Functional differences of photosystem II from Synechococcus elongatus and spinach characterized by flash induced oxygen evolution patterns

Detailed comparative studies of flash induced oxygen evolution patterns in thylakoids from the thermophilic cyanobacterium Synechococcus elongatus (S. elongatus; also referred to as Thermosynechococcus elongatus) and from spinach led to the following results: (i) the miss parameter alpha of S. elongatus thylakoids exhibits a pronounced temperature dependence with a minimum of 7% at 25 degrees C and values of 17 and 10% at 3 and 35 degrees C, respectively, while for spinach thylakoids alpha decreases continuously from 18% at 35 degrees C down to 8% at 3 degrees C; (ii) at all temperatures, the double hit probability beta exceeds in S. elongatus the corresponding values of spinach by an increment Delta beta of about 3%; (iii) at 20 degrees C the slow relaxation of the oxidation states S(2) and S(3) is about 15 and 30 times, respectively, slower in S. elongatus than in spinach, while the reduction of these S states by tyrosine Y(D) is 2-3 times faster; (iv) the reaction S(0)Y(D)(ox) --> S(1)Y(D) is slower by a factor of 4 in S. elongatus as compared to spinach; and (v) the activation energies of S state dark relaxations in S. elongatus are all within a factor of 1.5 as compared to the previously reported values from spinach thylakoids [Vass, I., Deak, Z., and Hideg, E. (1990) Biochim. Biophys. Acta 1017, 63-69; Messinger, J., Schr?der, W. P., and Renger, G. (1993) Biochemistry 32, 7658-7668], but the difference between the activation energies of the slow S(2) and S(3) decays is significantly larger in S. elongatus than in spinach. These results are discussed in terms of differences between cyanobacteria and higher plants on the acceptor side of PSII and a shift of the redox potential of the couple Y(D)/Y(D)(ox). The obtained data are also suitable to address questions about effects of the redox state of Y(D) on the miss probability and the possibility of an S state dependent miss parameter.     

Stability of the S₃and S₂state intermediates in photosystem II directly probed by EPR spectroscopy

The stability of the S(3) and S(2) states of the oxygen evolving complex in photosystem II (PSII) was directly probed by EPR spectroscopy in PSII membrane preparations from spinach in the presence of the exogenous electron acceptor PpBQ at 1, 10, and 20 °C. The decay of the S(3) state was followed in samples exposed to two flashes by measuring the split S(3) EPR signal induced by near-infrared illumination at 5 K. The decay of the S(2) state was followed in samples exposed to one flash by measuring the S(2) state multiline EPR signal. During the decay of the S(3) state, the S(2) state multiline EPR signal first increased and then decreased in amplitude. This shows that the decay of the S(3) state to the S(1) state occurs via the S(2) state. The decay of the S(3) state was biexponential with a fast kinetic phase with a few seconds decay half-time. This occurred in 10-20% of the PSII centers. The slow kinetic phase ranged from a decay half-time of 700 s (at 1 °C) to ~100 s (at 20 °C) in the remaining 80-90% of the centers. The decay of the S(2) state was also biphasic and showed quite similar kinetics to the decay of the S(3) state. Our experiments show that the auxiliary electron donor Y(D) was oxidized during the entire experiment. Thus, the reduced form of Y(D) does not participate to the fast decay of the S(2) and S(3) states we describe here. Instead, we suggest that the decay of the S(3) and S(2) states reflects electron transfer from the acceptor side of PSII to the donor side of PSII starting in the corresponding S state. It is proposed that this exists in equilibrium with Y(Z) according to S(3)Y(Z) ? S(2)Y(Z)(?) in the case of the S(3) state decay and S(2)Y(Z) ? S(1)Y(Z)(?) in the case of the S(2) state decay. Two kinetic models are discussed, both developed with the assumption that the slow decay of the S(3) and S(2) states occurs in PSII centers where Y(Z) is also a fast donor to P(680)(+) working in the nanosecond time regime and that the fast decay of the S(3) and S(2) states occurs in centers where Y(Z) reduces P(680)(+) with slower microsecond kinetics. Our measurements also demonstrate that the split S(3) EPR signal can be used as a direct probe to the S(3) state and that it can provide important information about the redox properties of the S(3) state.     

