The PSI-H subunit of photosystem I is essential for state transitions in plant photosynthesis

The PSI-O subunit of plant photosystem I is involved in balancing the excitation pressure between the two photosystems

PSI-O is a subunit of photosystem I in eukaryotes. The function of PSI-O was characterized in Arabidopsis plants using RNA interference. Several transformants with the psaO-RNAi construct were generated, and a high proportion of the plants contained only very little or virtually no residual PSI-O. Plants lacking PSI-O have a 50% reduction in state transitions indicating a role for PSI-O in the balancing of excitation energy between the two photosystems. PSI-H and -L have been shown previously to be involved in state transitions, and immunoblot analysis revealed that plants devoid of PSI-L or -H also have 80-90% reduction in the abundance of PSI-O. In contrast, down-regulation of PSI-O has no negative effect on the content of PSI-H and -L. The interaction between PSI-O and the PSI-L was confirmed by chemical cross-linking. A model of PSI is proposed in which PSI-L as the most ancient subunit is closest to the reaction center, and PSI-O is positioned close to PSI-L on the PSI-H/-L/-I side of the PSI complex. PSI-H, -L, -O, and possibly -I are all involved in forming a domain in PSI that is involved in the interaction with light-harvesting complex II.

Mutants for photosystem I subunit D of Arabidopsis thaliana: effects on photosynthesis, photosystem I stability and expression of nuclear genes for chloroplast functions

In Arabidopsis thaliana, the D-subunit of photosystem I (PSI-D) is encoded by two functional genes, PsaD1 and PsaD2, which are highly homologous. Knock-out alleles for each of the loci have been identified by a combination of forward and reverse genetics. The double mutant psad1-1 psad2-1 is seedling-lethal, high-chlorophyll-fluorescent and deficient for all tested PSI subunits, indicating that PSI-D is essential for photosynthesis. In addition, psad1-1 psad2-1 plants show a defect in the accumulation of thylakoid multiprotein complexes other than PSI. Of the single-gene mutations, psad2 plants behave like wild-type (WT) plants, whereas psad1-1 markedly affects the accumulation of PsaD mRNA and protein, and photosynthetic electron flow. Additional effects of the psad1-1 mutation include a decrease in growth rate under greenhouse conditions and downregulation of the mRNA expression of most genes involved in the light phase of photosynthesis. In the same mutant, a marked decrease in the levels of PSI and PSII polypeptides is evident, as well as a light-green leaf coloration and increased photosensitivity. Increased dosage of PsaD2 in the psad1-1 background restores the WT phenotype, indicating that PSI-D1 and PSI-D2 have redundant functions.

Metabolic Flexibility of the Green Alga Chlamydomonas reinhardtii as Revealed by the Link between State Transitions and Cyclic Electron Flow

In this Review we focus on the conversion of linear photosynthetic electron transport from water to NADP to the cyclic pathway around Photosystem I in the green alga Chlamydomonas reinhardtii. We discuss the strict relationship that exists between the changes in pathways of electron transport and state transitions, i.e., the reversible functional association of light harvesting proteins with one of the two photosystems of oxygenic photosynthesis. Such a link has not been reported in the case of other photosynthetic organisms, where the state transitions do not affect the pathway of electron transport. Rather, they provide a tool to optimise the rate of linear flow. We propose a kinetic-structural model that explains the mechanism of this particular relationship in Chlamydomonas, and discuss the advantages that this peculiar situation gives to the energetic metabolism of this alga.

Crystal structure of plant photosystem I

Oxygenic photosynthesis is the principal producer of both oxygen and organic matter on Earth. The conversion of sunlight into chemical energy is driven by two multisubunit membrane protein complexes named photosystem I and II. We determined the crystal structure of the complete photosystem I (PSI) from a higher plant (Pisum sativum var. alaska) to 4.4 A resolution. Its intricate structure shows 12 core subunits, 4 different light-harvesting membrane proteins (LHCI) assembled in a half-moon shape on one side of the core, 45 transmembrane helices, 167 chlorophylls, 3 Fe-S clusters and 2 phylloquinones. About 20 chlorophylls are positioned in strategic locations in the cleft between LHCI and the core. This structure provides a framework for exploration not only of energy and electron transfer but also of the evolutionary forces that shaped the photosynthetic apparatus of terrestrial plants after the divergence of chloroplasts from marine cyanobacteria one billion years ago.

