Ultraviolet B exposure of whole leaves of barley affects structure and functional organization of photosystem II

Synergistic effects of salt and ultraviolet radiation on the rice-field cyanobacterium Nostochopsis lobatus HKAR-21

Environmental variation has a significant impact on how organisms, including cyanobacteria, respond physiologically and biochemically. Salinity and ultraviolet radiation (UVR)-induced variations in the photopigments of the rice-field cyanobacterium Nostochopsis lobatus HKAR-21 and its photosynthetic performance was studied. We observed that excessive energy dissipation after UVR is mostly caused by Non-Photochemical Quenching (NPQ), whereas photochemical quenching is important for preventing photoinhibition. These findings suggest that ROS production may play an important role in the UVR-induced injury. To reduce ROS-induced oxidative stress, Nostochopsis lobatus HKAR-21 induces the effective antioxidant systems, which includes different antioxidant compounds like carotenoids and enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). The study indicates that Nostochopsis lobatus HKAR-21 exposed to photosynthetically active radiation + UV-A + UV-B (PAB) and PAB + NaCl (PABN) had significantly reduced photosynthetic efficiency. Furthermore, maximum ROS was detected in PAB exposed cyanobacterial cells. The induction of lipid peroxidation (LPO) has been investigated to evaluate the impact of UVR on the cyanobacterial membrane in addition to enzymatic defensive systems. The maximal LPO level was found in PABN treated cells. Based on the findings of this research, it was concluded that salinity and UVR had collegial effects on the major macromolecular components of the rice-field cyanobacterium Nostochopsis lobatus HKAR-21.

Spectral diversity of photosystem I from flowering plants

Photosystem I and II (PSI and PSII) work together to convert solar energy into chemical energy. Whilst a lot of research has been done to unravel variability of PSII fluorescence in response to biotic and abiotic factors, the contribution of PSI to in vivo fluorescence measurements has often been neglected or considered to be constant. Furthermore, little is known about how the absorption and emission properties of PSI from different plant species differ. In this study, we have isolated PSI from five plant species and compared their characteristics using a combination of optical and biochemical techniques. Differences have been identified in the fluorescence emission spectra and at the protein level, whereas the absorption spectra were virtually the same in all cases. In addition, the emission spectrum of PSI depends on temperature over a physiologically relevant range from 280 to 298 K. Combined, our data show a critical comparison of the absorption and emission properties of PSI from various plant species.

The role of light-harvesting complex I in excitation energy transfer from LHCII to photosystem I in Arabidopsis

Photosynthesis powers nearly all life on Earth. Light absorbed by photosystems drives the conversion of water and carbon dioxide into sugars. In plants, photosystem I (PSI) and photosystem II (PSII) work in series to drive the electron transport from water to NADP+. As both photosystems largely work in series, a balanced excitation pressure is required for optimal photosynthetic performance. Both photosystems are composed of a core and light-harvesting complexes (LHCI) for PSI and LHCII for PSII. When the light conditions favor the excitation of one photosystem over the other, a mobile pool of trimeric LHCII moves between both photosystems thus tuning their antenna cross-section in a process called state transitions. When PSII is overexcited multiple LHCIIs can associate with PSI. A trimeric LHCII binds to PSI at the PsaH/L/O site to form a well-characterized PSI-LHCI-LHCII supercomplex. The binding site(s) of the "additional" LHCII is still unclear, although a mediating role for LHCI has been proposed. In this work, we measured the PSI antenna size and trapping kinetics of photosynthetic membranes from Arabidopsis (Arabidopsis thaliana) plants. Membranes from wild-type (WT) plants were compared to those of the ΔLhca mutant that completely lacks the LHCI antenna. The results showed that "additional" LHCII complexes can transfer energy directly to the PSI core in the absence of LHCI. However, the transfer is about two times faster and therefore more efficient, when LHCI is present. This suggests LHCI mediates excitation energy transfer from loosely bound LHCII to PSI in WT plants.

