Photosystem I polypeptides

Photosystem I mediates light-induced electron transport from reduced plastocyanin in the thylakoid lumen to oxidized ferredoxin in the stroma. Photosystem I is located in the stroma lamellae of the thylakoid system and consists of a peripheral light-harvesting pigment-protein complex and a core complex carrying the electron transfer components and additional antenna pigments. The core complex consists of 11 different polypeptide subunits, five of which are chloroplast encoded and six of which are encoded by nuclear genes. The structure and function of the different subunits of the photosystem 1 core complex is discussed.     

The role of chlorophyll-protein complexes in the function and structure of chloroplast thylakoids

Summary The photosynthetic pigments of chloroplast thylakoid membranes are complexed with specific intrinsic polypeptides which are included in three supramolecular complexes, photosystem I complex, photosystem II complex and the light-harvesting complex. There is a marked lateral heterogeneity in the distribution of these complexes along the membrane with photosystem II complex and its associated light-harvesting complex being located mainly in the stacked membranes of the grana partitions, while photosystem I complex is found mainly in unstacked thylakoids together with ATP synthetase. In contrast, the intermediate electron transport complex, the cylochrome b-f complex, is rather uniformly distributed in these two membrane regions. The consequences of this lateral heterogeneity in the location of the thylakoid complexes are considered in relation to the function and structure of chloroplasts of higher plants.     

The structure and function of the chloroplast photosynthetic membrane — a model for the domain organization

Recent work on the domain organization of the thylakoid is reviewed and a model for the thylakoid of higher plants is presented. According to this model the thylakoid membrane is divided into three main domains: the stroma lamellae, the grana margins and the grana core (partitions). These have different biochemical compositions and have specialized functions. Linear electron transport occurs in the grana while cyclic electron transport is restricted to the stroma lamellae. This model is based on the following results and considerations. (1) There is no good candidate for a long-range mobile redox carrier between PS II in the grana and PS I in the stroma lamellae. The lateral diffusion of plastoquinone and plastocyanin is severely restricted by macromolecular crowding in the membrane and the lumen respectively. (2) There is an excess of 14±18% chlorophyll associated with PS I over that of PS II. This excess is assumed to be localized in the stroma lamellae where PS I drives cyclic electron transport. (3) For several plant species, the stroma lamellae account for 20±3% of the thylakoid membrane and the grana (including the appressed regions, margins and end membranes) for the remaining 80%. The amount of stroma lamellae (20%) corresponds to the excess (14–18%) of chlorophyll associated with PS I. (4) The model predicts a quantum requirement of about 10 quanta per oxygen molecule evolved, which is in good agreement with experimentally observed values. (5) There are at least two pools of each of the following components: PS I, PS II, cytochrome bf complex, plastocyanin, ATP synthase and plastoquinone. One pool is in the grana and the other in the stroma compartments. So far, it has been demonstrated that the PS I, PS II and cytochrome bf complexes each differ in their respective pools.Abbreviations PS I and PS II Photosystem I and II - P 700 reaction center of PS I - LHC II light-harvesting complex II     

Purification and properties of the alkaline phosphatase ofSerratia marcescens

1. A hypothesis based on the Hill-Bendall-model of photosynthetic electron transport is proposed to explain positive and negative photo-phobotaxis inPhormidium uncinatum. In the non-cyclic electron chain a pool is located into which photosystem II (e. g. by absorption by C-phycoerythrin, 561 nm) feeds electrons while photosystem I (e.g. 723 nm) drains electrons out of it.
2. Interruption of the electron flow into the pool causes a sudden decrease of the pool size and thus a positive phobic response. This happens e.g. when an organism leaves a trap which is illuminated by a wavelength absorbed by photosystem II pigments (e. g. 561 nm).
3. A negative reaction takes place when electrons are suddenly drained out of the pool; again the pool size decreases. This is the case when an organism enters a light trap illuminated by photosystem I light (723 nm).
4. The net flow of electrons into or out of the pool—and thus the reaction sense—can be manipulated by the relative excitation of the two photosystems or by blocking the electron influx by DCMU.