Kinetic analyses of state transitions of the cyanobacterium Synechococcus sp. PCC 7002 and its mutant strains impaired in electron transport

The state transitions of the cyanobacterium Synechococcus sp. PCC 7002 and of three mutant strains, which were impaired in PsaE-dependent cyclic electron transport (psaE(-)), respiratory electron transport (ndhF(-)) and both activities (psaE(-)ndhF(-)), were analyzed. Dark incubation of the wild type and psaE(-) cells led to a transition to state 2, while the ndhF(-) strains remained in state 1 after dark incubation. The ndhF(-) cells adapted to state 2 when the cells were incubated under anaerobic conditions or in the presence of potassium cyanide; these results suggest that the ndhF(-) cells were inefficient in performing state 1 to state 2 transitions in the dark unless cytochrome oxidase activity was inhibited. In the state 2 to state 1 transition of wild-type cells induced by light in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), there was still a significant reduction of the interphotosystem electron carriers by both respiration and cyclic electron flow around PSI. Kinetic analysis of the state 2 to state 1 transition shows that, in the absence of PSII activity, the relative contribution to the reduced state of the interphotosystem electron carriers by respiratory and cyclic electron transfer is about 72% and 28%, respectively. The state 2 to state 1 transition was prevented by the cytochrome b(6)f inhibitor 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB). On the other hand, the state 1 to state 2 transition was induced by DBMIB with half times of approximately 8 s in all strains. The externally added electron acceptor 2,5-dimethyl-benzoquinone (DMBQ) induced a state 2 to state 1 transition in the dark and this transition could be prevented by DBMIB. The light-induced oxidation of P700 showed that approximately 50% of PSI could be excited by 630-nm light absorbed by phycobilisomes (PBS) under state 2 conditions. P700 oxidation measurements with light absorbed by PBS also showed that the dark-induced state 1 to state 2 transition occurred in wild-type cells but not in the ndhF(-) cells. The possible mechanism for sensing an imbalanced light regime in cyanobacterial state transitions is discussed.

Light-harvesting complex II binds to several small subunits of photosystem I

Mobile light-harvesting complex II (LHCII) is implicated in the regulation of excitation energy distribution between Photosystem I (PSI) and Photosystem II (PSII) during state transitions. To investigate how LHCII interacts with PSI during state transitions, PSI was isolated from Arabidopsis thaliana plants treated with PSII or PSI light. The PSI preparations were made using digitonin. Chemical cross-linking using dithio-bis(succinimidylpropionate) followed by diagonal electrophoresis and immunoblotting showed that the docking site of LHCII (Lhcb1) on PSI is comprised of the PSI-H, -L, and -I subunits. This was confirmed by the lack of energy transfer from LHCII to PSI in the digitonin-PSI isolated from plants lacking PSI-H and -L. Digitonin-PSI was purified further to obtain an LHCII.PSI complex, and two to three times more LHCII was associated with PSI in the wild type in State 2 than in State 1. Lhcb1 was also associated with PSI from plants lacking PSI-K, but PSI from PSI-H, -L, or -O mutants contained only about 30% of Lhcb1 compared with the wild type. Surprisingly, a significant fraction of the LHCII bound to PSI in State 2 was not phosphorylated. Cross-linking prior to sucrose gradient purification resulted in copurification of phosphorylated LHCII in the wild type, but not with PSI from the PSI-H, -L, and -O mutants. The data suggest that migration of LHCII during state transitions cannot be explained sufficiently by different affinity of phosphorylated and unphosphorylated LHCII for PSI but is likely to involve structural changes in thylakoid organization.

Tracking the function of the cytochrome c6-like protein in higher plants

The contention that plastocyanin is the only mobile electron donor to photosystem I in higher plants was recently shaken by the discovery of a cytochrome c(6)-like protein in Arabidopsis and other flowering plants. However, the genetic and biochemical data presented in support of the idea that the cytochrome c(6) homologue can replace plastocyanin have now been challenged by two complementary studies. This re-opens the debate on the real function(s) of cytochrome c in the chloroplasts of higher plants.

