Architecture of the photosynthetic oxygen-evolving center

The 1.49 A resolution crystal structure of PsbQ from photosystem II of Spinacia oleracea reveals a PPII structure in the N-terminal region

We report the high-resolution structure of the spinach PsbQ protein, one of the main extrinsic proteins of higher plant photosystem II (PSII). The crystal structure shows that there are two well-defined regions in PsbQ, the C-terminal region (residues 46-149) folded as a four helix up-down bundle and the N-terminal region (residues 1-45) that is loosely packed. This structure provides, for the first time, insights into the crucial N-terminal region. First, two parallel beta-strands cross spatially, joining the beginning and the end of the N-terminal region of PsbQ. Secondly, the residues Pro9-Pro10-Pro11-Pro12 form a left-handed helix (or a polyproline type II (PPII) structure), which is stabilized by hydrogen bonds between the Pro peptide carbonyl groups and solvent water molecules. Thirdly, residues 14-33 are not visible in the electron density map, suggesting that this loop might be very flexible and presumably extended when PsbQ is free in solution. On the basis of the essential role of the N-terminal region of PsbQ in binding to PSII, we propose that both the PPII structure and the missing loop are key secondary structure elements in the recognition of specific protein-protein interactions between PsbQ and other oxygen-evolving complex extrinsic and/or intrinsic proteins of PSII. In addition, the PsbQ crystal coordinates two zinc ions, one of them is proposed to have a physiological role in higher plants, on the basis of the full conservation of the ligand protein residues in the sequence subfamily.

Structure and activity of the photosystem II manganese-stabilizing protein: role of the conserved disulfide bond

The 33-kDa manganese-stabilizing protein (MSP) of Photosystem II (PS II) maintains the functional stability of the Mn cluster in the enzyme's active site. This protein has been shown to possess characteristics similar to those of the intrinsically disordered, or natively unfolded proteins. Alternately it was proposed that MSP should be classified as a molten globule, based in part on the hypothesis that its lone disulfide bridge is necessary for structural stability and function in solution. A site-directed mutant MSP (C28A,C51A) that eliminates the disulfide bond reconstitutes O(2) evolution activity and binds to MSP-free PS II preparations at wild-type levels. This mutant was further characterized by incubation at 90 degrees C to determine the effect of loss of the disulfide bridge on MSP thermostability and solution structure. After heating at 90 degrees C for 20 min, C28A,C51A MSP was still able to bind to PS II preparations at molar stoichiometries similar to those of WT MSP and reconstitute O(2) evolution activity. A fraction of the protein aggregates upon heating, but after resolubilization, it regains the ability to bind to PS II and reconstitute O(2) evolution activity. Characterization of the solution structure of C28A,C51A MSP, using CD spectroscopy, UV absorption spectroscopy, and gel filtration chromatography, revealed that the mutant has a more disordered solution structure than WT MSP. The disulfide bond is therefore unnecessary for MSP function and the intrinsically disordered characteristics of MSP are not dependent on its presence. However, the disulfide bond does play a role in the solution structure of MSP in vivo, as evidenced by the lability of a C20S MSP mutation in Synechocystis 6803.

The mystery of oxygen evolution: analysis of structure and function of photosystem II, the water-plastoquinone oxido-reductase

Photosystem II (PS II) of thylakoid membrane of photosynthetic organisms has drawn attention of researchers over the years because it is the only system on Earth that provides us with oxygen that we breathe. In the recent past, structure of PS II has been the focus of research in plant science. The report of X-ray crystallographic structure of PS II complex by the research groups of James Barber and So Iwata in UK is a milestone in the area of research in photosynthesis. It follows the pioneering and elegant work from the laboratories of Horst Witt and W. Saenger in Germany, and J. Shen in Japan. It is time to analyze the historic events during the long journey made by the researchers to arrive at this point. This review makes an attempt to critically review the growth of the advancement of concepts and knowledge on the photosystem in the background of technological development. We conclude the review with perspectives on research and technology that should reveal the complete story of PS II of thylakoid in the future.

Dynamics of fluxes through photosynthetic complexes in response to changing light and inorganic carbon acclimation in Synechococcus elongatus

Cyanobacteria acclimate to environmental inorganic carbon (C(i)) concentrations through re-organisations of photosynthetic function and the induction of carbon concentrating mechanisms (CCMs), which alter and constrain their subsequent acclimation to changing light. We grew cells acclimated to high C(i) (4 mM) or low C(i) (0.02 mM), shifted them from 50 micromol m(-2) s(-1) to 500 micromol m(-2) s(-1), and quantified their photosynthetic performance in parallel with quantitation of allocations to key indicator macromolecules. Pigments cell(-1) declined, PsbA (PS II), AtpB (ATP Synthase), RbcL (Rubisco) and GlnA (Glutamine Synthetase) increased, and PsaC (PS I) remained stable through the light shift. The increase in these protein pools was slower and smaller in low C(i) cells, but acted in both cell types to re-normalise the electron fluxes through the catalytic complexes back toward values before the light shift (for PsbA and GlnA) or even below the initial flux per complex (for RbcL). In contrast, an increased electron flux per PsaC was sustained for at least 6 h after the increase in light. Initially, high levels of PS II cell(-1) and PS II connectivity in high C(i) cells caused a more rapid net photoinactivation of PS II in high C(i) cells than in low C(i) cells, depressing the rate of PS II-specific electron transport (PS II ETR) to levels similar to linear ETR (net O(2) evolution minus respiration). In low C(i) cells, PS II ETR remained in excess of linear ETR and may have helped maintain CCM activity. The pool sizes of PsbA, AtpB and GlnA correlated with cellular growth rate, and changed at similar rates in high C(i) and low C(i) cells when expressed on a generational rather than chronological timescale, which has implications for differing ecology of high and low C(i) cells under variable natural light.