     

Chloroplast-encoded small subunits of the three multiprotein complexes in photosynthetic electron transport

The photosystem I, photosystem II, and cytochromeb ₆ f complexes that are involved in electron transport of oxygenic photosynthesis consist of a number of subunits encoded by either the chloroplast or nuclear genomes. In addition to the major subunits that carry redox components or photosynthetic pigments, these complexes contain several to more than ten subunits with molecular masses of less than 10 kDa. Directed mutagenesis has served as a powerful tool for investigation of the roles of these small subunits in the organization or function of the complexes. Various chloroplast transformants of the green algaChlamydomonas reinhardtii and mutants of cyanobacteria in which a gene encoding a small subunit was deleted or altered have been constructed. Evidence has accumulated suggesting that these small subunits function in the assembly, stabilization, or protection from photoinhibition of the complexes or in the modulation or regulation of electron transport. This article presents an overview of the properties and functions of the chloroplast-encoded small subunits of the three multiprotein complexes of photosynthetic electron transport that have been mainly analyzed with chloroplast transformants ofC. reinhardtii and the corresponding cyanobacterial transformants. Recipient of the Botanical Society Award for Young Scientists, 1995.     

Structural changes and lateral redistribution of photosystem II during donor side photoinhibition of thylakoids

The structural and topological stability of thylakoid components under photoinhibitory conditions (4,500 microE.m-2.s-1 white light) was studied on Mn depleted thylakoids isolated from spinach leaves. After various exposures to photoinhibitory light, the chlorophyll-protein complexes of both photosystems I and II were separated by sucrose gradient centrifugation and analysed by Western blotting, using a set of polyclonals raised against various apoproteins of the photosynthetic apparatus. A series of events occurring during donor side photoinhibition are described for photosystem II, including: (a) lowering of the oligomerization state of the photosystem II core; (b) cleavage of 32-kD protein D1 at specific sites; (c) dissociation of chlorophyll-protein CP43 from the photosystem II core; and (d) migration of damaged photosystem II components from the grana to the stroma lamellae. A tentative scheme for the succession of these events is illustrated. Some effects of photoinhibition on photosystem I are also reported involving dissociation of antenna chlorophyll-proteins LHCI from the photosystem I reaction center.     

Effects of glyphosate [N-(phosphonomethyl)glycine] on photosynthetic pigments, stomatal response and photosynthetic electron transport in Medicago sativa and Trifolium pratense

Stomatal response, chlorophyll and carotenoid content, and photosynthetic electron transport were investigated in Medicago sativa L. (cv. Aragón) and Trifolium pratense L. (cv. Violeta) after plants were exposed to various glyphosate [ N -(phophonomethyl)glycine] concentrations. The herbicide decreased the content of photosynthetic pigments when applied at concentrations of 0.15, 1.5 and 15 m M , showing a greater effect on chlorophyll pigments than on carotenoids. The chlorophyll a / b ratio increased as a consequence of a major effect on chlorophyll b. Glyphosate treatment (15 m M ) led to a closure of stomates after seven days in both species. The results suggest that glyphosate acts as an electron transport inhibitor, because glyphosate pre-incubated chloroplasts, isolated from control plants, showed reduced photosystem II and photosystem I activities. In general, clover was more sensitive to glyphosate than glyphosate, lucerne.     

