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The recent success in the crystallization and crystal structure analysis of the Photosystem II (PS II) core complex from thermophilic cyanobacteria has contributed greatly to our understanding of the structure and function of PS II. We describe here the crystallization and crystal structure of the PS II core from , with comparisons to the structures from and the photosynthetic bacterial reaction center. The crystal structure of was analyzed at a 3.7 Å resolution and the structural model was built using sequences from the large subunits of PS II, while the structures of other subunits were represented either as polyalanines or alpha carbons (Cα). The crystal structure reveals a number of possible molecular interactions among different subunits on the lumenal side. The arrangement of the chlorophylls and other cofactors are also shown, including two (β-carotene molecules not previously identified. The location of these β-carotenes has provided new insights into a possible secondary electron transport pathway involving a peripheral chlorophyll molecule bound to the D2 subunit and cytochrome . In addition, the structure and environment surrounding the manganese cluster are described. In particular, the carboxyl-terminus of D1 polypeptide is shown to be connected to the manganese-cluster directly. The structural information obtained provides important insights into the mechanisms of the reactions taking place in PS II.

We review the details and significance of the 3.5 Å refined X-ray structure of Photosystem II isolated from (Ferreira NK, Iverson TM, Maghlaoui K, Barber J and Iwata S (2004) Science 303, 1831–1838) in the context of other studies. The complex contains 19 subunits and all but one have had their structures determined. Consequently the details of the protein environment of the 57 cofactors involved in light interception, energy transfer and charge separation have been revealed for the first time. Of particular importance are the details of the metal cluster of the oxygen evolving center (OEC). The X-ray analysis has shown the OEC to be composed of a cubane-like MnCaO cluster linked to a fourth Mn ion by one of the oxygens of the cubane. Six amino acid ligands have been identified for the four Mn ions and a number of other residues have been located close to the catalytic center. The organization of the OEC and its protein ligands suggest that only one Mn ion is involved directly in the binding of a substrate water molecule. This conclusion is compatible with a mechanism for O-O bond formation whereby a highly electrophilic Mn(V)=O or Mn(IV)-oxyl radical undergoes a nucleophilic reaction with the oxygen of a nearby water molecule located within the coordination sphere of Ca.

This chapter highlights some of the important, unresolved questions regarding the mechanism of energy transfer and trapping in Photosystem II (PS II), in particular, whether energy transfer is rate limiting and whether the primary acceptor pheophytin or the first bound plastoquinone Q traps the excitation energy in the reaction center. With these questions in mind, we review some of the results from spectroscopic studies made on isolated antenna, core, and reaction center complexes and discuss a number of models found in the literature that have been used to describe energy transfer and trapping. Although the rate-limiting step in PS II in vivo remains unknown, it is likely that the slow stabilization of the charge separation and consequent inefficiencies are related to the regulation of charge separation.

Photoprotection remains one of the most challenging and complex areas for research in photosynthesis. Coping with a wide range of adverse environmental conditions, especially full sunlight, is central to plant survival in nature, and understanding mechanisms of light acclimation is increasingly important in crop improvement. Excess light and other environmental stresses can result in prolonged lifetimes of excited state chlorophylls and enhanced triplet state yields that, left unchecked, markedly increase the rate of formation of reactive oxygen species such as HO, singlet oxygen (O*) and superoxide (O). Reactive oxygen species cause photo-oxidative damage such as bleaching and peroxidation to the photosystems and the whole plant. Carotenoids afford protection against photo-oxidative damage in complementary ways by stabilizing the pigment-protein complexes and quenching of excited state chlorophylls and reactive oxygen species.

The light-induced oxidation of water by Photosystem II (PS II) of higher plants, algae and cyanobacteria is the main source of atmospheric oxygen. The discovery of the flash-induced period-four oscillations in the O evolution [Joliot P, Barbieri G and Chabaud R (1969) Photochem Photobiol 10: 309–329] has had a lasting impact on current photosynthesis research. Such oscillations were explained by introducing the of flash-induced transitions of states of an oxygen evolving complex [Kok B, Forbush B and McGloin M (1970) Photochem Photobiol 11: 467–475]. In order to describe dampening of the oscillations in the O evolution the Kok model introduces , which characterize the failure to advance the S-states, and , which characterize the two-step advancement of S-states. While the Kok model has been successfully used for over 30 years for interpretation of experimental data in photosynthesis, until recently there was no simple analytical solution for it. A much overdue analytical solution is presented here. Correlation of S-states transitions at the donor side of the PS II and Q transitions at the acceptor side leads to the recognition of two different reaction sequence cycles of PS II, so called cycles and [Shinkarev VP and Wraight CA (1993a) Proc Natl Acad Sci USA 90:1834–1838]. In each of these cycles the quantitative description of binary oscillations of the Q semiquinone can be obtained from the analytical solution for individual S-states. Standard application of the Kok model consists in finding misses and double hits from measured sequence of flash-induced O evolution. In alternative approach known kinetic and thermodynamic data are used to reconstruct period-four oscillations of O evolution. This general kinetic model allows calculation of all transition probabilities in the Kok model from first principles. The model predicts that misses in the and cycles are different. The general character of the model allows simultaneous consideration of different flash-induced oscillation phenomena at the donor and acceptor sides of PS II, without limitations on the number of states needed to be considered.

