In vitro oligomerization of a membrane protein complex. liposome-based reconstitution of trimeric photosystem I from isolated monomers

Energy Transfer and Radical-Pair Dynamics in Photosystem I with Different Red Chlorophyll a Pigments

We establish a general kinetic scheme for the energy transfer and radical-pair dynamics in photosystem I (PSI) of Chlamydomonas reinhardtii, Synechocystis PCC6803, Thermosynechococcus elongatus and Spirulina platensis grown under white-light conditions. With the help of simultaneous target analysis of transient-absorption data sets measured with two selective excitations, we resolved the spectral and kinetic properties of the different species present in PSI. WL-PSI can be described as a Bulk Chl a in equilibrium with a higher-energy Chl a, one or two Red Chl a and a reaction-center compartment (WL-RC). Three radical pairs (RPs) have been resolved with very similar properties in the four model organisms. The charge separation is virtually irreversible with a rate of ≈900 ns-1. The second rate, of RP1 → RP2, ranges from 70-90 ns-1 and the third rate, of RP2 → RP3, is ≈30 ns-1. Since RP1 and the Red Chl a are simultaneously present, resolving the RP1 properties is challenging. In Chlamydomonas reinhardtii, the excited WL-RC and Bulk Chl a compartments equilibrate with a lifetime of ≈0.28 ps, whereas the Red and the Bulk Chl a compartments equilibrate with a lifetime of ≈2.65 ps. We present a description of the thermodynamic properties of the model organisms at room temperature.

SARS-CoV-2 virus-like-particles via liposomal reconstitution of spike glycoproteins

The SARS-CoV-2 virus, implicated in the COVID-19 pandemic, recognizes and binds host cells using its spike glycoprotein through an angiotensin converting enzyme 2 (ACE-2) receptor-mediated pathway. Recent research suggests that spatial distributions of the spike protein may influence viral interactions with target cells and immune systems. The goal of this study has been to develop a liposome-based virus-like particle (VLP) by reconstituting the SARS-CoV-2 spike glycoprotein within a synthetic nanoparticle membrane, aiming to eventually establish tunability in spike protein presentation on the nanoparticle surface. Here we report on first steps to this goal, wherein liposomal SARS-CoV-2 VLPs were successfully produced via detergent mediated spike protein reconstitution. The resultant VLPs are shown to successfully co-localize in vitro with the ACE-2 receptor on lung epithelial cell surfaces, followed by internalization into these cells. These VLPs are the first step toward the overall goal of this research which is to form an understanding of the relationship between spike protein surface density and cell-level immune response, eventually toward creating better vaccines and anti-viral therapeutics.

Socialization of Providencia stuartii Enables Resistance to Environmental Insults

Providencia stuartii is a highly social pathogen responsible for nosocomial chronic urinary tract infections. The bacterium indeed forms floating communities of cells (FCC) besides and prior-to canonical surface-attached biofilms (SAB). Within P. stuartii FCC, cells are riveted one to another owing to by self-interactions between its porins, viz. Omp-Pst1 and Omp-Pst2. In pathophysiological conditions, P. stuartii is principally exposed to high concentrations of urea, ammonia, bicarbonate, creatinine and to large variations of pH, questioning how these environmental cues affect socialization, and whether formation of SAB and FCC protects cells against those. Results from our investigations indicate that FCC and SAB can both form in the urinary tract, endowing cells with increased resistance and fitness. They additionally show that while Omp-Pst1 is the main gateway allowing penetration of urea, bicarbonate and ammonia into the periplasm, expression of Omp-Pst2 enables resistance to them.

Cryo-EM structure of a functional monomeric Photosystem I from Thermosynechococcus elongatus reveals red chlorophyll cluster

A high-resolution structure of trimeric cyanobacterial Photosystem I (PSI) from Thermosynechococcus elongatus was reported as the first atomic model of PSI almost 20 years ago. However, the monomeric PSI structure has not yet been reported despite long-standing interest in its structure and extensive spectroscopic characterization of the loss of red chlorophylls upon monomerization. Here, we describe the structure of monomeric PSI from Thermosynechococcus elongatus BP-1. Comparison with the trimer structure gave detailed insights into monomerization-induced changes in both the central trimerization domain and the peripheral regions of the complex. Monomerization-induced loss of red chlorophylls is assigned to a cluster of chlorophylls adjacent to PsaX. Based on our findings, we propose a role of PsaX in the stabilization of red chlorophylls and that lipids of the surrounding membrane present a major source of thermal energy for uphill excitation energy transfer from red chlorophylls to P700.

The in vitro synergistic denaturation effect of heat and surfactant on photosystem I isolated from Arthrospira Platensis

Photosystem I (PSI) generates the most negative redox potential found in nature, and the performance of solar energy conversion into alternative energy sources in artificial systems highly depends on the thermal stability of PSI. Thus, understanding thermal denaturation is an important prerequisite for the use of PSI at elevated temperatures. To assess the thermal stability of surfactant-solubilized PSI from cyanobacteria Arthrospira Platensis, the synergistic denaturation effect of heat and surfactant was studied. At room temperature, surfactant n-dodecyl-β-D-maltoside solubilized PSI trimer gradually disassembles into PSI monomers and free pigments over long time. In the solubilizing process of PSI particles, surfactant can uncouple pigments of PSI, and the high concentration of surfactant causes the pigment to uncouple more; after the surfactant-solubilizing process, the uncoupling is relatively slow. During the heating process, changes were monitored by transmittance T_800nm, ellipticity θ_686nm and θ_222nm, upon slow heating (1.5 °C per minute) of samples in Tris buffer (20 mM, pH 7.8) from 20 to 95 °C. The thermal denaturation of surfactant-solubilized PSI is a much more complicated process, which includes the uncoupling of pigments by surfactants, the disappearance of surrounding surfactants, and the unfolding of PSI α-helices. During the heating process, the uncoupling chlorophyll a (Chla) and converted pheophytin (Pheo) can form excitons of Chla-Pheo. The secondary structure α-helix of PSI proteins is stable up to 87-92 °C in the low-concentration surfactant solubilized PSI, and high-concentration surfactant and pigments uncoupling can accelerate the α-helical unfolding.

Remembering Navasard V. Karapetyan (1936-2015)

Navasard Vaganovich Karapetyan (September 6, 1936-March 6, 2015) began his scientific career at the Bach Institute of Biochemistry of the Russian Academy of Sciences, Moscow, and was associated with this institute for over 56 years. He worked in the area of biochemistry and biophysics of photosynthesis and was especially known for his studies on chlorophyll a fluorescence in higher plants and cyanobacteria, molecular organization of Photosystem I, photoprotective energy dissipation, and dynamics of energy migration in the two photosystems. We present here a brief biography and comments on the work of Navasard Karapetyan. We remember him as an enthusiastic person who had an unflagging curiosity, energy and profound sincere interest in many aspects of photosynthesis research.