Responses of a hot spring cyanobacterium under ultraviolet and photosynthetically active radiation: photosynthetic performance, antioxidative enzymes, mycosporine-like amino acid profiling and its antioxidative potentials

This study summarizes the response of a hot spring cyanobacterium Fischerella sp. strain HKAR-14, under simulated light conditions of ultraviolet radiation (UVR), photosynthetically active radiation (PAR), PAR + UV-A (PA) and PAR + UV-A + UV-B (PAB). Exposure to UVR caused a decline in growth and Chl a while total carotene content increased under PA and PAB. Maximum photochemical efficiency of photosystem II (F v /F m) and relative electron transport rate decreased significantly in PA and PAB exposure. Higher non-photochemical quenching and lower photochemical quenching values were observed in UVR-exposed samples as compared to the control. Levels of intracellular reactive oxygen species (ROS) increased significantly in PAB and PA. Fluorescence microscopic images showed an increase in green fluorescence, indicating the generation of ROS in UVR. The antioxidant machinery including superoxide dismutase, catalase and peroxidase showed an increase of 1.76-fold and 2.5-fold superoxide dismutase, 2.4-fold and 3.7-fold catalase, 1.83-fold and 2.5-fold peroxidase activities under PA and PAB, respectively. High-performance liquid chromatography equipped with photodiode array detector, electrospray ionization mass spectrometry, Fourier-transform infrared spectroscopy and nuclear magnetic resonance spectroscopy analyses reveal the occurrence of a single mycosporine-like amino acid, shinorine (λ max 332.3 ± 2 nm, m/z 333.1), with a retention time of 1.157 min. The electrochemical characterization of shinorine was determined by cyclic voltammetry. The shinorine molecule possesses electrochemical activity and represents diffusion-controlled process in 0.1 M (pH 7.0) phosphate buffer. An antioxidant assay of shinorine showed its efficient activity as antioxidant which increased in a dose-dependent manner.

Digitonin-sensitive LHCII enlarges the antenna of Photosystem I in stroma lamellae of Arabidopsis thaliana after far-red and blue-light treatment

Light drives photosynthesis. In plants it is absorbed by light-harvesting antenna complexes associated with Photosystem I (PSI) and photosystem II (PSII). As PSI and PSII work in series, it is important that the excitation pressure on the two photosystems is balanced. When plants are exposed to illumination that overexcites PSII, a special pool of the major light-harvesting complex LHCII is phosphorylated and moves from PSII to PSI (state 2). If instead PSI is over-excited the LHCII complex is dephosphorylated and moves back to PSII (state 1). Recent findings have suggested that LHCII might also transfer energy to PSI in state 1. In this work we used a combination of biochemistry and (time-resolved) fluorescence spectroscopy to investigate the PSI antenna size in state 1 and state 2 for Arabidopsis thaliana. Our data shows that 0.7 ± 0.1 unphosphorylated LHCII trimers per PSI are present in the stroma lamellae of state-1 plants. Upon transition to state 2 the antenna size of PSI in the stroma membrane increases with phosphorylated LHCIIs to a total of 1.2 ± 0.1 LHCII trimers per PSI. Both phosphorylated and unphosphorylated LHCII function as highly efficient PSI antenna.

Response of Synechocystis sp. PCC 6803 to UV radiations by alteration of polyamines associated with thylakoid membrane proteins

The responses of Synechocystis sp. PCC 6803 exposed to UVA, UVB and UVC for at least 3 h were investigated with the emphasis on the changes of polyamines (PAs) levels in whole cells, thylakoid membrane fraction, and thylakoid membrane-associated proteins fraction. All UV radiations caused a slight decrease on cell growth but a drastic reduction of photosynthetic efficiency of Synechocystis cells. UV radiations, especially UVB and UVC, severely decreased the levels of PAs associated with thylakoid membrane proteins. The decreased PAs levels as affected by UV radiation correlated well with the decrease of photosynthetic efficiency, suggesting the role of PAs for the maintenance of photosynthetic activity of Synechocystis. PAs, especially spermidine (Spd) and putrescine (Put), were found abundantly in the thylakoid membrane fraction, and these PAs were associated mainly with the PSI trimer complex. Importantly, the exposure of Synechocystis cells to all UV radiations for 3 h resulted in the increase of Spd associated with the PSII monomer and dimer complex, suggesting its protective role against UV radiations despite the overall decrease of PAs.