Effects of methanol on the S i -state transitions in photosynthetic water-splitting

From a chemical point of view methanol is one of the closest analogues of water. Consistent with this idea EPR spectroscopy studies have shown that methanol binds at-or at least very close to-the Mn(4)O(x)Ca cluster of photosystem II (PSII). In contrast, Clark-type oxygen rate measurements demonstrate that the O(2) evolving activity of PSII is surprisingly unaffected by methanol concentrations of up to 10%. Here we study for the first time in detail the effect of methanol on photosynthetic water-splitting by employing a Joliot-type bare platinum electrode. We demonstrate a linear dependence of the miss parameter for S( i ) state advancement on the methanol concentrations in the range of 0-10% (v/v). This finding is consistent with the idea that methanol binds in PSII with similar affinity as water to one or both substrate binding sites at the Mn(4)O(x)Ca cluster. The possibility is discussed that the two substrate water molecules bind at different stages of the cycle, one during the S(4) --> S(0) and the other during the S(2) --> S(3) transition.     

Identification of thylakoid membrane thermal transitions in Synechocystis sp. PCC6803 photosynthetic mutants

We used differential scanning calorimetry (DSC) as a technique capable of identifying photosynthetic complexes on the basis of their calorimetric transitions. Annotation of thermal transitions was carried out with thylakoid membranes isolated from various photosynthetic mutants of Synechocystis sp. PCC6803. The thylakoid membranes exhibited seven major DSC bands between 40 and 85°C. The heat sorption curves were analyzed both by mathematical deconvolution of the overall endotherms and by a subsequent annealing procedure. The successive annealing procedure proved to be more reliable technique than mathematical deconvolution in assigning thermal transitions. The main DSC band, around 47°C, resulting from the high enthalpy change that corresponds to non-interacting complex of PSII, was assigned using the PSI-less/apcE(-) mutant cells. Another band around 68-70°C relates to the denaturation of PSII surrounded by other proteins of the photosynthetic complexes in wild type and PSI-less/apcE(-) cells. A further major transition found at 82-84°C corresponds to the PSI core complex of wild type and PSII-deficient BE cells. Other transition bands between 50-67 and 65-75°C are believed to relate to ATP synthase and cytochrome b(6)f, respectively. These thermal transitions were obtained with thylakoids isolated from PSI(-)/PSII(-) mutant cells. Some minor bands determined at 59 and 83-84°C correspond to an unknown complex and NADH dehydrogenase, respectively. These annotations were done by PSI-less/apcE(-) and PSI(-)/PSII(-) mutants.     

pH dependence of the flash-induced S-state transitions in the oxygen-evolving center of photosystem II from Thermosynechoccocus elongatus as revealed by Fourier transform infrared spectroscopy