Association of High Light-Inducible HliA/HliB Stress Proteins with Photosystem 1 Trimers and Monomers of the Cyanobacterium Synechocystis PCC 6803

Hlip (high light-inducible proteins) are important for protection of the photosynthetic apparatus of cyanobacteria from light stress. However, the interaction of these proteins with chlorophyll-protein complexes of thylakoids remains unclear. The association of HliA/HliB stress proteins with photosystem 1 (PS1) complexes of the cyanobacterium Synechocystis PCC 6803 was studied to understand their function. Western blotting demonstrated that stress-induced HliA/HliB proteins are associated with PS1 trimers in wild-type cells grown under moderate light condition (40 µmol photons/m(2) per sec). The content of these proteins increased 1.7-fold after light stress (150 µmol photons/m(2) per sec) for 1 h. In the absence of PS1 trimers (ΔpsaL mutant), the HliA/HliB proteins are associated with PS1 monomers and the PS2 complex. HliA/HliB proteins are associated with PS1 monomers but not with PS1 trimers in Synechocystis PS2-deficient mutant grown at 5 µmol photons/m(2) per sec; the content of Hli proteins associated with PS1 monomers increased 1.2-fold after light stress. The HliA/HliB proteins were not detected in wild-type cells of cyanobacteria grown in glucose-supplemented medium at 5 µmol photons/m(2) per sec, but light stress induces the synthesis of stress proteins associated with PS1 trimers. Thus, for the first time, the association of HliA/HliB proteins not only with PS1 trimers, but also with PS1 monomers is shown, which suggests a universal role of these proteins in the protection of the photosynthetic apparatus from excess light.

Long-wavelength chlorophylls in photosystem I of cyanobacteria: origin, localization, and functions

The structural organization of photosystem I (PSI) complexes in cyanobacteria and the origin of the PSI antenna long-wavelength chlorophylls and their role in energy migration, charge separation, and dissipation of excess absorbed energy are discussed. The PSI complex in cyanobacterial membranes is organized preferentially as a trimer with the core antenna enriched with long-wavelength chlorophylls. The contents of long-wavelength chlorophylls and their spectral characteristics in PSI trimers and monomers are species-specific. Chlorophyll aggregates in PSI antenna are potential candidates for the role of the long-wavelength chlorophylls. The red-most chlorophylls in PSI trimers of the cyanobacteria Arthrospira platensis and Thermosynechococcus elongatus can be formed as a result of interaction of pigments peripherally localized on different monomeric complexes within the PSI trimers. Long-wavelength chlorophylls affect weakly energy equilibration within the heterogeneous PSI antenna, but they significantly delay energy trapping by P700. When the reaction center is open, energy absorbed by long-wavelength chlorophylls migrates to P700 at physiological temperatures, causing its oxidation. When the PSI reaction center is closed, the P700 cation radical or P700 triplet state (depending on the P700 redox state and the PSI acceptor side cofactors) efficiently quench the fluorescence of the long-wavelength chlorophylls of PSI and thus protect the complex against photodestruction.

Effect of surfactants on apparent oxygen consumption of photosystem I isolated from Arthrospira platensis

Surfactants play a significant role in solubilization of photosystem I (PSI) in vitro. Triton X-100 (TX), n-Dodecyl-β-D-maltoside (DDM), and sodium dodecyl sulfate (SDS) were employed to solubilize PSI particles in MES buffer to compare the effect of surfactant and its dosage on the apparent oxygen consumption rate of PSI. Through a combined assessment of sucrose density gradient centrifugation, Native PAGE and 77 K fluorescence with the apparent oxygen consumption, the nature of the enhancement of the apparent oxygen consumption activity of PSI by surfactants has been analyzed. Aggregated PSI particles can be dispersed by surfactant molecules into micelles, and the apparent oxygen consumption rate is higher for surfactant-solubilized PSI than for integral PSI particles. For DDM, PSI particles are solubilized mostly as the integral trimeric form. For TX, PSI particles are solubilized as incomplete trimeric and some monomeric forms. For the much harsher surfactant, SDS, PSI particles are completely solubilized as monomeric and its subunit forms. The enhancement of the oxygen consumption rate cannot be explained only by the effects of surfactant on the equilibrium between monomeric and trimeric forms of solubililized PSI. Care must be taken when the electron transfer activity of PSI is evaluated by methods based on oxygen consumption because the apparent oxygen consumption rate is influenced by uncoupled chlorophyll (Chl) from PSI, i.e., the larger the amount of uncoupled Chl, the higher the rate of apparent oxygen consumption. 77 K fluorescence spectra can be used to ensure that there is no uncoupled Chl present in the system. In order to eliminate the effect of trace uncoupled Chl, an efficient physical quencher of (1)O2, such as 1 mM NaN3, may be added into the mixture.

Red chlorophyll excitation dynamics in Arthrospira platensis photosystem I trimeric complexes as studied by femtosecond transient absorption spectroscopy

Femtosecond absorption spectroscopy was applied to study for the first time excitation dynamics in isolated photosystem I trimers from Arthrospira platensis, which display extremely long-wavelength absorption peaks. Pump-probe spectra observed at 77K in the timescale of dozens of picoseconds upon 70-fs excitation revealed two maxima near 710 and 730 nm, which correspond to red chlorophyll forms. Bleaching at 680 nm developed in ∼ 200 fs, whereas the bleaching kinetics at 710 and 730 nm exhibited two components with time constants of 1 and 5.5 ps. Comparison of the kinetics of bleaching development at 710 nm and 730 nm with that of bleaching decay at 680 nm indicated that both long-wavelength forms of trimers are populated mainly via direct energy transfer from bulk chlorophyll.

Triton X-100 as an effective surfactant for the isolation and purification of photosystem I from Arthrospira platensis

Surfactants play important roles in the preparation, structural, and functional research of membrane proteins, and solubilizing and isolating membrane protein, while keeping their structural integrity and activity intact is complicated. The commercial n-Dodecyl-β-D-maltoside (DDM) and Triton X-100 (TX) were used as solubilizers to extract and purify trimeric photosystem I (PSI) complex, an important photosynthetic membrane protein complex attracting broad interests. With an optimized procedure, TX can be used as an effective surfactant to isolate and purify PSI, as a replace of the much more expensive DDM. A mechanism was proposed to interpret the solubilization process at surfactant concentrations lower than the critical solubilization concentration. PSI-TX and PSI-DDM had identical polypeptide bands, pigment compositions, oxygen consumption, and photocurrent activities. This provides an alternative procedure and paves a way for economical and large-scale trimeric PSI preparation.