Loss of LHCI system affects LHCII re-distribution between thylakoid domains upon state transitions

LHCI, the peripheral antenna system of Photosystem I, includes four light-harvesting proteins (Lhca1-Lhca4) in higher plants, all of which are devoid in the Arabidopsis thaliana knock-out mutant ΔLhca. PSI absorption cross-section was reduced in the mutant, thus affecting the redox balance of the photosynthetic electron chain and resulting in a more reduced PQ with respect to the wild type. ΔLhca plants developed compensatory response by enhancing LHCII binding to PSI. However, the amplitude of state transitions, as measured from changes of chlorophyll fluorescence in vivo, was unexpectedly low than the high level of PSI-LHCII supercomplex established. In order to elucidate the reasons for discrepancy, we further analyzed state transition in ΔLhca plants. The STN7 kinase was fully active in the mutant as judged from up-regulation of LHCII phosphorylation in state II. Instead, the lateral heterogeneity of thylakoids was affected by lack of LHCI, with LHCII being enriched in stroma membranes with respect to the wild type. Re-distribution of this complex affected the overall fluorescence yield of thylakoids already in state I and minimized changes in RT fluorescence yield when LHCII did connect to PSI reaction center. We conclude that interpretation of chlorophyll fluorescence analysis of state transitions becomes problematic when applied to mutants whose thylakoid architecture is significantly modified with respect to the wild type.

Fingerprinting the macro-organisation of pigment-protein complexes in plant thylakoid membranes in vivo by circular-dichroism spectroscopy

Macro-organisation of the protein complexes in plant thylakoid membranes plays important roles in the regulation and fine-tuning of photosynthetic activity. These delicate structures might, however, undergo substantial changes during isolating the thylakoid membranes or during sample preparations, e.g., for electron microscopy. Circular-dichroism (CD) spectroscopy is a non-invasive technique which can thus be used on intact samples. Via excitonic and psi-type CD bands, respectively, it carries information on short-range excitonic pigment-pigment interactions and the macro-organisation (chiral macrodomains) of pigment-protein complexes (psi, polymer or salt-induced). In order to obtain more specific information on the origin of the major psi-type CD bands, at around (+)506, (-)674 and (+)690nm, we fingerprinted detached leaves and isolated thylakoid membranes of wild-type and mutant plants and also tested the effects of different environmental conditions in vivo. We show that (i) the chiral macrodomains disassemble upon mild detergent treatments, but not after crosslinking the protein complexes; (ii) in different wild-type leaves of dicotyledonous and monocotyledonous angiosperms the CD features are quite robust, displaying very similar excitonic and psi-type bands, suggesting similar protein composition and (macro-) organisation of photosystem II (PSII) supercomplexes in the grana; (iii) the main positive psi-type bands depend on light-harvesting protein II contents of the membranes; (iv) the (+)506nm band appears only in the presence of PSII-LHCII supercomplexes and does not depend on the xanthophyll composition of the membranes. Hence, CD spectroscopy can be used to detect different macro-domains in the thylakoid membranes with different outer antenna compositions in vivo.

High light-dependent phosphorylation of photosystem II inner antenna CP29 in monocots is STN7 independent and enhances nonphotochemical quenching

Phosphorylation of the photosystem II antenna protein CP29 has been reported to be induced by excess light and further enhanced by low temperature, increasing resistance to these stressing factors. Moreover, high light-induced CP29 phosphorylation was specifically found in monocots, both C3 and C4, which include the large majority of food crops. Recently, knockout collections have become available in rice (Oryza sativa), a model organism for monocots. In this work, we have used reverse genetics coupled to biochemical and physiological analysis to elucidate the molecular basis of high light-induced phosphorylation of CP29 and the mechanisms by which it exerts a photoprotective effect. We found that kinases and phosphatases involved in CP29 phosphorylation are distinct from those reported to act in State 1-State 2 transitions. In addition, we elucidated the photoprotective role of CP29 phosphorylation in reducing singlet oxygen production and enhancing excess energy dissipation. We thus established, in monocots, a mechanistic connection between phosphorylation of CP29 and nonphotochemical quenching, two processes so far considered independent from one another.