pH dependence of the efficiencies of the flash-induced S-state transitions in the oxygen-evolving center (OEC) was studied by means of Fourier transform infrared (FTIR) difference spectroscopy using photosystem II (PSII) core complexes from the thermophilic cyanobacterium Thermosynechoccocus elongatus. The PSII core complexes dark-adapted at different pHs in the presence of ferricyanide as an electron acceptor were excited by four consecutive saturating laser flashes, and FTIR difference spectra induced by each flash were recorded in the region of 1800-1200 cm(-1). Each difference spectrum was fitted with a linear combination of standard spectra measured at pH 6.0, which represent the spectra upon individual S-state transitions, and the transition efficiencies were estimated from the fitting parameters. It was found that the S1 --> S2 transition probability is independent of pH throughout the pH region of 3.5-9.5, while the S2 --> S3, S3 --> S0, and S0 --> S1 transition probabilities decrease at acidic pH with pK values of 3.6 +/- 0.2, 4.2 +/- 0.3, and 4.7 +/- 0.5, respectively. These findings, i.e., the pH-independent S1 --> S2 transition probability and the pK values for the inhibition in the acidic range of the other three transitions, were in good agreement with recent results obtained by electron paramagnetic resonance measurements for PSII-enriched membranes of spinach [Bernát, G., Morvaridi, F., Feyziyev, Y., and Styring, S. (2002) Biochemistry 41, 5830-5843]. On the basis of this correspondence for quite different types of PSII preparations exhibiting marked difference in the pH dependence of the apparent proton release pattern, it is concluded that the inhibition of the S2 --> S3, S3 --> S0, and S0 --> S1 transitions in the acidic region is an inherent property of the OEC. This feature probably reflects proton release from substrate water in these three transitions. On the other hand, all of the S-state transitions remained generally efficient up to pH 9.5 in the alkaline region, except for a slight decrease of the S3 --> S0 transition probability above pH 8 (pK approximately 10). This observation partly differs from the tendency reported for spinach preparations, suggesting that a mechanism different from that in the acidic region is responsible for the transition efficiencies in the alkaline region.     

Highly purified thermo-stable oxygen-evolving photosystem II core complex from the thermophilic cyanobacterium Synechococcus elongatus having His-tagged CP43

The carboxyl terminus of the CP43 subunit of photosystem II (PSII) in the thermophilic cyanobacterium, Synechococcus elongatus, was genetically tagged with six consecutive histidine residues to create a metal binding site on the PSII supramolecular complex. The histidine-tagging enabled rapid isolation of an intact cyanobacterial PSII core complex from dodecyl maltoside-solubilized thylakoids by a simple one-step Ni(2+)-affinity column chromatography. The isolated core complex was in a dimeric form with a molecular mass of about 580 kDa, consisting of five major intrinsic membrane proteins (CP47, CP43, D1, D2 and cytochrome b-559), three extrinsic proteins (33 kDa, 12 kDa, and cytochrome c-550), and a few low molecular mass membrane proteins, and evolved oxygen at a rate as high as 3,400 mumol (mg Chl)-1 h-1 at 45 degrees C with ferricyanide as an electron acceptor. The core complex emitted thermoluminescence B2-, B1- and Q-bands arising from S2QB-, S3QB- and S2QA- charge recombinations at respective emission temperatures of 45, 38 and 20 degrees C, all of which were higher by about 15 degrees C as compared with those in mesophilic spinach BBY membranes. These results indicated that the isolated core complex well retained the intact properties of thermoluminescence of thermophilic cyanobacterial cells, the deeper stabilization of PSII charge pairs. The isolated complex was extremely stable in terms of both protein composition and function, exhibiting no release of extrinsic proteins, no proteolytic degradation in any of its subunits, accompanied by only a slight (less than 10%) loss in oxygen evolution, after dark-incubation at 20 degrees C for 8 d. These properties of the thermophilic PSII core complex are highly useful for various types of studies on PSII.     

Efficiency and role of loss processes in light-driven water oxidation by PSII

Its superior quantum efficiency renders PSII a model for biomimetic systems. However, also in biological water oxidation by PSII, the efficiency is restricted by recombination losses. By laser-flash illumination, the secondary radical pair, P680(+)Q(-) (A) (where P680 is the primary Chl donor in PSII and Q(A), primary quinone acceptor of PSII), was formed in close to 100% of the PSII. Investigation of the quantum efficiency (or yield) of the subsequent steps by time-resolved delayed (10 micros to 60 ms) and prompt (70 micros to 700 ms) Chl fluorescence measurements on PSII membrane particles suggests that (1) the effective rate for P680(+) Q(-) (A) recombination is approximately 5 ms(-1) with an activation energy of approximately 0.34 eV, circumstantially confirming dominating losses by reformation of the primary radical pair followed by ground-state recombination. (2) Because of compensatory influences on recombination and forward reactions, the efficiency is only weakly temperature dependent. (3) Recombination losses are several-fold enhanced at lower pH. (4) Calculation based on delayed-fluorescence data suggests that the losses depend on the state of the water-oxidizing manganese complex, being low in the S(0)-->S(1) and S(1)-->S(2) transition, clearly higher in S(2)-->S(3) and S(3)-->S(4)-->S(0). (5) For the used artificial electron acceptor, the efficiency is limited by acceptor-side processes/S-state decay at high/low photon-absorption rates resulting in optimal efficiency at surprisingly low rates of approximately 0.15-15 photons s(-1) (per PSII). The pH and S-state dependence can be rationalized by the basic model of alternate electron-proton removal proposed elsewhere. A physiological function of the recombination losses could be limitation of the lifetime of the reactive donor-side tyrosine radical (Y(.) (Z)) in the case of low-pH blockage of water oxidation.     