Effects of solar UV radiation on morphology and photosynthesis of filamentous cyanobacterium Arthrospira platensis

To study the impact of solar UV radiation (UVR) (280 to 400 nm) on the filamentous cyanobacterium Arthrospira (Spirulina) platensis, we examined the morphological changes and photosynthetic performance using an indoor-grown strain (which had not been exposed to sunlight for decades) and an outdoor-grown strain (which had been grown under sunlight for decades) while they were cultured with three solar radiation treatments: PAB (photosynthetically active radiation [PAR] plus UVR; 280 to 700 nm), PA (PAR plus UV-A; 320 to 700 nm), and P (PAR only; 400 to 700 nm). Solar UVR broke the spiral filaments of A. platensis exposed to full solar radiation in short-term low-cell-density cultures. This breakage was observed after 2 h for the indoor strain but after 4 to 6 h for the outdoor strain. Filament breakage also occurred in the cultures exposed to PAR alone; however, the extent of breakage was less than that observed for filaments exposed to full solar radiation. The spiral filaments broke and compressed when high-cell-density cultures were exposed to full solar radiation during long-term experiments. When UV-B was screened off, the filaments initially broke, but they elongated and became loosely arranged later (i.e., there were fewer spirals per unit of filament length). When UVR was filtered out, the spiral structure hardly broke or became looser. Photosynthetic O(2) evolution in the presence of UVR was significantly suppressed in the indoor strain compared to the outdoor strain. UVR-induced inhibition increased with exposure time, and it was significantly lower in the outdoor strain. The concentration of UV-absorbing compounds was low in both strains, and there was no significant change in the amount regardless of the radiation treatment, suggesting that these compounds were not effectively used as protection against solar UVR. Self-shading, on the other hand, produced by compression of the spirals over adaptive time scales, seems to play an important role in protecting this species against deleterious UVR. Our findings suggest that the increase in UV-B irradiance due to ozone depletion not only might affect photosynthesis but also might alter the morphological development of filamentous cyanobacteria during acclimation or over adaptive time scales.

The PsbH protein is associated with the inner antenna CP47 and facilitates D1 processing and incorporation into PSII in the cyanobacterium Synechocystis PCC 6803

Analysis of a number of PSII complexes detectable in the wild-type and mutant cells of the cyanobacterium Synechocystis sp. PCC 6803 showed that the PsbH protein is present in the complexes containing CP47, including unassembled CP47. In a mutant lacking CP47, in which the PSII assembly is stopped at the level of the D1-D2-cytochrome b-559 reaction centre complex, a negligible amount of the PsbH protein was not bound to this complex but was detected in the free form. The results indicate that the PsbH protein has a high affinity for CP47 and during PSII assembly most probably first associates with CP47 and this pair is subsequently attached to the reaction centre complex. Similarly to CP47, the PsbH protein exhibits a slow light-induced degradation in the presence of protein synthesis inhibitor. The absence of the PsbH protein leads to a greatly increased D1 pool that is not associated with other PSII proteins or it is present as a part of the reaction centre complex. We conclude that PsbH is important for the prompt incorporation of the newly synthesized D1 protein into PSII complexes and for the fast D1 maturation.

A suppressor mutation in the alpha-phycocyanin gene in the light/glucose-sensitive phenotype of the psbK-disruptant of the cyanobacterium Synechocystis sp. PCC 6803

psbK encodes a small transmembrane component of PSII. Here we report that the psbK-disruptant of Synechocystis sp. PCC 6803 cannot survive under photomixotrophic conditions of light and glucose after transient growth, while the wild type is able to grow. A spontaneous yellow-green mutant that recovered the sustained growth under the same conditions was isolated from the psbK-disruptant. Instead of recovery, the mutant largely lost photoautotrophic growth. By phenotype complementation, the mutation was identified in cpcA as a sequence replacement with a close downstream segment, generating an inverted repeat of 23 bp. The mutant phenotype was characterized by (i) the complete loss of alpha- and beta-phycocyanin; (ii) increased accumulation of PSII; and (iii) greatly reduced transcripts harboring cpcA in abundance and in size. The inverted repeat generated in cpcA probably led to the early termination of transcription. A possible mechanism for such a mutation is discussed.

Synthesis, structure, and magnetism of heterometallic carboxylate complexes [MnIII2M(II)4O2(PhCOO)10(DMF)4], M = Mn(II), Co(II), Ni(II)

Three new homo- and heterometallic hexanuclear complexes [Mn(2)M(II)(4)O(2)(PhCOO)(10)(DMF)(4)] (with M = Mn (1), Co (2) or Ni (3) and DMF = dimethylformamide) have been synthesized by redox generation of benzoate ligands from benzaldehyde in a one-pot reaction. All of the compounds are isostructural and crystallize in the I-42d space group of the tetragonal system, data for 1: a = 27.2249(8) Angstroms, c = 25.5182(5) Angstroms, R1 = 0.0681. The crystal structure contains isolated molecules. Each molecule consists of 2 x Mn(III) surrounded by four M(II) ions to form two edge-sharing OMn(2)M(2) tetrahedra giving rise to the [Mn(2)M(4)O(2)] core. The coordination sphere of each metal is completed by the bridging benzoate ligands and DMF molecules. The magnetic susceptibilities of 1-3 have been measured in the 1.8 K < T < 300 K temperature range. The magnetic susceptibilities for 1 and 2 pass through a broad maximum at low temperature which is characteristic of the diamagnetic ground state, while for 3 no maximum is detected down to 1.8 K. The magnetic data have been interpreted quantitatively for 1 and 3 on the basis of spin exchange interactions between the metallic centers (spin Hamiltonian for a pair being H(AB) = -J(AB) S(A).S(B)). Single-crystal measurements on [Mn(6)O(2)(PhCOO)(10)(CH(3)CN)(4)] (4) show that significant magnetic anisotropy develops at low temperature.

Iron deficiency induces a chlorophyll d-binding Pcb antenna system around Photosystem I in Acaryochloris marina

The prochlorophyte-like cyanobacterium Acaryochloris marina contains two pcb genes, pcbA and pcbC, which encode chlorophyll (Chl) d-binding antenna proteins PcbA and PcbC, respectively. Using real-time reverse transcriptase polymerase chain reaction (RT-PCR), it is shown that when Acaryochloris cells are grown in an iron-deficient medium, the transcription of the pcbC gene is up-regulated compared to that of pcbA. Biochemical and immunological analyses indicated that under the same iron-deficient conditions, the level of Photosystem I (PSI) decreased compared with that of Photosystem II (PSII). Electron microscopy revealed that concomitant with these changes was the formation of Pcb-PSI supercomplexes which, in their largest form, were composed of 18 Pcb subunits forming a ring around the trimeric PSI reaction centre core. Mass spectrometry indicated that the PcbC protein is the main constituent of this outer PSI antenna system. It is therefore concluded that in Acaryochloris, the PcbC protein forms an antenna for PSI when iron levels become limiting and in this way compensates for the drop in the level of PSI relative to PSII which occurs under these conditions.

