its association with LHCI. During the process of acclimation, changes in thylakoid organization occurred in high light intensities with reduction of PSII supercomplex formation. This change may be attributed to alteration of protein-pigment complexes which are in agreement with circular dichoism spectra of high light-acclimatized cells, where decrease in the magnitude of psi-type bands indicates changes in ordered arrays of PSII-LHCII supercomplexes. These results specify that acclimation to high light stress through NPQ mechanism by expression of LHCSR3 and also observed changes in thylakoid protein profile/supercomplex formation lead to low photochemical yield and more biomass production in high light condition.

LHC-like proteins involved in stress responses and biogenesis/repair of the photosynthetic apparatus

LHC (light-harvesting complex) proteins of plants and algae are known to be involved both in collecting light energy for driving the primary photochemical reactions of photosynthesis and in photoprotection when the absorbed light energy exceeds the capacity of the photosynthetic apparatus. These proteins usually contain three transmembrane (TM) helices which span the thylakoid membranes and bind several chlorophyll, carotenoid and lipid molecules. In addition, the LHC protein family includes LHC-like proteins containing one, two, three or even four TM domains. One-helix proteins are not only present in eukaryotic photosynthetic organisms but also in cyanobacteria where they have been named high light-inducible proteins. These small proteins are probably the ancestors of the members of the extant LHC protein family which arouse through gene duplications, deletions and fusions. During evolution, some of these proteins have diverged and acquired novel functions. In most cases, LHC-like proteins are induced in response to various stress conditions including high light, high salinity, elevated temperature and nutrient limitation. Many of these proteins play key roles in photoprotection, notably in non-photochemical quenching of absorbed light energy. Moreover, some of these proteins appear to be involved in the regulation of chlorophyll synthesis and in the assembly and repair of Photosystem II and also of Photosystem I possibly by mediating the insertion of newly synthesized pigments into the photosynthetic reaction centers.

Spectroanalysis in native gels (SING): rapid spectral analysis of pigmented thylakoid membrane complexes separated by CN-PAGE

Photosynthetic organisms rapidly adjust the capture, transfer and utilization of light energy to optimize the efficiency of photosynthesis and avoid photodamage. These adjustments involve fine-tuning of expression levels and mutual interactions among electron/proton transfer components and their associated light-harvesting antenna. Detailed studies of these interactions and their dynamics have been hindered by the low throughput and resolution of currently available research tools, which involve laborious isolation, separation and characterization steps. To address these issues, we developed an approach that measured multiple spectroscopic properties of thylakoid preparations directly in native polyacrylamide gel electrophoresis gels, enabling unprecedented resolution of photosynthetic complexes, both in terms of the spectroscopic and functional details, as well as the ability to distinguish separate complexes and thus test their functional connections. As a demonstration, we explore the thylakoid membrane components of Chlamydomonas reinhardtii acclimated to high and low light, using a combination of room temperature absorption and 77K fluorescence emission to generate a multi-dimensional molecular and spectroscopic map of the photosynthetic apparatus. We show that low-light-acclimated cells accumulate a photosystem I-containing megacomplex that is absent in high-light-acclimated cells and contains distinct LhcII proteins that can be distinguished based on their spectral signatures.

Pyridine nucleotide transhydrogenase PntAB is essential for optimal growth and photosynthetic integrity under low-light mixotrophic conditions in Synechocystis sp. PCC 6803

Pyridine nucleotide transhydrogenase (PntAB) is an integral membrane protein complex participating in the regulation of NAD(P)⁺ :NAD(P)H redox homeostasis in various prokaryotic and eukaryotic organisms. In the present study we addressed the function and biological role of PntAB in oxygenic photosynthetic cyanobacteria capable of both autotrophic and heterotrophic growth, with support from structural three-dimensional (3D)-modeling. The pntA gene encoding the α subunit of heteromultimeric PntAB in Synechocystis sp. PCC 6803 was inactivated, followed by phenotypic and biophysical characterization of the ΔpntA mutant under autotrophic and mixotrophic conditions. Disruption of pntA resulted in phenotypic growth defects observed under low light intensities in the presence of glucose, whereas under autotrophic conditions the mutant did not differ from the wild-type strain. Biophysical characterization and protein-level analysis of the ΔpntA mutant revealed that the phenotypic defects were accompanied by significant malfunction and damage of the photosynthetic machinery. Our observations link the activity of PntAB in Synechocystis directly to mixotrophic growth, implicating that under these conditions PntAB functions to balance the NADH: NADPH equilibrium specifically in the direction of NADPH. The results also emphasize the importance of NAD(P)⁺ :NAD(P)H redox homeostasis and associated ATP:ADP equilibrium for maintaining the integrity of the photosynthetic apparatus under low-light glycolytic metabolism.