The significance of CP29 reversible phosphorylation in thylakoids of higher plants under environmental stresses

Reversible phosphorylation of proteins is a key event in many fundamental cellular processes. Under stressful conditions, many thylakoid membrane proteins in photosynthetic apparatus of higher plants undergo rapid phosphorylation and dephosphorylation in response to environmental changes. CP29 is the most frequently phosphorylated protein among three minor antennae complexes in higher plants. CP29 phosphorylation in dicotyledons has been known for several decades and is well characterized. However, CP29 phosphorylation in monocotyledons is less studied and appears to have a different phosphorylation pattern. In this review, we discuss recent advancements in CP29 phosphorylation and dephosphorylation studies and its physiological significance under environmental stresses in higher plants, especially in the monocotyledonous crops. Physiologically, the phosphorylation of CP29 is likely to be a prerequisite for state transitions and the disassembly of photosystem II supercomplexes, but not involved in non-photochemical quenching (NPQ). CP29 is phosphorylated in monocots exposed to environmental cues, with its subsequent lateral migration from grana stacks to stroma lamellae. However, neither CP29 phosphorylation nor its lateral migration occurs in dicotyledonous plants after drought, cold, or salt stress. Since the molecular mechanisms of differential CP29 phosphorylation under stresses are not fully understood, this review provides insights for future studies regarding the physiological function of CP29 reversible phosphorylation.

Short-term UV-B and UV-C radiations preferentially decrease spermidine contents and arginine decarboxylase transcript levels of Synechocystis sp. PCC 6803

To investigate the short term effect of ultraviolet (UV) radiations on changes in pigments and polyamine contents, Synechocystis sp. PCC 6803 cells after exposure to UV-radiation were extracted by dimethylformamide and perchloric acid for pigments and polyamines determination, respectively. Cell growth was slightly decreased after 1 h exposure to UV-A and UV-B radiations. UV-C had little effect on cell growth despite the decrease of photosynthetic rate by about 18%. UV-A and UV-B decreased the contents of chlorophyll a and carotenoids whereas UV-C decreased chlorophyll a but had no effect on carotenoids. Spermidine contents were unaffected by UV-A, in contrast to the reduction of 25 and 50% by UV-B and UV-C, respectively. All three types of UV-radiation particularly reduced perchloric acid-insoluble spermidine. Importantly, putrescine and spermine which accounted for less than 1% of intracellular polyamines were increased by about three- to eight-fold by UV-B and UV-C, respectively. The changes in polyamines contents by UV-B and UV-C were consistent with the changes in transcript levels of arginine decarboxylase mRNA, but not with the protein levels. The decrease in the transcripts of adc2 but not adc1 was observed with UV-B and UV-C treatments.

Complementary UV-absorption of mycosporine-like amino acids and scytonemin is responsible for the UV-insensitivity of photosynthesis in Nostoc flagelliforme

Mycosporine-like amino acids (MAAs) and scytonemin are UV-screening compounds that have presumably appeared early in the history of life and are widespread in cyanobacteria. Natural colonies of the UV-insensitive Nostoc flagelliforme were found to be especially rich in MAAs (32.1 mg g DW(-1)), concentrated in the glycan sheath together with scytonemin. MAAs are present in the form of oligosaccharide-linked molecules. Photosystem II activity, measured using PAM fluorescence and oxygen evolution, was used as a most sensitive physiological parameter to analyse the effectiveness of UV-protection. Laboratory experiments were performed under controlled conditions with a simulated solar radiation specifically deprived of UV-wavebands with cut-off filters (295, 305, 320, 345 and 395 nm). The UV-insensitivity of N. flagelliforme was found to cover the whole UV-A (315-400 nm) and UV-B (280-320 nm) range and is almost certainly due to the complementary UV-absorption of MAAs and scytonemin. The experimental approach used is proposed to be suitable for the comparison of the UV-protection ability in organisms that differ in their complement of UV-sunscreen compounds. Furthermore, this study performed with a genuinely terrestrial organism points to the relevance of marine photoprotective compounds for life on Earth, especially for the colonization of terrestrial environments.

