Light-dependent D1 protein synthesis and translocation is regulated by reaction center II. Reaction center II serves as an acceptor for the D1 precursor

The biogenesis and maintenance of PSII: Recent advances and current challenges

The growth of plants, algae, and cyanobacteria relies on the catalytic activity of the oxygen-evolving PSII complex, which uses solar energy to extract electrons from water to feed into the photosynthetic electron transport chain. PSII is proving to be an excellent system to study how large multi-subunit membrane-protein complexes are assembled in the thylakoid membrane and subsequently repaired in response to photooxidative damage. Here we summarize recent developments in understanding the biogenesis of PSII, with an emphasis on recent insights obtained from biochemical and structural analysis of cyanobacterial PSII assembly/repair intermediates. We also discuss how chlorophyll synthesis is synchronized with protein synthesis and suggest a possible role for PSI in PSII assembly. Special attention is paid to unresolved and controversial issues that could be addressed in future research.

Cyanobacterial Bioenergetics in Relation to Cellular Growth and Productivity

Cyanobacteria, the evolutionary originators of oxygenic photosynthesis, have the capability to convert CO2, water, and minerals into biomass using solar energy. This process is driven by intricate bioenergetic mechanisms that consist of interconnected photosynthetic and respiratory electron transport chains coupled. Over the last few decades, advances in physiochemical analysis, molecular genetics, and structural analysis have enabled us to gain a more comprehensive understanding of cyanobacterial bioenergetics. This includes the molecular understanding of the primary energy conversion mechanisms as well as photoprotective and other dissipative mechanisms that prevent photodamage when the rates of photosynthetic output, primarily in the form of ATP and NADPH, exceed the rates that cellular assimilatory processes consume these photosynthetic outputs. Despite this progress, there is still much to learn about the systems integration and the regulatory circuits that control expression levels for optimal cellular abundance and activity of the photosynthetic complexes and the cellular components that convert their products into biomass. With an improved understanding of these regulatory principles and mechanisms, it should be possible to optimally modify cyanobacteria for enhanced biotechnological purposes.

Photosystem II antenna modules CP43 and CP47 do not form a stable 'no reaction centre complex' in the cyanobacterium Synechocystis sp. PCC 6803

The repair of photosystem II is a key mechanism that keeps the light reactions of oxygenic photosynthesis functional. During this process, the PSII central subunit D1 is replaced with a newly synthesized copy while the neighbouring CP43 antenna with adjacent small subunits (CP43 module) is transiently detached. When the D2 protein is also damaged, it is degraded together with D1 leaving both the CP43 module and the second PSII antenna module CP47 unassembled. In the cyanobacterium Synechocystis sp. PCC 6803, the released CP43 and CP47 modules have been recently suggested to form a so-called no reaction centre complex (NRC). However, the data supporting the presence of NRC can also be interpreted as a co-migration of CP43 and CP47 modules during electrophoresis and ultracentrifugation without forming a mutual complex. To address the existence of NRC, we analysed Synechocystis PSII mutants accumulating one or both unassembled antenna modules as well as Synechocystis wild-type cells stressed with high light. The obtained results were not compatible with the existence of a stable NRC since each unassembled module was present as a separate protein complex with a mutually similar electrophoretic mobility regardless of the presence of the second module. The non-existence of NRC was further supported by isolation of the His-tagged CP43 and CP47 modules from strains lacking either D1 or D2 and their migration patterns on native gels.

Light-induced psbA translation in plants is triggered by photosystem II damage via an assembly-linked autoregulatory circuit

The D1 reaction center protein of photosystem II (PSII) is subject to light-induced damage. Degradation of damaged D1 and its replacement by nascent D1 are at the heart of a PSII repair cycle, without which photosynthesis is inhibited. In mature plant chloroplasts, light stimulates the recruitment of ribosomes specifically to psbA mRNA to provide nascent D1 for PSII repair and also triggers a global increase in translation elongation rate. The light-induced signals that initiate these responses are unclear. We present action spectrum and genetic data indicating that the light-induced recruitment of ribosomes to psbA mRNA is triggered by D1 photodamage, whereas the global stimulation of translation elongation is triggered by photosynthetic electron transport. Furthermore, mutants lacking HCF136, which mediates an early step in D1 assembly, exhibit constitutively high psbA ribosome occupancy in the dark and differ in this way from mutants lacking PSII for other reasons. These results, together with the recent elucidation of a thylakoid membrane complex that functions in PSII assembly, PSII repair, and psbA translation, suggest an autoregulatory mechanism in which the light-induced degradation of D1 relieves repressive interactions between D1 and translational activators in the complex. We suggest that the presence of D1 in this complex coordinates D1 synthesis with the need for nascent D1 during both PSII biogenesis and PSII repair in plant chloroplasts.

Pretilachlor toxicity is decided by discrete photo-acclimatizing conditions: Physiological and biochemical evidence from Anabaena sp. and Nostoc muscorum

The current study was undertaken to elucidate the impact of the herbicide pretilachlor (3 µg ml^-1 and 6 µg ml^-1) on cyanobacteria, Nostoc muscorum ATCC 27893 and Anabaena sp. PCC 7120 under three levels of photoacclimatization (suboptimum, 25 µmol photon m^-2 s^-1; optimum, 75 µmol photon m^-2 s^-1; and supra-optimum, 225 µmol photon m^-2 s^-1) by analyzing certain physiological (biomass accumulation, photosynthesis, Chl a fluorescence and respiration) and biochemical parameters (photosynthetic pigments‒ chlorophyll a, carotenoids and phycocyanin; reactive oxygen species‒ O₂^•¯, H₂O₂, lipid peroxidation; antioxidant system‒ superoxide dismutase, peroxidise, catalase and glutathione-S-transferase). The light conditioning played the most prominent role in deciding the extent of herbicide toxicity on both the tested cyanobacteria as the maximum toxicity was observed in suboptimum light acclimatized cyanobacterial cells corroborated by the least growth in the same cells. The impact of pretilachlor treatment on photosystem II photochemistry viz. φP₀, Ѱ₀, φE₀, PI_ABS, ABS/RC, TR₀/RC, ET₀/RC and DI₀/RC was also altered by light acclimatization. The percent rise in oxidative stress markers (SOR and H₂O₂) and consequent lipid peroxidation (MDA equivalents) were also highest in suboptimum light acclimatized cells exposed to pretilachlor which could not be prospered with compatible antioxidant performance. Conversely, supra-optimum light acclimatized cells of both the cyanobacteria was found to accelerate the activities of all the studied enzymes and thus able to counterbalance the pretilachlor toxicity and supported the healthier growth.