Differential accumulation of photosynthetic proteins regulates diurnal photochemical adjustments of PSII in common fig (Ficus carica L.) leaves

Molecular processes involved in photosystem II adaptation of woody species to diurnal changes in light and temperature conditions are still not well understood. Regarding this, here we investigated differences between young and mature leaves of common fig (Ficus carica L.) in photosynthetic performance as well as accumulation of the main photosynthetic proteins: light harvesting complex II, D1 protein and Rubisco large subunit. Investigated leaf types revealed different adjustment mechanisms to keep effective photosynthesis. Rather stable diurnal accumulation of light harvesting complex II in mature leaves enabled efficient excitation energy utilization (negative L-band) what triggered faster D1 protein degradation at high light. However, after photoinhibition, greater accumulation of D1 during the night enabled them faster recovery. So, the most photosynthetic parameters, as the maximum quantum yield for primary photochemistry, electron transport and overall photosynthetic efficiency in mature leaves successfully restored to their initial values at 1a.m. Reduced connectivity of light harvesting complexes II to its reaction centers (positive L-band) in young leaves increased dissipation of excess light causing less pressure to D1 and its slower degradation. Decreased electron transport in young leaves, due to reduced transfer beyond primary acceptor Q_A^- most probably additionally induced degradation of Rubisco large subunit what consequently led to the stronger decrease of overall photosynthetic efficiency in young leaves at noon.

Functional Accumulation of Antenna Proteins in Chlorophyll b-Less Mutants of Chlamydomonas reinhardtii

The green alga Chlamydomonas reinhardtii contains several light-harvesting chlorophyll a/b complexes (LHC): four major LHCIIs, two minor LHCIIs, and nine LHCIs. We characterized three chlorophyll b-less mutants to assess the effect of chlorophyll b deficiency on the function, assembly, and stability of these chlorophyll a/b binding proteins. We identified point mutations in two mutants that inactivate the CAO gene responsible for chlorophyll a to chlorophyll b conversion. All LHCIIs accumulated to wild-type levels in a CAO mutant but their light-harvesting function for photosystem II was impaired. In contrast, most LHCIs accumulated to wild-type levels in the mutant and their light-harvesting capability for photosystem I remained unaltered. Unexpectedly, LHCI accumulation and the photosystem I functional antenna size increased in the mutant compared with in the wild type when grown in dim light. When the CAO mutation was placed in a yellow-in-the-dark background (yid-BF3), in which chlorophyll a synthesis remains limited in dim light, accumulation of the major LHCIIs and of most LHCIs was markedly reduced, indicating that sustained synthesis of chlorophyll a is required to preserve the proteolytic resistance of antenna proteins. Indeed, after crossing yid-BF3 with a mutant defective for the thylakoid FtsH protease activity, yid-BF3-ftsh1 restored wild-type levels of LHCI, which defines LHCI as a new substrate for the FtsH protease.