UV-induced phycobilisome dismantling in the marine picocyanobacterium Synechococcus sp. WH8102

The marine picocyanobacterium Synechococcus sp. WH8102 was submitted to ultraviolet (UV-A and B) radiations and the effects of this stress on reaction center II and phycobilisome integrity were studied using a combination of biochemical, biophysical and molecular biology techniques. Under the UV conditions that were applied (4.3 W m(-2) UV-A and 0.86 W m(-2) UV-B), no significant cell mortality and little chlorophyll degradation occurred during the 5 h time course experiment. However, pulse amplitude modulated (PAM) fluorimetry analyses revealed a rapid photoinactivation of reaction centers II. Indeed, a dramatic decrease of the D1 protein amount was observed, despite a large and rapid increase in the expression level of the psbA gene pool. Our results suggest that D1 protein degradation was accompanied (or followed) by the disruption of the N-terminal domain of the anchor linker polypeptide LCM, which in turn led to the disconnection of the phycobilisome complex from the thylakoid membrane. Furthermore, time course analyses of in vivo fluorescence emission spectra suggested a partial dismantling of phycobilisome rods. This was confirmed by characterization of isolated antenna complexes by SDS-PAGE and immunoblotting analyses which allowed us to locate the disruption site of the rods near the phycoerythrin I-phycoerythrin II junction. In addition, genes encoding phycobilisome components, including alpha-subunits of all phycobiliproteins and phycoerythrin linker polypeptides were all down regulated in response to UV stress. Phycobilisome alteration could be the consequence of direct UV-induced photodamages and/or the result of a protease-mediated process.

Evidences for interaction of PsbS with photosynthetic complexes in maize thylakoids

The PsbS subunit of Photosystem II (PSII) has received much attention in the past few years, given its crucial role in photoprotection of higher plants. The exact location of this small subunit in thylakoids is also debated. In this work possible interaction partners of PsbS have been identified by immunoaffinity and immunoprecipitation, performed with mildly solubilized whole thylakoid membrane. The interacting proteins, as identified by mass spectrometry analysis of the immunoaffinity eluate, include CP29, some LHCII components, but also components of Photosystem I, of the cytochrome b(6)f complex as well as of ATP synthase. These proteins can be co-immunoprecipitated by using highly specific anti-PsbS antibodies and, vice-versa, PsbS is co-immunoprecipitated by antisera against components of the interacting complexes. We also find that PsbS co-migrates with bands containing PSII, ATP synthase and cytochrome b(6)f as well as with LHCII-containing bands on non-denaturing Deriphat PAGE. These results suggest multiple location of PsbS in the thylakoid membrane and point to an unexpected lateral mobility of this PSII subunit. As revealed by immunogold labelling with antibody against PsbS, the protein is associated either with granal membranes or prevalently with stroma lamellae in low or high-intensity light-treated intact leaves, respectively. This finding is consistent with the capability of PsbS to interact with complexes located in stroma lamellae, even though the exact physiological condition(s) under which these interactions may take place remain to be clarified.