Absence of the Lhcb1 and Lhcb2 proteins of the light-harvesting complex of photosystem II - effects on photosynthesis, grana stacking and fitness

We have constructed Arabidopsis thaliana plants that are virtually devoid of the major light-harvesting complex, LHC II. This was accomplished by introducing the Lhcb2.1 coding region in the antisense orientation into the genome by Agrobacterium-mediated transformation. Lhcb1 and Lhcb2 were absent, while Lhcb3, a protein present in LHC II associated with photosystem (PS) II, was retained. Plants had a pale green appearance and showed reduced chlorophyll content and an elevated chlorophyll a/b ratio. The content of PS II reaction centres was unchanged on a leaf area basis, but there was evidence for increases in the relative levels of other light harvesting proteins, notably CP26, associated with PS II, and Lhca4, associated with PS I. Electron microscopy showed the presence of grana. Photosynthetic rates at saturating irradiance were the same in wild-type and antisense plants, but there was a 10-15% reduction in quantum yield that reflected the decrease in light absorption by the leaf. The antisense plants were not able to perform state transitions, and their capacity for non-photochemical quenching was reduced. There was no difference in growth between wild-type and antisense plants under controlled climate conditions, but the antisense plants performed worse compared to the wild type in the field, with decreases in seed production of up to 70%.

Light stress-induced one-helix protein of the chlorophyll a/b-binding family associated with photosystem I

The superfamily of light-harvesting chlorophyll a/b-binding (Lhc) proteins in higher plants and green algae is composed of more than 20 different antenna proteins associated either with photosystem I (PSI) or photosystem II (PSII). Several distant relatives of this family with conserved chlorophyll-binding residues and proposed photoprotective functions are induced transiently under various stress conditions. Whereas "classical" Lhc proteins contain three-transmembrane alpha-helices, their distant relatives span the membrane with between one and four transmembrane segments. Here, we report the identification and isolation of a novel member of the Lhc family from Arabidopsis with one predicted transmembrane alpha-helix closely related to helix I of Lhc protein from PSI (Lhca4) that we named Ohp2 (for a second one-helix protein of Lhc family described from higher plants). We showed that the Ohp2 gene expression is triggered by light stress and that the Ohp2 transcript and protein accumulated in a light intensity-dependent manner. Other stress conditions did not up-regulate the expression of the Ohp2 gene. Localization studies revealed that Ohp2 is associated with PSI under low- or high-light conditions. Because all stress-induced Lhc relatives reported so far were found in PSII, we propose that the accumulation of Ohp2 might represent a novel photoprotective strategy induced within PSI in response to light stress.

Three-dimensional reconstruction of a light-harvesting complex I-photosystem I (LHCI-PSI) supercomplex from the green alga Chlamydomonas reinhardtii. Insights into light harvesting for PSI

A supercomplex containing the photosystem I (PSI) and chlorophyll a/b light-harvesting complex I (LHCI) has been isolated using a His-tagged mutant of Chlamydomonas reinhardtii. This LHCI-PSI supercomplex contained approximately 215 chlorophyll molecules of which 175 were estimated to be chlorophyll a and 40 to be chlorophyll b, based on P700 oxidation and chlorophyll a/b ratio measurements. Its room temperature long wavelength absorption peak was at 680 nm, and it emitted chlorophyll fluorescence maximally at 715 nm (77 K). The LHCI was composed of four or more different types of Lhca polypeptides including Lhca3. No LHCII proteins or other phosphoproteins were detected in the LHCI-PSI supercomplexes suggesting that the cells from which they were isolated were in State 1. Electron microscopy of negatively stained samples followed by image analysis revealed the LHCI-PSI supercomplex to have maximal dimensions of 220 A by 180 A and to be approximately 105 A thick. An averaged top view was used to model in x-ray and electron crystallographic data for PSI and Lhca proteins respectively. We conclude that the supercomplex consists of a PSI reaction center monomer with 11 Lhca proteins arranged along the side where the PSI proteins, PsaK, PsaJ, PsaF, and PsaG are located. The estimated molecular mass for the complex is 700 kDa including the bound chlorophyll molecules. The assignment of 11 Lhca proteins is consistent with a total chlorophyll level of 215 assuming that the PSI reaction center core binds approximately 100 chlorophylls and that each Lhca subunit binds 10 chlorophylls. There was no evidence for oligomerization of Chlamydomonas PSI in contrast to the trimerization of PSI in cyanobacteria.