Solubilization and stabilization of isolated photosystem I complex with lipopeptide detergents

It is difficult to maintain a target membrane protein in a soluble and functional form in aqueous solution without biological membranes. Use of surfactants can improve solubility, but it remains challenging to identify adequate surfactants that can improve solubility without damaging their native structures and biological functions. Here we report the use of a new class of lipopeptides to solubilize photosystem I (PS-I), a well known membrane protein complex. Changes in the molecular structure of these surfactants affected their amphiphilicity and the goal of this work was to exploit a delicate balance between detergency and biomimetic performance in PS-I solubilization via their binding capacity. Meanwhile, the effects of these surfactants on the thermal and structural stability and functionality of PS-I in aqueous solution were investigated by circular dichroism, fluorescence spectroscopy, SDS-PAGE analysis and O₂ uptake measurements, respectively. Our studies showed that the solubility of PS-I depended on both the polarity and charge in the hydrophilic head of the lipopeptides and the length of its hydrophobic tail. The best performing lipopeptides in favour of PS-I solubility turned out to be C14DK and C16DK, which were comparable to the optimal amphiphilicity of the conventional chemical surfactants tested. Lipopeptides showed obvious advantages in enhancing PS-I thermostability over sugar surfactant DDM and some full peptide amphiphiles reported previously. Fluorescence spectroscopy along with SDS-PAGE analysis demonstrated that lipopeptides did not undermine the polypeptide composition and conformation of PS-I after solubilization; instead they showed better performance in improving the structural stability and integrity of this multi-subunit membrane protein than conventional detergents. Furthermore, O₂ uptake measurements indicated that PS-I solubilized with lipopeptides maintained its functionality. The underlying mechanism for the favorable actions of lipopeptide in PS-I solubilization and stabilization is discussed.

Genetic decrease in fatty acid unsaturation of phosphatidylglycerol increased photoinhibition of photosystem I at low temperature in tobacco leaves

Leaves of transgenic tobacco plants with decreased levels of fatty acid unsaturation in phosphatidylglycerol (PG) exhibited a slightly lower level of the steady state oxidation of the photosystem I (PSI) reaction center P700 (P700(+)) than wild-type plants. The PSI photochemistry of wild-type plants was only marginally affected by high light treatments. Surprisingly, all plants of transgenic lines exhibited much higher susceptibility to photoinhibition of PSI than wild-type plants. This was accompanied by a 2.5-fold faster re-reduction rate of P700(+) in the dark, indicating a higher capacity for cyclic electron flow around PSI in high light treated transgenic leaves. This was associated with a much higher intersystem electron pool size suggesting over-reduction of the PQ pool in tobacco transgenic lines with altered PG unsaturation compared to wild-type plants. The physiological role of PG unsaturation in PSI down-regulation and modulation of the capacity of PSI-dependent cyclic electron flows and distribution of excitation light energy in tobacco plants under photoinhibitory conditions at low temperatures is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Red antenna states of Photosystem I trimers from Arthrospira platensis revealed by single-molecule spectroscopy

Single-molecule fluorescence spectroscopy at 1.4K was used to investigate the spectral properties of red (long-wavelength) chlorophylls in trimeric Photosystem I (PSI) complexes from the cyanobacterium Arthrospira platensis. Three distinct red antenna states could be identified in the fluorescence spectra of single PSI trimers from A. platensis in the presence of oxidized P700. Two of them are responsible for broad emission bands centered at 726 and 760nm. These bands are similar to those found in bulk fluorescence spectra measured at cryogenic temperatures. The broad fluorescence bands at ≅726 and ≅760nm belong to individual emitters that are broadened by strong electron-phonon coupling giving rise to a large Stokes-shift of about 20nm and rapid spectral diffusion. An almost perpendicular orientation of the transition dipole moments of F726 and F760 has to be assumed because direct excitation energy transfer does not occur between F726 and F760. For the first time a third red state assigned to the pool absorbing around 708nm could be detected by its zero-phonon lines. The center of the zero-phonon line distribution is found at ≅714nm. The spectral properties of the three red antenna states show a high similarity to the red antenna states found in trimeric PSI of Thermosynechoccocus elongatus. Based on these findings a similar organization of the red antenna states in PSI of these two cyanobacteria is discussed.

Structure and function of intact photosystem 1 monomers from the cyanobacterium Thermosynechococcus elongatus

Until now, the functional and structural characterization of monomeric photosystem 1 (PS1) complexes from Thermosynechococcus elongatus has been hampered by the lack of a fully intact PS1 preparation; for this reason, the three-dimensional crystal structure at 2.5 A resolution was determined with the trimeric PS1 complex [Jordan, P., et al. (2001) Nature 411 (6840), 909-917]. Here we show the possibility of isolating from this cyanobacterium the intact monomeric PS1 complex which preserves all subunits and the photochemical activity of the isolated trimeric complex. Moreover, the equilibrium between these complexes in the thylakoid membrane can be shifted by a high-salt treatment in favor of monomeric PS1 which can be quantitatively extracted below the phase transition temperature. Both monomers and trimers exhibit identical posttranslational modifications of their subunits and the same reaction centers but differ in the long-wavelength antenna chlorophylls. Their chlorophyll/P700 ratio (108 for the monomer and 112 for the trimer) is slightly higher than in the crystal structure, confirming mild preparation conditions. Interaction of antenna chlorophylls of the monomers within the trimer leads to a larger amount of long-wavelength chlorophylls, resulting in a higher photochemical activity of the trimers under red or far-red illumination. The dynamic equilibrium between monomers and trimers in the thylakoid membrane may indicate a transient monomer population in the course of biogenesis and could also be the basis for short-term adaptation of the cell to changing environmental conditions.

The mobility of PSI and PQ molecules in Spirulina platensis cells during state transition

Monomerization and trimerization of photosystem I (PSI) in cyanobacteria are reversible to response to light switched off and on, which leads to "energy spillover" to regulate excitation of the two photosystems in balance. Considering that PSI is a trans-membrane protein embedded in thylakoid membranes, the monomerization or trimerization must involve a movement of PSI in the membranes. In this work, the mobility of PSI was demonstrated by dependence of the monomerization and trimerization on temperature for intact Spirulina platensis cells undergoing a light-to-dark or a dark-to-light transition. Based on the characteristic absorbance of monomers and trimmers, it confirms that both monomerization and trimerization are temperature-sensitive. The relative populations of the monomers and trimmers are invariable above the phase transition temperature (T (PT)) while directly proportional to temperature below T (PT). On the other hand, the rate to reach the equilibrium population is proportional to temperature above T (PT) but invariable below T (PT). The PSI mobility and the temperature-dependent population are contrary to those of plastoquinone (PQ) molecules because PSI is a trans-membrane protein while PQ molecules are small diffusive electron carriers in thylakoid membranes as well as their distinctive sizes and environments. The less monomerization of PSI but the invariable time constant at lower temperature below T (PT) may be due to that accumulation of the reduced PQ molecules results in decrease of the stromal-side H(+) concentration which is a driving force of PSI monomerization.