Localization of a putative ClC chloride channel in spinach chloroplasts

Seven genes seem to encode for putative ClC chloride channels (AtClC-a to AtClC-g) in Arabidopsis thaliana. Their function and localization is still largely unknown. AtClC-f shares considerable sequence similarity with putative ClC channel proteins from Synechocystis, considered to represent the precursor of chloroplasts. We show by biochemical and mass spectrometry analysis that ClC-f is located in the outer envelope membrane of spinach chloroplasts. Consistent with the plastidial localization of ClC-f, p-chlorophenoxy-acetic acid (CPA) reduces photosynthetic activity and the protein is expressed in etioplasts and chloroplasts but not in root tissue. These findings may represent a step toward the molecular identification of ion channel activities in chloroplast membranes.

X-ray damage to the Mn4Ca complex in single crystals of photosystem II: a case study for metalloprotein crystallography

X-ray absorption spectroscopy was used to measure the damage caused by exposure to x-rays to the Mn(4)Ca active site in single crystals of photosystem II as a function of dose and energy of x-rays, temperature, and time. These studies reveal that the conditions used for structure determination by x-ray crystallography cause serious damage specifically to the metal-site structure. The x-ray absorption spectra show that the structure changes from one that is characteristic of a high-valent Mn(4)(III(2),IV(2)) oxo-bridged Mn(4)Ca cluster to that of Mn(II) in aqueous solution. This damage to the metal site occurs at a dose that is more than one order of magnitude lower than the dose that results in loss of diffractivity and is commonly considered safe for protein crystallography. These results establish quantitative x-ray dose parameters that are applicable to redox-active metalloproteins. This case study shows that a careful evaluation of the structural intactness of the active site(s) by spectroscopic techniques can validate structures derived from crystallography and that it can be a valuable complementary method before structure-function correlations of metalloproteins can be made on the basis of high-resolution x-ray crystal structures.

Cyclic endoperoxides of β-carotene, potential pro-oxidants, as products of chemical quenching of singlet oxygen

Photoprotection by carotenoids is generally considered to be based on the photophysical quenching of triplets and singlet oxygen. There is also accumulating evidence of an alternative, chemical quenching of triplets and singlet oxygen by carotenoids. We report the identification of relatively stable cyclic mono- and diendoperoxides as first products of such an alternative reaction. Nevertheless, these species remain reactive and in the dark cause autooxidation of beta-carotene in our model system. Their formation could explain the intriguing pro-oxidant and cytotoxic activity of carotenoids.

Protein Interactions Limit the Rate of Evolution of Photosynthetic Genes in Cyanobacteria

Using a bioinformatic approach, we analyzed the correspondence in genetic distance matrices between all possible pairwise combinations of 82 photosynthetic genes in 10 species of cyanobacteria. Our analysis reveals significant correlations between proteins linked in a conserved gene order and between structurally identified interacting protein scaffolds that coordinate the binding of cofactors involved in photosynthetic electron transport. Analyses of amino acid substitution rates suggest that the tempo of evolution of genes encoding core metabolic processes in the photosynthetic apparatus is highly constrained by protein-protein, protein-lipid, and protein-cofactor interactions (collectively called "protein interactions"). These interactions are critical for energy transduction, primary charge separation, and electron transport and effectively act as an internal selection pressure governing the conservation of clusters of photosynthetic genes in oxygenic prokaryotic photoautotrophs. Consequently, although several proteins within the photosynthetic apparatus are biophysically and physiologically inefficient, selection has not significantly altered the genes encoding these essential proteins over billions of years of evolution. In effect, these core proteins have become "frozen metabolic accidents."

Light and oxygenic photosynthesis: energy dissipation as a protection mechanism against photo-oxidation

Efficient photosynthesis is of fundamental importance for plant survival and fitness. However, in oxygenic photosynthesis, the complex apparatus responsible for the conversion of light into chemical energy is susceptible to photodamage. Oxygenic photosynthetic organisms have therefore evolved several protective mechanisms to deal with light energy. Rapidly inducible non-photochemical quenching (NPQ) is a short-term response by which plants and eukaryotic algae dissipate excitation energy as heat. This review focuses on recent advances in the elucidation of the molecular mechanisms underlying this protective quenching pathway in higher plants.

Extinction coefficients of cytochromes b559 and c550 of Thermosynechococcus elongatus and Cyt b559/PS II stoichiometry of higher plants

"Reduced minus oxidized" difference extinction coefficients Deltavarepsilon in the alpha-bands of Cyt b559 and Cyt c550 were determined by using functionally and structurally well-characterized PS II core complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus. Values of 25.1+/-1.0 mM(-1) cm(-1) and 27.0+/-1.0 mM(-1) cm(-1) were obtained for Cyt b559 and Cyt c550, respectively. Anaerobic redox titrations covering the wide range from -250 up to +450 mV revealed that the heme groups of both Cyt b559 and Cyt c550 exhibit homogenous redox properties in the sample preparation used, with E(m) values at pH 6.5 of 244+/-11 mV and -94+/-21 mV, respectively. No HP form of Cyt b559 could be detected. Experiments performed on PS II membrane fragments of higher plants where the content of the high potential form of Cyt b559 was varied by special treatments (pH, heat) have shown that the alpha-band extinction of Cyt b559 does not depend on the redox form of the heme group. Based on the results of this study the Cyt b559/PSII stoichiometry is inferred to be 1:1 not only in thermophilic cyanobacteria as known from the crystal structure but also in PSII of plants. Possible interrelationships between the structure of the Q(B) site and the microenvironment of the heme group of Cyt b559 are discussed.

Comparison of the light-harvesting networks of plant and cyanobacterial photosystem I

With the availability of structural models for photosystem I (PSI) in cyanobacteria and plants it is possible to compare the excitation transfer networks in this ubiquitous photosystem from two domains of life separated by over one billion years of divergent evolution, thus providing an insight into the physical constraints that shape the networks' evolution. Structure-based modeling methods are used to examine the excitation transfer kinetics of the plant PSI-LHCI supercomplex. For this purpose an effective Hamiltonian is constructed that combines an existing cyanobacterial model for structurally conserved chlorophylls with spectral information for chlorophylls in the Lhca subunits. The plant PSI excitation migration network thus characterized is compared to its cyanobacterial counterpart investigated earlier. In agreement with observations, an average excitation transfer lifetime of approximately 49 ps is computed for the plant PSI-LHCI supercomplex with a corresponding quantum yield of 95%. The sensitivity of the results to chlorophyll site energy assignments is discussed. Lhca subunits are efficiently coupled to the PSI core via gap chlorophylls. In contrast to the chlorophylls in the vicinity of the reaction center, previously shown to optimize the quantum yield of the excitation transfer process, the orientational ordering of peripheral chlorophylls does not show such optimality. The finding suggests that after close packing of chlorophylls was achieved, constraints other than efficiency of the overall excitation transfer process precluded further evolution of pigment ordering.