Role of chloroplast protein kinase Stt7 in LHCII phosphorylation and state transition in Chlamydomonas

Photosynthetic organisms adapt to changes in light quality by redistributing light excitation energy between two photosystems through state transition. This reorganization of antenna systems leads to an enhanced photosynthetic yield. Using a genetic approach in Chlamydomonas reinhardtii to dissect the signal transduction pathway of state transition, we identified a chloroplast thylakoid-associated serine-threonine protein kinase, Stt7, that has homologs in land plants. Stt7 is required for the phosphorylation of the major light-harvesting protein (LHCII) and for state transition.

A novel plant protein undergoing light-induced phosphorylation and release from the photosynthetic thylakoid membranes

The characteristics of a phosphoprotein with a relative electrophoretic mobility of 12 kDa have been unknown during two decades of studies on redox-dependent protein phosphorylation in plant photosynthetic membranes. Digestion of this protein from spinach thylakoid membranes with trypsin and subsequent tandem nanospray-quadrupole-time-of-flight mass spectrometry of the peptides revealed a protein sequence that did not correspond to any previously known protein. Sequencing of the corresponding cDNA uncovered a gene for a precursor protein with a transit peptide followed by a strongly basic mature protein with a molecular mass of 8,640 Da. Genes encoding homologous proteins were found on chromosome 3 of Arabidopsis and rice as well as in ESTs from 20 different plant species, but not from any other organisms. The protein can be released from the membrane with high salt and is also partially released in response to light-induced phosphorylation of thylakoids, in contrast to all other known thylakoid phosphoproteins, which are integral to the membrane. On the basis of its properties, this plant-specific protein is named thylakoid soluble phosphoprotein of 9 kDa (TSP9). Mass spectrometric analyses revealed the existence of non-, mono-, di-, and triphosphorylated forms of TSP9 and phosphorylation of three distinct threonine residues in the central part of the protein. The phosphorylation and release of TSP9 from the photosynthetic membrane on illumination favor participation of this basic protein in cell signaling and regulation of plant gene expression in response to changing light conditions.

From Genes to Photosynthesis in Arabidopsis thaliana

Although photosynthesis in higher plants is of cyanobacterial descent, it differs strikingly in organization and regulation from the prokaryotic process. Genomics, proteomics, and comparative genome analysis are now providing powerful new tools for the molecular dissection of photosynthesis in higher plants. Mutant screens and reverse genetics identify an increasing number of gene-function relationships that have a bearing on photosynthesis, revealing a marked interdependency between photosynthesis and other cellular processes. Photosynthesis-related functions are mostly located in the chloroplast, but can also be located in other compartments of the plant cell. The analysis by DNA-array hybridization of mRNA expression patterns both in the chloroplast and the nucleus, under various environmental conditions and/or in different genetic backgrounds that affect the function of the plastid, is rapidly improving our understanding of how photosynthesis is regulated, and it reveals that plastid-to-nucleus signaling plays a central role in its control.

Redox control, redox signaling, and redox homeostasis in plant cells

Redox chemistry is a key feature of life. Oxidized substrates are reduced to synthesize functional molecules; reduced substrates are oxidized for energy supply. In addition, cells must fight against uncontrolled oxidation of essential constituents, a process that continuously occurs in an atmosphere of 21% O2. The redox situation is further complicated in plants with their highly reactive photosynthetic metabolism. To this end it is now well established that redox regulation is a central element in adjusting plant metabolism and development to the prevailing environmental conditions. This review introduces general redox chemistry and the main components of the cellular redox network, namely pyridine nucleotides, ascorbate, glutathione, lipoic acid, tocopherol, thioredoxins, glutaredoxins, peroxiredoxins, and other thiol proteins. Examples for redox sensing, transduction, redox-regulated enzymes and transcription, and the function of regulatory circuits are presented. Emphasis is placed on redox regulation of photosynthesis, which is the best understood metabolism governed by redox control on essentially all levels, ranging from gene transcription to translation, assembly and turnover, as well as short-term adaptation by state transition and enzyme activity. Increasing evidence shows the importance of redox regulation in the context of transport, plant development, and programmed cell death.

