Two sites of primary degradation of the D1-protein induced by acceptor or donor side photo-inhibition in photosystem II core complexes

Rubredoxin 1 promotes the proper folding of D1 and is not required for heme b559 assembly in Chlamydomonas photosystem II

Photosystem II (PSII), the water:plastoquinone oxidoreductase of oxygenic photosynthesis, contains a heme b₅₅₉ iron whose axial ligands are provided by histidine residues from the α (PsbE) and β (PsbF) subunits. PSII assembly depends on accessory proteins that facilitate the step-wise association of its protein and pigment components into a functional complex, a process that is challenging to study due to the low accumulation of assembly intermediates. Here, we examined the putative role of the iron[1Fe-0S]-containing protein rubredoxin 1 (RBD1) as an assembly factor for cytochrome b₅₅₉, using the RBD1-lacking 2pac mutant from Chlamydomonas reinhardtii, in which the accumulation of PSII was rescued by the inactivation of the thylakoid membrane FtsH protease. To this end, we constructed the double mutant 2pac ftsh1-1, which harbored PSII dimers that sustained its photoautotrophic growth. We purified PSII from the 2pac ftsh1-1 background and found that α and β cytochrome b₅₅₉ subunits are still present and coordinate heme b₅₅₉ as in the WT. Interestingly, immunoblot analysis of dark- and low light-grown 2pac ftsh1-1 showed the accumulation of a 23-kDa fragment of the D1 protein, a marker typically associated with structural changes resulting from photodamage of PSII. Its cleavage occurs in the vicinity of a nonheme iron which binds to PSII on its electron acceptor side. Altogether, our findings demonstrate that RBD1 is not required for heme b₅₅₉ assembly and point to a role for RBD1 in promoting the proper folding of D1, possibly via delivery or reduction of the nonheme iron during PSII assembly.

Mitochondrial complex III Rieske Fe-S protein processing and assembly

Regulation of the mitochondrial respiratory chain biogenesis is a matter of great interest because of its implications for mitochondrial disease. One of the mitochondrial disease genes recently discovered associated to encephalopathy and mitochondrial complex III (cIII) deficiency is TTC19. Our study of TTC19-deficient human and mouse models, has led us to propose a post-assembly quality control role or 'husbandry' function for this factor that is linked to Rieske Fe-S protein (UQCRFS1). UQCRFS1 is the last incorporated cIII subunit, and its presence is essential for enzymatic activity. During UQCRFS1 assembly, the precursor is cleaved and its N-terminal part remains bound to the complex, between the two core subunits (UQCRC1 and UQCRC2). In the absence of TTC19 there is a prominent accumulation of these UQCRFS1-derived N-terminal fragments that proved to be detrimental for cIII function. In this article we will discuss some ideas around the UQCRFS1 processing and assembly and its importance for the regulation of cIII activity and biogenesis.

Amino acid oxidation of the D1 and D2 proteins by oxygen radicals during photoinhibition of Photosystem II

The Photosystem II reaction center is vulnerable to photoinhibition. The D1 and D2 proteins, lying at the core of the photosystem, are susceptible to oxidative modification by reactive oxygen species that are formed by the photosystem during illumination. Using spin probes and EPR spectroscopy, we have determined that both O₂^•- and HO^• are involved in the photoinhibitory process. Using tandem mass spectroscopy, we have identified a number of oxidatively modified D1 and D2 residues. Our analysis indicates that these oxidative modifications are associated with formation of HO^• at both the Mn₄O₅Ca cluster and the nonheme iron. Additionally, O₂^•- appears to be formed by the reduction of O₂ at either Pheo_D1 or Q_A Early oxidation of D1:³³²H, which is coordinated with the Mn1 of the Mn₄O₅Ca cluster, appears to initiate a cascade of oxidative events that lead to the oxidative modification of numerous residues in the C termini of the D1 and D2 proteins on the donor side of the photosystem. Oxidation of D2:²⁴⁴Y, which is a bicarbonate ligand for the nonheme iron, induces the propagation of oxidative reactions in residues of the D-de loop of the D2 protein on the electron acceptor side of the photosystem. Finally, D1:¹³⁰E and D2:²⁴⁶M are oxidatively modified by O₂^•- formed by the reduction of O₂ either by Pheo_D1^•- or Q_A^•- The identification of specific amino acid residues oxidized by reactive oxygen species provides insights into the mechanism of damage to the D1 and D2 proteins under light stress.

Photoprotective capacity of non-photochemical quenching in plants acclimated to different light intensities

Arabidopsis plants grown at low light were exposed to a gradually increasing actinic light routine. This method allows for the discerning of the photoprotective component of NPQ, pNPQ and photoinhibition. They exhibited lower values of Photosystem II (PSII) yield in comparison to high-light grown plants, and higher calculated dark fluorescence level (F'o calc.) than the measured one (F'o act.). As a result, in low-light grown plants, the values of qP measured in the dark appeared higher than 1. Normally, F'o act. and F'o calc. match well at moderate light intensities but F'o act. becomes higher at increasing intensities due to reaction centre (RCII) damage; this indicates the onset of photoinhibition. To explain the unusual increase of qP in the dark in low-light grown plants, we have undertaken an analysis of PSII antenna size using biochemical and spectroscopic approaches. Sucrose gradient separation of thylakoid membrane complexes and fast fluorescence induction experiments illustrated that the relative PSII cross section does not increase appreciably with the rise in PSII antenna size in the low-light grown plants. This suggests that part of the increased LHCII antenna is less efficiently coupled to the RCII. A model based upon the existence of an uncoupled population LHCII is proposed to explain the discrepancies in calculated and measured values of F'o.