Defining the Structural Basis for Assembly of a Transmembrane Cytochrome

To define the structural basis for cofactor binding to membrane proteins, we introduce a manageable model system, which allows us, for the first time, to study the influence of individual transmembrane helices and of single amino acid residues on the assembly of a transmembrane cytochrome. In vivo as well as in vitro analyses indicate central roles of single amino acid residues for either interaction of the transmembrane helices or for binding of the cofactor. The results clearly show that interaction of the PsbF transmembrane helix is independent from binding of the heme cofactor. On the other hand, binding of the cofactor highly depends on helix-helix interactions. By site-directed mutagenesis critical amino acid residues were identified, which are involved in the assembly of a functional transmembrane cytochrome. Especially, a highly conserved glycine residue is critical for interaction of the transmembrane helices and assembly of the cytochrome. Based on the two-stage-model of alpha-helical membrane protein folding, the presented results clearly indicate a third stage of membrane protein folding, in which a cofactor binds to a pre-assembled transmembrane protein.

Tocopherol as singlet oxygen scavenger in photosystem II

Singlet oxygen is formed in the photosystem II reaction center in the quench of P680 triplets, and the yield is dependent on light intensity and the reduction level of plastoquinone. Singlet oxygen in PS II triggers the degradation of the D1 protein. We investigated the participation of tocopherol as a singlet oxygen scavenger in this system. For this purpose, we inhibited tocopherol biosynthesis at the level of the HPP-dioxygenase in the alga Chlamydomonas reinhardtii under conditions in which plastoquinone did not limit the photosynthesis rate. In the presence of the inhibitor and in high light for 2 h, photosynthesis in vivo and photosystem II was inactivated, the D1 protein was degraded, and the tocopherol pool was depleted and fell below its turnover rate/h. The inhibited system could be fully resuscitated upon the addition of a chemical singlet oxygen quencher (diphenylamine), and partly by synthetic cell wall permeable short chain alpha- and gamma-tocopherol derivatives. We conclude that under conditions of photoinhibition and extensive D1 protein turnover tocopherol has a protective function as a singlet oxygen scavenger.

Oxygen-evolving Photosystem II core complexes: a new paradigm based on the spectral identification of the charge-separating state, the primary acceptor and assignment of low-temperature fluorescence

We review our recent low-temperature absorption, circular dichroism (CD), magnetic CD (MCD), fluorescence and laser-selective measurements of oxygen-evolving Photosystem II (PSII) core complexes and their constituent CP 4 3, CP 47 and D1/D2/cytb(559) sub-assemblies. Quantitative comparisons reveal that neither absorption nor fluorescence spectra of core complexes are simple additive combinations of the spectra of the sub-assemblies. The absorption spectrum of the D1/D2/cytb(559) component embedded within the core complex appears significantly better structured and red-shifted compared to that of the isolated sub-assembly. A characteristic MCD reduction or 'deficit' is a useful signature for the central chlorins in the reaction centre. We note a congruence of the MCD deficit spectra of the isolated D1/D2/cytb(559) sub-assemblies to their laser-induced transient bleaches associated with P 680. A comparison of spectra of core complexes prepared from different organisms helps distinguish features due to inner light-harvesting assemblies and the central reaction-centre chlorins. Electrochromic spectral shifts in core complexes that occur following low-temperature illumination of active core complexes arise from efficient charge separation and subsequent plastoquinone anion (Q(A)(-)) formation. Such measurements allow determinations of both charge-separation efficiencies and spectral characteristics of the primary acceptor, Pheo(D1). Efficient charge separation occurs with excitation wavelengths as long as 700 nm despite the illuminations being performed at 1.7 K and with an extremely low level of incident power density. A weak, homogeneously broadened, charge-separating state of PSII lies obscured beneath the CP 47 state centered at 690 nm. We present new data in the 690-760 nm region, clearly identifying a band extending to 730 nm. Active core complexes show remarkably strong persistent spectral hole-burning activity in spectral regions attributable to CP 43 and CP 47. Measurements of homogeneous hole-widths have established that, at low temperatures, excitation transfer from these inner light-harvesting assemblies to the reaction centre occurs with approximately 70-270 ps(-1) rates, when the quinone acceptor is reduced. The rate is slower for lower-energy sub-populations of an inhomogeneously broadened antenna (trap) pigment. The complex low-temperature fluorescence behaviour seen in PSII is explicable in terms of slow excitation transfer from traps to the weak low-energy charge-separating state and transfer to the more intense reaction-centre excitations near 685 nm. The nature and origin of the charge-separating state in oxygen-evolving PSII preparations is briefly discussed.

Pathways and timescales of primary charge separation in the photosystem II reaction center as revealed by a simultaneous fit of time-resolved fluorescence and transient absorption

We model the dynamics of energy transfer and primary charge separation in isolated photosystem II (PSII) reaction centers. Different exciton models with specific site energies of the six core pigments and two peripheral chlorophylls (Chls) in combination with different charge transfer schemes have been compared using a simultaneous fit of the absorption, linear dichroism, circular dichroism, steady-state fluorescence, transient absorption upon different excitation wavelengths, and time-resolved fluorescence. To obtain a quantitative fit of the data we use the modified Redfield theory, with the experimental spectral density including coupling to low-frequency phonons and 48 high-frequency vibrations. The best fit has been obtained with a model implying that the final charge separation occurs via an intermediate state with charge separation within the special pair (RP(1)). This state is weakly dipole-allowed, due to mixing with the exciton states, and can be populated directly or via 100-fs energy transfer from the core-pigments. The RP(1) and next two radical pairs with the electron transfer to the accessory Chl (RP(2)) and to the pheophytin (RP(3)) are characterized by increased electron-phonon coupling and energetic disorder. In the RP(3) state, the hole is delocalized within the special pair, with a predominant localization at the inactive-branch Chl. The intrinsic time constants of electron transfer between the three radical pairs vary from subpicoseconds to several picoseconds (depending on the realization of the disorder). The equilibration between RP(1) and RP(2) is reached within 5 ps at room temperature. During the 5-100-ps period the equilibrated core pigments and radical pairs RP(1) and RP(2) are slowly populated from peripheral chlorophylls and depopulated due to the formation of the third radical pair, RP(3). The effective time constant of the RP(3) formation is 7.5 ps. The calculated dynamics of the pheophytin absorption at 545 nm displays an instantaneous bleach (30% of the total amplitude) followed by a slow increase of the bleaching amplitude with time constants of 15 and 12 ps for blue (662 nm) and red (695 nm) excitation, respectively.