Chloroplast research in the genomic age

Chloroplast research takes significant advantage of genomics and genome sequencing, and a new picture is emerging of how the chloroplast functions and communicates with other cellular compartments. In terms of evolution, it is now known that only a fraction of the many proteins of cyanobacterial origin were rerouted to higher plant plastids. Reverse genetics and novel mutant screens are providing a growing catalogue of chloroplast protein-function relationships, and the characterization of plastid-to-nucleus signalling mutants reveals cell-organelle interactions. Recent advances in transcriptomics and proteomics of the chloroplast make this organelle one of the best understood of all plant cell compartments.

Protein-protein and protein-function relationships in Arabidopsis photosystem I: cluster analysis of PSI polypeptide levels and photosynthetic parameters in PSI mutants

In flowering plants, photosystem I (PSI) mediates electron transport across the thylakoid membrane and contains at least 14 proteins. The availability of co-suppression and/or mutant lines deficient for individual PSI polypeptides in Arabidopsis thaliana allows one to assign functions to PSI subunits. We have performed cluster analysis on an extensive set of data on PSI polypeptide levels in ten different PSI mutants. This type of analysis serves to group proteins that exhibit similar changes in amount in different genotypes, and also identifies genotypes which show similar PSI compositions. The interdependence of levels of PSI-C, -D and -E, of -H and -L, and of Lhca2 and 3, which was previously proposed based on the study of single genotypes or on cross-linking experiments, was confirmed by our analyses. In addition, the levels of the lumenal subunits F and N are found to be interdependent. The incorporation of photosynthetic parameters into the cluster analysis revealed that the level of photosynthetic state transitions correlates with the abundance of PSI-H in all 8 genotypes tested, supporting the hypothesis that PSI-H serves as a docking site for LHCII during state transitions.

Plants impaired in state transitions can to a large degree compensate for their defect

Arabidopsis thaliana plants lacking the PSI-H or PSI-L subunit of photosystem I have been shown to be severely affected in their ability to perform state transitions, but no visual phenotype was observed when these plants were grown under different light quantities and qualities. However, the chloroplasts in the PSI-H- and PSI-L-less plants contained fewer and more extended grana stacks. The plants lacking PSI-H or PSI-L were characterised with respect to their photosynthetic performance. Wild-type plants adjusted the non-photochemical fluorescence quenching to maintain constant levels of PSII quantum yield and reduction of the plastoquinone pool. In contrast, the plants deficient in state transitions had a more reduced plastoquinone pool and consequently, a less efficient PSII-photochemistry under growth-light conditions and in state 2. The maximal photosynthetic capacity and the quantum efficiency of oxygen evolution were diminished by 8-14% in the PSI-H-less plants. Under growth-light conditions, the stroma was similarly reduced in the PSI-H-less plants and the rate of cyclic electron transport was unchanged. Pigment analysis showed that the xanthophyll cycle was not upregulated in order to compensate for the lack of state transitions. In general, the plants lacking PSI-H and PSI-L showed a decreased ability to optimise photosynthesis according to the light conditions.

Will the real LHC II kinase please step forward?

Many laboratories have searched for the protein kinase responsible for phosphorylation of the chloroplast light-harvesting complex of photosynthesis, LHC II. The LHC II kinase provides a vital link in a redox signaling pathway of ecological, developmental, and evolutionary significance. Various candidates for the LHC II kinase, some stronger than others, have come and gone. Recently, a family of three thylakoid-associated kinases (TAKs) has been identified and purified; they too catalyze in vitro phosphorylation of LHC II. The LHC II kinase is part of an integrated network of signal transduction to which input is provided by a number of environmental factors. The implications of understanding these processes stretch beyond the important, central question of how plants adapt their photosynthetic machinery to changing wavelengths of light.

Chlamydomonas, a model system for studying the assembly and dynamics of photosynthetic complexes

The green unicellular alga Chlamydomonas reinhardtii has emerged as a powerful model system for studying the biosynthesis of the photosynthetic apparatus and the acclimation of this system to changes in light conditions. The assembly of the photosynthetic complexes involves the coordinate interaction between the nuclear and chloroplast genetic systems. Many factors involved in specific chloroplast post-transcriptional steps have been identified and characterized. Chlamydomonas is able to adapt to changes in light quality and in cellular ATP content by performing state transition, a process that leads to a redistribution of light excitation energy between photosystem II and photosystem I and that involves the redox state of the plastoquinone pool, the cytochrome b(6)f complex and one or several kinases specific for the light-harvesting system. Genetic approaches have provided new insights into this process.