Mutation of arginine 357 of the CP43 protein of photosystem II severely impairs the catalytic S-state cycle of the H2O oxidation complex

Basic amino acid side chains situated in active sites may mediate critical proton transfers during an enzymatic catalytic cycle. In the case of photosynthetic water oxidation, a strong base is postulated to facilitate the deprotonation of the active site Mn4-Ca cluster, thereby allowing the otherwise thermodynamically constrained transfer of an electron away from the Mn4-Ca cluster to the oxidized redox active tyrosine radical, YZ*, generated by photosynthetic charge separation. Arginine 357 of the CP43 polypeptide may be located in the second coordination shell of the O2-evolving Mn4-Ca cluster of photosystem II (PSII) according to current structural models. An ostensibly conservative substitution mutation, CP43-357K, was investigated using polarographic and fluorescence techniques in evaluating its potential impact on S-state cycling. Cells containing the CP43-357K mutation lost their capacity for autotrophic growth and exhibited a drastic reduction in O2 evolving activity ( approximately 15% of that of the wild type) despite the fact that mutant cells contained more than 80% of the concentration of charge-separating PSII reaction centers and more than half of these contained photooxidizable Mn. Fluorescence kinetics indicated that acceptor side electron transfer, dominated by the transfer of electrons from QA- to QB, was unaffected, but the fraction of centers containing Mn clusters capable of forming the S2 state was reduced to approximately 40% of that of the wild type. Analysis of O2 yields using a bare platinum electrode indicated a severe defect in the S-state cycling properties of the mutant H2O oxidation complexes. Although O2 evolution was delayed to the third flash during a train of single-turnover saturating flashes, the pattern of O2 emission did not exhibit a discernible periodicity indicating a very high miss factor, which was estimated to be approximately 45% compared to the wild-type value of approximately 10%. On the other hand, the multiflash fluorescence measurements indicate that the yield of formation of the S2 state from S1 is diminished by approximately 20%, although this latter estimate is complicated by the presence of damaged PSII centers. Taken together, the experiments indicate that the high miss factor observed during S-state cycling is likely due to a defect in the higher S-state transitions. These results are discussed in relation to the idea that CP43-R357 may serve as a ligand to bicarbonate or as the catalytic base proposed to mediate proton-coupled electron transfer (PCET) in the higher S states of the catalytic cycle of H2O oxidation.     

Role of novel dimeric Photosystem II (PSII)-Psb27 protein complex in PSII repair

The multisubunit membrane protein complex Photosystem II (PSII) catalyzes one of the key reactions in photosynthesis: the light-driven oxidation of water. Here, we focus on the role of the Psb27 assembly factor, which is involved in biogenesis and repair after light-induced damage of the complex. We show that Psb27 is essential for the survival of cyanobacterial cells grown under stress conditions. The combination of cold stress (30 °C) and high light stress (1000 μmol of photons × m(-2) × s(-1)) led to complete inhibition of growth in a Δpsb27 mutant strain of the thermophilic cyanobacterium Thermosynechococcus elongatus, whereas wild-type cells continued to grow. Moreover, Psb27-containing PSII complexes became the predominant PSII species in preparations from wild-type cells grown under cold stress. Two different PSII-Psb27 complexes were isolated and characterized in this study. The first complex represents the known monomeric PSII-Psb27 species, which is involved in the assembly of PSII. Additionally, a novel dimeric PSII-Psb27 complex could be allocated in the repair cycle, i.e. in processes after inactivation of PSII, by (15)N pulse-label experiments followed by mass spectrometry analysis. Comparison with the corresponding PSII species from Δpsb27 mutant cells showed that Psb27 prevented the release of manganese from the previously inactivated complex. These results indicate a more complex role of the Psb27 protein within the life cycle of PSII, especially under stress conditions.     

