The primary targets of thermal damage in plants are the oxygen evolving complex along with the associated cofactors in photosystem II (PSII), carbon fixation by Rubisco and the ATP generating system. Recent investigations on the combined action of moderate light intensity and heat stress suggest that moderately high temperatures do not cause serious PSII damage but inhibit the repair of PSII. The latter largely involves de novo synthesis of proteins, particularly the D1 protein of the photosynthetic machinery that is damaged due to generation of reactive oxygen species (ROS), resulting in the reduction of carbon fixation and oxygen evolution, as well as disruption of the linear electron flow. The attack of ROS during moderate heat stress principally affects the repair system of PSII, but not directly the PSII reaction center (RC). Heat stress additionally induces cleavage and aggregation of RC proteins; the mechanisms of such processes are as yet unclear. On the other hand, membrane linked sensors seem to trigger the accumulation of compatible solutes like glycinebetaine in the neighborhood of PSII membranes. They also induce the expression of stress proteins that alleviate the ROS-mediated inhibition of repair of the stress damaged photosynthetic machinery and are required for the acclimation process. In this review we summarize the recent progress in the studies of molecular mechanisms involved during moderate heat stress on the photosynthetic machinery, especially in PSII.

The evolution of O2-producing cyanobacteria that use water as terminal reductant transformed Earth's atmosphere to one suitable for the evolution of aerobic metabolism and complex life. The innovation of water oxidation freed photosynthesis to invade new environments and visibly changed the face of the Earth. We offer a new hypothesis for how this process evolved, which identifies two critical roles for carbon dioxide in the Archean period. First, we present a thermodynamic analysis showing that bicarbonate (formed by dissolution of CO 2) is a more efficient alternative substrate than water for O 2 production by oxygenic phototrophs. This analysis clarifies the origin of the long debated ''bicarbonate effect'' on photosynthetic O 2 production. We propose that bicarbonate was the thermodynamically preferred reductant before water in the evolution of oxygenic photosynthesis. Second, we have examined the speciation of manganese(II) and bicarbonate in water, and find that they form Mnbicarbonate clusters as the major species under conditions that model the chemistry of the Archean sea. These clusters have been found to be highly efficient precursors for the assembly of the tetramanganese-oxide core of the water-oxidizing enzyme during biogenesis. We show that these clusters can be oxidized at electrochemical potentials that are accessible to anoxygenic phototrophs and thus the most likely building blocks for assembly of the first O 2 evolving photoreaction center, most likely originating from green nonsulfur bacteria before the evolution of cyanobacteria.bicarbonate ͉ carbon dioxide ͉ cyanobacteria ͉ evolution ͉ manganese O xygen (O 2 ) production by photosynthesis is by far the dominant global process that replenishes atmospheric and oceanic oxygen essential to sustain all aerobic life. Geochemical records of terrestrial oxides indicate that O 2 evolution must have taken place in the precursors to cyanobacteria before ca. 2.8 billion years ago and led to the accumulation of O 2 in the atmosphere (1, 2). The creation of a photosynthetic apparatus capable of splitting water into O 2 , protons, and electrons was the pivotal innovation in the evolution of life on Earth. For the first time photosynthesis had an unlimited source of electrons and protons by using water as reductant. By freeing photosynthesis from the availability of reduced chemical substances, the global production of organic carbon could be enormously increased and opened new environments for photosynthesis to occur. This event literally changed the face of the Earth. The accumulation of O 2 in the atmosphere led to the biological innovation of aerobic respiration, which harnesses a more powerful metabolic energy source. Because aerobic metabolism generates 18 times more energy (ATP) per metabolic input (hexose sugar) than does anaerobic metabolism, the engine of life became supercharged. This sequence of evolutionary steps enabled the emergence of complex, multicellular, energy-efficient, eukaryotic organisms.Comparisons of cyanobacteria, green algae, ...

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Structural properties of the isolated extrinsic regulatory 33 kDa protein of the water-oxidizing complex were analyzed at different pH values. It was found that (a) titrations of the buffer capacity reveal a characteristic hysteresis effect that is unique for the 33 kDa subunit and is not observed for the other extrinsic proteins, (b) changes of the emission from the fluorescence probe 1,8-ANS are indicative of an increased accessibility of the hydrophobic core of the 33 kDa protein to the dye at lower pH, (c) the near-UV circular dichroism spectrum of the polypeptide is altered owing to a pH decrease from 6.8 to 3.8 and becomes drastically changed at pH 2.8, and (d) the content of secondary structure elements remains virtually constant in the range 3.8 < pH < 6.8, with the following values gathered from far-UV CD spectra: approximately 8% alpha-helix, approximately 33% beta-strand, approximately 15% turns, and approximately 44% random coil. Further acidification down to pH 2.8 gives rise to a decreased alpha-helix and increased beta-strand and random coil content. A theoretical model [Ptitsyn, O., & Finkelstein, A. (1983) Biopolymers 2, 15-22] was used to predict the probability and location of secondary structure elements within the protein sequence. On the basis of these calculations, an extended hydrophobic beta-sheet domain could exist in the center of the protein and an alpha-helix in the C-terminal region. From these data, the 33 kDa protein is inferred to change its tertiary structure in vitro upon acidification of the aqueous environment. Possible implications of these features are discussed.