This chapter deals with the mechanism of photosynthetic water oxidation that leads to O formation in Photosystem II (PS II). After a brief introduction to the structure and function of the PS II complex, the S-state cycle (Kok model) is outlined and the structure and oxidation states of the catalytic MnOCa complex are summarized. We then cover in detail the current information concerning substrate water binding and consider energetic and kinetic aspects of photosynthetic water oxidation. On that basis, we discuss several recent mechanistic proposals for O-O bond formation in PS II and summarize our current perceptions in a novel mechanistic proposal for photosynthetic water oxidation.

The biogenesis of the Photosystem II (PS II) water oxidizing complex is reviewed with emphasis on the sequence and location of assembly of the subunits within the internal membrane systems of prokaryotes and chloroplasts. The uptake and distribution of manganese in cyanobacterial cells are discussed. The role of individual subunits in isolated PS II in light-induced assembly of the inorganic core of the water splitting complex is discussed (photoactivation process). The roles of the inorganic cofactors (Mn, Ca, Cl and bicarbonate) in water splitting are revealed by examining the effects of their reconstitution with non-native cofactors. Novel instrumentation for the measurement of O concentration and the kinetics and mechanism of photoactivation are described.

Photoinactivation of Photosystem II (PS II) is unavoidable in oxygenic photosynthesis, but the photodamage is counteracted by an elaborate repair process without which the photosynthetic apparatus would soon perish. Photoinactivation of PS II depends on the light dosage; the quantum yield of photoinactivation is such that practically the entire population of PS II would be photoinactivated during the course of a sunny day if repair were inhibited. An agent predominantly and inadvertently responsible for the photoinactivation of PS II is likely to be P680, the strongest oxidant in photosynthesis needed for the oxidation of water molecules. Amelioration of photoinactivation of PS II occurs via many strategies, including one mechanism that appears to sustain activity in a small population of PS II during prolonged high-light stress. The majority of PS II complexes, however, have to be repaired following their photoinactivation. The repair process, culminating in the biosynthesis and insertion of a new copy of the D1 protein in a re-assembled PS II reaction center, consists of many steps. These include: (1) ‘triggering’ of the D1 protein, leading to monomerization and partial disassembly of the PS II dimer complex in granal appressed membranes in higher-plant chloroplasts; (2) migration of the PS II core monomer to stroma-exposed thylakoids; (3) D1 proteolysis catalyzed, for example, first by the proteases DegP2 followed by FtsHl, or directly by FtsH2; (4) targeting of the ribosome/nascent D1 chain complex to the thylakoid membrane; (5) translation elongation of D1 and insertion of new D1 into a D1-depleted PS II; (6) ligation of cofactors in the PS II reaction center; (7) termination of translation and carboxyl-terminal processing of D1; (8) post-translational assembly of PS II monomers; and (9) migration of re-assembled PS II monomers to granal membranes, where functional PS II dimers are formed. In this way, the functionality of PS II is maintained despite the inevitability of photoinactivation.

The plastid gene expression system employs a highly developed transcription and translation system that is an elaborated prokaryotic and eukaryotic chimera. Photosystem II (PS II) gene expression is regulated during transcription, post-transcriptionally, and translationally, and responds to environmental changes. Many regulatory factors and elements necessary for PS II biosynthesis and repair have been identified and characterized in the last decade. This chapter reviews recent progress on understanding PS II gene expression, introducing mechanisms common to algae and higher plants, as well as differences between these organisms. Transcriptional regulation is critical for chloroplast biogenesis and PS II accumulation. Unlike algae land plants have developed complex transcriptional regulatory systems for plastid differentiation, e.g., chloroplasts in leaves and amyloplasts in roots. Post-transcriptional RNA processing of primary transcripts is also an important step in the control of plastid gene expression, and is required for translation initiation of many plastid mRNAs. A set of chloroplast RNA-binding proteins and nucleases are involved in this process. Translational regulation is a key step for the rapid response to dark/light changes, and for PS II repair associated with photo-oxidation, in both algae and higher plants. Light-activated translation of several PS II encoding mRNAs requires -acting RNA elements, found in the 5′-untranslated region of the mRNA, as well as nuclear-encoded -acting protein factors. Genetic and biochemical analysis has identified a number of the components and mechanisms involved in regulating expression of PS II proteins.

The biogenesis, repair and homeostasis of Photosystem II (PS II) requires both protein targeting and post-translational modification. The various chloroplast targeting pathways for both nuclear-encoded and chloroplast-encoded proteins are described, with emphasis on the subunits of PS II. Also discussed are three post-translational processing events required for the development of PS II activity—removal of the stromal-targeting transit peptides from nuclear-encoded proteins by the stromal processing protease, removal of the thylakoid lumentargeting signal peptides by the thylakoid processing protease, and trimming of the carboxyl-terminus of the precursor D1 protein by the protease CtpA. Where appropriate, similarities and differences in PS II biogenesis and homeostasis between cyanobacteria and green plants are discussed.