Trimeric Photosystem I Research Articles

Presence of low-energy chlorophylls d in photosystem I trimer and monomer cores isolated from Acaryochloris sp. NBRC 102871.

Acaryochloris species belong to a special category of cyanobacteria possessing chlorophyll (Chl) d. One of the photosynthetic characteristics of Acaryochloris marina MBIC11017 is that the absorption spectra of photosystem I (PSI) showed almost no bands and shoulders of low-energy Chls d over 740nm. In contrast, the absorption spectra of other Acaryochloris species showed a shoulder around 740nm, suggesting that low-energy Chls d within PSI are diversified among Acaryochloris species. In this study, we purified PSI trimer and monomer cores from Acaryochloris sp. NBRC 102871 and examined their protein and pigment compositions and spectral properties. The protein bands and pigment compositions of the PSI trimer and monomer of NBRC102871 were virtually identical to those of MBIC11017. The absorption spectra of the NBRC102871 PSIs exhibited a shoulder around 740nm, whereas the fluorescence spectra of PSI trimer and monomer displayed maximum peaks at 754 and 767nm, respectively. These spectral properties were different from those of MBIC11017, indicating the presence of low-energy Chls d within the NBRC102871 PSIs. Moreover, we analyzed the NBRC102871 genome to identify amino acid sequences of PSI proteins and compared them with those of the A. marina MBIC11017 and MBIC10699 strains whose genomes are available. The results showed that some of the sequences in NBRC102871 were distinct from those in MBIC11017 and MBIC10699. These findings provide insights into the variety of low-energy Chls d with respect to the protein environments of PSI cores among the three Acaryochloris strains.

Function of iron-stress-induced protein A in cyanobacterial cells with monomeric and trimeric photosystem I.

The acclimation of cyanobacteria to iron deficiency is crucial for their survival in natural environments. In response to iron deficiency, many cyanobacterial species induce the production of a pigment-protein complex called iron-stress-induced protein A (IsiA). IsiA proteins associate with photosystem I (PSI) and can function as light-harvesting antennas or dissipate excess energy. They may also serve as chlorophyll storage during iron limitation. In this study, we examined the functional role of IsiA in cells of Synechocystis sp. PCC 6803 grown under iron limitation conditions by measuring the cellular IsiA content and its capability to transfer energy to PSI. We specifically tested the effect of the oligomeric state of PSI by comparing wild-type (WT) Synechocystis sp. PCC 6803 with mutants lacking specific subunits of PSI, namely PsaL/PsaI (PSI subunits XI/VIII) and PsaF/PsaJ (PSI subunits III/IX). Time-resolved fluorescence spectroscopy revealed that IsiA formed functional PSI3-IsiA18 supercomplexes, wherein IsiA effectively transfers energy to PSI on a timescale of 10 ps at room temperature-measured in isolated complexes and in vivo-confirming the primary role of IsiA as an accessory light-harvesting antenna to PSI. However, a notable fraction (40%) remained unconnected to PSI, supporting the notion of a dual functional role of IsiA. Cells with monomeric PSI under iron deficiency contained, on average, only 3 to 4 IsiA complexes bound to PSI. These results show that IsiA can transfer energy to trimeric and monomeric PSI but to varying degrees and that the acclimatory production of IsiA under iron stress is controlled by its ability to perform its light-harvesting function.

Isolation and characterization of trimeric and monomeric PSI cores from Acaryochloris marina MBIC11017.

Photosystem I (PSI) catalyzes light-induced electron-transfer reactions and has been observed to exhibit various oligomeric states and different energy levels of chlorophylls (Chls) in response to oligomerization. However, the biochemical and spectroscopic properties of a PSI monomer containing Chls d are not well understood. In this study, we successfully isolated and characterized PSI monomers from the cyanobacterium Acaryochloris marina MBIC11017, and compared their properties with those of the A. marina PSI trimer. The PSI trimers and monomers were prepared using trehalose density gradient centrifugation after anion-exchange and hydrophobic interaction chromatography. The polypeptide composition of the PSI monomer was found to be consistent with that of the PSI trimer. The absorption spectrum of the PSI monomer showed the Qy band of Chl d at 704nm, which was blue-shifted from the peak at 707nm observed in the PSI-trimer spectrum. The fluorescence-emission spectrum of the PSI monomer measured at 77K exhibited a peak at 730nm without a broad shoulder in the range of 745-780nm, which was clearly observed in the PSI-trimer spectrum. These spectroscopic properties of the A. marina PSI trimer and monomer suggest different formations of low-energy Chls d between the two types of PSI cores. Based on these findings, we discuss the location of low-energy Chls d in A. marina PSIs.