Biogenesis of thylakoid membranes

Thylakoids mediate photosynthetic electron transfer and represent one of the most elaborate energy-transducing membrane systems. Despite our detailed knowledge of its structure and function, much remains to be learned about how the machinery is put together. The concerted synthesis and assembly of lipids, proteins and low-molecular-weight cofactors like pigments and transition metal ions require a high level of spatiotemporal coordination. While increasing numbers of assembly factors are being functionally characterized, the principles that govern how thylakoid membrane maturation is organized in space are just starting to emerge. In both cyanobacteria and chloroplasts, distinct production lines for the fabrication of photosynthetic complexes, in particular photosystem II, have been identified. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

Subunit composition of CP43-less photosystem II complexes of Synechocystis sp. PCC 6803: implications for the assembly and repair of photosystem II

Photosystem II (PSII) mutants are useful experimental tools to trap potential intermediates involved in the assembly of the oxygen-evolving PSII complex. Here, we focus on the subunit composition of the RC47 assembly complex that accumulates in a psbC null mutant of the cyanobacterium Synechocystis sp. PCC 6803 unable to make the CP43 apopolypeptide. By using native gel electrophoresis, we showed that RC47 is heterogeneous and mainly found as a monomer of 220 kDa. RC47 complexes co-purify with small Cab-like proteins (ScpC and/or ScpD) and with Psb28 and its homologue Psb28-2. Analysis of isolated His-tagged RC47 indicated the presence of D1, D2, the CP47 apopolypeptide, plus nine of the 13 low-molecular-mass (LMM) subunits found in the PSII holoenzyme, including PsbL, PsbM and PsbT, which lie at the interface between the two momomers in the dimeric holoenzyme. Not detected were the LMM subunits (PsbK, PsbZ, Psb30 and PsbJ) located in the vicinity of CP43 in the holoenzyme. The photochemical activity of isolated RC47-His complexes, including the rate of reduction of P680⁺, was similar to that of PSII complexes lacking the Mn₄CaO₅ cluster. The implications of our results for the assembly and repair of PSII in vivo are discussed.

Photosystem II assembly: from cyanobacteria to plants

Photosystem II (PSII) is an integral-membrane, multisubunit complex that initiates electron flow in oxygenic photosynthesis. The biogenesis of this complex machine involves the concerted assembly of at least 20 different polypeptides as well as the incorporation of a variety of inorganic and organic cofactors. Many factors have recently been identified that constitute an integrative network mediating the stepwise assembly of PSII components. One recurring theme is the subcellular organization of the assembly process in specialized membranes that form distinct biogenesis centers. Here, we review our current knowledge of the molecular components and events involved in PSII assembly and their high degree of evolutionary conservation.

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.

HCF243 encodes a chloroplast-localized protein involved in the D1 protein stability of the arabidopsis photosystem II complex

Numerous auxiliary nuclear factors have been identified to be involved in the dynamics of the photosystem II (PSII) complex. In this study, we characterized the high chlorophyll fluorescence243 (hcf243) mutant of Arabidopsis (Arabidopsis thaliana), which shows higher chlorophyll fluorescence and is severely deficient in the accumulation of PSII supercomplexes compared with the wild type. The amount of core subunits was greatly decreased, while the outer antenna subunits and other subunits were hardly affected in hcf243. In vivo protein-labeling experiments indicated that the synthesis rate of both D1 and D2 proteins decreased severely in hcf243, whereas no change was found in the rate of other plastid-encoded proteins. Furthermore, the degradation rate of the PSII core subunit D1 protein is higher in hcf243 than in the wild type, and the assembly of PSII is retarded significantly in the hcf243 mutant. HCF243, a nuclear gene, encodes a chloroplast protein that interacts with the D1 protein. HCF243 homologs were identified in angiosperms with one or two copies but were not found in lower plants and prokaryotes. These results suggest that HCF243, which arose after the origin of the higher plants, may act as a cofactor to maintain the stability of D1 protein and to promote the subsequent assembly of the PSII complex.

Cooperation of LPA3 and LPA2 is essential for photosystem II assembly in Arabidopsis

Photosystem II (PSII) is a multisubunit membrane protein complex that is assembled in a sequence of steps. However, the molecular mechanisms responsible for the assembly of the individual subunits into functional PSII complexes are still largely unknown. Here, we report the identification of a chloroplast protein, Low PSII Accumulation3 (LPA3), which is required for the assembly of the CP43 subunit in PSII complexes in Arabidopsis (Arabidopsis thaliana). LPA3 interacts with LPA2, a previously identified PSII CP43 assembly factor, and a double mutation of LPA2 and LPA3 is more deleterious for assembly than either single mutation, resulting in a seedling-lethal phenotype. Our results indicate that LPA3 and LPA2 have overlapping functions in assisting CP43 assembly and that cooperation between LPA2 and LPA3 is essential for PSII assembly. In addition, we provide evidence that LPA2 and LPA3 interact with Albino3 (Alb3), which is essential for thylakoid protein biogenesis. Thus, the function of Alb3 in some PSII assembly processes is probably mediated through interactions with LPA2 and LPA3.

LPA19, a Psb27 homolog in Arabidopsis thaliana, facilitates D1 protein precursor processing during PSII biogenesis

The biogenesis and assembly of photosystem II (PSII) are mainly regulated by the nuclear-encoded factors. To further identify the novel components involved in PSII biogenesis, we isolated and characterized a high chlorophyll fluorescence low psii accumulation19 (lpa19) mutant, which is defective in PSII biogenesis. LPA19 encodes a Psb27 homolog (At1g05385). Interestingly, another Psb27 homolog (At1g03600) in Arabidopsis was revealed to be required for the efficient repair of photodamaged PSII. These results suggest that the Psb27 homologs play distinct functions in PSII biogenesis and repair in Arabidopsis. Chloroplast protein labeling assays showed that the C-terminal processing of D1 in the lpa19 mutant was impaired. Protein overlay assays provided evidence that LPA19 interacts with D1, and coimmunoprecipitation analysis demonstrated that LPA19 interacts with mature D1 (mD1) and precursor D1 (pD1). Moreover, LPA19 protein was shown to specifically interact with the soluble C terminus present in the precursor and mature D1 through yeast two-hybrid analyses. Thus, these studies suggest that LPA19 is involved in facilitating the D1 precursor protein processing in Arabidopsis.