Key role of conserved histidines in recombinant mouse beta-carotene 15,15'-monooxygenase-1 activity

Alignment of sequences of vertebrate beta-carotene 15,15'-monooxygenase-1 (BCMO1) and related oxygenases revealed four perfectly conserved histidines and five acidic residues (His172, His237, His308, His514, Asp52, Glu140, Glu314, Glu405, and Glu457 in mouse BCMO1). Because BCMO1 activity is iron-dependent, we propose that these residues participate in iron coordination and therefore are essential for catalytic activity. To test this hypothesis, we produced mutant forms of mouse BCMO1 by replacing the conserved histidines and acidic residues as well as four histidines and one glutamate non-conserved in the overall family with alanines by site-directed mutagenesis. Our in vitro and in vivo data showed that mutation of any of the four conserved histidines and Glu405 caused total loss of activity. However, mutations of non-conserved histidines or any of the other conserved acidic residues produced impaired although enzymatically active proteins, with a decrease in activity mostly due to changes in V(max). The iron bound to protein was determined by inductively coupled plasma atomic emission spectrometry. Bound iron was much lower in preparations of inactive mutants than in the wild-type protein. Therefore, the conserved histidines and Glu405 are absolutely required for the catalytic mechanism of BCMO1. Because the mutant proteins are impaired in iron binding, these residues are concluded to coordinate iron required for catalytic activity. These data are discussed in the context of the predicted structure for the related eubacterial apocarotenal oxygenase.

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.

Effect of exogenous histidine on restoration of electron transfer on the donor side of photosystem II depleted of Mn

It is shown that restoration of photoinduced electron flow with added Mn2+ (measured by photoreduction of DCPIP and photoinduced change of chlorophyll fluorescence yield) in Mn-depleted Photosystem II (PS II) membrane fragments isolated from spinach chloroplasts, is considerably increased by exogenous histidine (His). The stimulating effect of His is not observed if other electron donors (NH2OH or diphenylcarbazide) are used instead of Mn2+. His added alone does not induce electron transfer in Mn-depleted PS II preparations. Investigation of pH dependence of the stimulating effect of 2 mM His shows that the effect is observed only at pH > 5.0, it gives a 50% activation around pH 6.0 and saturates at pH 7.0-7.5. Nearly 200 microM His is required for a 50 effect at pH 7.0. It is suggested that the added His can be involved in stimulation of electron transfer on the donor side of PS II through direct interaction of Mn2+ with deprotonated form(s) of His resulting in formation of Mn-His complexes capable of efficient electron donation to PS II (though it is not excluded that His serves as a base that takes part in proton exchange coupled with redox reactions on the donor side of PS II or as an electron donor to the oxidized Mn).

Minor but key chlorophylls in photosystem II

A 'metal-free' chlorophyll (Chl) a, pheophytin (Phe) a, functions as the primary electron acceptor in PS II. On the basis of Phe a/PS II = 2, Phe a content is postulated as an index for estimation of the stoichiometry of pigments and photosystems. We found Phe a in a Chl d-dominant cyanobacterium Acaryochloris marina, whereas Phe d was absent. The minimum Chl a:Phe a ratio was 2:2, indicating that the primary electron donor is Chl a, accessory is Chl d, and the primary electron acceptor is Phe a in PS II of A. marina. Chl d was artificially formed by the treatment of Chl a with papain in aqueous organic solvents. Further, we will raise a key question on the mechanisms of water oxidation in PS II.

The origin of split EPR signals in the Ca2+-depleted photosystem II

A light-driven reaction model for the Ca2+-depleted Photosystem (PS) II is proposed to explain the split signal observed in electron paramagnetic resonance (EPR) spectra based on a comparison of EPR assignments with recent x-ray structural data. The split signal has a splitting linewidth of 160 G at around g = 2 and is seen upon illumination of the Ca2+-depleted PS II in the S2 state associated with complete or partial disappearance of the S2 state multiline signal. Another g=2 broad ESR signal with a 110 G linewidth was produced by 245 K illumination for a short period in the Ca2+-depleted PS II in S1 state. At the same time a normal YZ . radical signal was also efficiently trapped. The g=2 broad signal is attributed to an intermediate S1X. state in equilibrium with the trapped YZ . radical. Comparison with x-ray structural data suggests that one of the split signals (doublet signal) is attributable to interaction between His 190 and the YZ . radical, and other signals is attributable to interaction between His 337 and the manganese cluster, providing further clues as to the mechanism of water oxidation in photosynthetic oxygen evolution.

Search for intermediates of photosynthetic water oxidation

Photosystem II of cyanobacteria and plants incorporates the catalytic centre of water oxidation. Powered and clocked by quanta of light the centre accumulates four oxidising equivalents before oxygen is released. The first three oxidising equivalents are stored on the Mn(4)Ca-cluster, raising its formal oxidation state from S0 to S3 and the third on YZ, producing S3 YZ ox. From there on water oxidation proceeds in what appears as a single reaction step (S3 YZ ox(H2O)2 <==>O2 + 4H+ + S0. Intermediate oxidation products of bound water had not been detected, until our recent report on the stabilisation of such an intermediate by high oxygen pressure (NATURE 430, 2004, 480-483). Based on the oxygen titration (half-point 2.3 bar) the standard free-energy profile of a reaction sequence with a single intermediate was calculated. It revealed a rather small difference (-3 kJ mol-1) between the starting state [S3YZ OX(H2O2) and the product state S0YZ + O2 + 4H+ . Here we describe the tests for side effects of exposing core particles to high oxygen pressure. We found the reduction of P680 +* in ns and the reduction/dismutation of quinones at the acceptor side of PSII both unaffected, and the inhibition of the oxygen evolving reaction by exposure to high O2-pressure was fully reversible by decompression to atmospheric conditions.