Protein and lipid dynamics in photosynthetic thylakoid membranes investigated by in-situ solid-state NMR

Photosynthetic thylakoid membranes contain the protein machinery to convert sunlight in chemical energy and regulate this process in changing environmental conditions via interplay between lipid, protein and xanthophyll molecular constituents. This work addresses the molecular effects of zeaxanthin accumulation in thylakoids, which occurs in native systems under high light conditions through the conversion of the xanthophyll violaxanthin into zeaxanthin via the so called xanthophyll cycle. We applied biosynthetic isotope labeling and ¹³C solid-state NMR spectroscopy to simultaneously probe the conformational dynamics of protein, lipid and xanthophyll constituents of thylakoids isolated from wild type (cw15) and npq2 mutant of the green alga Chlamydomonas reinhardtii, that accumulates zeaxanthin constitutively. Results show differential dynamics of wild type and npq2 thylakoids. Ordered-phase lipids have reduced mobility and mobile-phase lipids have enlarged dynamics in npq2 membranes, together spanning a broader dynamical range. The fraction of ordered lipids is much larger than the fraction of mobile lipids, which explains why zeaxanthin appears to cause overall reduction of thylakoid membrane fluidity. In addition to the ordered lipids, also the xanthophylls and a subset of protein sites in npq2 thylakoids have reduced conformational dynamics. Our work demonstrates the applicability of solid-state NMR spectroscopy for obtaining a microscopic picture of different membrane constituents simultaneously, inside native, heterogeneous membranes.

Early folding events during light harvesting complex II assembly in vitro monitored by pulsed electron paramagnetic resonance

Efficient energy transfer in the major light harvesting complex II (LHCII) of green plants is facilitated by the precise alignment of pigments due to the protein matrix they are bound to. Much is known about the import of the LHCII apoprotein into the chloroplast via the TOC/TIC system and its targeting to the thylakoid membrane but information is sparse about when and where the pigments are bound and how this is coordinated with protein folding. In vitro, the LHCII apoprotein spontaneously folds and binds its pigments if the detergent-solubilized protein is combined with a mixture of chlorophylls a and b and carotenoids. In the present work, we employed this approach to study apoprotein folding and pigment binding in a time-resolved manner by using pulsed electron paramagnetic resonance (EPR). Intra-molecular distances were measured before folding, after 255 ms and 40 s folding time in the absence of cryoprotectant, and in the fully folded and assembled LHCII. In accordance with earlier results, the most of the folding of the three membrane-spanning alpha helices precedes their apposition into the final tertiary structure. However, their formation follows different kinetics, partially extending into the final phase of LHCII formation during which much of the condensation of the pigment-protein structure occurs, presumably governed by the binding of chlorophyll b. A rough timetable is proposed to sort partial events into the LHCII formation process.

The High Efficiency of Photosystem I in the Green Alga Chlamydomonas reinhardtii Is Maintained after the Antenna Size Is Substantially Increased by the Association of Light-harvesting Complexes II

Photosystems (PS) I and II activities depend on their light-harvesting capacity and trapping efficiency, which vary in different environmental conditions. For optimal functioning, these activities need to be balanced. This is achieved by redistribution of excitation energy between the two photosystems via the association and disassociation of light-harvesting complexes (LHC) II, in a process known as state transitions. Here we study the effect of LHCII binding to PSI on its absorption properties and trapping efficiency by comparing time-resolved fluorescence kinetics of PSI-LHCI and PSI-LHCI-LHCII complexes of Chlamydomonas reinhardtii. PSI-LHCI-LHCII of C. reinhardtii is the largest PSI supercomplex isolated so far and contains seven Lhcbs, in addition to the PSI core and the nine Lhcas that compose PSI-LHCI, together binding ∼ 320 chlorophylls. The average decay time for PSI-LHCI-LHCII is ∼ 65 ps upon 400 nm excitation (15 ps slower than PSI-LHCI) and ∼ 78 ps upon 475 nm excitation (27 ps slower). The transfer of excitation energy from LHCII to PSI-LHCI occurs in ∼ 60 ps. This relatively slow transfer, as compared with that from LHCI to the PSI core, suggests loose connectivity between LHCII and PSI-LHCI. Despite the relatively slow transfer, the overall decay time of PSI-LHCI-LHCII remains fast enough to assure a 96% trapping efficiency, which is only 1.4% lower than that of PSI-LHCI, concomitant with an increase of the absorption cross section of 47%. This indicates that, at variance with PSII, the design of PSI allows for a large increase of its light-harvesting capacities.