Exogenous calcium alleviates low night temperature stress on the photosynthetic apparatus of tomato leaves

The effect of exogenous CaCl2 on photosystem I and II (PSI and PSII) activities, cyclic electron flow (CEF), and proton motive force of tomato leaves under low night temperature (LNT) was investigated. LNT stress decreased the net photosynthetic rate (Pn), effective quantum yield of PSII [Y(II)], and photochemical quenching (qP), whereas CaCl2 pretreatment improved Pn, Y(II), and qP under LNT stress. LNT stress significantly increased the non-regulatory quantum yield of energy dissipation [Y(NO)], whereas CaCl2 alleviated this increase. Exogenous Ca2+ enhanced stimulation of CEF by LNT stress. Inhibition of oxidized PQ pools caused by LNT stress was alleviated by CaCl2 pretreatment. LNT stress reduced zeaxanthin formation and ATPase activity, but CaCl2 pretreatment reversed both of these effects. LNT stress caused excess formation of a proton gradient across the thylakoid membrane, whereas CaCl2 pretreatment decreased the said factor under LNT. Thus, our results showed that photoinhibition of LNT-stressed plants could be alleviated by CaCl2 pretreatment. Our findings further revealed that this alleviation was mediated in part by improvements in carbon fixation capacity, PQ pools, linear and cyclic electron transports, xanthophyll cycles, and ATPase activity.

Quality control of Photosystem II: the molecular basis for the action of FtsH protease and the dynamics of the thylakoid membranes

The reaction center-binding D1 protein of Photosystem II is damaged by excessive light, which leads to photoinhibition of Photosystem II. The damaged D1 protein is removed immediately by specific proteases, and a metalloprotease FtsH located in the thylakoid membranes is involved in the proteolytic process. According to recent studies on the distribution and organization of the protein complexes/supercomplexes in the thylakoid membranes, the grana of higher plant chloroplasts are crowded with Photosystem II complexes and light-harvesting complexes. For the repair of the photodamaged D1 protein, the majority of the active hexameric FtsH proteases should be localized in close proximity to the Photosystem II complexes. The unstacking of the grana may increase the area of the grana margin and facilitate easier access of the FtsH proteases to the damaged D1 protein. These results suggest that the structural changes of the thylakoid membranes by light stress increase the mobility of the membrane proteins and support the quality control of Photosystem II.

Assessing the Applicability of Singlet Oxygen Photosensitizers in Leaf Studies

Singlet oxygen ((1) O2 ) is of special interest in plant stress physiology. Studies focused on internal, chlorophyll-mediated production are often complemented with the use of artificial (1) O2 photosensitizers. Here, we report a comparative study on the effects of Rose Bengal (RB), Methylene Violet (MVI), Neutral Red (NR) and Indigo Carmine (IC). These were infiltrated into tobacco leaves at concentrations generating the same fluxes of (1) O2 in solution. Following green light-induced (1) O2 production from these dyes, leaf photosynthesis was characterized by Photosystem (PS) II and PSI electron transport and oxidative damage was monitored as degradation of D1, a PSII core protein. Cellular localizations were identified on the basis of the dyes' fluorescence using confocal laser scanning microscopy. We found that RB and NR were both localized in chloroplasts but the latter had very little effect, probably due to its pH-dependent photosensitizing. Both RB and intracellular, nonplastid MVI decreased PSII electron transport, but the effect of RB was stronger than that of MVI and only RB was capable of damaging the D1 protein. Intercellularly localized IC had no significant effect. Our results also suggest caution when using RB as photosensitizer because it affects PSII electron transport.

Expression, purification, crystallization and preliminary X-ray diffraction analysis of Arabidopsis thaliana Deg8

Arabidopsis thaliana Deg8, an ATP-independent serine endopeptidase, is involved in the repair of photosystem II (PSII), specifically the degradation of the photo-damaged PSII reaction centre D1 protein. To understand the molecular mechanism underlying the participation of Deg8 in the degradation of the photo-damaged D1 protein, the structure of Deg8 is needed. Until recently, however, no structure of Deg8 had been solved. In this study, Deg8 from A. thaliana was cloned, overexpressed and purified in Escherichia coli. Crystallization was performed at 277 K using tribasic sodium citrate as the precipitant and the crystals diffracted to 2.0 Å resolution, belonging to space group C2 with unit-cell parameters a = 129.5, b = 124.2, c = 93.3 Å, α = γ = 90, β = 132.4°. Assuming one trimer in the asymmetric unit, the Matthews coefficient and the solvent content were calculated to be 2.35 Å(3) Da(-1) and 47.6%, respectively.

Damage to photosystem II due to heat stress without light-driven electron flow: involvement of enhanced introduction of reducing power into thylakoid membranes

Under a moderately heat-stressed condition, the photosystems of higher plants are damaged in the dark more easily than they are in the presence of light. To obtain a better understanding of this heat-derived damage mechanism that occurs in the dark, we focused on the involvement of the light-independent electron flow that occurs at 40 °C during the damage. In various plant species, the maximal photochemical quantum yield of photosystem (PS) II (Fv/Fm) decreased as a result of heat treatment in the dark. In the case of wheat, the most sensitive plant species tested, both Fv/Fm and oxygen evolution rapidly decreased by heat treatment at 40 °C for 30 min in the dark. In the damage, specific degradation of D1 protein was involved, as shown by immunochemical analysis of major proteins in the photosystem. Because light canceled the damage to PSII, the light-driven electron flow may play a protective role against PSII damage without light. Light-independent incorporation of reducing power from stroma was enhanced at 40 °C but not below 35 °C. Arabidopsis mutants that have a deficit of enzymes which mediate the incorporation of stromal reducing power into thylakoid membranes were tolerant against heat treatment at 40 °C in the dark, suggesting that the reduction of the plastoquinone pool may be involved in the damage. In conclusion, the enhanced introduction of reducing power from stroma into thylakoid membranes that occurs around 40 °C causes over-reduction of plastoquinone, resulting in the damage to D1 protein under heat stress without linear electron flow.