How fast can photosystem II split water? Kinetic performance at high and low frequencies

Molecular oxygen evolution from water is a universal signature of oxygenic photosynthesis. Detection of the presence, speed and efficiency of the enzymatic machinery that catalyzes this process in vivo has been limited. We describe a laser-based fast repetition rate fluorometer (FRRF) that allows highly accurate and rapid measurements of these properties via the kinetics of Chl-a variable fluorescence yield (Fv) in living cells and leaves at repetition rates up to 10 kHz. Application to the detection of quenching of Fv is described and compared to flash-induced O2 yield data. Period-four oscillations in both Fv and O2, caused by stimulation of primary charge recombination by the O2 evolving complex (WOC) within Photosystem II (PS II), are directly compared. The first quantitative calculations of the enzymatic parameters of the Kok model (alpha - miss; beta - double hit; S-state populations) are reported from Fv data over a 5 kHz range of flash frequencies that is 100-fold wider than previously examined. Comparison of a few examples of cyanobacteria, green algae and spinach reveals that Arthrospira m., a cyanobacterium that thrives in alkaline carbonate lakes, exhibits the fastest water-splitting rates ever observed thus farin vivo. In all oxygenic phototrophs examined thus far, an unprecedented large increase in the Kok alpha and beta parameters occur at both high and low flash frequencies, which together with their strong correlation, indicates that PS II-WOC centers split water at remarkably lower efficiencies and possibly by different mechanisms at these extreme flash frequencies. Revisions to the classic Kok model are anticipated.

Towards a spin coupling model for the Mn4 cluster in Photosystem II

The X-band EPR spectra of the IR sensitive untreated PSII and of MeOH- and NH(3)-treated PSII from spinach in the S(2)-state are simulated with collinear and rhombic g- and Mn-hyperfine tensors. The obtained principal values indicate a 1Mn(III)3Mn(IV) composition for the Mn(4) cluster. The four isotropic components of the Mn-hyperfine tensors are found in good agreement with the previously published values determined from EPR and (55)Mn-ENDOR data. Assuming intrinsic isotropic components of the Mn-hyperfine interactions identical to those of the Mn-catalase, spin density values are calculated. A Y-shape 4J-coupling scheme is explored to reproduce the spin densities for the untreated PSII. All the required criteria such as a S=1/2 ground state with a low lying excited spin state (30 cm(-1)) and an easy conversion to a S=5/2 system responsible for the g=4.1 EPR signal are shown to be satisfied with four antiferromagnetic interactions lying between -290 and -130 cm(-1).

Cyanobacterial photosystem II at 3.2 A resolution - the plastoquinone binding pockets

Photosystem II from thylakoid membranes of the thermophilic cyanobacterium Thermosynechococcus elongatus was solubilized with n-beta-dodecylmaltoside and purified using anion exchange chromatography. Molecular weight, pigment stoichiometry and subunit composition were assayed using various techniques. The holocomplex is dimeric with a molecular mass of 756 +/- 18 kDa and functionally fully active. Crystals obtained from these samples showed significantly improved quality leading to a 3D structure at 3.2 A resolution. Several loop regions of the principal protein subunits are now defined that were not interpretable at lower (3.8 A) resolution, thus resulting in a more complete model. The head groups of the cofactors of the electron transfer chain and of the antennae have been modeled, coordinating and hydrogen bonding amino acids identified and the nature of the binding pockets derived. The orientations of these cofactors resemble those of the reaction centre from anoxygenic purple bacteria. For the two plastoquinones, electron density was only found for the head group of QA and none for QB indicating low or even no occupancy of this site in the crystal structure. Both binding pockets and problems related to the QB site are discussed here and compared to the situation in the purple bacterial reaction centre.

Proteins, chlorophylls and lipids: X-ray analysis of a three-way relationship

Photosynthetic reaction centres and light harvesting complexes have been at the forefront of crystallographic studies of integral membrane proteins. In recent years, there have been spectacular advances in our understanding of the structure of (bacterio)chlorophyll-containing membrane proteins from oxygenic and anoxygenic phototrophs. In these complex structures, the protein scaffold encases different combinations of cofactors and interacts with several tightly bound lipid species that play a variety of hitherto unrecognized structural roles. Some of these lipids have relevance to the physiological function of the protein, whereas others are important for the formation of highly ordered crystals. The first site-directed mutagenesis studies of individual lipid binding sites have now underlined the importance of the lipid component for the structural stability of protein-cofactor-lipid complexes.

EPR properties of a g=2 broad signal trapped in an S1 state in Ca2+-depleted Photosystem II

We investigated a new EPR signal that gives a broad line shape around g=2 in Ca(2+)-depleted Photosystem (PS) II. The signal was trapped by illumination at 243 K in parallel with the formation of Y(Z)*. The ratio of the intensities between the g=2 broad signal and the Y(Z)* signal was 1:3, assuming a Gaussian line shape for the former. The g=2 broad signal and the Y(Z)* signal decayed together in parallel with the appearance of the S(2) state multiline at 243 K. The g=2 broad signal was assigned to be an intermediate S(1)X* state in the transition from the S(1) to the S(2) state, where X* represents an amino acid radical nearby manganese cluster, such as D1-His337. The signal is in thermal equilibrium with Y(Z)*. Possible reactions in the S state transitions in Ca(2+)-depleted PS II were discussed.

Pigment exchange of photosystem II reaction center by chlorophyll d

Pigment exchanges among photosystem reaction centers (RCs) are useful for the identification and functional analysis of chromophores in photosynthetic organisms. Pigment replacement within the spinach Photosystem II RC was performed with Chl d derived from the oxygenic alga Acaryochloris marina, using a protocol similar to that reported previously [Gall et al. (1998) FEBS Lett 434: 88-92] based on the incubation of reaction centers with an excess of other pigments. In this study, we analyzed Chl d-modified monomeric RC which was separated from Chl d-modified dimeric RC by size-exclusion chromatography. Based on the assumption of a constant ratio of two Pheo a molecules per RC, the number of Chl a molecules in Chl d-modified monomeric RCs was found to decrease from six to four. The absorption spectrum of the Chl d-modified monomeric RC at room temperature showed a large peak at 699.5 nm originating from Chl d and a small peak at 672.5 nm orignating from Chl a. Photoaccumulation of the Pheo a- in Chl d-modified monomeric RC, in the presence of sodium dithionate and methyl viologen, did not differ significantly from that in control RC, showing that the Chl d-modified monomeric RC retains its charge separation activity and photochemically active Pheo a.

Assignment of the low-temperature fluorescence in oxygen-evolving photosystem II

Low-temperature absorption and fluorescence spectra of fully active cores and membrane-bound PS II preparations are compared. Detailed temperature dependence of fluorescence spectra between 5 and 70 K are presented as well as 1.7-K fluorescence line-narrowed (FLN) spectra of cores, confirming that PS II emission is composite. Spectra are compared to those reported for LHCII, CP43, CP47 and D1/D2/cytit b559 subunits of PS II. A combination of subunit spectra cannot account for emission of active PS II. The complex temperature dependence of PS II fluorescence is interpretable by noting that excitation transfer from CP43 and CP47 to the reaction centre is slow, and strongly dependent on the precise energy at which a 'slow-transfer' pigment in CP43 or CP47 is located within its inhomogeneous distribution. PS II fluorescence arises from CP43 and CP47 'slow-transfer' states, convolved by this dependence. At higher temperatures, thermally activated excitation transfer to the PS II charge-separating system bypasses such bottlenecks. As the charge-separating state of active PS II absorbs at >700 nm, PS II emission in the 680-700 nm region is unlikely to arise from reaction centre pigments. PS II emission at physiological temperatures is discussed in terms of these results.