Assembly of the inorganic core (Mn(4)O(x)Ca(1)Cl(y)) of the water oxidizing enzyme of oxygenic photosynthesis generates O(2) evolution capacity via the photodriven binding and photooxidation of the free inorganic cofactors within the cofactor-depleted enzyme (apo-WOC-PSII) by a process called photoactivation. Using in vitro photoactivation of spinach PSII membranes, we identify a new lower affinity site for bicarbonate interaction in the WOC. Bicarbonate addition causes a 300% stimulation of the rate and a 50% increase in yield of photoassembled PSII centers when using Mn(2+) and Ca(2+) concentrations that are 10-50-fold larger range than previously examined. Maintenance of a fixed Mn(2+)/Ca(2+) ratio (1:500) produces the fastest rates and highest yields of photoactivation, which has implications for intracellular cofactor homeostasis. A two-step (biexponential) model is shown to accurately fit the assembly kinetics over a 200-fold range of Mn(2+) concentrations. The first step, the binding and photooxidation of Mn(2+) to Mn(3+), is specifically stimulated via formation of a ternary complex between Mn(2+), bicarbonate, and apo-WOC-PSII, having a proposed stoichiometry of [Mn(2+)(HCO(3)(-))]. This low-affinity bicarbonate complex is thermodynamically easier to oxidize than the aqua precursor, [Mn(2+)(OH(2))]. The photooxidized intermediate, [Mn(3+)(HCO(3)(-))], is longer lived and increases the photoactivation yield by suppressing irreversible photodamage to the cofactor-free apo-WOC-PSII (photoinhibition). Bicarbonate does not affect the second (rate-limiting) dark step of photoactivation, attributed to a protein conformational change. Together with the previously characterized high-affinity site, these results reveal that bicarbonate is a multifunctional "native" cofactor important for photoactivation and photoprotection of the WOC-PSII complex.

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We show for the first time that Cah3, a carbonic anhydrase associated with the photosystem II (PSII) donor side in Chlamydomonas reinhardtii, regulates the water oxidation reaction. The mutant cia3, lacking Cah3 activity, has an impaired water splitting capacity, as shown for intact cells, thylakoids and PSII particles. To compensate this impairment, the mutant overproduces PSII reaction centres (1.6 times more than wild type). We present compelling evidence that the mutant has an average of two manganese atoms per PSII reaction centre. When bicarbonate is added to mutant thylakoids or PSII particles, the O2 evolution rates exceed those of the wild type by up to 50%. The donor side of PSII in the mutant also exhibits a much higher sensitivity to overexcitation than that of the wild type. We therefore conclude that Cah3 activity is necessary to stabilize the manganese cluster and maintain the water-oxidizing complex in a functionally active state. The possibility that two manganese atoms are enough for water oxidation if bicarbonate ions are available is discussed.

Photoreduction of the intermediary electron acceptor, pheophytin (Pheo), in photosystem II reaction centers of spinach chloroplasts or subchloroplast particles (TSF-II and TSF-IIa) at 220 K and redox potential Eh = -450 mV produces an EPR doublet centered at g = 2.00 with a splitting of 52 G at 7 K in addition to a narrow signal attributed to Pheo (g = 2.0033, AH -13 G). The doublet is eliminated after extraction of lyophilized TSF-II with hexane containing 0. Phototrapping of Pheo-in PS II at 295 K is accompanied by the appearance of an EPR signal with g k 2.0035 and AH 13 G (1 G = 10-4 tesla) (6, 7), similar to that of the monomeric The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §17734 solely to indicate this fact. 7227anion radical of Pheo in vitro (18). After phototrapping of Pheo at 220 K, however, an EPR "doublet" centered at g = 2.00 with a splitting of m52 G is also observed (7). In analogy with a similar observation in bacterial reaction centers (19)(20)(21)(22), it was suggested that the doublet is probably the result of interaction of Pheo' with Q, which includes Fe or some other transition metal (7). Here we report data supporting the idea that a PQ-Fe complex acts as the stable "primary" electron acceptor in PS II reaction centers and that an exchange interaction of its singly reduced form with Pheo-accounts for the split EPR signal. MATERIALS AND METHODSSpinach chloroplasts and subchloroplast particles (Tritonfractionated subchloroplast fragments, TSF-II and TSF-IIa), highly enriched in PS II reaction center components and free of P700, were isolated as described (23- Iron was extracted by incubating PS II preparations (Chl at 50 mg/ml) in 10 mM Tris-HCI (pH 8.0) containing 0.3-1 M LiCl04 with or without 2.5 mM o-phenanthroline, for 2 hr at 20C, followed by centrifugation (200,000 X g for 40 min) and dialysis (12 hr at 2°C) of the pellet against 10 mM Tris-HCl (pH 8.0)/50 mM NaCI/10 AM EDTA (David Knaff, personal communication; also ref. 27). In the reconstitution experiments the extracted material was additionally dialyzed for 12 hr at 20C under anaerobic conditions against 10 mM acetate buffer (pH 5.5) containing 0.2 M LiCl04 and 1 MM EDTA, with or without 0.2 mM Fe(NH4)2(SO4)2, MnSO4, or MgSO4.Nonheme iron content was determined by the bathophenanthroline method (28). Changes in absorbance and fluorescence yield, induced by actinic light (intensity t0.05 Jcm2-s-l) from a 1000-W incandescent lamp filtered by 5 cm of CuSO4 solution, were measured in a phosphoroscopic pho-