Cooperative D1 degradation in the photosystem II repair mediated by chloroplastic proteases in Arabidopsis

Light energy constantly damages photosynthetic apparatuses, ultimately causing impaired growth. Particularly, the sessile nature of higher plants has allowed chloroplasts to develop unique mechanisms to alleviate the irreversible inactivation of photosynthesis. Photosystem II (PSII) is known as a primary target of photodamage. Photosynthetic organisms have evolved the so-called PSII repair cycle, in which a reaction center protein, D1, is degraded rapidly in a specific manner. Two proteases that perform processive or endopeptidic degradation, FtsH and Deg, respectively, participate in this cycle. To examine the cooperative D1 degradation by these proteases, we engaged Arabidopsis (Arabidopsis thaliana) mutants lacking FtsH2 (yellow variegated2 [var2]) and Deg5/Deg8 (deg5 deg8) in detecting D1 cleaved fragments. We detected several D1 fragments only under the var2 background, using amino-terminal or carboxyl-terminal specific antibodies of D1. The appearance of these D1 fragments was inhibited by a serine protease inhibitor and by deg5 deg8 mutations. Given the localization of Deg5/Deg8 on the luminal side of thylakoid membranes, we inferred that Deg5/Deg8 cleaves D1 at its luminal loop connecting the transmembrane helices C and D and that the cleaved products of D1 are the substrate for FtsH. These D1 fragments detected in var2 were associated with the PSII monomer, dimer, and partial disassembly complex but not with PSII supercomplexes. It is particularly interesting that another processive protease, Clp, was up-regulated and appeared to be recruited from stroma to the thylakoid membrane in var2, suggesting compensation for FtsH deficiency. Together, our data demonstrate in vivo cooperative degradation of D1, in which Deg cleavage assists FtsH processive degradation under photoinhibitory conditions.

The structural basis of mode of activation and functional diversity: a case study with HtrA family of serine proteases

HtrA (High temperature requirement protease A) proteins that are primarily involved in protein quality control belong to a family of serine proteases conserved from bacteria to humans. HtrAs are oligomeric proteins that share a common trimeric pyramidal architecture where each monomer comprises a serine protease domain and one or two PDZ domains. Although the overall structural integrity is well maintained and they exhibit similar mechanism of activation, subtle conformational changes and structural plasticity especially in the flexible loop regions and domain interfaces lead to differences in their active site conformation and hence in their specificity and functions.

Cytochrome b₅₅₉ and cyclic electron transfer within photosystem II

Cytochrome b₅₅₉ (Cyt b₅₅₉), β-carotene (Car), and chlorophyll (Chl) cofactors participate in the secondary electron-transfer pathways in photosystem II (PSII), which are believed to protect PSII from photodamage under conditions in which the primary electron-donation pathway leading to water oxidation is inhibited. Among these cofactors, Cyt b₅₅₉ is preferentially photooxidized under conditions in which the primary electron-donation pathway is blocked. When Cyt b₅₅₉ is preoxidized, the photooxidation of several of the 11 Car and 35 Chl molecules present per PSII is observed. In this review, the discovery of the secondary electron donors, their structures and electron-transfer properties, and progress in the characterization of the secondary electron-transfer pathways are discussed. This article is part of a Special Issue entitled: Photosystem II.

Assay of photoinhibition and heat inhibition of photosystem II in higher plants

When thylakoids of higher plant chloroplasts are exposed to excessive light or moderate heat stress, photosystem II reaction center-binding protein D1 is damaged. The photodamage of the D1 protein is caused by reactive oxygen species, mostly singlet oxygen, and also by endogenous cationic radicals generated by the photochemical reactions of photosystem II. Moreover, it was shown recently that the damage to the D1 protein by moderate heat stress is due to reactive oxygen species produced by lipid peroxidation near photosystem II. To maintain photosystem II activity, the oxidatively damaged D1 protein must be replaced by a newly synthesized copy, and thus degradation and removal of the photo- or heat-damaged D1 protein are essential for maintaining the viability of photosystem II. In this chapter, we describe the methods for assaying photoinhibition and heat inhibition of photosystem II in higher plant materials.

D1-protein dynamics in photosystem II: the lingering enigma

The D1/D2 heterodimer core is the heart of the photosystem II reaction center. A characteristic feature of this heterodimer is the differentially rapid, light-dependent degradation of the D1 protein. The D1 protein is possibly the most researched photosynthetic polypeptide, with aspects of structure-function, gene, messenger and protein regulation, electron transport, reactive oxygen species, photoinhibition, herbicide binding, stromal-granal translocations, reversible phosphorylation, and specific proteases, all under intensive investigation more than three decades after the protein's debut in the literature. This review will touch on some treaded areas of D1 research that have, so far, defied clear resolution, as well as cutting edge research on mechanisms and consequences of D1 protein degradation.

Photosynthesis in aquatic adventitious roots of the halophytic stem-succulent Tecticornia pergranulata (formerly Halosarcia pergranulata)

In flood-tolerant species, a common response to inundation is growth of adventitious roots into the water column. The capacity for these roots to become photosynthetically active has received scant attention. The experiments presented here show the aquatic adventitious roots of the flood-tolerant, halophytic stem-succulent, Tecticornia pergranulata (subfamily Salicornioideae, Chenopodiaceae) are photosynthetic and quantify for the first time the photosynthetic capacity of aquatic roots for a terrestrial species. Fluorescence microscopy was used to determine the presence of chloroplasts within cells of aquatic roots. Net O(2) production by excised aquatic roots, when underwater, was measured with varying light and CO(2) regimes; the apparent maximum capacity (P(max)) for underwater net photosynthesis in aquatic roots was 0.45 micromol O(2) m(-2) s(-1). The photosynthetic potential of these roots was supported by the immunolocalization of PsbA, the major protein of photosystem II, and ribulose-1-5-bisphosphate carboxylase/oxygenase (Rubisco) in root protein extracts. Chlorophyllous aquatic roots of T. pergranulata are photosynthetically active, and such activity is a previously unrecognized source of O(2), and potentially carbohydrates, in flooded and submerged plants.

Quality control of photosystem II. Cleavage of reaction center D1 protein in spinach thylakoids by FtsH protease under moderate heat stress

When spinach thylakoids were subjected to moderate heat stress (40 degrees C for 30 min), oxygen evolution was inhibited, and cleavage of the reaction center-binding protein D1 of photosystem II took place, producing 23-kDa N-terminal fragments. The D1 cleavage was greatly facilitated by the addition of 0.15 mM ZnCl2 and 1 mM ATP and was completely inhibited by 1 mM EDTA, indicating the participation of an ATP-dependent metalloprotease(s) in the D1 cleavage. Herbicides 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, bromoxynil, and ioxynil, all of which bind to the Q(B) site, inhibited the D1 cleavage, suggesting that the DE-loop of the D1 protein is the heat-sensitive cleavage site. We solubilized the protease by treating the thylakoids with 2 M KSCN and detected a protease activity in the supernatant by gelatin activity gel electrophoresis in the 70-80-kDa region. The antibodies against tobacco FtsH and Arabidopsis FtsH2 reacted with a 70-80-kDa band of the KSCN-solubilized fraction, which suggests the presence of FtsH in the fraction. In accordance with this finding, we identified the homolog to Arabidopsis FtsH8 in the 70-80-kDa region by matrix-assisted laser desorption ionization time-of-flight mass analysis of the thylakoids. The KSCN-solubilized fraction was successively reconstituted with thylakoids to show heat-induced cleavage of the D1 protein and production of the D1 fragment. These results strongly suggest that an FtsH protease(s) is involved in the primary cleavage of the D1 protein under moderate heat stress.