Control of misses in oxygen evolution by the oxido-reduction state of plastoquinone in Dunaliella tertiolecta

The way misses happen in oxygen evolution is subject to debate (Govindjee et al. 1985). We recently observed a linear lowering of the miss probability with the flash number (Meunier and Popovic 1989). Therefore, we investigated in Dunaliella tertiolecta the link between the average miss probability and the redox state of plastoquinone after n flashes. The effect of flashes was to oxidize the plastoquinone pool; we found that the oxidation of plastoquinone highly correlated (linear regression: R ²=0.996) with the lowering of the miss probability. The flash frequency was found to affect both the miss probability and the redox state of plastoquinone. When pre-flashes were given using a high flash frequency (10 Hz), the plastoquinone pool was oxidized and misses were low; however, if long dark intervals between flashes were used, the oxidizing effect of flashes was lost and the misses were high. We could not explain our results by assuming equal misses over all S-states; but unequal misses, caused by deactivations, were coherent with our results. We deduced that chlororespiration was responsible for the reduction of plastoquinone in the dark interval between flashes. We compared oxygen evolution with and without benzoquinone, using a low flash frequency (0.5 Hz) for maximum misses. Benzoquinone lowered the misses from 34% to 3%, and raised the amplitude of oxygen evolution by more than a factor of two (2). From this we deduced that the charge carrier C postulated to explain misses (Lavorel and Maison-Peteri 1983) did not account for more than 3% of miss probability in Dunaliella tertiolecta. These results indicate that the misses in oxygen evolution are controlled by the redox state of plastoquinone, through deactivations.     

Detection of structural changes upon S1-to-S2 transition in the oxygen-evolving manganese cluster in photosystem II by light-induced Fourier transform infrared difference spectroscopy.

The light-induced Fourier transform infrared (FT-IR) difference spectrum between the S1 and S2 states of the O2-evolving photosystem II (PSII) was obtained for the first time. Detection of an S2/S1 difference spectrum virtually free from contributions by the acceptor-side signals was achieved by employing an exogenous electron acceptor, potassium ferricyanide, to trypsin-treated PSII membranes and using one-flash excitation at 250 K. A synthetic difference spectrum obtained by adding this S2/S1 spectrum to the QA-/QA spectrum measured with Mn-depleted PSII was almost identical with the difference spectrum of the S2QA-/S1QA charge separation measured with untreated PSII. This successful simulation verifies the correctness of the S2/S1 spectrum thus obtained. The observed S2/S1 spectrum reflects the structural changes within the water-oxidizing Mn cluster upon the S1-to-S2 transition, most probably changes in vibrational modes of ligands coordinating to the Mn ion(s) that is (are) oxidized upon the S2 formation and/or changes in protein conformation. The present results demonstrate that FT-IR difference spectroscopy is a promising method to investigate the structure of the intermediates of the Mn cluster involved in photosynthetic water oxidation.     

The S3 state of photosystem II: differences between the structure of the manganese complex in the S2 and S3 states determined by X-ray absorption spectroscopy