Mass spectrometry and spectroscopic characterization of a tetrameric photosystem I supercomplex from Leptolyngbya ohadii, a desiccation-tolerant cyanobacterium

Cyanobacteria inhabiting desert biological soil crusts face the harsh conditions of the desert. They evolved a suite of strategies toward desiccation-hydration cycles mixed with high light irradiations, etc. In this study we purified and characterized the structure and function of Photosystem I (PSI) from Leptolyngbya ohadii, a desiccation-tolerant desert cyanobacterium. We discovered that PSI forms tetrameric (PSI-Tet) aggregate. We investigated it by using sucrose density gradient centrifugation, clear native PAGE, high performance liquid chromatography, mass spectrometry (MS), time-resolved fluorescence (TRF) and time-resolved transient absorption (TA) spectroscopy. MS analysis identified the presence of two PsaB and two PsaL proteins in PSI-Tet and uniquely revealed that PsaLs are N-terminally acetylated in contrast to non-modified PsaL in the trimeric PSI from Synechocystis sp. PCC 6803. Chlorophyll (Chl) a fluorescence decay profiles of the PSI-Tet performed at 77 K revealed two emission bands at ∼690 nm and 725 nm with the former appearing only at early delay time. The main fluorescence emission peak, associated with emission from the low energy Chls a, decays within a few nanoseconds. TA studies demonstrated that the 725 nm emission band is associated with low energy Chls a with absorption band clearly resolved at ∼710 nm at 77 K. In summary, our work suggests that the heterogenous composition of PsaBs and PsaL in PSI-Tet is related with the adaptation mechanisms needed to cope with stressful conditions under which this bacterium naturally grows.

Thermal site energy fluctuations in photosystem I: new insights from MD/QM/MM calculations.

Cyanobacterial photosystem I (PSI) is one of the most efficient photosynthetic machineries found in nature. Due to the large scale and complexity of the system, the energy transfer mechanism from the antenna complex to the reaction center is still not fully understood. A central element is the accurate evaluation of the individual chlorophyll excitation energies (site energies). Such an evaluation must include a detailed treatment of site specific environmental influences on structural and electrostatic properties, but also their evolution in the temporal domain, because of the dynamic nature of the energy transfer process. In this work, we calculate the site energies of all 96 chlorophylls in a membrane-embedded model of PSI. The employed hybrid QM/MM approach using the multireference DFT/MRCI method in the QM region allows to obtain accurate site energies under explicit consideration of the natural environment. We identify energy traps and barriers in the antenna complex and discuss their implications for energy transfer to the reaction center. Going beyond previous studies, our model also accounts for the molecular dynamics of the full trimeric PSI complex. Via statistical analysis we show that the thermal fluctuations of single chlorophylls prevent the formation of a single prominent energy funnel within the antenna complex. These findings are also supported by a dipole exciton model. We conclude that energy transfer pathways may form only transiently at physiological temperatures, as thermal fluctuations overcome energy barriers. The set of site energies provided in this work sets the stage for theoretical and experimental studies on the highly efficient energy transfer mechanisms in PSI.

Structural basis for the absence of low-energy chlorophylls in a photosystem I trimer from Gloeobacter violaceus.

Photosystem I (PSI) is a multi-subunit pigment-protein complex that functions in light-harvesting and photochemical charge-separation reactions, followed by reduction of NADP to NADPH required for CO2 fixation in photosynthetic organisms. PSI from different photosynthetic organisms has a variety of chlorophylls (Chls), some of which are at lower-energy levels than its reaction center P700, a special pair of Chls, and are called low-energy Chls. However, the sites of low-energy Chls are still under debate. Here, we solved a 2.04-Å resolution structure of a PSI trimer by cryo-electron microscopy from a primordial cyanobacterium Gloeobacter violaceus PCC 7421, which has no low-energy Chls. The structure shows the absence of some subunits commonly found in other cyanobacteria, confirming the primordial nature of this cyanobacterium. Comparison with the known structures of PSI from other cyanobacteria and eukaryotic organisms reveals that one dimeric and one trimeric Chls are lacking in the Gloeobacter PSI. The dimeric and trimeric Chls are named Low1 and Low2, respectively. Low2 is missing in some cyanobacterial and eukaryotic PSIs, whereas Low1 is absent only in Gloeobacter. These findings provide insights into not only the identity of low-energy Chls in PSI, but also the evolutionary changes of low-energy Chls in oxyphototrophs.