EPR study of electron transport in the cyanobacterium Synechocystis sp. PCC 6803: oxygen-dependent interrelations between photosynthetic and respiratory electron transport chains

In this work, we investigated electron transport processes in the cyanobacterium Synechocystis sp. PCC 6803, with a special emphasis focused on oxygen-dependent interrelations between photosynthetic and respiratory electron transport chains. Redox transients of the photosystem I primary donor P700 and oxygen exchange processes were measured by the EPR method under the same experimental conditions. To discriminate between the factors controlling electron flow through photosynthetic and respiratory electron transport chains, we compared the P700 redox transients and oxygen exchange processes in wild type cells and mutants with impaired photosystem II and terminal oxidases (CtaI, CydAB, CtaDEII). It was shown that the rates of electron flow through both photosynthetic and respiratory electron transport chains strongly depended on the transmembrane proton gradient and oxygen concentration in cell suspension. Electron transport through photosystem I was controlled by two main mechanisms: (i) oxygen-dependent acceleration of electron transfer from photosystem I to NADP(+), and (ii) slowing down of electron flow between photosystem II and photosystem I governed by the intrathylakoid pH. Inhibitor analysis of P700 redox transients led us to the conclusion that electron fluxes from dehydrogenases and from cyclic electron transport pathway comprise 20-30% of the total electron flux from the intersystem electron transport chain to P700(+).     

Lateral distribution and diffusion of plastocyanin in chloroplast thylakoids

The lateral distribution of plastocyanin in the thylakoid lumen of spinach and pea chloroplasts was studied by combining immunocytochemical localization and kinetic measurements of P700+ reduction at high time resolution. In dark-adapted chloroplasts, the concentration of plastocyanin in the photosystem I containing stroma membranes exceeds that in photosystem II containing grana membranes by a factor of about two. Under these conditions, the reduction of P700+ with a halftime of 12 microseconds after a laser flash of saturating intensity indicates that to greater than 95% of total photosystem I a plastocyanin molecule is bound. An analysis of the labeling densities, the length of the different lumenal regions, and the total amounts of plastocyanin and P700 shows that most of the remaining presumable mobile plastocyanin is found in the granal lumen. This distribution of plastocyanin is consistent with a more negative surface charge density in the stromal than in the granal lumen. During illumination the concentration of plastocyanin in grana increases at the expense of that in stroma lamellae, indicating a light-driven diffusion from stroma to grana regions. Our observations provide evidence that a high concentration of plastocyanin in grana in the light favors the lateral electron transport from cytochrome b6/f complexes in appressed grana across the long distance to photosystem I in nonappressed stroma membranes.     

Regulation of electron-transport pathways in cells of Chlamydomonas reinhardtii under stress conditions

The kinetics of interactions between electron-transport pathways in the thylakoid membrane was examined. A mathematical model was proposed to describe the kinetics of redox transitions in photosystem II, proton concentration changes in the chloroplast stroma, and the plastoquinone pool reduction due to photosynthetic and chlororespiratory pathways. A kinetic mechanism is considered that redirects electron flows between photosynthetic and chlororespiratory pathways in response to the increased NADPH content under mineral deficiency. According to the simulation model, the electron transport flows via different routes are switched over in a stepwise manner. The results of numerical simulations are qualitatively consistent with experimental data for Chlamydomonas reinhardtii cells subjected to mineral deprivation.     

Analysis of the polypeptide composition of grana and stroma thylakoids by two-dimensional gel electrophoresis. Distribution of photosystem II between grana and stroma lamellae

The polypeptide composition of whole thylakoids and membrane subfragments was studied by using a modified two-dimensional gel electrophoresis technique of O'Farrell [J. Biol. Chem. 250, 4007-4021 (1975)]. The modifications were lithium dodecyl sulphate solubilization instead instead of SDS, reverse isofocusing and sensitive silver staining procedure. This high-resolution technique allowed us to separate and identify about 170 polypeptides of thylakoid membranes. After separating grana and stroma thylakoids it was found that both types of lamellae contained nearly equal amounts of polypeptides, but about 70 polypeptides were different in the two preparations. In grana thylakoids, 54 polypeptides out of 95 were found to be mainly present in grana and 31 of them were only present in grana preparations. In stroma membranes, 43 polypeptides out of 99 were mainly present in stroma lamellae and 38 of these polypeptides were exclusively present in stroma lamellae. In a functional photosystem II preparation, 61 individual polypeptides could be distinguished. Most of these polypeptides were present in both grana and stroma lamellae, but 22 of them were more pronounced in grana than in stroma lamellae. 9 polypeptides of photosystem II were distinctly different in grana and stroma lamellae, and these differences may connect closely with the functional differences of photosystem II in the two types of thylakoids.     