Pigment organization and energy transfer dynamics in isolated photosystem I (PSI) complexes from Arabidopsis thaliana depleted of the PSI-G, PSI-K, PSI-L, or PSI-N subunit

Green plant photosystem I (PSI) consists of at least 18 different protein subunits. The roles of some of these protein subunits are not well known, in particular those that do not occur in the well characterized PSI complexes from cyanobacteria. We investigated the spectroscopic properties and excited-state dynamics of isolated PSI-200 particles from wild-type and mutant Arabidopsis thaliana plants devoid of the PSI-G, PSI-K, PSI-L, or PSI-N subunit. Pigment analysis and a comparison of the 5 K absorption spectra of the various particles suggests that the PSI-L and PSI-H subunits together bind approximately five chlorophyll a molecules with absorption maxima near 688 and 667 nm, that the PSI-G subunit binds approximately two red-shifted beta-carotene molecules, that PSI-200 particles without PSI-K lack a part of the peripheral antenna, and that the PSI-N subunit does not bind pigments. Measurements of fluorescence decay kinetics at room temperature with picosecond time resolution revealed lifetimes of ~0.6, 5, 15, 50, 120, and 5000 ps in all particles. The 5- and 15-ps phases could, at least in part, be attributed to the excitation equilibration between bulk and red chlorophyll forms, though the 15-ps phase also contains a contribution from trapping by charge separation. The 50- and 120-ps phases predominantly reflect trapping by charge separation. We suggest that contributions from the core antenna dominate the 15-ps trapping phase, that those from the peripheral antenna proteins Lhca2 and Lhca3 dominate the 50-ps phase, and that those from Lhca1 and Lhca4 dominate the 120-ps phase. In the PSI-200 particles without PSI-K or PSI-G protein, more excitations are trapped in the 15-ps phase and less in 50- and 120-ps phases, which is in agreement with the notion that these subunits are involved in the interaction between the core and peripheral antenna proteins.

Supramolecular organization of photosystem I and light-harvesting complex I in Chlamydomonas reinhardtii

We report a structural characterization by electron microscopy and image analysis of a supramolecular complex consisting of photosystem I and light-harvesting complex I from the unicellular green alga Chlamydomonas reinhardtii. The complex is a monomer, has longest dimensions of 21.3 and 18.2 nm in projection, and is significantly larger than the corresponding complex in spinach. Comparison with photosystem I complexes from other organisms suggests that the complex contains about 14 light-harvesting proteins, two or three of which bind at the side of the PSI-H subunit. We suggest that special light-harvesting I proteins play a role in the binding of phosphorylated light-harvesting complex II in state 2.

The location of plastocyanin in vascular plant photosystem I

We have studied the binding sites of the electron donor and acceptor proteins of vascular plant photosystem I by electron microscopy/crystallography. Previously, we identified the binding site for the electron acceptor (ferredoxin). In this paper we complete these studies with the characterization of the electron donor (plastocyanin) binding site. After cross-linking, plastocyanin is detected using Fourier difference analysis of two dimensionally ordered arrays of photosystem I located at the periphery of chloroplast grana. Plastocyanin binds in a small cavity on the lumenal surface of photosystem I, close to the center and with a slight bias toward the PsaL subunit of the complex. The recent release of the full coordinates for the cyanobacterial photosystem I reaction center has allowed a detailed comparison between the structures of the eukaryotic and prokaryotic systems. This reveals a very close homology, which is particularly striking for the lumenal side of photosystem I.