Protective dissipation of excess absorbed energy by photosynthetic apparatus of cyanobacteria: role of antenna terminal emitters

Two mechanisms of photoprotective dissipation of the excessively absorbed energy by photosynthetic apparatus of cyanobacteria are described that divert energy from reaction centers. Energy dissipation, monitored as nonphotochemical fluorescence quenching, occurs at different steps of energy transfer within the phycobilisomes or core antenna of photosystem I. Although these mechanisms differ significantly, in both cases, energy dissipates mainly from terminal emitters: allophycocyanin B or core membrane linker protein (L(CM)) in phycobilisomes, or the longest-wavelength chlorophylls in photosystem I antenna. It is supposed that carotenoid-induced energy dissipation in phycobilisomes is triggered by light-induced transformation of the nonquenched state of antenna into quenched state due to conformation changes caused by orange carotinoid-binding protein (OCP)-phycobilisome interaction. Fluorescence of the longest-wavelength chlorophylls of photosystem I antenna is strongly quenched by P700 cation radical or by P700 triplet state, dependent on redox state of the acceptor side cofactors of photosystem I.

Quantum yield of P700+ photodestruction in isolated photosystem I complexes of the cyanobacterium Arthrospira platensis

The photostability of P700 cation radical (P700+) was studied by evaluating the quantum yields of P700(+) photodestruction in photosystem I (PSI) complexes of the cyanobacterium Arthrospira platensis. The time courses of P700+ photodestruction in PSI trimers and monomers have been measured in aerobic conditions under selective excitation of far-red absorption band of P700+ by intense light of laser diodes. Long-term exposure of PSI complexes to 808 or 870 nm laser light caused destruction of P700+ and antenna chlorophylls. The true integral quantum yield of P700+ photodestruction calculated from these data was less than 0.7-1.4 x 10(-8). Illumination of PSI complexes by 650 nm light caused destruction of antenna chlorophylls with true quantum yield of about 6-7 x 10(-6) and damage of P700 with apparent quantum yield 2-3 x 10(-8). Preferential photodestruction of the long-wavelength antenna chlorophyll absorbing at 710 nm as compared with bulk chlorophylls was observed. About three orders of difference in magnitude between quantum yields of P700+ and bulk chlorophyll photodestruction indicates that P700+ is extremely photostable for functioning as an efficient quencher of singlet excitation energy in PSI.

Overexpression of glycerol-3-phosphate acyltransferase gene improves chilling tolerance in tomato

A tomato (Lycopersicon esculentum Mill.) glycerol-3-phosphate acyltransferase gene (LeGPAT) was isolated. The deduced amino acid sequence revealed that LeGPAT contained four acyltransferase domains, showing high identities with GPAT in other plant species. A GFP fusion protein of LeGPAT was targeted to chloroplast in cowpea mesophyll protoplast. RNA gel blot showed that the mRNA accumulation of LeGPAT in the wild type (WT) was induced by chilling temperature. Higher expression levels were observed when tomato leaves were exposed to 4 degrees C for 4 h. RNA gel and western blot analysis confirmed that the sense gene LeGPAT was transferred into the tomato genome and overexpressed under the control of 35S-CaMV. Although tomato is classified as a chilling-sensitive plant, LeGPAT exhibited selectivity to 18:1 over 16:0. Overexpression of LeGPAT increased total activity of LeGPAT and cis-unsaturated fatty acids in PG in thylakoid membrane. Chilling treatment induced less ion leakage from the transgenic plants than from the WT. The photosynthetic rate and the maximal photochemical efficiency of PS II (Fv/Fm) in transgenic plants decreased more slowly during chilling stress and recovered faster than in WT under optimal conditions. The oxidizable P700 in both WT and transgenic plants decreased obviously at chilling temperature under low irradiance, but the oxidizable P700 recovered faster in transgenic plants than in the WT. These results indicate that overexpression of LeGPAT increased the levels of PG cis-unsaturated fatty acids in thylakoid membrane, which was beneficial for the recovery of chilling-induced PS I photoinhibition in tomato.

The state transition mechanism—simply depending on light-on and -off in Spirulina platensis

The state transition in cyanobacteria is a long-discussed topic of how the photosynthetic machine regulates the excitation energy distribution in balance between the two photosystems. In the current work, whether the state transition is realized by "mobile phycobilisome (PBS)" or "energy spillover" has been clearly answered by monitoring the spectral responses of the intact cells of the cyanobacterium Spirulina platensis. Firstly, light-induced state transition depends completely on a movement of PBSs toward PSI or PSII while the redox-induced one on not only the "mobile PBS" but also an "energy spillover". Secondly, the "energy spillover" is triggered by dissociation of PSI trimers into the monomers which specially occurs under a case from light to dark, while the PSI monomers will re-aggregate into the trimers under a case from dark to light, i.e., the PSI oligomerization is reversibly regulated by light switch on and off. Thirdly, PSI oligomerization is regulated by the local H(+) concentration on the cytosol side of the thylakoid membranes, which in turn is regulated by light switch on and off. Fourthly, PSI oligomerization change is the only mechanism for the "energy spillover". Thus, it can be concluded that the "mobile PBS" is a common rule for light-induced state transition while the "energy spillover" is only a special case when dark condition is involved.

Genome-wide analysis of light sensing in Prochlorococcus

Prochlorococcus MED4 has, with a total of only 1,716 annotated protein-coding genes, the most compact genome of a free-living photoautotroph. Although light quality and quantity play an important role in regulating the growth rate of this organism in its natural habitat, the majority of known light-sensing proteins are absent from its genome. To explore the potential for light sensing in this phototroph, we measured its global gene expression pattern in response to different light qualities and quantities by using high-density Affymetrix microarrays. Though seven different conditions were tested, only blue light elicited a strong response. In addition, hierarchical clustering revealed that the responses to high white light and blue light were very similar and different from that of the lower-intensity white light, suggesting that the actual sensing of high light is mediated via a blue-light receptor. Bacterial cryptochromes seem to be good candidates for the blue-light sensors. The existence of a signaling pathway for the redox state of the photosynthetic electron transport chain was suggested by the presence of genes that responded similarly to red and blue light as well as genes that responded to the addition of DCMU [3-(3,4-dichlorophenyl)-1,1-N-N'-dimethylurea], a specific inhibitor of photosystem II-mediated electron transport.