The thermodynamics and kinetics of electron transfer between cytochrome b6f and photosystem I in the chlorophyll d-dominated cyanobacterium, Acaryochloris marina

We have investigated the photosynthetic properties of Acaryochloris marina, a cyanobacterium distinguished by having a high level of chlorophyll d, which has its absorption bands shifted to the red when compared with chlorophyll a. Despite this unusual pigment content, the overall rate and thermodynamics of the photosynthetic electron flow are similar to those of chlorophyll a-containing species. The midpoint potential of both cytochrome f and the primary electron donor of photosystem I (P(740)) were found to be unchanged with respect to those prevailing in organisms having chlorophyll a, being 345 and 425 mV, respectively. Thus, contrary to previous reports (Hu, Q., Miyashita, H., Iwasaki, I. I., Kurano, N., Miyachi, S., Iwaki, M., and Itoh, S. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 13319-13323), the midpoint potential of the electron donor P(740) has not been tuned to compensate for the decrease in excitonic energy in A. marina and to maintain the reducing power of photosystem I. We argue that this is a weaker constraint on the engineering of the oxygenic photosynthetic electron transfer chain than preserving the driving force for plastoquinol oxidation by P(740), via the cytochrome b(6)f complex. We further show that there is no restriction in the diffusion of the soluble electron carrier between cytochrome b(6)f and photosystem I in A. marina, at variance with plants. This difference probably reflects the simplified ultrastructure of the thylakoids of this organism, where no segregation into grana and stroma lamellae is observed. Nevertheless, chlorophyll fluorescence measurements suggest that there is energy transfer between adjacent photosystem II complexes but not from photosystem II to photosystem I, indicating spatial separation between the two photosystems.     

Characterization of a non-detergent PS II-cytochrome b/f preparation (BS)

A non-detergent photosystem II preparation, named BS, has been characterized by countercurrent distribution, light saturation curves, absorption spectra and fluorescence at room and at low temperature (–196°C). The BS fraction is prepared by a sonication-phase partitioning procedure (Svensson P and Albertsson P-Å, Photosynth Res 20: 249–259, 1989) which removes the stroma lamellae and the margins from the grana and leaves the appressed partition region intact in the form of vesicles. These are closed structures of inside-out conformation. They have a chlorophyll a/b ratio of 1.8–2.0, have a high oxygen evolving capacity (295 mol O₂ per mg chl h), are depleted in P700 and enriched in the cytochrome b/f complex. They have about 2 Photosystem II reaction centers per 1 cytochrome b/f complex.The plastoquinone pool available for PS II in the BS vesicles is 6–7 quinones per reaction center, about the same as for the whole thylakoid. It is concluded, therefore, that the plastoquinone of the stroma lamellae is not available to the PS II in the grana and that plastoquinone does not act as a long range electron transport shuttler between the grana and stroma lamellae.Compared with Photosystem II particles prepared by detergent (Triton X-100) treatment, the BS vesicles retain more cytochrome b/f complex and are more homogenous in their surface properties, as revealed by countercurrent distribution, and they have a more efficient energy transfer from the antenna pigments to the reaction center.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_v variable fluorescence - LHC light-harvesting complex - PpBQ phenyl-p-benzoquinone - PQ plastoquinone pool - P700 reaction center of PS I - PS I, PS II Photosystem I, II - Q_A first bound plastoquinone accepter - RC reaction centre     

Effect of cadmium on photosystem II activity in <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>: alteration of O–J–I–P fluorescence transients indicating the change of apparent activation energies within photosystem II