The thylakoid protease Deg1 is involved in photosystem-II assembly in Arabidopsis thaliana

DegP proteases have been shown to possess both chaperone and protease activities. The proteolytic activities of chloroplast DegP-like proteases have been well documented. However, whether chloroplast Deg proteases also have chaperone activities has remained unknown. Here we show that chloroplast Deg1 also has chaperone activities, like its Escherichia coli ortholog DegP. Transgenic plants with reduced levels of Deg1 accumulated normal levels of different subunits of the major photosynthetic protein complexes, but their levels of photosystem-II (PSII) dimers and supercomplexes were reduced. In vivo pulse-chase protein labeling experiments showed that the assembly of newly synthesized proteins into PSII dimers and supercomplexes was impaired, although the synthesis rate of chloroplast proteins was unaffected in the transgenic lines. Protein overlay assays provided direct evidence that Deg1 interacts with the PSII reaction center protein D2. These results suggest that Deg1 assists the assembly of the PSII complex, probably through interaction with the PSII reaction center D2 protein.

The different effects of chilling stress under moderate light intensity on photosystem II compared with photosystem I and subsequent recovery in tropical tree species

Tropical plants are sensitive to chilling temperatures above zero but it is still unclear whether photosystem I (PSI) or photosystem II (PSII) of tropical plants is mainly affected by chilling temperatures. In this study, the effect of 4 degrees C associated with various light densities on PSII and PSI was studied in the potted seedlings of four tropical evergreen tree species grown in an open field, Khaya ivorensis, Pometia tomentosa, Dalbergia odorifera, and Erythrophleum guineense. After 8 h chilling exposure at the different photosynthetic flux densities of 20, 50, 100, 150 micromol m(-2) s(-1), the maximum quantum yield of PSII (F (v) /F (m)) in all of the four species decreased little, while the quantity of efficient PSI complex (P (m)) remained stable in all species except E. guineense. However, after chilling exposure under 250 micromol m(-2) s(-1) for 24 h, F (v) /F (m) was severely photoinhibited in all species whereas P (m) was relative stable in all plants except E. guineense. At the chilling temperature of 4 degrees C, electron transport from PSII to PSI was blocked because of excessive reduction of primary electron acceptor of PSII. F (v) /F (m) in these species except E. guineense recovered to approximately 90% after 8 h recovery in low light, suggesting the dependence of the recovery of PSII on moderate PSI and/or PSII activity. These results suggest that PSII is more sensitive to chilling temperature under the moderate light than PSI in tropical trees, and the photoinhibition of PSII and closure of PSII reaction centers can serve to protect PSI.

The stromal chloroplast Deg7 protease participates in the repair of photosystem II after photoinhibition in Arabidopsis

Light is the ultimate source of energy for photosynthesis; however, excessive light leads to photooxidative damage and hence reduced photosynthetic efficiency, especially when combined with other abiotic stresses. Although the photosystem II (PSII) reaction center D1 protein is the primary target of photooxidative damage, other PSII core proteins are also damaged and degraded. However, it is still largely unknown whether degradation of D1 and other PSII proteins involves previously uncharacterized proteases. Here, we show that Deg7 is peripherally associated with the stromal side of the thylakoid membranes and that Deg7 interacts directly with PSII. Our results show that Deg7 is involved in the primary cleavage of photodamaged D1, D2, CP47, and CP43 and that this activity is essential for its function in PSII repair. The double mutants deg5 deg7 and deg8 deg7 showed no obvious phenotypic differences under normal growth conditions, but additive effects were observed under high light. These results suggest that Deg proteases on both the stromal and luminal sides of the thylakoid membranes are important for the efficient PSII repair in Arabidopsis (Arabidopsis thaliana).

Mechanism of REP27 protein action in the D1 protein turnover and photosystem II repair from photodamage

The function of the REP27 protein (GenBank accession no. EF127650) in the photosystem II (PSII) repair process was elucidated. REP27 is a nucleus-encoded and chloroplast-targeted protein containing two tetratricopeptide repeat (TPR) motifs, two putative transmembrane domains, and an extended carboxyl (C)-terminal region. Cell fractionation and western-blot analysis localized the REP27 protein in the Chlamydomonas reinhardtii chloroplast thylakoids. A folding model for REP27 suggested chloroplast stroma localization for amino- and C-terminal regions as well as the two TPRs. A REP27 gene knockout strain of Chlamydomonas, termed the rep27 mutant, was employed for complementation studies. The rep27 mutant was aberrant in the PSII-repair process and had substantially lower than wild-type levels of D1 protein. Truncated REP27 cDNA constructs were made for complementation of rep27, whereby TPR1, TPR2, TPR1+TPR2, or the C-terminal domains were deleted. rep27-complemented strains minus the TPR motifs showed elevated levels of D1 in thylakoids, comparable to those in the wild type, but the PSII photochemical efficiency of these strains was not restored, suggesting that the functionality of the PSII reaction center could not be recovered in the absence of the TPR motifs. It is suggested that TPR motifs play a role in the functional activation of the newly integrated D1 protein in the PSII reaction center. rep27-complemented strains missing the C-terminal domain showed low levels of D1 protein in thylakoids as well as low PSII photochemical efficiency, comparable to those in the rep27 mutant. Therefore, the C-terminal domain is needed for a de novo biosynthesis and/or assembly of D1 in the photodamaged PSII template. We conclude that REP27 plays a dual role in the regulation of D1 protein turnover by facilitating cotranslational biosynthesis insertion (C-terminal domain) and activation (TPR motifs) of the nascent D1 during the PSII repair process.