Modulating energy arriving at photochemical reaction centers: orange carotenoid protein-related photoprotection and state transitions

Photosynthetic organisms tightly regulate the energy arriving to the reaction centers in order to avoid photodamage or imbalance between the photosystems. To this purpose, cyanobacteria have developed mechanisms involving relatively rapid (seconds to minutes) changes in the photosynthetic apparatus. In this review, two of these processes will be described: orange carotenoid protein(OCP)-related photoprotection and state transitions which optimize energy distribution between the two photosystems. The photoactive OCP is a light intensity sensor and an energy dissipater. Photoactivation depends on light intensity and only the red-active OCP form, by interacting with phycobilisome cores, increases thermal energy dissipation at the level of the antenna. A second protein, the "fluorescence recovery protein", is needed to recover full antenna capacity under low light conditions. This protein accelerates OCP conversion to the inactive orange form and plays a role in dislodging the red OCP protein from the phycobilisome. The mechanism of state transitions is still controversial. Changes in the redox state of the plastoquinone pool induce movement of phycobilisomes and/or photosystems leading to redistribution of energy absorbed by phycobilisomes between PSII and PSI and/or to changes in excitation energy spillover between photosystems. The different steps going from the induction of redox changes to movement of phycobilisomes or photosystems remain to be elucidated.

Sulphur responsiveness of the Chlamydomonas reinhardtii LHCBM9 promoter

A 44-base-pair region in the Chlamydomonas reinhardtii LHCBM9 promoter is essential for sulphur responsiveness. The photosynthetic light-harvesting complex (LHC) proteins play essential roles both in light capture, the first step of photosynthesis, and in photoprotective mechanisms. In contrast to the other LHC proteins and the majority of photosynthesis proteins, the Chlamydomonas reinhardtii photosystem II-associated LHC protein, LHCBM9, was recently reported to be up-regulated under sulphur deprivation conditions, which also induce hydrogen production. Here, we examined the sulphur responsiveness of the LHCBM9 gene at the transcriptional level, through promoter deletion analysis. The LHCBM9 promoter was found to be responsive to sulphur deprivation, with a 44-base-pair region between nucleotide positions -136 and -180 relative to the translation start site identified as essential for this response. Anaerobiosis was found to enhance promoter activity under sulphur deprivation conditions, however, alone was unable to induce promoter activity. The study of LHCBM9 is of biological and biotechnological importance, as its expression is linked to photobiological hydrogen production, theoretically the most efficient process for biofuel production, while the simplicity of using an S-deprivation trigger enables the development of a novel C. reinhardtii-inducible promoter system based on LHCBM9.

Dynamic regulation of photosynthesis in Chlamydomonas reinhardtii

Plants and algae have acquired the ability to acclimatize to ever-changing environments to survive. During photosynthesis, light energy is converted by several membrane protein supercomplexes into electrochemical energy, which is eventually used to assimilate CO2 . The efficiency of photosynthesis is modulated by many environmental factors, including temperature, drought, CO2 concentration, and the quality and quantity of light. Recently, our understanding of such regulators of photosynthesis and the underlying molecular mechanisms has increased considerably. The photosynthetic supercomplexes undergo supramolecular reorganizations within a short time after receiving environmental cues. These reorganizations include state transitions that balance the excitation of the two photosystems: qE quenching, which thermally dissipates excess energy at the level of the light-harvesting antenna, and cyclic electron flow, which supplies the increased ATP demanded by CO2 assimilation and the pH gradient to activate qE quenching. This review focuses on the recent findings regarding the environmental regulation of photosynthesis in model organisms, paying particular attention to the unicellular green alga Chlamydomonas reinhardtii, which offer a glimpse into the dynamic behavior of photosynthetic machinery in nature.