Protein Extraction Efficiency and Selectivity of Esterified Styrene-Maleic Acid Copolymers in Thylakoid Membranes.

Amphiphilic styrene-maleic acid copolymers (SMAs) have been shown to effectively extract membrane proteins surrounded by an annulus of native membrane lipids via the formation of nanodiscs. Recent reports have shown that 2-butoxyethanol-functionalized SMA derivatives promote the extraction of membrane proteins from thylakoid membranes, whereas unfunctionalized SMA is essentially ineffective. However, it is unknown how the extent of functionalization and identity of sidechains impact protein solubilization and specificity. Herein, we show that the monoesterification of an SMA polymer with hydrophobic alkoxy ethoxylate sidechains leads to an increased solubilization efficiency (SE) of trimeric photosystem I (PSI) from the membranes of cyanobacterium Thermosynechococcus elongatus. The specific SMA polymer used in this study, PRO 10235, cannot encapsulate single PSI trimers from this cyanobacterium; however, as it is functionalized with alkoxy ethoxylates of increasing alkoxy chain length, a clear increase in the trimeric PSI SE is observed. Furthermore, an exponential increase in the SE is observed when >50% of the maleic acid repeat units are monoesterified with long alkoxy ethoxylates, suggesting that the PSI extraction mechanism is highly dependent on both the number and length of the attached side chains.

Closing the Gap for Electronic Short-Circuiting: Photosystem I Mixed Monolayers Enable Improved Anisotropic Electron Flow in Biophotovoltaic Devices.

Well‐defined assemblies of photosynthetic protein complexes are required for an optimal performance of semi‐artificial energy conversion devices, capable of providing unidirectional electron flow when light‐harvesting proteins are interfaced with electrode surfaces. We present mixed photosystem I (PSI) monolayers constituted of native cyanobacterial PSI trimers in combination with isolated PSI monomers from the same organism. The resulting compact arrangement ensures a high density of photoactive protein complexes per unit area, providing the basis to effectively minimize short‐circuiting processes that typically limit the performance of PSI‐based bioelectrodes. The PSI film is further interfaced with redox polymers for optimal electron transfer, enabling highly efficient light‐induced photocurrent generation. Coupling of the photocathode with a [NiFeSe]‐hydrogenase confirms the possibility to realize light‐induced H2 evolution.

Structure of a cyanobacterial photosystem I surrounded by octadecameric IsiA antenna proteins

Iron-stress induced protein A (IsiA) is a chlorophyll-binding membrane-spanning protein in photosynthetic prokaryote cyanobacteria, and is associated with photosystem I (PSI) trimer cores, but its structural and functional significance in light harvesting remains unclear. Here we report a 2.7-Å resolution cryo-electron microscopic structure of a supercomplex between PSI core trimer and IsiA from a thermophilic cyanobacterium Thermosynechococcus vulcanus. The structure showed that 18 IsiA subunits form a closed ring surrounding a PSI trimer core. Detailed arrangement of pigments within the supercomplex, as well as molecular interactions between PSI and IsiA and among IsiAs, were resolved. Time-resolved fluorescence spectra of the PSI–IsiA supercomplex showed clear excitation-energy transfer from IsiA to PSI, strongly indicating that IsiA functions as an energy donor, but not an energy quencher, in the supercomplex. These structural and spectroscopic findings provide important insights into the excitation-energy-transfer and subunit assembly mechanisms in the PSI–IsiA supercomplex.

Structural basis for energy and electron transfer of the photosystem I-IsiA-flavodoxin supercomplex.

Under iron-deficiency stress, which occurs frequently in natural aquatic environments, cyanobacteria reduce the amount of iron-enriched proteins, including photosystem I (PSI) and ferredoxin (Fd), and upregulate the expression of iron-stress-induced proteins A and B (IsiA and flavodoxin (Fld)). Multiple IsiAs function as the peripheral antennae that encircle the PSI core, whereas Fld replaces Fd as the electron receptor of PSI. Here, we report the structures of the PSI3-IsiA18-Fld3 and PSI3-IsiA18 supercomplexes from Synechococcus sp. PCC 7942, revealing features that are different from the previously reported PSI structures, and a sophisticated pigment network that involves previously unobserved pigment molecules. Spectroscopic results demonstrated that IsiAs are efficient light harvesters for PSI. Three Flds bind symmetrically to the trimeric PSI core-we reveal the detailed interaction and the electron transport path between PSI and Fld. Our results provide a structural basis for understanding the mechanisms of light harvesting, energy transfer and electron transport of cyanobacterial PSI under stressed conditions.