The synechocystis sp PCC 6803 oxa1 homolog is essential for membrane integration of reaction center precursor protein pD1

Synechocystis sp PCC 6803 Slr1471p, an Oxa1p/Alb3/YidC homolog, is an essential protein for cell viability for which functions in thylakoid membrane biogenesis and cell division have been proposed. Using a fusion of green fluorescent protein to the C terminus of Slr1471p, we found that the mutant slr1471-gfp is photochemically inhibited when light intensities increase to 80 micromol x m(-2) x s(-1). We show that photoinhibition correlates with an increased redox potential of the reaction center quinone Q(A)(-) and a decreased redox potential of Q(B)(-). Analysis reveals that membrane integration of the D1 precursor protein is affected, leading to the accumulation of pD1 in the membrane phase. We show that Slr1471p interacts directly with the D1 protein and discuss why the accumulation of pD1 in two reaction center assembly intermediates is dependent on Slr1471p.

Iron deficiency in cyanobacteria causes monomerization of photosystem I trimers and reduces the capacity for state transitions and the effective absorption cross section of photosystem I in vivo

The induction of the isiA (CP43') protein in iron-stressed cyanobacteria is accompanied by the formation of a ring of 18 CP43' proteins around the photosystem I (PSI) trimer and is thought to increase the absorption cross section of PSI within the CP43'-PSI supercomplex. In contrast to these in vitro studies, our in vivo measurements failed to demonstrate any increase of the PSI absorption cross section in two strains (Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803) of iron-stressed cells. We report that iron-stressed cells exhibited a reduced capacity for state transitions and limited dark reduction of the plastoquinone pool, which accounts for the increase in PSII-related 685 nm chlorophyll fluorescence under iron deficiency. This was accompanied by lower abundance of the NADP-dehydrogenase complex and the PSI-associated subunit PsaL, as well as a reduced amount of phosphatidylglycerol. Nondenaturating polyacrylamide gel electrophoresis separation of the chlorophyll-protein complexes indicated that the monomeric form of PSI is favored over the trimeric form of PSI under iron stress. Thus, we demonstrate that the induction of CP43' does not increase the PSI functional absorption cross section of whole cells in vivo, but rather, induces monomerization of PSI trimers and reduces the capacity for state transitions. We discuss the role of CP43' as an effective energy quencher to photoprotect PSII and PSI under unfavorable environmental conditions in cyanobacteria in vivo.

Light Harvesting in Photosystem I Supercomplexes,

In photosynthetic membranes of cyanobacteria, algae, and higher plants, photosystem I (PSI) mediates light-driven transmembrane electron transfer from plastocyanin or cytochrome c6 to the ferredoxin-NADP complex. The oxidoreductase function of PSI is sensitized by a reversible photooxidation of primary electron donor P700, which launches a multistep electron transfer via a series of redox cofactors of the reaction center (RC). The excitation energy for the functioning of the primary electron donor in the RC is delivered via the chlorophyll core antenna in the complex with peripheral light-harvesting antennas. Supermolecular complexes of the PSI acquire remarkably different structural forms of the peripheral light-harvesting antenna complexes, including distinct pigment types and organizational principles. The PSI core antenna, being the main functional unit of the supercomplexes, provides an increased functional connectivity in the chlorophyll antenna network due to dense pigment packing resulting in a fast spread of the excitation among the neighbors. Functional connectivity within the network as well as the spectral overlap of antenna pigments allows equilibration of the excitation energy in the depth of the whole membrane within picoseconds and loss-free delivery of the excitation to primary donor P700 within 20-40 ps. Low-light-adapted cyanobacteria under iron-deficiency conditions extend this capacity via assembly of efficiently energy coupled rings of CP43-like complexes around the PSI trimers. In green algae and higher plants, less efficient energy coupling in the eukaryotic PSI-LHCI supercomplexes is probably a result of the structural adaptation of the Chl a/b binding LHCI peripheral antenna that not only extends the absorption cross section of the PSI core but participates in regulation of excitation flows between the two photosystems as well as in photoprotection.

Molecular reorganization induced by Ca2+ of plant photosystem I reconstituted into phosphatidylglycerol liposomes

The interaction of divalent cations with biomembranes is important for a number of biological processes. In this study, the regulatory effect of Ca2+ on the interaction between plant spinach photosystem I (PSI) particles and negatively charged lipid phosphatidylglycerol (PG) was investigated by circular dichroism (CD) spectroscopy. It was found that in the absence of CaCl2, PG causes an increase in alpha-helix and a decrease in disordered conformations of protein secondary structures of PSI, the beta-sheet and turns being almost unaffected. Meanwhile, the same effect also enhances the excitonic interactions relating to Chl a and Chl b from the PSI core complex and external antenna light-harvesting complex (LHCI). By contrast, in the presence of CaCl2, PG hardly interferes with the structure of the proteins' skeleton of PSI, but it can depress the excitonic interactions for Chl b of LHCI and for PSI core complex Chl a at (-) 433.5 nm of the CD signal which is accompanied by a blue shift of its peak. It is most likely that the neutralization of the phosphate groups in the PSI-PG complex and the negative surface charges of PSI, and partial dehydration in the vicinity of the ester CO region of the PG polar head group by the Ca-ions modify the interaction between PSI and PG, thereby inducing molecular reorganization of protein and pigments within both the external antenna LHCI and PSI core complex in proteoliposomes.

Effect of phosphatidylglycerol on molecular organization of photosystem I

Phosphatidylglycerol (PG) is the only anionic phospholipid in photosynthetic membrane. In this study, photosystem I (PSI) particles obtained from plant spinach were reconstituted into PG liposomes at a relatively high concentration. The results from visible absorption, fluorescence emission, and circular dichroism (CD) spectra reveal an existence of the interactions of PSI with PG. PG effect causes blue-shift and intensity decrease of Chl a peak bands in the absorption and 77 K fluorescence emission. The visible CD spectra indicate that the excitonic interactions for Chl a and Chl b molecules were enhanced upon reconstitution. Furthermore, more or less blue- or red-shift of the peaks characterized by Chl a, Chl b, and carotenoid molecules are also occurred. Simultaneously, an increase in alpha-helix and a decrease particularly in the disordered conformations of protein secondary structures are observed. In addition, the same effect also leads to somewhat more tryptophan (Trp) residues exposed to the polar environment. These results demonstrate that some alteration of molecular organization occurs within both the external antenna LHCI and PSI core complex after PSI reconstitution.

Interaction of pigment-protein complexes within aggregates stimulates dissipation of excess energy

Pigment-protein complexes in photosynthetic membranes exist mainly as aggregates that are functionally active as monomers but more stable due to their ability to dissipate excess energy. Dissipation of energy in the photosystem I (PSI) trimers of cyanobacteria takes place with a contribution of the long-wavelength chlorophylls whose excited state is quenched by cation radical of P700 or P700 in its triplet state. If P700 in one of the monomer complexes within a PSI trimer is oxidized, energy migration from antenna of other monomer complexes to cation radical of P700 via peripherally localized long-wavelength chlorophylls results in energy dissipation, thus protecting PSI complex of cyanobacteria against photodestruction. It is suggested that dissipation of excess absorbed energy in aggregates of the light-harvesting complex LHCII of higher plants takes place with a contribution of peripherally located chlorophylls and carotenoids.