Near-infrared-induced transitions in the manganese cluster of photosystem II: action spectra for the S2 and S3 redox states

The Mn4Ca complex that is involved in water oxidation in PSII is affected by near-infrared (NIR) light in certain redox states and these phenomena can be monitored by electron paramagnetic resonance (EPR) at low temperature. Here we report the action spectra of the NIR effects in the S2 and S3 states in PSII from plants and the thermophilic cyanobacterium Thermosynechococcus elongatus. The action spectra obtained are very similar in both S states, indicating the presence of the same photoactive form of the Mn4Ca complex in both states. Since the chemical nature of the photoactive species is not known, an unequivocal interpretation of this result cannot be made; however, it appears to be more easily reconciled with the view that the redox state of the Mn4Ca cluster does not change from the S2 to the S3 transition, at least in those centers sensitive to NIR light. The temperature dependence of the NIR effect and the action spectra for S2 indicate the presence of structural heterogeneity in the Mn4Ca cluster.

Origin of the F685 and F695 fluorescence in photosystem II

The emission spectra of CP47-RC and core complexes of Photosystem II (PS II) were measured at different temperatures and excitation wavelengths in order to establish the origin of the emission and the role of the core antenna in the energy transfer and charge separation processes in PS II. Both types of particles reveal strong dependences of spectral shape and yield on temperature. The results indicate that the well-known F-695 emission at 77 K arises from excitations that are trapped on a red-absorbing CP47 chlorophyll, whereas the F-685 nm emission at 77 K arises from excitations that are transferred slowly from 683 nm states in CP47 and CP43 to the RC, where they are trapped by charge separation. We conclude that F-695 at 77 K originates from the low-energy part of the inhomogeneous distribution of the 690 nm absorbing chlorophyll of CP47, while at 4 K the fluorescence originates from the complete distribution of the 690 nm chlorophyll of CP47 and from the low-energy part of the inhomogeneous distribution of one or more CP43 chlorophylls.

Specific loss of the extrinsic 18 KDa protein from photosystem II upon heating to 47 degrees C causes inactivation of oxygen evolution likely due to Ca release from the Mn-complex

Exposure of Photosystem II (PS II) membrane particles from spinach to a temperature of 47 degrees C caused the rapid release of the 18 kDa protein in parallel to inactivation of oxygen evolution. Previously, it has been suggested that the first heat-jump response involves rapid Ca release from the Mn complex of O2-evolution, followed by the slower release of (2 + 2) MnII ions [Pospisil P et al. (2003) Biophys J 84: 1370-1386]. Here, the predicted biphasic MnII release to the bulk was verified by atomic absorption spectroscopy (AAS). Analysis of laser flash-induced delayed fluorescence transients suggests that the loss of the essential Ca ion from the Mn4Ca complex in the dark is due to the loss of the 18 kDa protein. The S2-state multiline EPR signal of the Mn complex was still generated in heat-treated PS II presumably lacking Ca, but retaining four Mn ions.

Key cofactors of photosystem II cores from four organisms identified by 1.7-K absorption, CD and MCD

Active Photosystem II (PS II) cores were prepared from spinach, pea, Synechocystis PCC 6803, and Thermosynechococcus vulcanus, the latter of which has been structurally determined [Kamiya and Shen (2003) Proc Natl Acad Sci USA 100: 98-103]. Electrochromic shifts resulting from QA reduction by 1.7-K illumination were recorded, and the Qx and Qy absorption bands of the redox-active pheophytin a thus identified in the different organisms. The Qx transition is approximately 3 nm (100 cm-1) to higher energy in cyanobacteria than in the plants. The predominant Qy shift appears in the range 683-686 nm depending on species, and does not appear to have a systematic shift. Low-temperature absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of the chlorophyll Qy region are very similar in spinach and pea, but vary in cyanobacteria. We assigned CP43 and CP47 trap-chlorophyll absorption features in all species, as well as a P680 transition. Each absorption identified has an area of one chlorophyll a. The MCD deficit, introduced previously for spinach as an indicator of P680 activity, occurs in the same spectral region and has the same area in all species, pointing to a robustness of this as a signature for P680. MCD and CD characteristics point towards a significant variance in P680 structure between cyanobacteria, thermophilic cyanobacteria, and higher plants.

Evidence for localisation of accumulated chlorophyll cation on the D1-accessory chlorophyll in the reaction centre of photosystem II

Absorption and circular dichroism spectra of Photosystem II (PS II) reaction centres (RC) were studied and compared with spectra calculated on the basis of point-dipole approximation. Chlorophyll cation was accumulated during a light treatment of PS II RC in the presence of artificial electron acceptor silicomolybdate. Light-induced difference spectra and their calculated counterparts revealed the location of accumulated cation at the accessory chlorophyll of the D1 protein subunit.

Pulse EPR, 55Mn-ENDOR and ELDOR-detected NMR of the S2-state of the oxygen evolving complex in photosystem II

Pulse EPR, 55Mn-ENDOR and ELDOR-detected NMR experiments were performed on the S2-state of the oxygen-evolving complex from spinach Photosystem II. The novel technique of random acquisition in ENDOR was used to suppress heating artefacts. Our data unambiguously shows that four Mn ions have significant hyperfine coupling constants. Numerical simulation of the 55Mn-ENDOR spectrum allowed the determination of the principal values of the hyperfine interaction tensors for all four Mn ions of the oxygen-evolving complex. The results of our 55Mn-ENDOR experiments are in good agreement with previously published data [Peloquin JM et al. (2000) J Am Chem Soc 122: 10926-10942]. For the first time ELDOR-detected NMR was applied to the S2-state and revealed a broad peak that can be simulated numerically with the same parameters that were used for the simulation of the 55Mn-ENDOR spectrum. This provides strong independent support for the assigned hyperfine parameters.