Physiological and evolutionary implications of tetrameric photosystem I in cyanobacteria.

Photosystem I (PSI) is present as trimeric complexes in most characterized cyanobacteria and as monomers in plants and algae. Recent reports of tetrameric PSI have raised questions regarding its structural basis, physiological role, phylogenetic distribution and evolutionary significance. Here, we examined PSI in 61 cyanobacteria, showing that tetrameric PSI, which correlates with the psaL gene and a distinct genomic structure, is widespread among heterocyst-forming cyanobacteria and their close relatives. Physiological studies revealed that expression of tetrameric PSI is favoured under high light, with an increased content of novel PSI-bound carotenoids (myxoxanthophyll, canthaxanthan and echinenone). In sum, this work suggests that tetrameric PSI is an adaptation to high light intensity, and that change in PsaL leads to monomerization of trimeric PSI, supporting the hypothesis of tetrameric PSI being the evolutionary intermediate in the transition from cyanobacterial trimeric PSI to monomeric PSI in plants and algae.

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 T800nm, ellipticity θ686nm and θ222nm, upon slow heating (1.5°C per minute) of samples in Tris buffer (20mM, 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.

Redox-state dependent blinking of single photosystem I trimers at around liquid-nitrogen temperature

Efficient light harvesting in a photosynthetic antenna system is disturbed by a ragged and fluctuating energy landscape of the antenna pigments in response to the conformation dynamics of the protein. This situation is especially pronounced in Photosystem I (PSI) containing red shifted chlorophylls (red Chls) with the excitation energy much lower than the primary donor. The present study was conducted to clarify light-harvesting dynamics of PSI isolated from Synechocystis sp. PCC6803 by using single-molecule spectroscopy at liquid‑nitrogen temperatures. Fluorescence emission at around 720 nm from the red Chls in single PSI trimers was monitored at 80–100 K. Intermittent variations in the emission intensities, so-called blinking, were frequently observed. Its time scale lay in several tens of seconds. The blinking amplitude depended on the redox state of the phylloquinone (A1). Electrochromic shifts of Chls induced by the negative charge on A1 were calculated based on the X-ray crystallographic structure. A Chl molecule, Chl-A839 (numbering according to PDB 5OY0), bound near A1 was found to have a large electrochromic shift. This Chl has strong exciton coupling with neighboring Chl (A838) whose site energy was predicted to be determined by interaction with an arginine residue (ArgF84) [Adolphs et al., 2010]. A possible scenario of the blinking was proposed. Conformational fluctuations of ArgF84 seesaw the excitation-energy of Chl-A838, which perturbs the branching ratio of excitation-energy between the red Chl and the cationic form of P700 as a quencher. The electrochromic shift of Chl-A839 enhances the effect of the conformation dynamics of ArgF84.

X-ray structure of an asymmetrical trimeric ferredoxin-photosystem I complex.

Photosystem I (PSI), a large protein complex located in the thylakoid membrane, mediates the final step in light-driven electron transfer to the stromal electron carrier protein ferredoxin (Fd). Here, we report the first structural description of the PSI-Fd complex from Thermosynechococcus elongatus. The trimeric PSI complex binds three Fds in a non-equivalent manner. While each is recognized by a PSI protomer in a similar orientation, the distances between Fds and the PSI redox centres differ. Fd binding thus entails loss of the exact three-fold symmetry of the PSI's soluble subunits, inducing structural perturbations which are transferred to the lumen through PsaF. Affinity chromatography and nuclear magnetic resonance analyses of PSI-Fd complexes support the existence of two different Fd-binding states, with one Fd being more tightly bound than the others. We propose a dynamic structural basis for productive complex formation, which supports fast electron transfer between PSI and Fd.