Photochemical activities of plant photosystem I particles reconstituted into phosphatidylglycerol liposomes

Phosphatidylglycerol (PG) is the only anionic phospholipid in photosynthetic membrane and the important component of photosystem I (PSI). In this study, the interaction of PG with PSI particle from spinach was investigated by using reconstitution method. The results from the properties of electron transport, fluorescence emission, turbidity, and protein secondary structures in PSI complex incorporated into PG liposomes revealed the existence of PSI-PG interactions. A stimulation and an inhibition of oxygen uptake in PSI particle at a low and higher PG/chlorophyll mass ratio, respectively, were observed. Moreover, an additional enhancement and depression of electron flow in the PSI-PG complexes were occurred in the reaction medium containing CaCl2 at concentrations below and above 5 mM, the aggregation threshold of the reconstituted membranes, respectively. The results demonstrated that the maintenance of the structural optimization was needed for a stimulation of electron transport at a low PG/PSI mass ratio, while a decay of this PSI activity at high PG/PSI ratio was the result of inhibition of the energy transfer from LHCI to PSI reaction center induced by the dissociation of LHCI-680.

P700+- and 3P700-induced quenching of the fluorescence at 760 nm in trimeric Photosystem I complexes from the cyanobacterium Arthrospira platensis

The 5 K absorption spectrum of Photosystem I (PS I) trimers from Arthrospira platensis (old name: Spirulina platensis) exhibits long-wavelength antenna (exciton) states absorbing at 707 nm (called C707) and at 740 nm (called C740). The lowest energy state (C740) fluoresces around 760 nm (F760) at low temperature. The analysis of the spectral properties (peak position and line width) of the lowest energy transition (C740) as a function of temperature within the linear electron-phonon approximation indicates a large optical reorganization energy of approximately 110 cm(-1) and a broad inhomogeneous site distribution characterized by a line width of approximately 115 cm(-1). Linear dichroism (LD) measurements indicate that the transition dipole moment of the red-most state is virtually parallel to the membrane plane. The relative fluorescence yield at 760 nm of PS I with P700 oxidized increases only slightly when the temperature is lowered to 77 K, whereas in the presence of reduced P700 the fluorescence yield increases nearly 40-fold at 77 K as compared to that at room temperature (RT). A fluorescence induction effect could not be resolved at RT. At 77 K the fluorescence yield of PS I trimers frozen in the dark in the presence of sodium ascorbate decreases during illumination by about a factor of 5 due to the irreversible formation of (P700+)F(A/B-) in about 60% of the centers and the reversible accumulation of the longer-lived state (P700+)FX-. The quenching efficiency of different functionally relevant intermediate states of the photochemistry in PS I has been studied. The redox state of the acceptors beyond A(0) does not affect F760. Direct kinetic evidence is presented that the fluorescence at 760 nm is strongly quenched not only by P700+ but also by 3P700. Similar kinetics were observed for flash-induced absorbance changes attributed to the decay of 3P700 or P700+, respectively, and flash-induced fluorescence changes at 760 nm measured under identical conditions. A nonlinear relationship between the variable fluorescence around 760 nm and the [P700red]/[P700total] ratio was derived from titration curves of the absorbance change at 826 nm and the variable fluorescence at 760 nm as a function of the redox potential imposed on the sample solution at room temperature before freezing. The result indicates that the energy exchange between the antennae of different monomers within a PS I trimer stimulates quenching of F760 by P700+.

Responsibility of phosphatidylglycerol for biogenesis of the PSI complex

Phosphatidylglycerol (PG) ubiquitous in thylakoid membranes of photosynthetic organisms was previously shown to contribute to accumulation of chlorophyll through analysis of the cdsA- mutant of a cyanobacterium Synechocystis sp. PCC6803 defective in PG synthesis (SNC1). Here, we characterized effects of manipulation of the PG content in thylakoid membranes of Synechocystis sp. PCC6803 on the photosystem complexes to specify roles of PG in biogenesis of thylakoid membranes. SNC1 cells with PG deprivation in vivo, together with the chlorophyll decrease, exhibited a decline not in PSII, but in PSI, at the complex level as well as the subunit levels. On the other hand, the decrease in the PSI complex was accounted for by a remarkable decrease in the PSI trimer with an increase in the monomer. These symptoms of SNC1 cells were complemented in vivo by supplementation of PG. Besides, a reduction in the PG content of thylakoid membranes isolated from the wild type in vitro on treatment with phospholipase A2 (PLA2), similar to the PG-deprivation in SNC1 in vivo, brought about a decrease in the trimer population of PSI with accumulation of the monomer. These results demonstrated that PG contributes to the synthesis and/or stability of the PSI complex for maintenance of the cellular content of chlorophyll, and also to construction of the PSI trimer from the monomer at least through stabilization of the trimerized conformation.

Phosphatidylglycerol is essential for oligomerization of photosystem I reaction center

Our earlier studies with the pgsA mutant of Synechocystis PCC6803 demonstrated the important role of phosphatidylglycerol (PG) in PSII dimer formation and in electron transport between the primary and secondary electron-accepting plastoquinones of PSII. Using a long-term depletion of PG from pgsA mutant cells, we could induce a decrease not only in PSII but also in PSI activity. Simultaneously with the decrease in PSI activity, dramatic structural changes of the PSI complex were detected. A 21-d PG depletion resulted in the degradation of PSI trimers and concomitant accumulation of monomer PSI. The analyses of PSI particles isolated by MonoQ chromatography showed that, following the 21-d depletion, PSI trimers were no longer detectable in the thylakoid membranes. Immunoblot analyses revealed that the PSI monomers accumulating in the PG-depleted mutant cells do not contain PsaL, the protein subunit thought to be responsible for the trimer formation. Nevertheless, the trimeric structure of PSI reaction center could be restored by readdition of PG, even in the presence of the protein synthesis inhibitor lincomycin, indicating that free PsaL was present in thylakoid membranes following the 21-d PG depletion. Our data suggest an indispensable role for PG in the PsaL-mediated assembly of the PSI reaction center.