UVB Effects on the Photosystem II-D1 Protein of Phytoplankton and Natural Phytoplankton Communities

The reaction center of photosystem II is susceptible to photodamage. In particular the D1 protein located in the photosystem II core has a rapid, light-dependent turnover termed the photosystem II repair cycle that, under illumination, degrades and resynthesizes D1 protein to limit accumulation of photodamaged photosystem II. Most studies concerning the effects of UVB (280-320 nm) on this cycle have been on cyanobacteria or specific phytoplankton species rather than on natural communities of phytoplankton. During a 5-year multidisciplinary project on the effects of UV radiation (200-400 nm) on natural systems, the effects of UVB on the D1 protein of natural phytoplankton communities were assessed. This review provides an overview of photoinhibitory effects of light on cultured and natural phytoplankton, with an emphasis on the interrelation of UVB exposure, D1 protein degradation and the repair of photosystem II through D1 resynthesis. Although the UVB component of the solar spectrum contributes to the primary photoinactivation of photosystem II, we conclude that, in natural communities, inhibition of the rate of the photosystem II repair cycle is a more important influence of UVB on primary productivity. Indeed, exposing tropical and temperate phytoplankton communities to supplemented UVB had more inhibitory effect on D1 synthesis than on the D1 degradation process itself. However, the rate of net D1 damage was faster for the tropical communities, likely because of the effects of high ambient light and water temperature on mechanisms of protein degradation and synthesis.

Quality control of Photosystem II: an FtsH protease plays an essential role in the turnover of the reaction center D1 protein in Synechocystis PCC 6803 under heat stress as well as light stress conditions

The role of an AAA protease FtsH (slr0228) in the turnover of the D1 protein was studied under moderate heat stress conditions using wild-type cells of the cyanobacterium Synechocystis PCC 6803 and the mutant cells lacking a homologue of FtsH (slr0228). When the growth temperature of the wild-type was shifted from 30 degrees C to 40 degrees C, growth and oxygen-evolving activity were partially inhibited. Under the same heat stress, growth of the mutant was inhibited more significantly (63% inhibition after 5 days heat stress, compared with 26% inhibition with the wild-type cells) and the oxygen-evolving activity was also impaired in parallel. With heat stress at 42 degrees C, the level of the D1 protein of wild type cells was decreased, whereas that in mutant cells was not. The responses of cyanobacterial cells to heat stress observed here are quite similar to those to light stress that were reported previously. From these results, we suggest that the FtsH protease (slr0228) is responsible for both the heat-induced and light-induced degradation of the D1 protein. Notably, the amount of FtsH increased when the wild-type cells were exposed to heat stress or light stress, indicating that the up-regulation of the FtsH protease in the thylakoids is crucial for the cyanobacterial cells to cope with these abiotic stresses.

Formation of radicals from singlet oxygen produced during photoinhibition of isolated light-harvesting proteins of photosystem II

Electron spin resonance spectroscopy and liquid chromatography have been used to detect radical formation and fragmentation of polypeptides during photoinhibition of purified major antenna proteins, free of protease contaminants. In the absence of oxygen and light, no radicals were observed and there was no damage to the proteins. Similarly illumination of the apoproteins did not induce any polypeptide fragmentation, suggesting that chlorophyll, light and atmospheric oxygen are all participating in antenna degradation. The use of TEMP and DMPO as spin traps showed that protein damage initiates with generation of (1)O(2), presumably from a triplet chlorophyll, acting as a Type II photosensitizer which attacks directly the amino acids causing a complete degradation of protein into small fragments, without the contribution of proteases. Through the use of scavengers, it was shown that superoxide and H(2)O(2) were not involved initially in the reaction mechanism. A higher production of radicals was observed in trimers than in monomeric antenna, while radical production is strongly reduced when antennae were organized in the photosystem II (PSII) complex. Thus, monomerization of antennae as well as their incorporation into the PSII complex seem to represent physiologically protected forms. A comparison is made of the photoinhibition mechanisms of different photosynthetic systems.

Dissecting a cyanobacterial proteolytic system: efficiency in inducing degradation of the D1 protein of photosystem II in cyanobacteria and plants

A chromatography fraction, prepared from isolated thylakoids of a fatty acid desaturation mutant (Fad6/desA Colon, two colons Km(r)) of the cyanobacterium Synechocystis 6803, could induce an initial cleavage of the D1 protein in Photosystem II (PSII) particles of Synechocystis 6803 mutant and Synechococcus 7002 wild type as well as in supercomplexes of PSII-light harvesting complex II of spinach. Proteolysis was demonstrated both in darkness and in light as a reduction in the amount of full-length D1 protein or as a production of C-terminal initial degradation fragments. In the Synechocystis mutant, the main degradation fragment was a 10-kDa C-terminal one, indicating an initial cleavage occurring in the cytoplasmic DE-loop of the D1 protein. A protein component of 70-90 kDa isolated from the chromatographic fraction was found to be involved in the production of this 10-kDa fragment. In spinach, only traces of the corresponding fragment were detected, whereas a 24-kDa C-terminal fragment accumulated, indicating an initial cleavage in the lumenal AB-loop of the D1 protein. Also in Synechocystis the 24-kDa fragment was detected as a faint band. An antibody raised against the Arabidopsis DegP2 protease recognized a 35-kDa band in the proteolytically active chromatographic fraction, suggesting the existence of a lumenal protease that may be the homologue DegP of Synechocystis. The identity of the other protease cleaving the D1 protein in the DE-loop exposed on the stromal (cytoplasmic) side of the membrane is discussed.