Chloroplast protein targeting involves localized translation in Chlamydomonas

The compartmentalization of eukaryotic cells requires that newly synthesized proteins be targeted to the compartments in which they function. In chloroplasts, a few thousand proteins function in photosynthesis, expression of the chloroplast genome, and other processes. Most chloroplast proteins are synthesized in the cytoplasm, imported, and then targeted to a specific chloroplast compartment. The remainder are encoded by the chloroplast genome, synthesized within the organelle, and targeted by mechanisms that are only beginning to be elucidated. We used fluorescence confocal microscopy to explore the targeting mechanisms used by several chloroplast proteins in the green alga Chlamydomonas. These include the small subunit of ribulose bisphosphate carboxylase (rubisco) and the light-harvesting complex II (LHCII) subunits, which are imported from the cytoplasm, and 2 proteins synthesized in the chloroplast: the D1 subunit of photosystem II and the rubisco large subunit. We determined whether the targeting of each protein involves localized translation of the mRNA that encodes it. When this was the case, we explored whether the targeting sequence was in the nascent polypeptide or in the mRNA, based on whether the localization was translation-dependent or -independent, respectively. The results reveal 2 novel examples of targeting by localized translation, in LHCII subunit import and the targeting of the rubisco large subunit to the pyrenoid. They also demonstrate examples of each of the three known mechanisms-posttranslational, cotranslational (signal recognition particle-mediated), and mRNA-based-in the targeting of specific chloroplast proteins. Our findings can help guide the exploration of these pathways at the biochemical level.

Community-level analysis of psbA gene sequences and irgarol tolerance in marine periphyton

This study analyzes psbA gene sequences, predicted D1 protein sequences, species relative abundance, and pollution-induced community tolerance in marine periphyton communities exposed to the antifouling compound Irgarol 1051. The mechanism of action of Irgarol is the inhibition of photosynthetic electron transport at photosystem II by binding to the D1 protein. The metagenome of the communities was used to produce clone libraries containing fragments of the psbA gene encoding the D1 protein. Community tolerance was quantified with a short-term test for the inhibition of photosynthesis. The communities were established in a continuous flow of natural seawater through microcosms with or without added Irgarol. The selection pressure from Irgarol resulted in an altered species composition and an inducted community tolerance to Irgarol. Moreover, there was a very high diversity in the psbA gene sequences in the periphyton, and the composition of psbA and D1 fragments within the communities was dramatically altered by increased Irgarol exposure. Even though tolerance to this type of compound in land plants often depends on a single amino acid substitution (Ser(264)-->Gly) in the D1 protein, this was not the case for marine periphyton species. Instead, the tolerance mechanism likely involves increased degradation of D1. When we compared sequences from low and high Irgarol exposure, differences in nonconserved amino acids were found only in the so-called PEST region of D1, which is involved in regulating its degradation. Our results suggest that environmental contamination with Irgarol has led to selection for high-turnover D1 proteins in marine periphyton communities at the west coast of Sweden.

Stress induces the assembly of RNA granules in the chloroplast of Chlamydomonas reinhardtii

Eukaryotic cells under stress repress translation and localize these messenger RNAs (mRNAs) to cytoplasmic RNA granules. We show that specific stress stimuli induce the assembly of RNA granules in an organelle with bacterial ancestry, the chloroplast of Chlamydomonas reinhardtii. These chloroplast stress granules (cpSGs) form during oxidative stress and disassemble during recovery from stress. Like mammalian stress granules, cpSGs contain poly(A)-binding protein and the small, but not the large, ribosomal subunit. In addition, mRNAs are in continuous flux between polysomes and cpSGs during stress. Localization of cpSGs within the pyrenoid reveals that this chloroplast compartment functions in this stress response. The large subunit of ribulosebisphosphate carboxylase/oxygenase also assembles into cpSGs and is known to bind mRNAs during oxidative stress, raising the possibility that it plays a role in cpSG assembly. This discovery within such an organelle suggests that mRNA localization to granules during stress is a more general phenomenon than currently realized.

Photosystem II assembly and repair are differentially localized in Chlamydomonas

Many proteins of the photosynthesis complexes are encoded by the genome of the chloroplast and synthesized by bacterium-like ribosomes within this organelle. To determine where proteins are synthesized for the de novo assembly and repair of photosystem II (PSII) in the chloroplast of Chlamydomonas reinhardtii, we used fluorescence in situ hybridization, immunofluorescence staining, and confocal microscopy. These locations were defined as having colocalized chloroplast mRNAs encoding PSII subunits and proteins of the chloroplast translation machinery specifically under conditions of PSII subunit synthesis. The results revealed that the synthesis of the D1 subunit for the repair of photodamaged PSII complexes occurs in regions of the chloroplast with thylakoids, consistent with the current model. However, for de novo PSII assembly, PSII subunit synthesis was detected in discrete regions near the pyrenoid, termed T zones (for translation zones). In two PSII assembly mutants, unassembled D1 subunits and incompletely assembled PSII complexes localized around the pyrenoid, where we propose that they mark an intermediate compartment of PSII assembly. These results reveal a novel chloroplast compartment that houses de novo PSII biogenesis and the regulated transport of newly assembled PSII complexes to thylakoid membranes throughout the chloroplast.

Cool temperatures interfere with D1 synthesis in tomato by causing ribosomal pausing

Photodamage occurs when leaves are exposed to light in excess of what can be used for photosynthesis and in excess of the capacity of ancillary photoprotective as well as repair mechanisms. An important site of photodamage is the chloroplast encoded D1 protein, a component of the photosystem II (PSII) reaction center. Even under optimal growth irradiance, D1 is photodamaged necessitating rapid turnover to prevent the accumulation of photodamaged PSII reaction centers and consequent inhibition of photosynthesis. However, this on-going process of D1 turnover and replacement was impeded in the chilling-sensitive tomato (Solanum lycopersicum) plants when exposed to high-growth light at cool temperature. The decrease in D1 turnover and replacement was found not to be due to changes in the steady-state level of the psbA message. While the recruitment of ribosomes to psbA transcript, initiation of D1 translation, and the association of polysomes with the thylakoid membrane occurred normally, chilling temperatures caused ribosomal pausing during D1 peptide elongation in tomato. The pause locations were non-randomly located on the D1 transcript. The interference with translation caused by ribosomal pausing allowed photodamaged PSII centers to accumulate leading to the consequent inhibition of photosynthesis.