In this study, we evaluated how cadmium inhibitory effect on photosystem II and I electron transport may affect light energy conversion into electron transport by photosystem II. To induce cadmium effect on the photosynthetic apparatus, we exposed Chlamydomonas reinhardtii 24 h to 0–4.62 μM Cd²⁺. By evaluating the half time of fluorescence transients O–J–I–P at different temperatures (20–30°C), we were able to determine the photosystem II apparent activation energies for different reduction steps of photosystem II, indicated by the O–J–I–P fluorescence transients. The decrease of the apparent activation energies for PSII electron transport was found to be strongly related to the cadmium-induced inhibition of photosynthetic electron transport. We found a strong correlation between the photosystem II apparent activation energies and photosystem II oxygen evolution rate and photosystem I activity. Different levels of cadmium inhibition at photosystem II water-splitting system and photosystem I activity showed that photosystem II apparent activation energies are strongly dependent to photosystem II donor and acceptor sides. Therefore, the oxido-reduction state of whole photosystem II and I electron transport chain affects the conversion of light energy from antenna complex to photosystem II electron transport.     

EPR study of electron transport in the cyanobacterium Synechocystis sp. PCC 6803: Oxygen-dependent interrelations between photosynthetic and respiratory electron transport chains

In this work, we investigated electron transport processes in the cyanobacterium Synechocystis sp. PCC 6803, with a special emphasis focused on oxygen-dependent interrelations between photosynthetic and respiratory electron transport chains. Redox transients of the photosystem I primary donor P700 and oxygen exchange processes were measured by the EPR method under the same experimental conditions. To discriminate between the factors controlling electron flow through photosynthetic and respiratory electron transport chains, we compared the P700 redox transients and oxygen exchange processes in wild type cells and mutants with impaired photosystem II and terminal oxidases (CtaI, CydAB, CtaDEII). It was shown that the rates of electron flow through both photosynthetic and respiratory electron transport chains strongly depended on the transmembrane proton gradient and oxygen concentration in cell suspension. Electron transport through photosystem I was controlled by two main mechanisms: (i) oxygen-dependent acceleration of electron transfer from photosystem I to NADP⁺, and (ii) slowing down of electron flow between photosystem II and photosystem I governed by the intrathylakoid pH. Inhibitor analysis of P700 redox transients led us to the conclusion that electron fluxes from dehydrogenases and from cyclic electron transport pathway comprise 20-30% of the total electron flux from the intersystem electron transport chain to P700⁺.     

Fractionation of the thylakoid membranes from tobacco. A tentative isolation of 'end membrane' and purified 'stroma lamellae' membranes

Thylakoids isolated from tobacco were fragmented by sonication and the vesicles so obtained were separated by partitioning in aqueous polymer two-phase systems. By this procedure, grana vesicles were separated from stroma exposed membrane vesicles. The latter vesicles could be further fractionated by countercurrent distribution, with dextran-polyethylene glycol phase systems, and divided into two main populations, tentatively named 'stroma lamellae' and 'end membrane'. Both these vesicle preparations have high chlorophyll a/b ratio, high photosystem (PS) I and low PS II content, suggesting their origin from stroma exposed regions of the thylakoid. The two vesicle populations have been compared with respect to biochemical composition and photosynthetic activity. The 'end membrane' has a higher chlorophyll a/b ratio (5.7 vs. 4.7), higher P700 content (4.7 vs. 3.3 mmol/mol of chlorophyll). The 'end membrane' has the lowest PS II content, the ratio PS I/PS II being more than 10, as shown by EPR measurements. The PS II in both fractions is of the beta-type. The decay of fluorescence is different for the two populations, the 'stroma lamellae' showing a very slow decay even in the presence of K3Fe(CN)6 as an acceptor. The two vesicle populations have very different surface properties: the end membranes prefer the upper phase much more than the stroma lamellae, a fact which was utilized for their separation. Arguments are presented which support the suggestion that the two vesicle populations originate from the grana end membranes and the stroma lamellae, respectively.     