O2-evolving photosystem II (PSII) membranes from spinach have been cryogenically stabilized in the S3 state of the oxygen-evolving complex. The cryogenic trapping of the S3 state was achieved using a double-turnover illumination of dark-adapted PSII preparations maintained at 240 K. A double turnover of PSII was accomplished using the high-potential acceptor, Q400, which is the high-spin iron of the iron-quinone acceptor complex. EPR spectroscopy was the principal tool establishing the S-state composition and defining the electron-transfer events associated with a double turnover of PSII. The inflection point energy of the Mn X-ray absorption K-edge of PSII preparations poised in the S3 state is the same as for those poised in the S2 state. This is surprising in light of the loss of the multiline EPR signal upon advancing to the S3 state. This indicates that the oxidative equivalent stored within the oxygen-evolving complex (OEC) during this transition resides on another intermediate donor which must be very close to the manganese complex. An analysis of the Mn extended X-ray absorption fine structure (EXAFS) of PSII preparations poised in the S2 and S3 states indicates that a small structural rearrangement occurs during this photoinduced transition. A detailed comparison of the Mn EXAFS of these two S states with the EXAFS of four multinuclear mu-oxo-bridged manganese compounds indicates that the photosynthetic manganese site most probably consists of a pair of binuclear di-mu-oxo-bridged manganese structures. However, we cannot rule out, on the basis of the EXAFS analysis alone, a complex containing a mononuclear center and a linear trinuclear complex. The subtle differences observed between the S states are best explained by an increase in the spread of Mn-Mn distances occurring during the S2----S3 state transition. This increased disorder in the manganese distances suggests the presence of two inequivalent di-mu-oxo-bridged binuclear structures in the S3 state.     

Acceptor side effects on the electron transfer at cryogenic temperatures in intact photosystem II

In intact PSII, both the secondary electron donor (Tyr(Z)) and side-path electron donors (Car/Chl(Z)/Cyt(b)(559)) can be oxidized by P(680)(+) at cryogenic temperatures. In this paper, the effects of acceptor side, especially the redox state of the non-heme iron, on the donor side electron transfer induced by visible light at cryogenic temperatures were studied by EPR spectroscopy. We found that the formation and decay of the S(1)Tyr(Z) EPR signal were independent of the treatment of K(3)Fe(CN)(6), whereas formation and decay of the Car(+)/Chl(Z)(+) EPR signal correlated with the reduction and recovery of the Fe(3+) EPR signal of the non-heme iron in K(3)Fe(CN)(6) pre-treated PSII, respectively. Based on the observed correlation between Car/Chl(Z) oxidation and Fe(3+) reduction, the oxidation of non-heme iron by K(3)Fe(CN)(6) at 0 degrees C was quantified, which showed that around 50-60% fractions of the reaction centers gave rise to the Fe(3+) EPR signal. In addition, we found that the presence of phenyl-p-benzoquinone significantly enhanced the yield of Tyr(Z) oxidation. These results indicate that the electron transfer at the donor side can be significantly modified by changes at the acceptor side, and indicate that two types of reaction centers are present in intact PSII, namely, one contains unoxidizable non-heme iron and another one contains oxidizable non-heme iron. Tyr(Z) oxidation and side-path reaction occur separately in these two types of reaction centers, instead of competition with each other in the same reaction centers. In addition, our results show that the non-heme iron has different properties in active and inactive PSII. The oxidation of non-heme iron by K(3)Fe(CN)(6) takes place only in inactive PSII, which implies that the Fe(3+) state is probably not the intermediate species for the turnover of quinone reduction.     

Flash-induced Fourier transform infrared detection of the structural changes during the S-state cycle of the oxygen-evolving complex in photosystem II

Fourier transform infrared (FTIR) difference spectra of all flash-induced S-state transitions of the oxygen-evolving complex were measured using photosystem II (PSII) core complexes of Synechococcus elongatus. The PSII core sample was given eight successive flashes with 1 s intervals at 10 degrees C, and FTIR difference spectra upon individual flashes were measured. The obtained difference spectra upon the first to fourth flashes showed considerably different spectral features from each other, whereas the fifth, sixth, seventh, and eighth flash spectra were similar to the first, second, third, and fourth flash spectra, respectively. The intensities at the wavenumbers of prominent peaks of the first and second flash spectra showed clear period four oscillation patterns. These oscillation patterns were well fitted with the Kok model with 13% misses. These results indicate that the first, second, third, and fourth flash spectra represent the difference spectra upon the S(1) --> S(2), S(2) --> S(3), S(3) --> S(0), and S(0) --> S(1) transitions, respectively. In these spectra, prominent bands were observed in the symmetric (1300-1450 cm(-)(1)) and asymmetric (1500-1600 cm(-)(1)) stretching regions of carboxylate groups and in the amide I region (1600-1700 cm(-)(1)). Comparison of the band features suggests that the drastic coordination changes of carboxylate groups and the protein conformational changes in the S(1) --> S(2) and S(2) --> S(3) transitions are reversed in the S(3) --> S(0) and S(0) --> S(1) transitions. The flash-induced FTIR measurements during the S-state cycle will be a promising method to investigate the detailed molecular mechanism of photosynthetic oxygen evolution.     