Spectral differences between monomers and trimers of photosystem I depend on the interaction between peripheral chlorophylls of neighboring monomers in trimer

The spatial position of the long-wavelength chlorophylls in trimer of pigment-protein complex of photosystem I (PSI) have been determined bymodeling the optical fluorescence absorption and emission spectra for two hypothetical models of PSI trimer. The calculation has been performed using X-ray diffraction data on the spatial position of chlorophylls in PSI monomer; the pigment site energies were taken from the studies of other researchers, while interactions between monomers in trimer are considered as fitting parameters. The interaction energy between the chlorophylls spaced by a distance smaller than 10 A was estimated based on the concept of extended dipole−dipole interaction. The model under study allowed us to evaluate the influence of the exciton interaction between peripheral pigments on the optical response of PSI trimer. The intensity and shape of stationary fluorescence line turned out to be sensitive to the PSI monomer packing in trimer. A visualization of the density matrix for low-energy exciton states has made it possible to estimate the localization of long-wavelength chlorophylls in PSI trimer.

Lateral Segregation of Photosystem I in Cyanobacterial Thylakoids.

Photosystem I (PSI) is the dominant photosystem in cyanobacteria and it plays a pivotal role in cyanobacterial metabolism. Despite its biological importance, the native organization of PSI in cyanobacterial thylakoid membranes is poorly understood. Here, we use atomic force microscopy (AFM) to show that ordered, extensive macromolecular arrays of PSI complexes are present in thylakoids from Thermosynechococcus elongatus, Synechococcus sp PCC 7002, and Synechocystis sp PCC 6803. Hyperspectral confocal fluorescence microscopy and three-dimensional structured illumination microscopy of Synechocystis sp PCC 6803 cells visualize PSI domains within the context of the complete thylakoid system. Crystallographic and AFM data were used to build a structural model of a membrane landscape comprising 96 PSI trimers and 27,648 chlorophyll a molecules. Rather than facilitating intertrimer energy transfer, the close associations between PSI primarily maximize packing efficiency; short-range interactions with Complex I and cytochrome b6f are excluded from these regions of the membrane, so PSI turnover is sustained by long-distance diffusion of the electron donors at the membrane surface. Elsewhere, PSI-photosystem II contact zones provide sites for docking phycobilisomes and the formation of megacomplexes. PSI-enriched domains in cyanobacteria might foreshadow the partitioning of PSI into stromal lamellae in plants, similarly sustained by long-distance diffusion of electron carriers.

Variety in excitation energy transfer processes from phycobilisomes to photosystems I and II.

The light-harvesting antennas of oxygenic photosynthetic organisms capture light energy and transfer it to the reaction centers of their photosystems. The light-harvesting antennas of cyanobacteria and red algae, called phycobilisomes (PBSs), supply light energy to both photosystem I (PSI) and photosystem II (PSII). However, the excitation energy transfer processes from PBS to PSI and PSII are not understood in detail. In the present study, the energy transfer processes from PBS to PSs in various cyanobacteria and red algae were examined in vivo by selectively exciting their PSs or PBSs, and measuring the resulting picosecond to nanosecond time-resolved fluorescences. By observing the delayed fluorescence spectrum of PBS-selective excitation in Arthrospira platensis, we demonstrated that energy transfer from PBS to PSI via PSII (PBS→PSII→PSI transfer) occurs even for PSI trimers. The contribution of PBS→PSII→PSI transfer was species dependent, being largest in the wild-type of red alga Pyropia yezoensis (formerly Porphyra yezoensis) and smallest in Synechococcus sp. PCC 7002. Comparing the time-resolved fluorescence after PSs- and PBS-selective excitation, we revealed that light energy flows from CP43 to CP47 by energy transfer between the neighboring PSII monomers in PBS-PSII supercomplexes. We also suggest two pathways of energy transfer: direct energy transfer from PBS to PSI (PBS→PSI transfer) and indirect transfer through PSII (PBS→PSII→PSI transfer). We also infer that PBS→PSI transfer conveys light energy to a lower-energy red chlorophyll than PBS→PSII→PSI transfer.

Thermostability of photosystem I trimers and monomers from the cyanobacterium Thermosynechococcus elongatus