Species-specific differences of the spectroscopic properties of P700: analysis of the influence of non-conserved amino acid residues by site-directed mutagenesis of photosystem I from Chlamydomonas reinhardtii

We applied optical spectroscopy, magnetic resonance techniques, and redox titrations to investigate the properties of the primary electron donor P700 in photosystem I (PS I) core complexes from cyanobacteria (Thermosynechococcus elongatus, Spirulina platensis, and Synechocystis sp. PCC 6803), algae (Chlamydomonas reinhardtii CC2696), and higher plants (Spinacia oleracea). Remarkable species-specific differences of the optical properties of P700 were revealed monitoring the (3P700-P700) and (P700+.-P700) absorbance and CD difference spectra. The main bleaching band in the Qy region differs in peak position and line width for the various species. In cyanobacteria the absorbance of P700 extends more to the red compared with algae and higher plants which is favorable for energy transfer from red core antenna chlorophylls to P700 in cyanobacteria. The amino acids in the environment of P700 are highly conserved with two distinct deviations. In C. reinhardtii a Tyr is found at position PsaB659 instead of a Trp present in all other organisms, whereas in Synechocystis a Phe is found instead of a Trp at the homologous position PsaA679. We constructed several mutants in C. reinhardtii CC2696. Strikingly, no PS I could be detected in the mutant YW B659 indicating steric constraints unique to this organism. In the mutants WA A679 and YA B659 significant changes of the spectral features in the (3P700-P700), the (P700+.-P700) absorbance difference and in the (P700+.-P700) CD difference spectra are induced. The results indicate structural differences among PS I from higher plants, algae, and cyanobacteria and give further insight into specific protein-cofactor interactions contributing to the optical spectra.

Iron stress responses in the cyanobacterium Synechococcus sp. PCC7942

In the present study, we describe the sequential events by which the cyanobacterium Synechococcus sp. PCC 7942 adapts to iron deficiency. In doing so, we have tried to elucidate both short and long-term acclimation to low iron stress in order to understand how the photosynthetic apparatus adjusts to low iron conditions. Our results show that after an initial step, where CP43' is induced and where ferredoxin is partly replaced by flavodoxin, the photosynthetic unit starts to undergo major rearrangements. All measured components of Photosystem I (PSI), PSII and cytochrome (Cyt) f decrease relative to chlorophyll (Chl) a. The photochemical efficiencies of the two photosystems also decline during this phase of acclimation. The well-known drop in phycobilisome content measured as phycocyanin (PC)/Chl was not due to an increased degradation, but rather to a decreased rate of synthesis. The largest effects of iron deficiency were observed on PSI, the most iron-rich structure of the photosynthetic apparatus. In the light of the recent discovery of an iron deficiency induced CP43' ring around PSI a possible dual function of this protein as both an antenna and a quencher is discussed. We also describe the time course of a blue shift in the low temperature Chl emission peak around 715 nm, which originates in PSI. The shift might reflect the disassembly and/or degradation of PSI during iron deficiency and, as a consequence, PSI might under these conditions be found predominantly in a monomeric form. We suggest that the observed functional and compositional alterations represent cellular acclimation enabling growth and development under iron deficiency, and that growth ceases when the acclimation capacity is exhausted. However, the cells remain viable even after growth has ceased, since they resumed growth once iron was added back to the culture.

Chlorophyll b inhibits the formation of photosystem I trimer in Synechocystis sp. PCC6803

Chlorophyllide a oxygenase (CAO) catalyzes two-step oxygenation reactions and converts chlorophyllide a to chlorophyllide b. When CAO was introduced into the Synechocystis sp. PCC6803 genome, chlorophyll b was synthesized and incorporated into P700-chlorophyll a-protein complexes. Curve analysis of photosystem I particles showed that Ca687 was decreased with a concomitant increase in Cb652 suggesting that chlorophyll b was incorporated into Ca687-binding sites. When the level of chlorophyll b was high, Ca704, which is known as red chlorophyll and photosystem I trimers were decreased. Formation of photosystem I trimers is discussed in relation to red chlorophyll and chlorophyll b accumulation.

Electric field effects on red chlorophylls, beta-carotenes and P700 in cyanobacterial Photosystem I complexes

We have probed the absorption changes due to an externally applied electric field (Stark effect) of Photosystem I (PSI) core complexes from the cyanobacteria Synechocystis sp. PCC 6803, Synechococcus elongatus and Spirulina platensis. The results reveal that the so-called C719 chlorophylls in S. elongatus and S. platensis are characterized by very large polarizability differences between the ground and electronically excited states (with Tr(Deltaalpha) values up to about 1000 A(3) f(-2)) and by moderately high change in permanent dipole moments (with average Deltamu values between 2 and 3 D f(-1)). The C740 chlorophylls in S. platensis and, in particular, the C708 chlorophylls in all three species give rise to smaller Stark shifts, which are, however, still significantly larger than those found before for monomeric chlorophyll. The results confirm the hypothesis that these states originate from strongly coupled chlorophyll a molecules. The absorption and Stark spectra of the beta-carotene molecules are almost identical in all complexes and suggest similar or slightly higher values for Tr(Deltaalpha) and Deltamu than for those of beta-carotene in solution. Oxidation of P700 did not significantly change the Stark response of the carotenes and the red antenna states C719 and C740, but revealed in all PSI complexes changes around 700-705 and 690-693 nm, which we attribute to the change in permanent dipole moments of reduced P700 and the chlorophylls responsible for the strong absorption band at 690 nm with oxidized P700, respectively.

Preparation of luciferase-bacterial magnetic particle complex by artificial integration of MagA-luciferase fusion protein into the bacterial magnetic particle membrane

MagA is an iron-translocating protein found in the membranes of magnetic bacterium. Luciferase-bacterial magnetic particle (BMP) complexes were prepared by artificially inserting MagA-luciferase fusion proteins into the membranes of BMPs from Magnetospirillum magneticum strain AMB-1. Fusion proteins were from recombinant Escherichia coli membranes. MagA-Luc fusion proteins were integrated by sonication in vitro. Successful integration of fusion proteins was confirmed by luciferase luminescence on BMPs. Maximum luminescence was obtained after sonication for 3 min with a solution containing 300 mM NaCl, and is 18 times higher compared with recombinant Luc-BMPs generated using previously reported gene fusion techniques.

Interaction of phycobilisomes with photosystem II dimers and photosystem I monomers and trimers in the cyanobacterium Spirulina platensis

Distribution of phycobilisomes between photosystem I (PSI) and photosystem II (PSII) complexes in the cyanobacterium Spirulina platensis has been studied by analysis of the action spectra of H2 and O2 photoevolution and by analysis of the 77 K fluorescence excitation and emission spectra of the photosystems. PSI monomers and trimers were spectrally discriminated in the cell by the unique 760 nm low-temperature fluorescence, emitted by the trimers under reductive conditions. The phycobilisome-specific 625 nm peak was observed in the action spectra of both PSI and PSII, as well as in the 77 K fluorescence excitation spectra for chlorophyll emission at 695 nm (PSII), 730 nm (PSI monomers), and 760 nm (PSI trimers). The contributions of phycobilisomes to the absorption, action, and excitation spectra were derived from the in vivo absorption coefficients of phycobiliproteins and of chlorophyll. Analyzing the sum of PSI and PSII action spectra against the absorption spectrum and estimating the P700:P680 reaction center ratio of 5.7 in Spirulina, we calculated that PSII contained only 5% of the total chlorophyll, while PSI carried the greatest part, about 95%. Quantitative analysis of the obtained data showed that about 20% of phycobilisomes in Spirulina cells are bound to PSII, while 60% of phycobilisomes transfer the energy to PSI trimers, and the remaining 20% are associated with PSI monomers. A relevant model of organization of phycobilisomes and chlorophyll pigment-protein complexes in Spirulina is proposed. It is suggested that phycobilisomes are connected with PSII dimers, PSI trimers, and coupled PSI monomers.