Single and double knockouts of the genes for photosystem I subunits G, K, and H of Arabidopsis. Effects on photosystem I composition, photosynthetic electron flow, and state transitions

Photosystem I (PSI) of higher plants contains 18 subunits. Using Arabidopsis En insertion lines, we have isolated knockout alleles of the genes psaG, psaH2, and psaK, which code for PSI-G, -H, and -K. In the mutants psak-1 and psag-1.4, complete loss of PSI-K and -G, respectively, was confirmed, whereas the residual H level in psah2-1.4 is due to a second gene encoding PSI-H, psaH1. Double mutants, lacking PSI-G, and also -K, or a fraction of -H, together with the three single mutants were characterized for their growth phenotypes and PSI polypeptide composition. In general, the loss of each subunit has secondary, in some cases additive, effects on the abundance of other PSI polypeptides, such as D, E, H, L, N, and the light-harvesting complex I proteins Lhca2 and 3. In the G-less mutant psag-1.4, the variation in PSI composition suggests that PSI-G stabilizes the PSI-core. Levels of light-harvesting complex I proteins in plants, which lack simultaneously PSI-G and -K, indicate that PSI subunits other than G and K can also bind Lhca2 and 3. In the same single and double mutants, psag-1.4, psak-1, psah2-1.4, psag-1.4/psah2-1.4, and psag-1.4/psak-1 photosynthetic electron flow and excitation energy quenching were analyzed to address the roles of the various subunits in P700 reduction (mediated by PSI-F and -N) and oxidation (PSI-E), and state transitions (PSI-H). Based on the results, we also suggest for PSI-K a role in state transitions.

Photosystem I activity is increased in the absence of the PSI-G subunit

PSI-G is a subunit of photosystem I in eukaryotes. The function of PSI-G was characterized in Arabidopsis plants transformed with a psaG cDNA in antisense orientation. Several plants with significantly decreased PSI-G protein content were identified. Plants with reduced PSI-G content were indistinguishable from wild type when grown under optimal conditions, despite a 40% reduction of photosystem I. This decrease of photosystem I was correlated with a similar reduction in state transitions. Surprisingly, the reduced photosystem I content was compensated for by a more effective photosystem I because the light-dependent reduction of NADP(+) in vitro was 48% higher. Photosystem I antenna size determined from flash-induced P700 absorption changes did not reveal any significant effect on the size of the photosystem I antenna in the absence of PSI-G, whereas a 17% reduction was seen in the absence of PSI-K. However, nondenaturing green gels revealed that the interaction between photosystem I and the light-harvesting complex I was less stable in the absence of PSI-G. Thus, PSI-G plays a role in stabilizing the binding of the peripheral antenna. The increased activity in the absence of PSI-G suggests that PSI-G could have an important role in regulation of photosystem I.

Peroxidative reactions attenuate oxygen effect on spectroscopic properties of isolated chloroplasts

Steady-state absorption and fluorescence excitation spectra have been measured at 25 degrees C in order to elucidate the differences between isolated chloroplasts from pea (chilling-sensitive plant) and bean (chilling-tolerant plant) and their response to oxygen treatment. A weaker light harvesting in bean in comparison with pea chloroplasts is related to higher free fatty acids level and extended peroxidation activities of bean chloroplasts. Peroxidation of free fatty acids in bean chloroplasts results in an accumulation of oxygenated forms of fatty acids demonstrated by a large negative band around 400 nm in absorption difference spectra, while the excitation spectra are not significantly altered. Similar changes have been observed in the lipase-treated pea chloroplasts. In contrast, in both pea and bean chloroplasts exhibiting no peroxidation due to antimycin A treatment, oxygen induces a pronounced absorbance increase in the regions around 435, 470 and 674 nm indicating the chloroplast swelling. A decline of chlorophyll fluorescence excitation caused by oxygen, may result from a decrease in energy transfer from antennae complexes to chlorophyll species emitting at both 680 and 740 nm. The oxygen-induced changes are partially reversed upon restoration of anaerobic conditions. The presented data show for the first time, that in contrast to pea chloroplasts the peroxidation abolishes an oxygen-induced decrease in light harvesting in bean chloroplasts, i.e., a chilling-sensitive plant.

Role of subunits in eukaryotic Photosystem I

Photosystem I (PSI) of eukaryotes has a number of features that distinguishes it from PSI of cyanobacteria. In plants, the PSI core has three subunits that are not found in cyanobacterial PSI. The remaining 11 subunits of the core are conserved but several of the subunits have a different role in eukaryotic PSI. A distinguishing feature of eukaryotic PSI is the membrane-imbedded peripheral antenna. Light-harvesting complex I is composed of four different subunits and is specific for PSI. Light-harvesting complex II can be associated with both PSI and PSII. Several of the core subunits interact with the peripheral antenna proteins and are important for proper function of the peripheral antenna. The review describes the role of the different subunits in eukaryotic PSI. The emphasis is on features that are different from cyanobacterial PSI.