LOW PSII ACCUMULATION1 is involved in efficient assembly of photosystem II in Arabidopsis thaliana

To gain insight into the processes involved in photosystem II (PSII) biogenesis and maintenance, we characterized the low psii accumulation1 (lpa1) mutant of Arabidopsis thaliana, which generally accumulates lower than wild-type levels of the PSII complex. In vivo protein labeling experiments showed that synthesis of the D1 and D2 proteins was greatly reduced in the lpa1 mutant, while other plastid-encoded proteins were translated at rates similar to the wild type. In addition, turnover rates of the PSII core proteins CP47, CP43, D1, and D2 were higher in lpa1 than in wild-type plants. The newly synthesized PSII proteins were assembled into functional protein complexes, but the assembly was less efficient in the mutant. LPA1 encodes a chloroplast protein that contains two tetratricopeptide repeat domains and is an intrinsic membrane protein but not an integral subunit of PSII. Yeast two-hybrid studies revealed that LPA1 interacts with D1 but not with D2, cytochrome b6, or Alb3. Thus, LPA1 appears to be an integral membrane chaperone that is required for efficient PSII assembly, probably through direct interaction with the PSII reaction center protein D1.

Chloroplast sulfate transport in green algae--genes, proteins and effects

This review summarizes evidence at the molecular genetic, protein and regulatory levels concerning the existence and function of a putative ABC-type chloroplast envelope-localized sulfate transporter in the model unicellular green alga Chlamydomonas reinhardtii. From the four nuclear genes encoding this sulfate permease holocomplex, two are coding for chloroplast envelope-targeted transmembrane proteins (SulP and SulP2), a chloroplast stroma-targeted ATP-binding protein (Sabc) and a substrate (sulfate)-binding protein (Sbp) that is localized on the cytosolic side of the chloroplast envelope. The sulfate permease holocomplex is postulated to consist of a SulP-SulP2 chloroplast envelope transmembrane heterodimer, flanked by the Sabc and the Sbp proteins on the stroma side and the cytosolic side of the inner envelope, respectively. The mature SulP and SulP2 proteins contain seven transmembrane domains and one or two large hydrophilic loops, which are oriented toward the cytosol. The corresponding prokaryotic-origin genes (SulP and SulP2) probably migrated from the chloroplast to the nuclear genome during the evolution of Chlamydomonas reinhardtii. These genes, or any of its homologues, have not been retained in vascular plants, e.g. Arabidopsis thaliana, although they are encountered in the chloroplast genome of a liverwort (Marchantia polymorpha). The function of the SulP protein was probed in antisense transformants of C. reinhardtii having lower expression levels of the SulP gene. Results showed that cellular sulfate uptake capacity was lowered as a consequence of attenuated SulP gene expression in the cell, directly affecting rates of de novo protein biosynthesis in the chloroplast. The antisense transformants exhibited phenotypes of sulfate-deprived cells, displaying slow rates of light-saturated oxygen evolution, low levels of Rubisco in the chloroplast and low steady-state levels of the Photosystem II D1 reaction center protein. The role of the chloroplast sulfate transport in the uptake and assimilation of sulfate in Chlamydomonas reinhardtii is discussed along with its impact on the repair of Photosystem II from a frequently occurring photo-oxidative damage and H2-evolution related metabolism in this green alga.

Psb29, a conserved 22-kD protein, functions in the biogenesis of Photosystem II complexes in Synechocystis and Arabidopsis

Photosystem II (PSII), the enzyme responsible for photosynthetic oxygen evolution, is a rapidly turned over membrane protein complex. However, the factors that regulate biogenesis of PSII are poorly defined. Previous proteomic analysis of the PSII preparations from the cyanobacterium Synechocystis sp PCC 6803 detected a novel protein, Psb29 (Sll1414), homologs of which are found in all cyanobacteria and vascular plants with sequenced genomes. Deletion of psb29 in Synechocystis 6803 results in slower growth rates under high light intensities, increased light sensitivity, and lower PSII efficiency, without affecting the PSII core electron transfer activities. A T-DNA insertion line in the PSB29 gene in Arabidopsis thaliana displays a phenotype similar to that of the Synechocystis mutant. This plant mutant grows slowly and exhibits variegated leaves, and its PSII activity is light sensitive. Low temperature fluorescence emission spectroscopy of both cyanobacterial and plant mutants shows an increase in the proportion of uncoupled proximal antennae in PSII as a function of increasing growth light intensities. The similar phenotypes observed in both plant and cyanobacterial mutants demonstrate that the function of Psb29 has been conserved throughout the evolution of oxygenic photosynthetic organisms and suggest a role for the Psb29 protein in the biogenesis of PSII.

The MntC crystal structure suggests that import of Mn2+ in cyanobacteria is redox controlled

The MntC protein is the periplasmic solute-binding protein component of the high-affinity manganese ATP-binding cassette-type transport system in the cyanobacterium Synechocytis PCC sp. 6803. We have determined the structure of recombinant MntC at 2.9 A resolution by X-ray crystallography using a combination of multi-wavelength anomalous diffraction and molecular replacement. The presence of Mn2+ in the metal ion-binding site was ascertained by use of anomalous difference electron density maps using diffraction data collected at the Mn absorption edge. The MntC protein is similar to previously determined metal ion-binding, solute-binding proteins with two globular domains connected by an extended alpha-helix. However, the metal ion-binding site is asymmetric, with two of the four ligating residues (Glu220 and Asp295) situated closer to the ion than the two histidine residues (His89 and His154). A unique characteristic of the MntC is the existence of a disulfide bond between Cys219 and Cys268. Analysis of amino acid sequences of homologous proteins shows that conservation of the cysteine residues forming the disulfide bond occurs only in cyanobacterial manganese solute-binding proteins. One of the monomers in the MntC asymmetric unit trimer is disordered significantly in the globular domain containing the disulfide bond. The electron density on the manganese ion and on the disulfide bond in this monomer indicates that reduction of this bond changes the relative position of the lower domain and of the Glu220 ligand, potentially lowering the affinity towards Mn2+. This is confirmed by reduction of the disulfide bond in vitro, showing the release of bound Mn2+. We propose that the reduction or oxidation state of the disulfide bond can alter the binding affinity of the protein towards Mn2+ and thus determine whether these ions will be transported into the cytoplasm, or be available for photosystem II biogenesis in the periplasm.