Evidence that the C-terminus of the D1 polypeptide of photosystem II is ligated to the manganese ion that undergoes oxidation during the S1 to S2 transition: an isotope-edited FTIR study

Isotope-edited FTIR difference spectroscopy was employed to determine if the C-terminal alpha-COO(-) group of the D1 polypeptide ligates the (Mn)(4) cluster in photosystem II (PSII) and, if so, if it ligates the Mn ion that undergoes an oxidation during the S(1) --> S(2) transition. Wild-type and mutant cells of the cyanobacterium Synechocystis sp. PCC 6803 were propagated photoautotrophically in the presence of L-[1-(13)C]alanine or unlabeled ((12)C) L-alanine. In wild-type cells, both the C-terminal alpha-COO(-) group of the D1 polypeptide at D1-Ala344 and all alanine-derived peptide carbonyl groups will be labeled. In D1-A344G and D1-A344S mutant cells, the C-terminal alpha-COO(-) group of the D1 polypeptide will not be labeled because this group is no longer provided by alanine. The resultant S(2)-minus-S(1) FTIR difference spectra of purified wild-type and mutant PSII particles showed that one symmetric carboxylate stretching mode that is altered during the S(1) --> S(2) transition is sensitive to L-[1-(13)C]alanine-labeling in wild-type PSII particles but not in D1-A344G and D1-A344S PSII particles. Because the only carboxylate group that can be labeled in the wild-type PSII particles but not in the mutant PSII particles is the C-terminal alpha-COO(-) group of the D1 polypeptide, we assign the L-[1-(13)C]alanine-sensitive symmetric carboxylate stretching mode to the alpha-COO(-) group of D1-Ala344. In unlabeled wild-type PSII particles, this mode appears at approximately 1356 cm(-1) in the S(1) state and at approximately 1339 or approximately 1320 cm(-1) in the S(2) state. These frequencies are consistent with unidentate ligation of the (Mn)(4) cluster by the alpha-COO(-) group of D1-Ala344 in both the S(1) and S(2) states. The apparent 17-36 cm(-1) downshift in frequency in response to the S(1) --> S(2) transition is consistent with the alpha-COO(-) group of D1-Ala344 ligating a Mn ion whose charge increases during the S(1) --> S(2) transition. Accordingly, we propose that the alpha-COO(-) group of D1-Ala344 ligates the Mn ion that undergoes an oxidation during the S(1) --> S(2) transition. Control experiments were conducted with Mn-depleted wild-type PSII particles. These experiments showed that tyrosine Y(D) may be structurally coupled to the carbonyl oxygen of an alanine-derived peptide carbonyl group.     

Fast structural changes (200-900ns) may prepare the photosynthetic manganese complex for oxidation by the adjacent tyrosine radical