Disruption of thylakoid-associated kinase 1 leads to alteration of light harvesting in Arabidopsis

To survive fluctuations in quality and intensity of light, plants and algae are able to preferentially direct the absorption of light energy to either one of the two photosystems, PSI or PSII. This rapid process is referred to as a state transition and has been correlated with the phosphorylation and migration of the light-harvesting complex protein (LHCP) between PSII and PSI. We show here that thylakoid protein kinases (TAKs) are required for state transitions in Arabidopsis. Antisense TAK1 expression leads to a loss of LHCP phosphorylation and a reduction in state transitions. Preferential activation of PSII causes LHCP to accumulate with PSI, and TAK1 mutants disrupt this process. Finally, TAKs also influence the phosphorylation of multiple thylakoid proteins.

State transitions reveal the dynamics and flexibility of the photosynthetic apparatus

The chloroplast-based photosynthetic apparatus of plants and algae associates various redox cofactors and pigments with approximately 70 polypeptides to form five major transmembrane protein complexes. Among these are two photosystems that have distinct light absorption properties but work in series to produce reducing equivalents aimed at the fixation of atmospheric carbon. A short term chromatic adaptation known as 'State transitions' was discovered thirty years ago that allows photosynthetic organisms to adapt to changes in light quality and intensity which would otherwise compromise the efficiency of photosynthetic energy conversion. A two-decade research effort has finally unraveled the major aspects of the molecular mechanism responsible for State transitions, and their physiological significance has been revisited. This review describes how a-still elusive-regulatory kinase senses the physiological state of the photosynthetic cell and triggers an extensive supramolecular reorganization of the photosynthetic membranes. The resulting picture of the photosynthetic apparatus is that of a highly flexible energy convertor that adapts to the ever-changing intracellular demand for ATP and/or reducing power.

Molecular recognition in thylakoid structure and function

In photosynthesis, light-harvesting chlorophyll molecules are shunted between photosystems by phosphorylation of the protein to which they are bound. An anchor for the phosphorylated chlorophyll-protein complex has now been identified in the reaction centre of chloroplast photosystem I. This finding supports the idea that molecular recognition, not membrane surface charge, governs the architecture of the chloroplast thylakoid membrane. We describe a model for the chloroplast thylakoid membrane that is consistent with recent structural data that specify the relative dimensions of intrinsic protein complexes and their dispositions within the membrane. Control of molecular recognition accommodates membrane stacking, lateral heterogeneity and regulation of light-harvesting function by means of protein phosphorylation during state transitions--adaptations that compensate for selective excitation of photosystem I or photosystem II. High-resolution structural description of membrane protein-protein interactions is now required to understand thylakoid structure and regulation of photosynthesis.

Balance of power: a view of the mechanism of photosynthetic state transitions

Photosynthesis in plants involves photosystem I and photosystem II, both of which use light energy to drive redox processes. Plants can balance the distribution of absorbed light energy between the two photosystems. When photosystem II is favoured, a mobile pool of light harvesting complex II moves from photosystem II to photosystem I. This short-term and reversible redistribution is known as a state transition. It is associated with changes in the phosphorylation of light harvesting complex II but the regulation is complex. Redistribution of energy during state transitions depends on an altered binding equilibrium between the light harvesting complex II-photosystem II and light harvesting complex II-photosystem I complexes.

PHOTOSYSTEM I: Function and Physiology

Photosystem I is the light-driven plastocyanin-ferredoxin oxidoreductase in the thylakoid membranes of cyanobacteria and chloroplasts. In recent years, sophisticated spectroscopy, molecular genetics, and biochemistry have been used to understand the light conversion and electron transport functions of photosystem I. The light-harvesting complexes and internal antenna of photosystem I absorb photons and transfer the excitation energy to P700, the primary electron donor. The subsequent charge separation and electron transport leads to the reduction of ferredoxin. The photosystem I proteins are responsible for the precise arrangement of cofactors and determine redox properties of the electron transfer centers. With the availability of genomic information and the structure of photosystem I, one can now probe the functions of photosystem I proteins and cofactors. The strong reductant produced by photosystem I has a central role in chloroplast metabolism, and thus photosystem I has a critical role in the metabolic networks and physiological responses in plants.