Localization and function of SulP, a nuclear-encoded chloroplast sulfate permease in Chlamydomonas reinhardtii

Recent work [H.-C. Chen et al. (2003) Planta 218:98-106] reported on the genomic, proteomic, phylogenetic and evolutionary aspects of a putative nuclear gene ( SulP) encoding a chloroplast sulfate permease in the model green alga Chlamydomonas reinhardtii. In this article, evidence is provided for the envelope localization of the SulP protein and its function in the uptake and assimilation of sulfate by the chloroplast. Localization of the SulP protein in the chloroplast envelope was concluded upon isolation of C. reinhardtii chloroplasts, followed by fractionation into envelope and thylakoid membranes and Western blotting of these fractions with specific polyclonal antibodies raised against the recombinant SulP protein. The function of the SulP protein was probed in antisense transformants of C. reinhardtii having lower expression levels of the SulP gene. Results showed that cellular sulfate uptake capacity was lowered as a consequence of attenuated SulP gene expression in the cell, directly affecting rates of de novo protein biosynthesis in the chloroplast. The antisense transformants exhibited phenotypes of sulfate-deprived cells, displaying slow rates of light-saturated oxygen evolution, low levels of Rubisco in the chloroplast and low steady-state levels of the photosystem-II D1 reaction-center protein. The role of the chloroplast sulfate transport in the uptake and assimilation of sulfate in C. reinhardtii is discussed along with its impact on the repair of photosystem-II from a frequently occurring photo-oxidative damage and potential use for the elucidation of the H(2)-evolution-related metabolism in this green alga.

Redox regulation of thylakoid protein phosphorylation

The photosystem II of chloroplast thylakoid membranes contains several proteins phosphorylated by redox-activated protein kinases. The mechanism of the reversible activation of the light-harvesting antenna complex II (LHCII) kinase(s) is one of the best understood and related to the regulation of energy transfer to photosystem II or I, thereby optimizing their relative excitation (state transition). The deactivated LHCII protein kinase(s) is associated with cytochrome b(6)f and dissociates from the complex upon activation. Activation of the LHCII protein kinase occurs via dynamic conformational changes in the cytochrome b(6)f complex taking place during plastoquinol oxidation. Deactivation of the kinase involves its reassociation with an oxidized cytochrome complex. A fine-tuning redox-dependent regulatory loop inhibits the activation of the kinase via reduction of protein disulfide groups, possibly involving the thioredoxin complex. Phosphorylation of LHCII is further modulated by light-induced conformational changes of the LHCII substrate. The reversible phosphorylation of LHCII and other thylakoid phosphoproteins, catalyzed by respective kinases and phosphatases, is under strict regulation in response to environmental changes.

Biogenesis, assembly and turnover of photosystem II units

Assembly of photosystem II, a multiprotein complex embedded in the thylakoid membrane, requires stoichiometric production of over 20 protein subunits. Since part of the protein subunits are encoded in the chloroplast genome and part in the nucleus, a signalling network operates between the two genetic compartments in order to prevent wasteful production of proteins. Coordinated synthesis of proteins also takes place among the chloroplast-encoded subunits, thus establishing a hierarchy in the protein components that allows a stepwise building of the complex. In addition to this dependence on assembly partners, other factors such as the developmental stage of the plastid and various photosynthesis-related parameters exert a strict control on the accumulation, membrane targeting and assembly of the PSII subunits. Here, we briefly review recent results on this field obtained with three major approaches: biogenesis of photosystem II during the development of chloroplasts from etioplasts, use of photosystem II-specific mutants and photosystem II turnover during its repair cycle.

A chloroplast RNA binding protein from stromal thylakoid membranes specifically binds to the 5' untranslated region of the psbA mRNA

The intrachloroplastic localization of post-transcriptional gene expression steps represents one key determinant for the regulation of chloroplast development. We have characterized an RNA binding protein of 63 kDa (RBP63) from Chlamydomonas reinhardtii chloroplasts, which cofractionates with stromal thylakoid membranes. Solubility properties suggest that RBP63 is a peripheral membrane protein. Among RNA probes from different 5' untranslated regions of chloroplast transcripts, RBP63 preferentially binds to the psbA leader. This binding is dependent on a region comprising seven consecutive A residues, which is required for D1 protein synthesis. A possible role for this newly discovered RNA binding protein in membrane targeting of psbA gene expression is discussed.

Synthesis, membrane insertion and assembly of the chloroplast-encoded D1 protein into photosystem II

Rapid light-dependent turnover of the chloroplast-encoded D1 protein maintains photosystem II (PS II) functional over a wide range of light intensities. Following initiation of psbA mRNA translation, the elongating D1 is targeted, possibly by chloroplast signal recognition particle 54 (cpSRP54), to the thylakoid cpSecY translocation channel. Transmembrane domains of nascent D1 start interacting with other PS II core proteins already during the translocation process to ensure an efficient assembly of the multiprotein membrane complex. Here we review the progress recently made concerning the synthesis, targeting, membrane insertion and assembly to PS II of the chloroplast-encoded D1 protein and discuss the possible convergence of targeting and translocation of chloroplast- and nuclear-encoded thylakoid proteins.

Photosystem II damage and repair cycle in the green alga Dunaliella salina: involvement of a chloroplast-localized HSP70

The involvement of HSP70B in the photosystem II damage and repair process in Dunaliella salina was investigated. A full-length cDNA of the D. salina hsp70B gene was cloned and sequenced. Expression patterns of the hsp70B gene were investigated upon shifting a D. salina culture from low-light to high-light growth conditions, designed to significantly accelerate the rate of PSII photodamage. Northern blot analyses and nuclear run-on transcription assays revealed a significant but transient induction of hsp70B gene transcription, followed by a subsequent increase in HSP70B protein synthesis and accumulation. Mild detergent solubilization of photoinhibited thylakoid membranes, in which photodamaged PSII centers had accumulated, followed by native gel electrophoresis revealed the formation of a 320 kDa protein complex that contained, in addition to the HSP70B, the photodamaged but as yet undegraded D1 protein as well as D2 and CP47. Evidence suggested that the 320 kDa complex is a transiently forming PSII repair intermediate. Denaturing solubilization of the 320 kDa PSII repair intermediate by SDS-urea resulted in cross-linking of its polypeptide constituents, yielding a 160 kDa protein complex. The role of the HSP70B in the repair of photodamaged PSII centers, e.g. in stabilizing the disassembled PSII-core complex and in facilitating the D1 degradation and replacement process, is discussed.