The Mn complex of photosystem II (PSII) cycles through 4 semi-stable states (S(0) to S(3)). Laser-flash excitation of PSII in the S(2) or S(3) state induces processes with time constants around 350ns, which have been assigned previously to energetic relaxation of the oxidized tyrosine (Y(Z)(ox)). Herein we report monitoring of these processes in the time domain of hundreds of nanoseconds by photoacoustic (or 'optoacoustic') experiments involving pressure-wave detection after excitation of PSII membrane particles by ns-laser flashes. We find that specifically for excitation of PSII in the S(2) state, nuclear rearrangements are induced which amount to a contraction of PSII by at least 30?(3) (time constant of 350ns at 25°C; activation energy of 285+/-50meV). In the S(3) state, the 350-ns-contraction is about 5 times smaller whereas in S(0) and S(1), no volume changes are detectable in this time domain. It is proposed that the classical S(2)=>S(3) transition of the Mn complex is a multi-step process. The first step after Y(Z)(ox) formation involves a fast nuclear rearrangement of the Mn complex and its protein-water environment (~350ns), which may serve a dual role: (1) The Mn- complex entity is prepared for the subsequent proton removal and electron transfer by formation of an intermediate state of specific (but still unknown) atomic structure. (2) Formation of the structural intermediate is associated (necessarily) with energetic relaxation and thus stabilization of Y(Z)(ox) so that energy losses by charge recombination with the Q(A)(-) anion radical are minimized. The intermediate formed within about 350ns after Y(Z)(ox) formation in the S(2)-state is discussed in the context of two recent models of the S(2)=>S(3) transition of the water oxidation cycle. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: From Natural to Artificial.     

Electron paramagnetic resonance study of the electron transfer reactions in photosystem II membrane preparations from Arabidopsis thaliana

Arabidopsis thaliana is widely used as a model organism in plant biology as its genome has been sequenced and transformation is known to be efficient. A large number of mutant lines and genomic resources are available for Arabidopsis. All this makes Arabidopsis a useful tool for studies of photosynthetic reactions in higher plants. In this study, photosystem II (PSII) enriched membranes were successfully isolated from thylakoids of Arabidopsis plants and for the first time the electron transfer cofactors in PSII were systematically studied using electron paramagnetic resonance (EPR) spectroscopy. EPR signals from both of the donor and acceptor sides of PSII, as well as from auxiliary electron donors were recorded. From the acceptor side of PSII, EPR signals from Q(A)- Fe2(+) and Phe- Q(A)- Fe2(+) as well as from the free Phe- radical were observed. The multiline EPR signals from the S?- and S?-states of CaMn?O(x)-cluster in the water oxidation complex were characterized. Moreover, split EPR signals, the interaction signals from Y(Z) and CaMn?O(x)-cluster in the S?-, S?-, S?-, and the S?-state were induced by illumination of the PSII membranes at 5K and characterized. In addition, EPR signals from auxiliary donors Y(D), Chl(+) and cytochrome b??? were observed. In total, we were able to detect about 20 different EPR signals covering all electron transfer components in PSII. Use of this spectroscopic platform opens a possibility to study PSII reactions in the library of mutants available in Arabidopsis.     

The Regulation of Photosynthetic Electron Transport during Nutrient Deprivation in Chlamydomonas reinhardtii

The light-saturated rate of photosynthetic O₂ evolution in Chlamydomonas reinhardtii declined by approximately 75% on a per-cell basis after 4 d of P starvation or 1 d of S starvation. Quantitation of the partial reactions of photosynthetic electron transport demonstrated that the light-saturated rate of photosystem (PS) I activity was unaffected by P or S limitation, whereas light-saturated PSII activity was reduced by more than 50%. This decline in PSII activity correlated with a decline in both the maximal quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers (PSII centers capable of performing a charge separation but unable to reduce the plastoquinone pool). In addition to a decline in the light-saturated rate of O₂ evolution, there was reduced efficiency of excitation energy transfer to the reaction centers of PSII (because of dissipation of absorbed light energy as heat and because of a transition to state 2). These findings establish a common suite of alterations in photosynthetic electron transport that results in decreased linear electron flow when C. reinhardtii is limited for either P or S. It was interesting that the decline in the maximum quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers were regulated specifically during S-limited growth by the SacI gene product, which was previously shown to be critical for the acclimation of C. reinhardtii to S limitation (J.P. Davies, F.H. Yildiz, and A.R. Grossman [1996] EMBO J 15: 2150–2159).