Photosynthesis Research最新文献

Phycobilisomes (PBSs) are the major light-harvesting complexes in the cyanobacteria and red algae and they consist of a central core and peripheral rods that are attached to the core. The PBS cores contain 2-5 allophycocyanin cylinders that are organized by ApcE. At the present, structures of PBS with tricylindrical and pentacylindrical cores have been determined while the structure of the PBS with a bicylindrical core is yet to be revealed. Here we report the cryo-EM structure of PBS with bicylindrical core from Synechococcus elongatus PCC 7942 (Synechococcus 7942) at an overall resolution of approximately 3 Å. Similar to the PBS with a tricylindrical core, six peripheral rods are attached to the core by the rod-core linker protein CpcG in the PBS of Synechococcus 7942 even though the core lacks the top AP cylinder, which is important for the attachment of peripheral rods to the tricylindrical cores. We found that the C-terminus of ApcE in the Synechococcus 7942 was involved in interacting with both CpcG and CpcB of a top peripheral rod, compensating for the absence of the top AP cylinder of the core and maintaining PBS stability. Analysis of the bilin distribution reveals that distance of excitation energy transfer from top peripheral rods to the terminal emitters is approximately 15% shorter compared to the PBS with tricylindrical cores. Although there are 30% fewer bilin chromophores in the Synechococcus 7942 PBS core compared with the tricylindrical core, the aromatic residue ring in the Synechococcus 7942 PBS core is conserved, supporting the suggestion that these aromatic residues from AP and linker proteins are critical to the energy transfer of PBS.

The generation of voltage (ΔV) by purified photosynthetic protein-pigment complex of photosystem I (PS I) from the cyanobacterium Synechocystis sp. PCC 6803 immobilized onto a porous Millipore nitrocellulose membrane filter (MF) and contacting with indium tin oxide (ITO) semiconductor glass slides, termed ITO|PS I - MF|ITO have been directly measured. In the presence of the ascorbate/2,6-dichlorophenolindophenol redox couple and disaccharide trehalose, photoinduced generation of ΔV with a maximum amplitude was observed upon light irradiation for more than 1 h. After storing the ITO|PS I - MF|ITO system in a desiccator at 22 ± 1 °C and ~ 65% relative humidity for 7 days in the dark, the photovoltage amplitude decreased by ~ 15%. A significant drop in the ΔV amplitude was observed on the 14th day of storage under these conditions. The amplitude of the stable ΔV signal was fully restored after re-injecting fresh buffer directly into the system, although maximum amplitude was reached more slowly. It is clear that longer preservation of PS I functional activity during storage of the system can be achieved at a higher relative humidity in the desiccator, since the photovoltage was completely suppressed within 24 h when the system was stored in the dark in the laboratory room. The protective role of trehalose is also discussed.

Based on earlier data concerning the reaction enthalpy [Formula: see text] of Ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO) from spinach (Frank et al. Phys Chem Chem Phys 2:1301-1304, 2000), it is shown that the specificity constant [Formula: see text], indicating the ability of RUBISCO to discriminate between CO₂ and O₂ as substrate, can be determined from an isothermal titration calorimetric (ITC) measurement of [Formula: see text] as a function of the substrate concentration ratio [Formula: see text]. The approach does not need any radioactive materials and might be a cost-effective alternative for RUBISCO engineering to screen for variants with a high specificity constant.

Photosystem I (PSI) from the siphonous green alga Bryopsis corticulans carries ten subunits of Light-Harvesting Complex I (LHCI) that enlarge its absorption cross-section yet must still deliver excitations rapidly to the P₇₀₀ reaction center. Here we investigate the light-harvesting function of PSI-LHCI, the excitation energy transfer within the supercomplex and how it affects the overall photochemical trapping lifetime. We performed picosecond time-resolved fluorescence spectroscopy at room temperature and 77 K and femtosecond transient absorption spectroscopy on purified PSI-LHCI supercomplex. Ultrafast spectroscopy showed that spectral equilibration in the antenna occurs mainly in the first 1 ps after excitation and is virtually complete within 10 ps. Nonetheless, the fluorescence lifetime of the supercomplex was 49-55 (±1) ps, depending on excitation wavelength, compared to 22 ps in the core complex. This result suggests that energy transfer continues over tens of ps, long after thermal equilibration between high- and low-energy excited states. The fluorescence analysis suggests that two factors contribute to the increased excitation lifetime - a thermodynamic factor, directly stemming from the increased antenna size as well as a kinetic factor, i.e. a rate limitation of trapping imposed by the energy transfer from LHCI to PSI. The average time of excitation transfer from LHCI to the reaction center was estimated to be close to 30 ps. Therefore, energy transfer from the enlarged antenna does not impose a sizeable kinetic limitation to the photochemical quantum efficiency of the photosystem.

Acaryochloris marina (A. marina) is a distinctive cyanobacterium that contains rod-shaped phycobiliproteins (PBPs) and uses chlorophyll (Chl) d as its principal photosynthetic pigment, enabling it to absorb far-red light for photosynthesis. Under varying environmental light conditions, several cyanobacteria have been reported to exhibit decreased PBP content under red light and increased levels under green light. For A. marina, a reduction in PBP has also been observed under red and far-red light. However, the changes in light-harvesting and energy-transfer processes of A. marina in response to different light qualities remain insufficiently characterized. In the present study, A. marina cells were cultured under white, blue, green, yellow, red, or far-red LEDs. Compared with cultures grown under white-LEDs, blue-, red-, and far-red-LED-grown cells showed a decrease in the PBP/Chl d ratio, whereas green- and yellow-LED-grown cells showed an increase. The energy-transfer process of green-LED-grown cells at 77 K exhibited pronounced alterations in photosystem II. Based on these findings, we discuss the variations in light-harvesting and energy-transfer processes of A. marina under different light conditions.

We model the energy transfers between two B850 antenna units using the hybrid hierarchical equation (HEOM) method, where intra-B850 dynamics is evaluated nonperturbatively using HEOM, whereas inter-B850 transfers are treated by the generalized Förster theory. To account for a conformation-induced energetic disorder we use a realistic model emerging from a quantitative explanation of the single molecule spectral fluctuations in the B850 antenna. The disorder produces a variety of configurations including realizations where the exciton states are predominantly localized at the red-shifted sites. A slow detrapping of excitation from the red states can compete with the transfers to the adjacent B850 unit, thus slowing down the whole equilibration dynamics. In order to highlight the interplay between intra-B850 dynamics and migration to neighboring B850 complex we analyze the diversity of scenarios produced by specific disorder patterns within both donor and acceptor units.