Twenty years of biophysics of photosynthesis in Padova, Italy (1984-2005): a tale of two brothers

This paper tells the history of two brothers, almost a generation apart in age, who met again, after having followed different academic paths, to introduce biophysical research in photosynthesis at the University of Padova. The development of two research groups, one in the Chemistry Department, the other in the Biology Department led to a comprehensive interdisciplinary group across academic barriers. The group of Giovanni Giacometti developed in Physical Chemistry, during the years before his retirement, with some roots which can be traced to the famous Linus Pauling school of the mid 1950s, and made possible, by the work of many students (especially Donatella Carbonera and Marilena Di Valentin) and of an older associate (Giancarlo Agostini). The group participated quite actively with a number of European and American laboratories in the application of physical techniques, especially Electron Spin Resonance (EPR) associated with Optical Spectroscopy (Optically Detected Magnetic Resonance; ODMR), and contributed to the development of the understanding of the structure-function relationships in photosynthetic membrane complexes, stimulated by the determination of the X-ray structure of the purple photosynthetic reaction center in the mid 1980s ( J. Deisenhofer, H. Michel, R. Huber and others). The younger brother of Giovanni, Giorgio Mario Giacometti, came to Padova after obtaining biochemical knowledge from the Rossi-Fanelli school in Rome, where Jeffries Wyman, Eraldo Antonini and Maurizio Brunori were the world masters of hemoglobin research. In Padova, together with a group of young scientists (at first Roberto Bassi and Roberto Barbato, now leaders of their own groups in Verona and in Alessandria respectively, followed soon by brilliant coworkers such as Fernanda Rigoni, Elisabetta Bergantino and more recently Ildikò Szabò and Paola Costantini), Giorgio approached more biochemical themes of oxygenic photosynthesis, such as purification and characterization of antenna chlorophyll-protein complexes, Photosystem II (PS II) particles and subunits, having always in mind structural and molecular problems at the level of the largest integrated particles, which are more difficult to investigate in detail by the spectroscopic techniques.

Photosynthesis research in Italy: a review

This historical review was compiled and edited by Giorgio Forti, whereas the other authors of the different sections are listed alphabetically after his name, below the title of the paper; they are also listed in the individual sections. This review deals with the research on photosynthesis performed in several Italian laboratories during the last 50 years; it includes research done, in collaboration, at several international laboratories, particularly USA, UK, Switzerland, Hungary, Germany, France, Finland, Denmark, and Austria. Wherever pertinent, references are provided, especially to other historical papers in Govindjee et al. [Govindjee, Beatty JT, Gest H, Allen JF (eds) (2005) Discoveries in Photosynthesis. Springer, Dordrecht]. This paper covers the physical and chemical events starting with the absorption of a quantum of light by a pigment molecule to the conversion of the radiation energy into the stable chemical forms of the reducing power and of ATP. It describes the work done on the structure, function and regulation of the photosynthetic apparatus in higher plants, unicellular algae and in photosynthetic bacteria. Phenomena such as photoinhibition and the protection from it are also included. Research in biophysics of photosynthesis in Padova (Italy) is discussed by G.M. Giacometti and G. Giacometti (2006).

Effect of moderate UV-B irradiation on Synechocystis PCC 6803 biliproteins

In the present study, we investigated the mechanism of UV-B radiation induced damage to the light harvesting apparatus of the cyanobacterium Synechocystis 6803. Liquid chromatography analysis and spectroscopy investigations performed on phycobilisomes or isolated biliproteins irradiated with moderate UV-B intensity (1.3 W/m(2)) revealed rapid destruction of beta-phycocyanin and a slower damage of the other biliproteins, alpha-phycocyanin and both alpha and beta-allophycocyanin. EPR spin trapping measurements revealed that carbon centered adducts of the spin trap DMPO were formed. This evidence indicates that free radicals produced from bilins probably attack the polypeptide chain of protein inducing its degradation. Our results show that the bilin chromophore is the main target of UV-B irradiation, causing structural changes, which in turn induce reaction of the chromophore with atmospheric oxygen and lead to production of reactive radicals. Our results also demonstrate that beta-phycocyanin is the most affected biliprotein, probably due to the presence of two bilins as chromophore.

