A highly active oxygen-evolving photosystem II (PSII) complex was purified from the HT-3 strain of the widely used cyanobacterium Synechocystis sp. PCC 6803, in which the CP47 polypeptide has been genetically engineered to contain a polyhistidine tag at its carboxyl terminus [Bricker, T. M., Morvant, J., Masri, N., Sutton, H. M., and Frankel, L. K. (1998) Biochim. Biophys. Acta 1409, 50-57]. These purified PSII centers had four manganese atoms, one calcium atom, and two cytochrome b(559) hemes each. Optical absorption and fluorescence emission spectroscopy as well as western immunoblot analysis demonstrated that the purified PSII preparation was devoid of any contamination with photosystem I and phycobiliproteins. A comprehensive proteomic analysis using a system designed to enhance resolution of low-molecular-weight polypeptides, followed by MALDI mass spectrometry and N-terminal amino acid sequencing, identified 31 distinct polypeptides in this PSII preparation. We propose a new nomenclature for the polypeptide components of PSII identified after PsbZ, which proceeds sequentially from Psb27. During this study, the polypeptides PsbJ, PsbM, PsbX, PsbY, PsbZ, Psb27, and Psb28 proteins were detected for the first time in a purified PSII complex from Synechocystis 6803. Five novel polypeptides were also identified in this preparation. They included the Sll1638 protein, which shares significant sequence similarity to PsbQ, a peripheral protein of PSII that was previously thought to be present only in chloroplasts. This work describes newly identified proteins in a highly purified cyanobacterial PSII preparation that is being widely used to investigate the structure, function, and biogenesis of this photosystem.

Regulated thermal dissipation of absorbed light energy within the photosystem II antenna system helps protect photosystem II from damage in excess light. This reversible photoprotective process decreases the maximum quantum yield of photosystem II (Fv)/Fm) and CO2 assimilation (phiCO2), and decreases the convexity of the non-rectangular hyperbola describing the response of leaf CO2 assimilation to photon flux (theta). At high light, a decrease in phiCO2 has minimal impact on carbon gain, while high thermal energy dissipation protects PSII against oxidative damage. Light in leaf canopies in the field is continually fluctuating and a finite period of time is required for recovery of phiCO2 and when light drops below excess levels. Low phiCO2) and can limit the rate of photosynthetic carbon assimilation on transfer to low light, an effect prolonged by low temperature. What is the cost of this delayed reversal of thermal energy dissipation and phiCO2 recovery to potential CO2 uptake by a canopy in the field? To address this question a reverse ray-tracing algorithm for predicting the light dynamics of 120 randomly selected individual points in a model canopy was used to describe the discontinuity and heterogeneity of light flux within the canopy. Because photoprotection is at the level of the cell, not the leaf, light was simulated for small points of 10(4) micro m rather than as an average for a leaf. The predicted light dynamics were combined with empirical equations simulating the dynamics of the light-dependent decrease and recovery of phiCO2 and and their effects on the integrated daily canopy carbon uptake (A'c). The simulation was for a model canopy of leaf area index 3 with random inclination and orientation of foliage, on a clear sky day (latitude 44 degrees N, 120th day of the year). The delay in recovery of photoprotection was predicted to decrease A'c by 17% at 30 degrees C and 32% at 10 degrees C for a chilling-susceptible species, and by 12.8% at 30 degrees C and 24% at 10 degrees C for a chilling-tolerant species. These predictions suggest that the selection, or engineering, of genotypes capable of more rapid recovery from the photoprotected state would substantially increase carbon uptake by crop canopies in the field.

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The flash-induced electrochromic shift, measured by the amplitude of the rapid absorbance increase at 518 nanometers (AA518), was used to determine the amount of charge separation within photosystems 11 and I in spinach (Spinacia oleracea L.) leaves. The recovery time of the reaction centers was determined by comparing the amplitudes of AA518 induced by two flashes separated by a variable time interval. The recovery of the AA518 on the second flash revealed that 20% of the reaction centers exhibited a recovery half-time of 1.7 ± 0.3 seconds, which is 1000 times slower than normally active reaction centers. Measurements using isolated thylakoid membranes showed that photosystem I constituted 38% of the total number of reaction centers, and that the photosystem I reaction centers were nearly fully active, indicating that the slowly tuming over reaction centers were due solely to photosystem I. The results demonstrate that in spinach leaves approximately 32% of the photosystem 11 complexes are effectively inactive, in that their contribution to energy conversion is negligible. Additional evidence for inactive photosystem 11 complexes in spinach leaves was provided by fluorescence induction measurements, used to monitor the oxidation kinetics of the primary quinone acceptor of photosystem 11, QA, after a short flash. The measurements showed that in a fraction of the photosystem 11 complexes the oxidation of QA was slow, displaying a half-time of 1.5 ± 0.3 seconds. The kinetics of QOA oxidation were virtually identical to the kinetics of the recovery of photosystem 11 determined from the electrochromic shift. The key difference between active and inactive photosystem 11 centers is that in the inactive centers the oxidation rate of QOA is slow compared to active centers. Measurements of the electrochromic shift in detached leaves from several different species of plants revealed a significant fraction of slowly tuming over reaction centers, raising the possibility that reaction centers that are inefficient in energy conversion may be a common feature in plants.In normally functioning PSII complexes, bound plastoquinone is reduced by electrons from water, and subsequently released into the thylakoid membrane (reviewed in Crofts and Wraight [4] two bound plastoquinone molecules, QA2 and QB, operating in series. QA acts as a single electron carrier and is permanently bound in PSII. The plastoquinone molecule at the QB site differs from QA in that it becomes fully reduced to plastoquinol after two turnovers of the reaction center, and it exchanges rapidly with the freely mobile plastoquinone in the membrane. In reaction centers in which QA and QB are initially oxidized, the first light reaction drives an electron from P680, the primary donor of PSII, to pheophytin, which in turn reduces QA. The electron is then transferred from QA to QB, enabling the reaction center to turn over a second time. In the second light reaction an electron is transferred over the same path to QB-, and together with two protons results i...