Characterization of the major light-harvesting complexes (LHCBM) of the green alga Chlamydomonas reinhardtii

Nine genes (LHCBM1-9) encode the major light-harvesting system of Chlamydomonas reinhardtii. Transcriptomic and proteomic analyses have shown that those genes are all expressed albeit in different amounts and some of them only in certain conditions. However, little is known about the properties and specific functions of the individual gene products because they have never been isolated. Here we have purified several complexes from native membranes and/or we have reconstituted them in vitro with pigments extracted from C. reinhardtii. It is shown that LHCBM1 and -M2/7 represent more than half of the LHCBM population in the membrane. LHCBM2/7 forms homotrimers while LHCBM1 seems to be present in heterotrimers. Trimers containing only type I LHCBM (M3/4/6/8/9) were also observed. Despite their different roles, all complexes have very similar properties in terms of pigment content, organization, stability, absorption, fluorescence and excited-state lifetimes. Thus the involvement of LHCBM1 in non-photochemical quenching is suggested to be due to specific interactions with other components of the membrane and not to the inherent quenching properties of the complex. Similarly, the overexpression of LHCBM9 during sulfur deprivation can be explained by its low sulfur content as compared with the other LHCBMs. Considering the highly conserved biochemical and spectroscopic properties, the major difference between the complexes may be in their capacity to interact with other components of the thylakoid membrane.

Light-Harvesting Complex Protein LHCBM9 Is Critical for Photosystem II Activity and Hydrogen Production in Chlamydomonas reinhardtii

Photosynthetic organisms developed multiple strategies for balancing light-harvesting versus intracellular energy utilization to survive ever-changing environmental conditions. The light-harvesting complex (LHC) protein family is of paramount importance for this function and can form light-harvesting pigment protein complexes. In this work, we describe detailed analyses of the photosystem II (PSII) LHC protein LHCBM9 of the microalga Chlamydomonas reinhardtii in terms of expression kinetics, localization, and function. In contrast to most LHC members described before, LHCBM9 expression was determined to be very low during standard cell cultivation but strongly increased as a response to specific stress conditions, e.g., when nutrient availability was limited. LHCBM9 was localized as part of PSII supercomplexes but was not found in association with photosystem I complexes. Knockdown cell lines with 50 to 70% reduced amounts of LHCBM9 showed reduced photosynthetic activity upon illumination and severe perturbation of hydrogen production activity. Functional analysis, performed on isolated PSII supercomplexes and recombinant LHCBM9 proteins, demonstrated that presence of LHCBM9 resulted in faster chlorophyll fluorescence decay and reduced production of singlet oxygen, indicating upgraded photoprotection. We conclude that LHCBM9 has a special role within the family of LHCII proteins and serves an important protective function during stress conditions by promoting efficient light energy dissipation and stabilizing PSII supercomplexes.

Engineering the assembly of heme cofactors in man-made proteins

Timely ligation of one or more chemical cofactors at preselected locations in proteins is a critical preamble for catalysis in many natural enzymes, including the oxidoreductases and allied transport and signaling proteins. Likewise, ligation strategies must be directly addressed when designing oxidoreductase and molecular transport functions in man-made, first-principle protein constructs intended to operate in vitro or in vivo. As one of the most common catalytic cofactors in biology, we have chosen heme B, along with its chemical analogues, to determine the kinetics and barriers to cofactor incorporation and bishistidine ligation in a range of 4-α-helix proteins. We compare five elementary synthetic designs (maquettes) and the natural cytochrome b562 that differ in oligomeric forms, apo- and holo-tertiary structural stability; qualities that we show can either assist or hinder assembly. The cofactor itself also imposes an assembly barrier if amphiphilicity ranges toward too hydrophobic or hydrophilic. With progressive removal of identified barriers, we achieve maquette assembly rates as fast as native cytochrome b562, paving the way to in vivo assembly of man-made hemoprotein maquettes and integration of artificial proteins into enzymatic pathways.

Regulation and dynamics of the light-harvesting system

Photosynthetic organisms are continuously subjected to changes in light quantity and quality, and must adjust their photosynthetic machinery so that it maintains optimal performance under limiting light and minimizes photodamage under excess light. To achieve this goal, these organisms use two main strategies in which light-harvesting complex II (LHCII), the light-harvesting system of photosystem II (PSII), plays a key role both for the collection of light energy and for photoprotection. The first is energy-dependent nonphotochemical quenching, whereby the high-light-induced proton gradient across the thylakoid membrane triggers a process in which excess excitation energy is harmlessly dissipated as heat. The second involves a redistribution of the mobile LHCII between the two photosystems in response to changes in the redox poise of the electron transport chain sensed through a signaling chain. These two processes strongly diminish the production of damaging reactive oxygen species, but photodamage of PSII is unavoidable, and it is repaired efficiently.