Functional heterogeneity of photosystem II in domain specific regions of the thylakoid membrane of spinach (Spinacia oleracea L.)

A mild sonication and phase fractionation method has been used to isolate five regions of the thylakoid membrane in order to characterize the functional lateral heterogeneity of photosynthetic reaction centers and light harvesting complexes. Low-temperature fluorescence and absorbance spectra, absorbance cross-section measurements, and picosecond time-resolved fluorescence decay kinetics were used to determine the relative amounts of photosystem II (PSII) and photosystem I (PSI), to determine the relative PSII antenna size, and to characterize the excited-state dynamics of PSI and PSII in each fraction. Marked progressive increases in the proportion of PSI complexes were observed in the following sequence: grana core (BS), whole grana (B3), margins (MA), stroma lamellae (T3), and purified stromal fraction (Y100). PSII antenna size was drastically reduced in the margins of the grana stack and stroma lamellae fractions as compared to the grana. Picosecond time-resolved fluorescence decay kinetics of PSII were characterized by three exponential decay components in the grana fractions, and were found to have only two decay components with slower lifetimes in the stroma. Results are discussed in the framework of existing models of chloroplast thylakoid membrane lateral heterogeneity and the PSII repair cycle. Kinetic modeling of the PSII fluorescence decay kinetics revealed that PSII populations in the stroma and grana margin fractions possess much slower primary charge separation rates and decreased photosynthetic efficiency when compared to PSII populations in the grana stack.     

Regulation of photosynthetic electron transport

The photosynthetic electron transport chain consists of photosystem II, the cytochrome b(6)f complex, photosystem I, and the free electron carriers plastoquinone and plastocyanin. Light-driven charge separation events occur at the level of photosystem II and photosystem I, which are associated at one end of the chain with the oxidation of water followed by electron flow along the electron transport chain and concomitant pumping of protons into the thylakoid lumen, which is used by the ATP synthase to generate ATP. At the other end of the chain reducing power is generated, which together with ATP is used for CO(2) assimilation. A remarkable feature of the photosynthetic apparatus is its ability to adapt to changes in environmental conditions by sensing light quality and quantity, CO(2) levels, temperature, and nutrient availability. These acclimation responses involve a complex signaling network in the chloroplasts comprising the thylakoid protein kinases Stt7/STN7 and Stl1/STN7 and the phosphatase PPH1/TAP38, which play important roles in state transitions and in the regulation of electron flow as well as in thylakoid membrane folding. The activity of some of these enzymes is closely connected to the redox state of the plastoquinone pool, and they appear to be involved both in short-term and long-term acclimation. This article is part of a Special Issue entitled "Regulation of Electron Transport in Chloroplasts".     

Reprint of: Regulation of photosynthetic electron transport

The photosynthetic electron transport chain consists of photosystem II, the cytochrome b(6)f complex, photosystem I, and the free electron carriers plastoquinone and plastocyanin. Light-driven charge separation events occur at the level of photosystem II and photosystem I, which are associated at one end of the chain with the oxidation of water followed by electron flow along the electron transport chain and concomitant pumping of protons into the thylakoid lumen, which is used by the ATP synthase to generate ATP. At the other end of the chain reducing power is generated, which together with ATP is used for CO(2) assimilation. A remarkable feature of the photosynthetic apparatus is its ability to adapt to changes in environmental conditions by sensing light quality and quantity, CO(2) levels, temperature, and nutrient availability. These acclimation responses involve a complex signaling network in the chloroplasts comprising the thylakoid protein kinases Stt7/STN7 and Stl1/STN7 and the phosphatase PPH1/TAP38, which play important roles in state transitions and in the regulation of electron flow as well as in thylakoid membrane folding. The activity of some of these enzymes is closely connected to the redox state of the plastoquinone pool, and they appear to be involved both in short-term and long-term acclimation. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.