Formate binding near the redox-active tyrosineD in photosystem II: consequences on the properties of tyrD

Formate and phosphate affect substantially the rate of tyrosine D (TyrD) oxidation and the stability of the radical TyrD* in Photosystem II [Hienerwadel R, Boussac A, Breton J and Berthomieu C (1996) Biochemistry 35: 15447-15460]. This observation prompted us to analyze the influence of formate and phosphate on the environment of TyrD using FTIR spectroscopy. The nu (CO) IR mode of TyrD* at 1503 cm-1 remains unchanged whatever the buffer used at pH 6 and whether formate is present or not in the sample. Similarly, the main IR mode of reduced TyrD remains at approximately 1250 cm-1 in all tested conditions. We thus conclude that formate does not modify the hydrogen-bonded interactions of TyrD and TyrD* with neighbouring D2His189 and D2Gln164. In the TyrD-state, an IR mode of formate significantly different from that observed in solution, is detected using 13C-formate, showing that formate forms a strong electrostatic interaction within PS II. The presence of formate affects also IR bands that may be assigned to an arginine side chain. Upon TyrD* formation, formate does not protonate but its binding interaction weakens. A proton uptake by Mes or phosphate buffer is detected, which is not observed when BisTris is used as a buffer. In these latter conditions, IR bands characteristic of the protonation of a carboxylate group of the protein are detected instead. The present IR data and the recent structural model of the TyrD environment proposed by Ferreira KN, Iverson TM, Maghlaoui K, Barber J and Iwata S [(2004) Science 303: 1831-1838], suggest that the proton released upon TyrD* formation is shared within a hydrogen bonding network including D2Arg294, and CP47Glu364 and that perturbation of this network by formate - possibly binding near D2Arg294 - substantially affects the properties of TyrD.

Antenna ring around trimeric Photosystem I in chlorophyll b containing cyanobacterium Prochlorothrix hollandica

Prochlorothrix hollandica is one of the three known species of an unusual clade of cyanobacteria (formerly called "prochlorophytes") that contain chlorophyll a and b molecules bound to intrinsic light-harvesting antenna proteins. Here, we report the structural characterization of supramolecular complex consisting of Photosystem I (PSI) associated with the chlorophyll a/b-binding Pcb proteins. Electron microscopy and single particle image analysis of negatively stained preparations revealed that the Pcb-PSI supercomplex consists of a central trimeric PSI surrounded by a ring of 18 Pcb subunits. We conclude that the formation of the Pcb ring around trimeric PSI represents a mechanism for increasing the light-harvesting efficiency in chlorophyll b-containing cyanobacteria.

Reaction pattern of photosystem II: oxidative water cleavage and protein flexibility

This short communication addresses three topics of photosynthetic water cleavage in Photosystem II (PS II): (a) effect of protonation in the acidic range on the extent of the 'fast' ns kinetics of P680+. reduction by YZ, (b) mechanism of O-O bond formation and (c) role of protein flexibility in the functional integrity of PS II. Based on measurements of light-induced absorption changes and quasielastic neutron scattering in combination with mechanistic considerations, evidence is presented for the protein acting as a functionally active constituent of the water cleavage machinery, in particular, for directed local proton transfer. A specific flexibility emerging above a threshold of about 230 K is an indispensable prerequisite for oxygen evolution and plastoquinol formation.

Extinction coefficients and critical solubilisation concentrations of photosystems I and II from Thermosynechococcus elongatus

The absorption properties of chlorophyll a (Chla) in active core complexes of photosystems I (PSI) and II (PSII) isolated in high purity from the thermophilic cyanobacterium Thermosynechococcus elongatus were correlated with those of extracts in 80% acetone to determine effective extinction coefficients of protein-bound Chla and molar extinction coefficients of core complexes and reaction centers (RC). These coefficients allow a quick determination of Chla and protein concentrations from steady-state absorption spectra of intact samples without the need for pigment extraction and protein destruction. In the visible range, epsilon(680)(p) = 57 mM(-1) cm(-1) for trimeric PSI (PSIt) and epsilon(674)(p) = 70 mM(-1) cm(-1) for dimeric (PSIId) and monomeric (PSIIm) PSII (error +/-6%; superscript "p" refers to Chla bound to intact protein, subscripts are the peak maxima in nm). The integral extinction coefficient phi(p) = 2.8 nm microM(-1) cm(-1) for the wavelength interval between 550 and 800 nm and the extinction coefficient epsilon(B)(p) = 14 mM(-1) cm(-1) for the smaller absorption maximum (B = 632 nm for PSI and 627 nm for PSII) were found to be essentially the same for both types of PS. The coefficients of PSIt are shown to remain unaltered when 65% (v/v) of the buffer is replaced with glycerol. Molar extinction coefficients of core complexes were determined using Chla/RC ratios of 96+/-1 for PSI and 35+/-2 for PSII based on X-ray data. In addition, the critical solubilisation concentration of n-dodecyl-beta-d-maltoside (betaDM), necessary to keep the core complexes in solution, was determined by turbidimetric titrations. It was found that at least approximately 500 betaDM molecules per PSIt ( approximately 2 betaDM per Chla) and 190 betaDM molecules per PSIIm ( approximately 5 betaDM per Chla, also for PSIId) in excess of the critical micelle concentration of 0.16 +/- 0.03 mM are necessary for a complete solubilisation of the core complexes.

Investigation of a requirement for the PsbP-like protein in Synechocystis sp. PCC 6803

The sll1418 gene encodes a PsbP-like protein in Synechocystis sp. PCC 6803. Expression of sll1418 was similar in BG-11 and in Cl- - or Ca2+ -limiting media, and inactivation of sll1418 did not prevent photoautotrophic growth in normal or nutrient-limiting conditions. Also the wild-type and DeltaPsbP strains exhibited similar oxygen evolution and assembly of Photosystem II (PS II) centers. Inactivation of sll1418 in the DeltaPsbO: DeltaPsbP, DeltaPsbQ:DeltaPsbP, DeltaPsbU:DeltaPsbP and DeltaPsbV:DeltaPsbP mutants did not prevent photoautotrophy or alter PS II assembly and oxygen evolution in these strains. Moreover, the absence of PsbP did not affect the ability of alkaline pH to restore photoautotrophic growth in the DeltaPsbO:DeltaPsbU strain. The PsbO, PsbU and PsbV proteins are required for thermostability of PS II and thermal acclimation in Synechocystis sp. PCC 6803 [Kimura et al. (2002) Plant Cell Physiol 43: 932-938]. However, thermostability and thermal acclimation in DeltaPsbP cells were similar to wild type. These results are consistent with the conclusion that PsbP is associated with approximately 3 of PS II centers, and may play a regulatory role in PS II [Thornton et al. (2004) Plant Cell 16: 2164-2175].