The performance of solar energy conversion into alternative energy sources in artificial systems highly depends on the thermostability of photosystem I (PSI) complexes Terasaki et al. (2007), Iwuchukwu et al. (2010), Kothe et al. (2013) . To assess the thermostability of PSI complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus heating induced perturbations on the level of secondary structure of the proteins were studied. Changes were monitored by Fourier transform infrared (FT-IR) spectra in the mid-IR region upon slow heating (1°C per minute) of samples in D2O phosphate buffer (pD 7.4) from 20°C to 100°C. These spectra showed distinct changes in the Amide I region of PSI complexes as a function of the rising temperature. Absorbance at the Amide I maximum of PSI monomers (centered around 1653cm−1), gradually dropped in two temperature intervals, i.e. 60–75 and 80–90°C. In contrast, absorbance at the Amide I maximum of PSI trimers (around 1656cm−1) dropped only in one temperature interval 80–95°C. The thermal profile of the spectral shift of α-helices bands in the region 1656–1642cm−1 confirms the same two temperature intervals for PSI monomers and only one interval for trimers. Apparently, the observed absorbance changes at the Amide I maximum during heating of PSI monomers and trimers are caused by deformation and unfolding of α-helices. The absence of absorbance changes in the interval of 20–65°C in PSI trimers is probably caused by a greater stability of protein secondary structure as compared to that in monomers. Upon heating above 80°C a large part of α-helices both in trimers and monomers converts to unordered and aggregated structures. Spectral changes of PSI trimers and monomers heated up to 100°C are irreversible due to protein denaturation and non-specific aggregation of complexes leading to new absorption bands at 1618–1620cm−1. We propose that monomers shield the denaturation sensitive sides at the monomer/monomer interface within a trimer, making the oligomeric structure more stable against thermal stress.

Light-induced hydrogen production by photosystem I–Pt nanoparticle conjugates immobilized in porous glass plate nanopores

In recent years, a number of light-induced hydrogen production systems composed of photosystem I (PSI) and hydrogen production catalysts (e.g. hydrogenases and Pt nanoparticles) have been reported. However, the utility of these systems under aerobic conditions is limited due to their poor stability in the presence of oxygen. The development of light-induced hydrogen production systems that work under aerobic conditions is, therefore, of great importance to establish artificial photosynthetic devices. Ideally, these systems should utilise water as an electron source, via water splitting by photosystem II (PSII). We report the construction of a novel light-induced hydrogen production system composed of PSI-platinum nanoparticle conjugates and cytochrome c 6 (cyt c 6) immobilised in nanoporous glass plates (PGP50, 50-nm pore diameter). PSI trimer (PSIt) from Thermosynechococcus elongatus and Pt nanoparticles (PtNPs) were conjugated via electrostatic interactions (PSIt-PtNP). PSIt-PtNP and cyt c 6 were spontaneously absorbed in nanopores of PGP50 without denaturation. Upon irradiation in the presence of ascorbate as a sacrificial electron donor, catalytic H2 evolution was observed for PSIt-PtNP immobilised in the pores of PGP50 (PSIt-PtNP/PGP50) under both anaerobic and aerobic conditions, indicating that an effective photoinduced electron transfer system had been established. PSIt-PtNP/PGP50 was found to exhibit improved oxygen resistivity over the homogeneous solution system consisting of PSIt-PtNP, cyt c 6, and ascorbate, suggesting that the PSIt-PtNP/PGP50 system could be a potential candidate for artificial photosynthetic systems. The distribution of the components, PSIt-PtNP and cyt c 6, in PGP50 was characterised to discuss the efficiency of light-induced hydrogen production.

Cryo-EM structure of a tetrameric cyanobacterial photosystem I complex reveals novel subunit interactions

Photosystem I (PSI) of the thermophilic cyanobacterium Chroococcidiopsis sp. TS-821 (TS-821) forms tetramers Li et al. (2014). Two-dimensional maps obtained by single particle electron microscopy (EM) clearly show that the tetramer lacks four-fold symmetry and is actually composed of a dimer of dimers with C2 symmetry. The resolution of these negative stain 2D maps did not permit the placement of most of the small PSI subunits, except for PsaL. Therefore cryo-EM was used for 3D reconstruction of the PSI tetramer complex. A 3D model at ~11.5Å resolution was obtained and a 2D map within the membrane plane of ~6.1Å. This data was used to build a model that was compared with the high-resolution structure of the PSI of Thermosynechococcus elongatus (T. elongatus) at 2.5Å. This comparison reveals key differences in which subunits are involved in the two different interfaces, interface type 1 within a dimer and interface type 2 between dimers. The type 1 interface in TS-821 is similar to the monomer interface in the trimeric PSI from T. elongatus, with interactions between subunits PsaA, -B, -I, -L and M. In type 2 the interaction is only between PsaA, -B and -L. Unlike the trimeric PSI, the central cavity of the complex is not filled with the PsaL-derived helical bundle, but instead seems filled with lipids. The physiological or evolutionary advantage of the tetramer is unknown. However, the presence of both dimers and tetramers in the thylakoid membrane suggest a dynamic equilibrium that shifts towards the tetramers in high light.