A reaction-induced FT-IR study of cyanobacterial photosystem I

In oxygenic photosynthesis, photosystem I (PSI) conducts light-driven electron transfer from plastocyanin to ferredoxin. The reactions are initiated when the primary chlorophyll donor, P(700), is photooxidized. P(700) is a chlorophyll dimer ligated by the core subunits psaA and psaB. A difference Fourier transform infrared spectrum, associated with P(700)(+)-minus-P(700), can be acquired using PSI from the cyanobacterium Synechocystis sp. PCC 6803. This spectrum reflects contributions from oxidation-sensitive modes of chlorophyll, as well as from oxidation-induced structural changes in amino acid residues and the peptide backbone. Oxidation-induced structural changes may play a role in the facilitation and control of electron-transfer reactions involving the primary donor. In this paper, we report that photooxidation of P(700) in cyanobacterial PSI perturbs a cysteine residue. At 264 and 80 K, a downshift of a SH stretching vibration from 2560 to 2551 cm(-1) is observed. Such a downshift is consistent with an increase in hydrogen bonding, with a change in C-S-H conformation, or with an electric field effect. Deuterium exchange experiments were also performed. While the perturbed cysteine is in a protein region that is resistant to exchange, other (2)H-sensitive vibrational chl and amino acid bands were observed. From the (2)H exchange experiments, we conclude that photooxidation of P(700) perturbs internal or bound water molecules in PSI and that the P(700)(+)-minus-P(700) spectrum is (2)H exchange-sensitive. The results are consistent with structural complexity in the PSI primary donor, as previously suggested [Kim, S., and Barry, B. A. (2000) J. Am. Chem. Soc. 122, 4980-4981]. Possible explanations, including a partial enolization of P(700)(+), are discussed.

Time-resolved fluorescence emission measurements of photosystem I particles of various cyanobacteria: a unified compartmental model

Photosystem I (PS-I) contains a small fraction of chlorophylls (Chls) that absorb at wavelengths longer than the primary electron donor P700. The total number of these long wavelength Chls and their spectral distribution are strongly species dependent. In this contribution we present room temperature time-resolved fluorescence data of five PS-I core complexes that contain different amounts of these long wavelength Chls, i.e., monomeric and trimeric photosystem I particles of the cyanobacteria Synechocystis sp. PCC 6803, Synechococcus elongatus, and Spirulina platensis, which were obtained using a synchroscan streak camera. Global analysis of the data reveals considerable differences between the equilibration components (3.4-15 ps) and trapping components (23-50 ps) of the various PS-I complexes. We show that a relatively simple compartmental model can be used to reproduce all of the observed kinetics and demonstrate that the large kinetic differences are purely the result of differences in the long wavelength Chl content. This procedure not only offers rate constants of energy transfer between and of trapping from the compartments, but also well-defined room temperature emission spectra of the individual Chl pools. A pool of red shifted Chls absorbing around 702 nm and emitting around 712 nm was found to be a common feature of all studied PS-I particles. These red shifted Chls were found to be located neither very close to P700 nor very remote from P700. In Synechococcus trimeric and Spirulina monomeric PS-I cores, a second pool of red Chls was present which absorbs around 708 nm, and emits around 721 nm. In Spirulina trimeric PS-I cores an even more red shifted second pool of red Chls was found, absorbing around 715 nm and emitting at 730 nm.

Evolutionarily related insertion pathways of bacterial, mitochondrial, and thylakoid membrane proteins

The inner membranes of eubacteria and mitochondria, as well as the chloroplast thylakoid membrane, contain essential proteins that function in oxidative phosphorylation and electron transport processes or in photosynthesis. Because most of the organellar proteins are nuclear encoded, they are synthesized in the cytoplasm and subsequently imported into the organelle before they are inserted into the membrane. This review focuses on the pathways of protein insertion into the inner membrane of eubacteria and mitochondria and into the chloroplast thylakoid membrane. In many respects, insertion of proteins into the inner membrane of bacteria is a process similar to that used by proteins of the thylakoid membrane. In both of these systems a signal recognition particle (SRP) and a SecYE-translocase are involved, as in translocation into the endoplasmic reticulum. The pathway of proteins into the mitochondrial membranes appears to be different in that it involves no SecYE-like components. A conservative pathway, recently identified in mitochondria, involves the Oxa1 protein for the insertion of proteins from the matrix. The presence of Oxa1 homologues in eubacteria and chloroplasts suggests that this pathway is evolutionarily conserved.

Towards structural determination of the water-splitting enzyme. Purification, crystallization, and preliminary crystallographic studies of photosystem II from a thermophilic cyanobacterium

A photosystem II preparation from the thermophilic cyanobacterium Synechococcus elongatus, which is especially suitable for three-dimensional crystallization in a fully active form was developed. The efficient purification method applied here yielded 10 mg of protein of a homogenous dimeric complex of about 500 kDa within 2 days. Detailed characterization of the preparation demonstrated a fully active electron transport chain from the manganese cluster to plastoquinone in the Q(B) binding site. The oxygen-evolving activity, 5000-6000 micromol of O(2)/(h.mg of chlorophyll), was the highest so far reported and is maintained even at temperatures as high as 50 degrees C. The crystals obtained by the vapor diffusion method diffracted to a resolution of 4.3 A. The space group was determined to be P2(1)2(1)2(1) with four photosystem II dimers per unit cell. Analysis of the redissolved crystals revealed that activity, supramolecular organization, and subunit composition were maintained during crystallization.

Novel, rapid purification of the membrane protein photosystem I by high-performance liquid chromatography on porous materials

New porous materials have been tested for their potential to speed up purification of membrane proteins. As an example the purification of photosystem I, a light-driven electron pump from the cyanobacterium Synechocystis PCC6803, was optimized. The combination of two HPLC steps (an anion-exchange chromatography followed by a hydrophobic interaction chromatography) yields homogeneous monomeric or trimeric photosystem I as determined by gel filtration and gel electrophoresis. In comparison to traditional purification schemes our method is at least three-times faster and allows for easy scale-up.