Thylakoid-associated Polyamines Adjust the UV-B Sensitivity of the Photosynthetic Apparatus by Means of Light-harvesting Complex II Changes¶

The sensitivity of the photosynthetic apparatus to ultraviolet-B (UV-B) irradiation was studied in cultures of unicellular green alga Scenedesmus obliquus incubated in low light (low photosynthetically active radiation intensity [LL]) and high light (high photosynthetically active radiation intensity [HL]) conditions, treated or not with exogenous polyamines. Biochemical and physicochemical measurements showed that UV-B radiation induces a decrease in the thylakoid-associated putrescine (Put) and an increase in spermine (Spm), so that the reduction of Put/Spm ratio leads to the increase of light-harvesting complex II (LHCII) size per active reaction center and, consequently, the amplification of UV-B effects on the photosynthetic apparatus. The separation of oligomeric and monomeric forms of LHCII from isolated thylakoids showed that UV-B induces an increase in the oligomeric forms of LHCII, which was more intense in LL than in HL. By manipulating the LHCII size with exogenous polyamines, the sensitivity degree of the photosynthetic apparatus to UV-B changed significantly. Specifically, the addition of Put decreased highly the sensitivity of LL culture to UV-B because of the inhibitory effect of Put on the LHCII size increasing, whereas the addition of Spm enhanced the UV-B injury induced in HL culture because of the increasing of LHCII size. The ability of the photosynthetic apparatus to recover the UV-B induced changes was also investigated.

Light- and pH-dependent structural changes in the PsbS subunit of photosystem II

In higher plants, the PsbS subunit of photosystem II (PSII) plays a crucial role in pH- and xanthophyll-dependent nonphotochemical quenching of excess absorbed light energy, thus contributing to the defense mechanism against photoinhibition. We determined the amino acid sequence of Zea mays PsbS and produced an antibody that recognizes with high specificity a region of the protein located in the stroma-exposed loop between the second and third putative helices. By means of this antiserum, the thylakoid membranes of various higher plant species revealed the presence of a 42-kDa protein band, indicating the formation of a dimer of the 21-kDa PsbS protein. Crosslinking experiments and immunoblotting with other antisera seem to exclude the formation of a heterodimer with other PSII protein components. The PsbS monomer/dimer ratio in isolated thylakoid membranes was found to vary with luminal pH in a reversible manner, the monomer being the prevalent form at acidic and the dimer at alkaline pH. In intact chloroplasts and whole plants, dimer-to-monomer conversion is reversibly induced by light, known to cause luminal acidification. Sucrose-gradient centrifugation revealed a prevalent association of the PsbS monomer and dimer with light-harvesting complex and PSII core complexes, respectively. The finding of the existence of a light-induced change in the quaternary structure of the PsbS subunit may contribute to understanding the mechanism of PsbS action during nonphotochemical quenching.

An alternative model for photosystem II/light harvesting complex II in grana membranes based on cryo-electron microscopy studies

The photosynthetic protein complexes in plants are located in the chloroplast thylakoid membranes. These membranes have an ultrastructure that consists of tightly stacked 'grana' regions interconnected by unstacked membrane regions. The structure of isolated grana membranes has been studied here by cryo-electron microscopy. The data reveals an unusual arrangement of the photosynthetic protein complexes, staggered over two tightly stacked planes. Chaotrope treatment of the paired grana membranes has allowed the separation and isolation of two biochemically distinct membrane fractions. These data have led us to an alternative model of the ultrastructure of the grana where segregation exists within the grana itself. This arrangement would change the existing view of plant photosynthesis, and suggests potential links between cyanobacterial and plant photosystem II light harvesting systems.

Determination of photosystem II subunits by matrix-assisted laser desorption/ionization mass spectrometry

Photosystem II of higher plants and cyanobacteria is composed of more than 20 polypeptide subunits. The pronounced hydrophobicity of these proteins hinders their purification and subsequent analysis by mass spectrometry. This paper reports the results obtained by application of matrix-assisted laser desorption/ionization mass spectrometry directly to isolated complexes and thylakoid membranes prepared from cyanobacteria and spinach. Changes in protein contents following physiopathological stimuli are also described. Good correlations between expected and measured molecular masses allowed the identification of the main, as well as most of the minor, low molecular weight components of photosystem II. These results open up new perspectives for clarifying the functional role of the various polypeptide components of photosystems and other supramolecular integral membrane complexes.