Dynamic reorganization of photosynthetic supercomplexes during environmental acclimation of photosynthesis

Plants and algae have acquired the ability to acclimate to ever-changing environments in order to survive. During photosynthesis, light energy is converted by several membrane protein supercomplexes into electrochemical energy, which is eventually used to assimilate CO2. The efficiency of photosynthesis is modulated by many environmental factors such as quality and quantity of light, temperature, drought, and CO2 concentration, among others. Accumulating evidence indicates that photosynthetic supercomplexes undergo supramolecular reorganization within a short time frame during acclimation to an environmental change. This reorganization includes state transitions that balance the excitation of photosystem I and II by shuttling peripheral antenna proteins between the two, thermal energy dissipation that occurs at energy-quenching sites within the light-harvesting antenna generated for negative feedback when excess light is absorbed, and cyclic electron flow that is facilitated between photosystem I and the cytochrome bf complex when cells demand more ATP and/or need to activate energy dissipation. This review will highlight the recent findings regarding these environmental acclimation events in model organisms with particular attention to the unicellular green alga C. reinhardtii and with reference to the vascular plant A. thaliana, which offers a glimpse into the dynamic behavior of photosynthetic machineries in nature.

Characterization of the decline and recovery of heat-treated Scenedesmus vacuolatus

BACKGROUND

To find out how algal cells cope with and recover from heat stress, the small vegetative cells of the synchronous Scenedesmus vacuolatus culture were subjected to a heat pretreatment (46.5°C for 1 h) followed by dark recultivation. The changes in physiological activities and morphology of Scenedesmus cells were continuously monitored throughout the course of decline and recovery.

RESULTS

It was found that the heat treatment, though completely inhibited photosynthesis, did not kill Scenedesmus cells. These cells, during dark recultivation, could make a fast repair and regained the ability of proliferation. We suggest that they entered a 'stand-by' state, which was characterized by condensed chromatin, partially functional but morphologically altered chloroplasts, disappeared vacuoles, slightly shrunk protoplast and intact plasma membranes. These stressed cells, on the surface, seemingly were undergoing some kind of disintegration, could readily and quickly return to normal cells upon illumination. Cell death occurred only after a long period of darkness (>48 h).

CONCLUSIONS

Our results suggest that the recovery of algal cells from stress damage may actually proceed in two steps. The middle "stand-by' stage normally is gone through too rapidly to be detected unless cells are kept in the dark.

Spectral and functional studies on siphonaxanthin-type light-harvesting complex of photosystem II from Bryopsis corticulans

Carotenoids with conjugated carbonyl groups possess special photophysical properties which have been studied in some water-soluble light-harvesting proteins (Polívka and Sundström, Chem Rev 104:2021-2071, 2004). However, siphonaxanthin-type light-harvesting complexes of photosystem II (LHCII) in siphonous green alga have received fewer studies. In the present study, we determined sequences of genes for several Bryopsis corticulans Lhcbm proteins, which showed that they belong to the group of major LHCII and diverged early from green algae and higher plants. Analysis of pigment composition indicated that this siphonaxanthin-type LHCII contained in total 3 siphonaxanthin and siphonein but no lutein and violaxanthin. In addition, 2 chlorophylls a in higher plant LHCII were replaced by chlorophyll b. These changes led to an increased absorption in green and blue-green light region compared with higher plant LHCII. The binding sites for chlorophylls, siphonaxanthin, and siphonein were suggested based on the structural comparison with that of higher plant LHCII. All of the ligands for the chlorophylls were completely conserved, suggesting that the two chlorophylls b were replaced by chlorophyll a without changing their binding sites in higher plant LHCII. Comparisons of the absorption spectra of isolated siphonaxanthin and siphonein in different organic solutions and the effect of heat treatment suggested that these pigments existed in a low hydrophobic protein environment, leading to an enhancement of light harvesting in the green light region. This low hydrophobic protein environment was maintained by the presence of more serine and threonine residues in B. corticulans LHCII. Finally, esterization of siphonein may also contribute to the enhanced harvesting of green light.