A SecY homologue is involved in chloroplast-encoded D1 protein biogenesis

We have used the photosystem II reaction center D1 protein as a model to study the mechanisms of targeting and insertion of chloroplast-encoded thylakoid membrane proteins. The unusually high turnover rate and distinct pausing intermediates during translation make the D1 protein biogenesis particularly suitable for these purposes. Here we show that cpSecY, a chloroplast homologue of bacterial essential translocon component SecY, interacts tightly with thylakoid membrane-bound ribosomes, suggesting its involvement in protein translocation and insertion. Co-immunoprecipitation and cross-linking experiments indicated that cpSecY resides in the vicinity of D1 elongation intermediates and provided evidence for a transient interaction of cpSecY with D1 elongation intermediates during the biogenesis of D1. After termination of translation, such interactions no longer existed. Our results indicate that, in addition to a well characterized role of cpSecY in posttranslational translocation of nuclear-encoded proteins, it seems to be also involved in cotranslational membrane protein translocation and insertion in chloroplasts.

Regulation of translation elongation in cyanobacteria: membrane targeting of the ribosome nascent-chain complexes controls the synthesis of D1 protein

The photosystem II reaction centre protein D1 is encoded by the psbA gene. The D1 protein is stable in darkness but undergoes rapid turnover in the light. Here, we show that, in cyanobacterium Synechocystis sp. PCC6803, the synthesis of the D1 protein is regulated at the level of translation elongation in addition to the previously known transcriptional regulation. When Synechocystis sp. PCC6803 cells were transferred from light to darkness, the psbA mRNA remained abundant for hours. Cytosolic ribosomes were attached to psbA transcripts in the dark, and translation continued up to a distinct pausing site. However, ribosome nascent D1 chain complexes were not targeted to the thylakoid membrane, and no full-length D1 protein was produced in darkness. The arrest in translation elongation was released in the light, concomitantly with targeting of ribosome D1 nascent-chain complexes to the thylakoid membrane, allowing the synthesis of the full-length D1 protein. Downregulation of membrane targeting of ribosome complexes was also observed in the light if damage to the D1 protein was slow. This novel type of regulation of prokaryotic translation functions to balance the synthesis and degradation of the rapidly turning over photosystem II D1 protein in Synechocystis sp. PCC6803.

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

This study examines the effects of ecologically important levels of ultraviolet B radiation on protein D1 turnover and stability and lateral redistribution of photosystem II. It is shown that ultraviolet B light supported only limited synthesis of protein D1, one of the most important components of photosystem II, whereas it promoted significant degradation of proteins D1 and D2. Furthermore, dephosphorylation of photosystem II subunits was specifically elicited upon exposure to ultraviolet B light. Structural modifications of photosystem II and changes in its lateral distribution between granum membranes and stroma-exposed lamellae were found to be different from those observed after photoinhibition by strong visible light. In particular, more complete dismantling of photosystem II cores was observed. Altogether, the data reported here suggest that ultraviolet B radiation alone fails to activate the photosystem II repair cycle, as hypothesized for visible light. This failure may contribute to the toxic effect of ultraviolet B radiation, which is increasing as a consequence of depletion of stratospheric ozone.

GTP enhances the degradation of the photosystem II D1 protein irrespective of its conformational heterogeneity at the Q(B) site

The light exposure history and/or binding of different herbicides at the Q(B) site may induce heterogeneity of photosystem II acceptor side conformation that affects D1 protein degradation under photoinhibitory conditions. GTP was recently found to stimulate the D1 protein degradation of photoinactivated photosystem II (Spetea, C. , Hundal, T., Lohmann, F., and Andersson, B. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6547-6552). Here we report that GTP enhances the cleavage of the D1 protein D-E loop following exposure of thylakoid membranes to either high light, low light, or repetitive single turnover flashes but not to trypsin. GTP does not stimulate D1 protein degradation in the presence of herbicides known to affect the accessibility of the cleavage site to proteolysis. However, GTP stimulates degradation that can be induced even in darkness in some photosystem II conformers following binding of the PNO8 herbicide (Nakajima, Y., Yoshida, S., Inoue, Y., Yoneyama, K., and Ono, T. (1995) Biochim. Biophys. Acta 1230, 38-44). Both the PNO8- and the light-induced primary cleavage of the D1 protein occur in the grana membrane domains. The subsequent migration of photosytem II containing the D1 protein fragments to the stroma domains for secondary proteolysis is light-activated. We conclude that the GTP effect is not confined to a specific photoinactivation pathway nor to the conformational state of the photosystem II acceptor side. Consequently, GTP does not interact with the site of D1 protein cleavage but rather enhances the activity of the endogenous proteolytic system.

Recovery of photosystem II activity in photoinhibited synechocystis cells: light-dependent translation activity is required besides light-independent synthesis of the D1 protein

Irreversible photoinactivation of photosystem II (PSII) results in the degradation of the reaction center II D1 protein. In Synechocystis PCC 6714 cells, recovery of PSII activity requires illumination. The rates of photoinactivation and recovery of PSII activity in the light are similar in cells grown in minimal (MM) or glucose-containing medium (GM). Reassembly of PSII with newly synthesized proteins requires degradation of the D1 protein of the photoinactivated PSII. This process may occur in darkness in both types of cells. The degraded D1 protein is, however, only partially replaced by newly synthesized protein in MM cells in darkness while a high level of D1 protein synthesis occurs in darkness in the GM cells. The newly synthesized D1 protein in darkness appears to be assembled with other PSII proteins. However, PSII activity is not recovered in such cells. Illumination of the cells in absence but not in the presence of protein synthesis inhibitors allows recovery of PSII activity.

Damage to photosystem II in symbiotic dinoflagellates: a determinant of coral bleaching

Coral bleaching has been defined as a general phenomenon, whereby reef corals turn visibly pale because of the loss of their symbiotic dinoflagellates and/or algal pigments during periods of exposure to elevated seawater temperatures. During the summer of 1997, seawater temperatures in the Florida Keys remained at or above 30 degrees C for more than 6 weeks, and extensive coral bleaching was observed. Bleached colonies of the dominant Caribbean reef-building species, Montastrea faveolata and Montastrea franksi, were sampled over a depth gradient from 1 to 17 m during this period of elevated temperature and contained lower densities of symbiotic dinoflagellates in deeper corals than seen in previous "nonbleaching" years. Fluorescence analysis by pulse-amplitude modulation fluorometry revealed severe damage to photosystem II (PSII) in remaining symbionts within the corals, with greater damage indicated at deeper depths. Dinoflagellates with the greatest loss in PSII activity also showed a significant decline in the D1 reaction center protein of PSII, as measured by immunoblot analysis. Laboratory experiments on the temperature-sensitive species Montastrea annularis, as well as temperature-sensitive and temperature-tolerant cultured symbiotic dinoflagellates, confirmed the temperature-dependent loss of PSII activity and concomitant decrease in D1 reaction center protein seen in symbionts collected from corals naturally bleached on the reef. In addition, variation in PSII repair was detected, indicating that perturbation of PSII protein turnover rates during photoinhibition at elevated temperatures underlies the physiological collapse of symbionts in corals susceptible to heat-induced bleaching.