Histidine residue 252 of the Photosystem II D1 polypeptide is involved in a light-induced cross-linking of the polypeptide with the alpha subunit of cytochrome b-559: study of a site-directed mutant of Synechocystis PCC 6803

Properties of the Photosystem II (PSII) complex were examined in the wild-type (control) strain of the cyanobacterium Synechocystis PCC 6803 and its site-directed mutant D1-His252Leu in which the histidine residue 252 of the D1 polypeptide was replaced by leucine. This mutation caused a severe blockage of electron transfer between the PSII electron acceptors Q(A) and Q(B) and largely inhibited PSII oxygen evolving activity. Strong illumination induced formation of a D1-cytochrome b-559 adduct in isolated, detergent-solubilized thylakoid membranes from the control but not the mutant strain. The light-induced generation of the adduct was suppressed after prior modification of thylakoid proteins either with the histidine modifier platinum-terpyridine-chloride or with primary amino group modifiers. Anaerobic conditions and the presence of radical scavengers also inhibited the appearance of the adduct. The data suggest that the D1-cytochrome adduct is the product of a reaction between the oxidized residue His(252) of the D1 polypeptide and the N-terminal amino group of the cytochrome alpha subunit. As the rate of the D1 degradation in the control and mutant strains is similar, formation of the adduct does not seem to represent a required intermediary step in the D1 degradation pathway.

Engineering of the protein environment around the redox-active TyrZ in photosystem II. The role of F186 and P162 in the D1 protein of Synechocystis 6803

The photosystem II reaction centre protein D1 is encoded by the psbA gene. By activation of the silent and divergent psbA1 gene in the cyanobacterium Synechocystis 6803, a novel D1 protein, D1', was produced [Salih, G. & Jansson, C. (1997) Plant Cell 9, 869-878]. The D1' protein was found to be fully operational although it deviates from the normal D1 protein in 54 out of 360 amino acids. Two notable amino-acid substitutions in D1' are the replacements of F186 by a leucine and P162 by a serine. The F186 and P162 positions are located in the vicinity of the reaction centre chlorophyll dimer P680 and the redox-active Y161 (TyrZ), and F186 has been implicated in the electron transfer between Y161 and P680. The importance of F186 was addressed by construction of engineered D1 proteins in Synechocystis 6803. F186 was replaced by leucine, serine, alanine, tyrosine or tryptophan. Only the leucine replacement yielded a functional D1 protein. Other substitutions did not support photoautotrophic growth and the corresponding mutants showed no or very poor oxygen evolving activity. In the F186Y and F186W mutants, the D1 protein failed to accumulate to appreciable levels in the thylakoid membrane. The F186S mutation severely increased the light sensitivity of the D1 protein, as indicated by the presence of a 16-kDa proteolytic degradation product. We conclude that the hydrophobicity and van der Waals volume are the most important features of the residue at position 186. Exchanging P162 for a serine yielded no observable phenotype.

The quenching of photosystem II fluorescence does not protect the D1 protein against light induced degradation in thylakoids

In spinach thylakoids, the quenching of the singlet excited state in the photosystem II antenna by m-dinitrobenzene does not change the rate of the light induced degradation of the D1 reaction centre protein and offers only limited protection against photoinhibition itself. These results are discussed in terms of the role of non-photochemical quenching as a photoprotective strategy.

The light sensitivity of ATP synthase mutants of Chlamydomonas reinhardtii

Chlamydomonas reinhardtii mutants defective in the chloroplast ATP synthase are highly sensitive to light. The ac46 mutant is affected in the MDH1 gene, required for production or stability of the monocistronic atpH mRNA encoding CF(O)-III. In this and other ATP synthase mutants, we show that short-term exposure to moderate light intensities-a few minutes-induces an inhibition of electron transfer after the primary quinone acceptor of photosystem II (PSII), whereas longer exposure-several hours-leads to a progressive loss of PSII cores. An extensive swelling of thylakoids accompanies the initial inhibition of electron flow. Thylakoids deflate as PSII cores are lost. The slow process of PSII degradation involves the participation of ClpP, a chloroplast-encoded peptidase that is part of a major stromal protease Clp. In the light of the above findings, we discuss the photosensitivity of ATP synthase mutants with respect to the regular photoinhibition process that affects photosynthetic competent strains at much higher light intensities.

Quality control of photosystem II

Photosystem II is particularly vulnerable to excess light. When illuminated with strong visible light, the reaction center D1 protein is damaged by reactive oxygen molecules or by endogenous cationic radicals generated by photochemical reactions, which is followed by proteolytic degradation of the damaged D1 protein. Homologs of prokaryotic proteases, such as ClpP, FtsH and DegP, have been identified in chloroplasts, and participation of the thylakoid-bound FtsH in the secondary degradation steps of the photodamaged D1 protein has been suggested. We found that cross-linking of the D1 protein with the D2 protein, the alpha-subunit of cytochrome b(559), and the antenna chlorophyll-binding protein CP43, occurs in parallel with the degradation of the D1 protein during the illumination of intact chloroplasts, thylakoids and photosystem II-enriched membranes. The cross-linked products are then digested by a stromal protease(s). These results indicate that the degradation of the photodamaged D1 protein proceeds through membrane-bound proteases and stromal proteases. Moreover, a 33-kDa subunit of oxygen-evolving complex (OEC), bound to the lumen side of photosystem II, regulates the formation of the cross-linked products of the D1 protein in donor-side photoinhibition of photosystem II. Thus, various proteases and protein components in different compartments in chloroplasts are implicated in the efficient turnover of the D1 protein, thus contributing to the control of the quality of photosystem II under light stress conditions.

Turnover of the aggregates and cross-linked products of the D1 protein generated by acceptor-side photoinhibition of photosystem II

It is known that the reaction-center binding protein D1 in photosystem (PS) II is degraded significantly during photoinhibition. The D1 protein also cross-links covalently or aggregates non-covalently with the nearby polypeptides in PS II complexes by illumination. In the present study, we detected the adducts between the D1 protein and the other reaction-center binding protein D2 (D1/D2), the alpha-subunit of cyt b(559) (D1/cyt b(559)), and the antenna chlorophyll-binding protein CP43 (D1/CP43) by SDS/urea-polyacrylamide gel electrophoresis and Western blotting with specific antibodies. The adducts were observed by weak and strong illumination (light intensity: 50-5000 microE m(-2) s(-1)) of PS II membranes, thylakoids and intact chloroplasts from spinach, under aerobic conditions. These results indicate that the cross-linking or aggregation of the D1 protein is a general phenomenon which occurs in vivo as well as in vitro with photodamaged D1 proteins. We found that the formation of the D1/D2, D1/cyt b(559) and D1/CP43 adducts is differently dependent on the light intensity; the D1/D2 heterodimers and D1/cyt b(559) were formed even by illumination with weak light, whereas generation of the D1/CP43 aggregates required strong illumination. We also detected that these D1 adducts were efficiently removed by the addition of stromal components, which may contain proteases, molecular chaperones and the associated proteins. By two-dimensional SDS/urea-polyacrylamide gel electrophoresis, we found that several stromal proteins, including a 15-kDa protein are effective in removing the D1/CP43 aggregates, and that their activity is resistant to SDS.