Co-translational assembly of the D1 protein into photosystem II

Assembly of multi-subunit membrane protein complexes is poorly understood. In this study, we present direct evidence that the D1 protein, a multiple membrane spanning protein, assembles co-translationally into the large membrane-bound complex, photosystem II. During pulse-chase studies in intact chloroplasts, incorporation of the D1 protein occurred without transient accumulation of free labeled protein in the thylakoid membrane, and photosystem II subcomplexes contained nascent D1 intermediates of 17, 22, and 25 kDa. These N-terminal D1 intermediates could be co-immunoprecipitated with antiserum directed against the D2 protein, suggesting co-translational assembly of the D1 protein into PS II complexes. Further evidence for a co-translational assembly of the D1 protein into photosystem II was obtained by analyzing ribosome nascent chain complexes liberated from the thylakoid membrane after a short pulse labeling. Radiolabeled D1 intermediates could be immunoprecipitated under nondenaturing conditions with antisera raised against the D1 and D2 protein as well as CP47. However, when the ribosome pellets were solubilized with SDS, the interaction of these intermediates with CP47 was completely lost, but strong interaction of a 25-kDa D1 intermediate with the D2 protein still remained. Taken together, our results indicate that during the repair of photosystem II, the assembly of the newly synthesized D1 protein into photosystem II occurs co-translationally involving direct interaction of the nascent D1 chains with the D2 protein.

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.

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.

Transcriptional and translational adjustments of psbA gene expression in mature chloroplasts during photoinhibition and subsequent repair of photosystem II

The D1 reaction centre protein of photosystem II (PSII), encoded by the plastid psbA gene, has the highest turnover rate of all thylakoid proteins, due to light-induced damage to D1. The expression of the psbA gene was studied in chloroplasts of fully developed pea (Pisum sativum L.) leaves during high-light photoinhibitory treatment and subsequent restoration of PSII function at low light. psbA transcript levels were determined and the translational activity was followed by in vivo pulse-labelling, by in vitro translations with intact chloroplasts, and by run-off translations on isolated thylakoid membranes. PSII photochemical efficiency was determined in vivo by monitoring the ratio of variable fluorescence to maximal fluorescence (F(V)/F(M)). Enhanced D1 synthesis in pea leaves, upon a shift first from darkness to growth light and subsequently to high light, was accompanied by a substantial increase in the total number of pshA transcripts and by the accumulation of psbA mRNA x initiation complexes on thylakoid membrane. This suggested that high-light illumination increased the transcriptional activity of the psbA gene in mature leaves, and that enhanced translational initiation of psbA mRNA was followed by docking of the initiation complexes to the thylakoid membrane. The high-light-induced increase in the number of thylakoid-associated psbA mRNA x initiation complexes, occurred in parallel with enhanced in vivo D1 synthesis. This, however, did not result in an enhanced accumulation of D1 translation products in in vitro run-off translations when pea leaves were shifted from growth light to high light. This may suggest that at high light only a portion of thylakoid-associated psbA mRNA can be under translational elongation at a given moment. When the leaves were shifted from high light to low light to allow repair of PSII, thylakoid-associated psbA mRNA was rapidly released from the membrane and the high-light-induced pool of psbA transcripts was degraded. The synthesis of the D1 protein decreased on the same time scale. However, the restoration of PSII photochemical function, measured as F(V)/F(M), took a substantially longer time. It is concluded that during changing light conditions, mature leaves are able to adjust psbA gene expression both at the transcriptional and at the translational level.

Synthesis and assembly of the D1 protein into photosystem II: processing of the C-terminus and identification of the initial assembly partners and complexes during photosystem II repair

In previous studies [van Wijk, K. J., Bingsmark, S., Aro, E.-M., & Andersson, B. (1995) J. Biol. Chem. 270, 25685-25695; van Wijk, K. J., Andersson, B., & Aro, E.-M. (1996) J. Biol. Chem 271, 9627-9636], we have demonstrated that D1 protein synthesized in isolated chloroplasts and thylakoids is incorporated into the photosystem II (PSII) core complex. By pulse-chase experiments in these in vitro systems, followed by sucrose gradient fractionation of solubilized thylakoid membranes, it was shown that this assembly proceeded stepwise; first the D1 protein was incorporated to form a PSII reaction center complex (PSII rc), and through additional assembly steps the PSII core complex was formed. In this study, we have analyzed this assembly process in more detail, with special emphasis on the initial events, through further purification and analysis of the assembly intermediates by nondenaturing Deriphat-PAGE and by flatbed isoelectric focusing. The D2 protein was found to be the dominant PSII reaction center protein initially associating with the new D1 protein. This strongly suggests that the D2 protein is the primary "receptor" or stabilizing component during or directly after synthesis of the D1 protein. After formation of the D1-D2 heterodimer, cyt b559 became attached, whereas the psbI gene product was assembled as a subsequent step, thereby forming a PSII reaction center complex. Subsequent formation of the PSII core occurred by binding of CP47 and then CP43 to the PSII rc. The rapid radiolabeling of a minor population of a PSII core subcomplex without CP43 indicated that an association of newly synthesized D1 protein with a preexisting complex consisting of D2/cyt b55q/psbI gene product/CP47 was possibly occurring, in parallel to the predominant sequential assembly pathway. The kinetics of synthesis and processing of the precursor D1 protein were followed in isolated chloroplasts and were compared with its incorporation into PSII assembly intermediates. No precursor D1 protein was found in PSII core complexes, indicating either that incorporation into the PSII core complex facilitates the cleavage of the C-terminus or, more likely, that processing is more rapid than the assembly into the PSII core.