D1-D2 protein degradation in the chloroplast. Complex light saturation kinetics

The D1 and D2 proteins of the photosystem II (PSII) reaction center are stable in the dark, while rapid degradation occurs in the light. Thus far, a quantitative correlation between degradation and photon fluences has not been determined. In Spirodela oligorrhiza, D1-D2 degradation increases with photon flux. We find that kinetics for D2 degradation mirror those for D1, except that the actual half-life times of the D2 protein are about three times larger than those of the D1. The degradation ratio, D2/D1, is fluence independent, supporting the proposal [Jansen, M.A.K., Greenberg, B.M., Edelman, M., Mattoo, A.K. & Gaba, V. (1996), Photochem. Photobiol. 63, 814-817] that degradation of the two proteins is coupled. It is commonly conceived that D1 degradation is predominantly associated with photon fluences that are supersaturating for photosynthesis. We now show that a fluence as low as 5 mumol.m-2.s-1 elicited a reaction constituting > 25% of the total degradation response, while > 90% of the degradation potential was attained at intensities below saturation for photosynthesis (approximately 750 mumol.m-2.s-1). Thus, in intact plants, D1 degradation is overwhelmingly associated with fluences limiting for photosynthesis. D1 degradation increases with photon flux in a complex, multiphasic manner. Four phases were uncovered over the fluence range from 0-1600 mumol.m-2.s-1. The multiphasic saturation kinetics underscore that the D1 and D2 degradation response is complex, and emanates from more than one parameter. The physiological processes associated with each phase remain to be determined.

Photosystem II activity and turnover of the D1 protein are impaired in the psbA Y112L mutant of Synechocystis PCC6803 sp

Site-directed psbA mutants at the tyrosine Y112 position have been generated in Synechocystis PCC6803 cells. The mutation Y112F does not affect photosystem II (PSII) activity as compared with control 4 delta 1K cells. However, the Y112L mutant exhibits a photosynthetically impaired phenotype. PSII activity is not detectable in this mutant when grown at 30 mumol photons m-2 s-1, while low levels of the D1 and D2 proteins and oxygen evolution activity are present in the mutant cells grown at a low light intensity (0.5-1 mumol m-2 s-1). The recombination of the QB-/S2,3 states of PSII in the Y112L mutant cells as detected by thermoluminescence (TL) is altered. The TL signal emission maximum of these cells due to charge recombination of the S2,3/QB- occurs at 20 degrees C as compared to 35-40 degrees C for the wild-type cells, indicating a possible change in the S2,3/Yz equilibrium. The Y112L mutant cells are rapidly photoinactivated and impaired in the recovery of the PSII activity. These results suggest that replacement of the aromatic residue at position Y112 by a hydrophobic amino acid may alter the function of the donor-side activity and affects the degradation and replacement of the PSII core proteins.

Photosystem two

Photosystem two (PSII) is unique among hte various types of photosynthetic systems in that it produces a very high redox potential so as to oxidise water. As a consequence it is unable to protect itself completely against singlet oxygen production generated by chlorophyll triplets. Mass spectrometry has shown that this leads to successive light induced oxidations of the D1, and to a lesser extent, the D2 proteins which constitute the PSII reaction centre. It seems likely that it is these detrimental side reactions that underlie the requirement to degrade and replace the D1 protein at a relatively high rate. Recent structural studies of various forms of isolated PSII using electron micrographical techniques have revealed the relative positioning of the major proteins and emphasise that D1/CP43 and D2/CP47 are related through a pseudo-twofold symmetry axis which is consistent with our current understanding of the disassembly/reassembly processes involved in D1 protein turnover and with the proposed structural relationship between PSII and photosystem one.

The nuclear-encoded PsbW protein subunit of photosystem II undergoes light-induced proteolysis

The repair of photoinhibitory damage to photosystem II involves the rapid degradation and turnover of the D1 reaction center subunit. Additional protein subunits which show a limited degradation at high light intensities are the complementary reaction center subunit, D2, and the two chlorophyll a binding proteins, CP 47 and CP 43. In this work, we provide the first evidence for light-induced degradation of a nuclear-encoded subunit of photosystem II, the recently discovered PsbW protein. This 6.1 kDa protein is predicted to have a single membrane span and was found to be closely associated with the photosystem II reaction center. The degradation of the PsbW protein was demonstrated by photoinhibitory experiments, both in vitro, using thylakoid membranes and photosystem II core particles, and in vivo using leaf discs. The PsbW protein showed almost the same rate and extent of degradation as the D1 protein, and its degradation was more pronounced compared to the D2 and CP 43 proteins. The degradation of the PsbW protein was shown to share many mechanistic similarities with the more well characterized D1 protein degradation, such as oxygen dependence, sensitivity to serine protease inhibitors, and high light triggering while the actual degradation could readily occur in total darkness. The degradation of the PsbW protein was impaired by protein phosphorylation, although this protein was not itself phosphorylated. This impairment was correlated to the phosphorylation of the D1 protein which has been shown to block its degradation during photoinhibitory conditions. It is concluded that the PsbW protein is not degraded as a direct consequence of primary photodamage but due to a general destabilization of the photosystem II complex under conditions were the D1 protein becomes degraded in the absence of a sufficient repair system. The results are discussed in terms of a requirement for coordination between degradation and protein synthesis/integration during the repair process of photodamaged photosystem II reaction centers.

Characterization of the low molecular weight photosystem II reaction center subunits and their light-induced modifications by mass spectrometry

A sensitive and simple reverse phase HPLC purification scheme was developed for the rapid separation of the small protein subunits from photosystem II reaction center preparations. The precise molecular masses of the alpha- and beta-subunits of cytochrome b559 and the psbI gene product from pea plants, found to be 4394.6 +/- 0. 6, 9283.6 +/- 0.7, and 4209.5 +/- 0.5 Da, respectively, were then successfully determined for the first time by electrospray- and fast atom bombardment-mass spectrometry. Discrepancies between the molecular weights assigned and those calculated from the respective DNA sequences were observed for alpha- and beta-subunits of cytochrome b559. Currently, the nucleotide sequence of the psbI gene product from pea plants is not available. Application of novel mapping and sequencing strategies has assured the elucidation of full primary structures of all of the purified subunits. The modifications identified here include the post-translational processing of the initiating methionine on both subunits of cytochrome b559, NH2-terminal acetylation and an mRNA editing site at residue 26 (Ser --> Phe) on the beta-subunit, and retention of the NH2-terminal formyl-Met on the psbI gene product. In addition, specific oxidation of a single amino acid residue was identified on the psbI gene product and the beta-subunit purified from light-treated reaction center preparations. Overall, these studies provide the first detailed primary structural characterization of the small subunits of the reaction center complex and their associated light-induced modifications.

Reduced levels of cytochrome b 6/f in transgenic tobacco increases the excitation pressure on Photosystem II without increasing sensitivity to photoinhibition in vivo

We have examined tobacco transformed with an antisense construct against the Rieske-FeS subunit of the cytochromeb 6 f complex, containing only 15 to 20% of the wild-type level of cytochrome f. The anti-Rieske-FeS leaves had a comparable chlorophyll and Photosystem II reaction center stoichiometry and a comparable carotenoid profile to the wild-type, with differences of less than 10% on a leaf area basis. When exposed to high irradiance, the anti-Rieske-FeS leaves showed a greatly increased closure of Photosystem II and a much reduced capacity to develop non-photochemical quenching compared with wild-type. However, contrary to our expectations, the anti-Rieske-FeS leaves were not more susceptible to photoinhibition than were wild-type leaves. Further, when we regulated the irradiance so that the excitation pressure on photosystem II was equivalent in both the anti-Rieske-FeS and wild-type leaves, the anti-Rieske-FeS leaves experienced much less photoinhibition than wild-type. The evidence from the anti-Rieske-FeS tobacco suggests that rapid photoinactivation of Photosystem II in vivo only occurs when closure of Photosystem II coincides with lumen acidification. These results suggest that the model of photoinhibition in vivo occurring principally because of limitations to electron withdrawal from photosystem II does not explain photoinhibition in these transgenic tobacco leaves, and we need to re-evaluate the twinned concepts of photoinhibition and photoprotection.

Photoinhibition of photosystem II from higher plants. Effect of copper inhibition

Strong illumination of Cu(II)-inhibited photosystem II membranes resulted in a faster loss of oxygen evolution activity compared with that of the intact samples. The phenomenon was oxygen- and temperature-dependent. However, D1 protein degradation rate was similar in both preparations and slower than that found in non-oxygen evolving PSII particles (i.e. Mn-depleted photosystem II). These results seem to indicate that during illumination Cu(II)-inhibited samples do not behave as a typical non-oxygen evolving photosystem II. Cytochrome b559 was functional in the presence of Cu(II). The effect of Cu(II) inhibition decreased the amount of photoreduced cytochrome b559 and slowed down the rate of its photoreduction. The presence of Cu(II) during illumination seems to protect P680 against photodamage as occurs in photosystem II reaction centers when the acceptor side is protected. The data were consistent with the finding that production of singlet oxygen was highly reduced in the preparations treated with Cu(II). EPR spin trapping experiments showed that inactivation of Cu(II)-treated samples was dominated by hydroxyl radical, and the loss of oxygen evolution activity was diminished by the presence of superoxide dismutase and catalase. These results indicate that the rapid loss of oxygen evolution activity in the presence of Cu(II) is mainly due to the formation of .OH radicals from superoxide ion via a Cu(II)-catalyzed Haber-Weiss mechanism. Considering that this inactivation process was oxygen-dependent, we propose that the formation of superoxide occurs in the acceptor side of photosystem II by interaction of molecular oxygen with reduced electron acceptor species, and thus, the primarily Cu(II)-inhibitory site in photosystem II is on the acceptor side.

Occupation of the QB-binding pocket by a photosystem II inhibitor triggers dark cleavage of the D1 protein subjected to brief preillumination

The D1 protein of the photosystem (PS) II reaction center turns over very rapidly in a light-dependent manner initiated by its selective and specific cleavage. The cleavage of D1 was studied by using a PS II inhibitor, N-octyl-3-nitro-2,4,6-trihydroxybenzamide (PNO8), as a molecular probe. The following results were obtained. (i) D1 was selectively cleaved into 23-kDa N-terminal and 9-kDa C-terminal fragments in complete darkness by PNO8 at a single site in a D-E loop connecting membrane-spanning helices D and E. (ii) The cleavage was markedly enhanced when PS II was illuminated for a brief period before the addition of PNO8 in darkness. (iii) The effect of preillumination was slowly lost during incubation in the dark, with a decay half-time of approximately 1 h at 25 degrees C. (iv) The light intensity of preillumination required for the cleavage was much lower than that required for O2 evolution. (v) The light-triggered cleavage of D1 was observed in thylakoids, PS II membranes, and PS II core particles, but not in purified PS II reaction centers. More than 60% of D1 was cleaved into the two fragments with no other by-products. (vi) The cleavage reaction revealed a marked pH dependence that was considerably different from that for inhibition of PS II activity. The results are interpreted as indicating that the binding of PNO8 to the QB-binding pocket triggers proteolytic cleavage of D1 that has been previously modified during illumination.

pH sensitivity of the redox state of cytochrome b559 may regulate its function as a protectant against donor and acceptor side photoinhibition

A series of experiments have been conducted with isolated reaction centers of photosystem two (PS II) with the aim to elucidate the functional role of cytochrome (Cyt b 559). At pH 6.5 it was found that Cyt b 559 was reversibly photoreduced by red actinic light when Mn(2+) was present as an electron donor while at pH 8.5 a photo-oxidation was observed under the same lighting conditions, which was dark reversible in the presence of hydroquinone. These pH dependent light induced changes were measured under anaerobic conditions and correlated with changes in the relative levels of high (HP) and low (LP) potential forms of the cytochrome. At pH 6.5 the cytochrome was mainly in its LP form while at pH 8.5 a significant proportion was converted to the HP form as detected by dark titrations with hydroquinone. This pH dependent difference in the levels of HP and LP Cyt b 559 was also detected when bright white light was used to monitor the level of the LP form using a novel reaction involving direct electron donation from the flavin of glucose oxidase (present in the medium and used together with glucose and catalase as an oxygen trap). The results suggest that PS II directly oxidises and reduces the HP and LP forms, respectively and that the extent of these photo-reactions is dependent on the relative levels of the two forms, which are in turn governed by the pH. This conclusion is interpreted in terms of the model presented previously (Barber J and De Las Rivas J (1993) Proc Natl Acad Sci USA 90: 10942-10946) whereby the pH induced effect is considered as a possible mechanism by which interconversion of LP and HP forms of Cyt b 559 is achieved. In agreement with this was the finding that as the extent of photo-oxidisable HPCyt b 559 increases, with increasing pH, the rate of irreversible photo-oxidation of β-carotene decreases, a result expected if the HP form protects against donor side photoinhibition.

The cyanobacterium Synechococcus modulates Photosystem II function in response to excitation stress through D1 exchange

In this minireview we discuss effects of excitation stress on the molecular organization and function of PS II as induced by high light or low temperature in the cyanobacterium Synechococcus sp. PCC 7942. Synechococcus displays PS II plasticity by transiently replacing the constitutive D1 form (D1:1) with another form (D1:2) upon exposure to excitation stress. The cells thereby counteract photoinhibition by increasing D1 turn over and modulating PS II function. A comparison between the cyanobacterium Synechococcus and plants shows that in cyanobacteria, with their large phycobilisomes, resistance to photoinhibition is mainly through the dynamic properties (D1 turnover and quenching) of the reaction centre. In contrast, plants use antenna quenching in the light-harvesting complex as an important means to protect the reaction center from excessive excitation.

Characterization of the light-induced cross-linking of the alpha-subunit of cytochrome b559 and the D1 protein in isolated photosystem II reaction centers

Illumination of the isolated reaction center of photosystem II generates a protein of 41 kDa molecular mass. Using immunoblotting, it is confirmed that the protein is an adduct of the D1 protein and the alpha-subunit of cytochrome b559. Its formation seems to be photochemically induced, being independent of temperature between 4 and 20 degrees C and unaffected by a mixture of protease inhibitors. The maximum levels are detected when the pH is in the region 6.5-8.5 and when illumination intensities are moderate. Although higher light intensities induce a higher rate of formation, the accumulation of elevated levels of the 41-kDa protein does not occur due to light-induced degradation. This degradation is also unaffected by the presence of protease inhibitors. Proteolytic mapping and N-terminal sequencing indicates that the cross-linking process involves the N-terminal serine of the alpha-subunit of cytochrome b559 and D1 residues in the 239-244 FGQEEE motif close to the QB binding site. In conclusion, the results indicate that the N terminus of the alpha-subunit is exposed on the stromal side of photosystem II in such a way as to undergo light-induced cross-linking in the QB region of the D1 protein. They also suggest that the 41-kDa adduct may be an intermediate before the light-induced cleavage of the D1 protein in the FGQEEE region.

Specific degradation of the D1 protein of photosystem II by treatment with hydrogen peroxide in darkness: implications for the mechanism of degradation of the D1 protein under illumination

The D1 protein of the photosystem II (PSII) reaction center has a rapid turnover and is specifically degraded under illumination in vivo. When isolated PSII membranes were treated in darkness with 10 mM hydrogen peroxide (H2O2), an active form of oxygen that is generated at the acceptor side of PSII under illumination, proteins of the PSII reaction center were specifically damaged in almost the same way as observed under illumination with strong light. The D1 protein and, to a lesser extent, the D2 protein were degraded to specific fragments, and cross-linked products (the covalently linked adduct of the D1 protein and the alpha subunit of cytochrome b559 and the heterodimer of the D1 and D2 proteins) were generated concomitantly. The site of cleavage of the D1 protein that gave rise to a major fragment of 22 kDa was located in the loop that connects membrane-spanning helixes IV and V. Treatment with H2O2 caused the same damage to proteins in isolated thylakoids and in core complexes that contained the non-heme iron at the acceptor side, but not in isolated reaction centers depleted of the iron. From these observations and the effects of reagents that are known to interact with the non-heme iron, it is suggested that the damage to proteins is caused by oxygen radicals generated by the non-heme iron in the Fe(II) state in a reaction with H2O2. It is proposed, moreover, that a similar mechanism is operative during the selective and specific degradation of the D1 protein under illumination.

Comparison of psbO and psbH deletion mutants of Synechocystis PCC 6803 indicates that degradation of D1 protein is regulated by the QB site and dependent on protein synthesis

Mutants of the cyanobacterium Synechocystis PCC 6803 lacking the psbO or psbH gene are more vulnerable to photoinhibition than the wild type (WT). In the case of the psbO-less mutant, the increased sensitivity to photodamage is also accompanied by accelerated turnover of the D1 protein and a rapid rate of recovery on transfer to non-photoinhibitory conditions. In contrast, in low light the psbH-less mutant has a poor ability to recover after photoinhibition and has a reduced rate of D1 turnover as compared with WT. Since the psbO gene encodes the 33 kDa manganese-stabilizing protein associated with the water-splitting reaction, the increased sensitivity to photoinduced damage is attributed to perturbation of electron transfer processes on the donor side of photosystem II (PSII). In contrast, the absence of H protein, encoded by the psbH gene, affects the acceptor side of PSII with preferential photoinhibitory damage occurring at the QB site. The apparent consequence of this is that the psbH-less mutant, unlike the psbO-less mutant, is not able to regulate the rate of turnover of the D1 protein. In all cases it was shown that chloramphenicol, which blocks protein synthesis, enhances the rate of photoinhibition as judged by a decrease in oxygen evolution but slows down the rate of degradation of D1 protein compared to that observed during normal turnover.(ABSTRACT TRUNCATED AT 250 WORDS)