Thylakoid grana stacking revealed by multiplex genome editing of LHCII encoding genes

Land plant chloroplasts differ from algal ones for their thylakoid membranes being organized in grana: piles of vesicles paired by their stromal surface, forming domains including Photosystem (PS) II and its antenna while excluding PS I and ATPase to stroma membranes, connecting grana stacks. The molecular basis of grana stacking remain unclear. We obtained genotypes lacking the trimeric antenna complex (Lhcb1-2-3), the monomeric Lhcb4-5-6, or both. Full deletion caused loss of grana, while either monomers or trimers support 50% stacking. The expression of Lhcb5 alone restored stacking at 50%, while Lhcb2 alone produced huge grana which broke down upon light exposure. Cyclic electron transport was maintained in the lack of stacking, while excitation energy balance between photosystems and the repair efficiency of damaged Photosystem II were affected. We conclude that grana evolved for need of regulating energy balance between photosystems under terrestrial canopy involving rapid changes in photon spectral distribution.

Generation of Hydrogen and Oxygen from Water by Solar Energy Conversion

Photosynthesis is considered to be one of the promising areas of cheap and environmentally friendly energy. Photosynthesis involves the process of water oxidation with the formation of molecular oxygen and hydrogen as byproducts. The aim of the present article is to review the energy (light) phase of photosynthesis based on the published X-ray studies of photosystems I and II (PS-I and PS-II). Using modern ideas about semiconductors and biological semiconductor structures, the mechanisms of H+, O2↑, e− generation from water are described. At the initial stage, PS II produces hydrogen peroxide from water as a result of the photoenzymatic reaction, which is oxidized in the active center of PS-II on the Mn4CaO5 cluster to form O2↑, H+, e−. Mn4+ is reduced to Mn2+ and then oxidized to Mn4+ with the transfer of reducing the equivalents of PS-I. The electrons formed are transported to PS-I (P 700), where the electrochemical reaction of water decomposition takes place in a two-electrode electrolysis system with the formation of gaseous oxygen and hydrogen. The proposed functioning mechanisms of PS-I and PS-II can be used in the development of environmentally friendly technologies for the production of molecular hydrogen.

Oxidative Modification of LHC II Associated with Photosystem II and PS I-LHC I-LHC II Membranes

Under aerobic conditions the production of Reactive Oxygen Species (ROS) by electron transport chains is unavoidable, and occurs in both autotrophic and heterotrophic organisms. In photosynthetic organisms both Photosystem II (PS II) and Photosystem I (PS I), in addition to the cytochrome b6/f complex, are demonstrated sources of ROS. All of these membrane protein complexes exhibit oxidative damage when isolated from field-grown plant material. An additional possible source of ROS in PS I and PS II is the distal, chlorophyll-containing light-harvesting array LHC II, which is present in both photosystems. These serve as possible sources of 1O2 produced by the interaction of 3O2 with 3chl* produced by intersystem crossing. We have hypothesized that amino acid residues close to the sites of ROS generation will be more susceptible to oxidative modification than distant residues. In this study, we have identified oxidized amino acid residues in a subset of the spinach LHC II proteins (Lhcb1 and Lhcb2) that were associated with either PS II membranes (i.e. BBYs) or PS I-LHC I-LHC II membranes, both of which were isolated from field-grown spinach. We identified oxidatively modified residues by high-resolution tandem mass spectrometry. Interestingly, two different patterns of oxidative modification were evident for the Lhcb1 and Lhcb2 proteins from these different sources. In the LHC II associated with PS II membranes, oxidized residues were identified to be located on the stromal surface of Lhcb1 and, to a much lesser extent, Lhcb2. Relatively few oxidized residues were identified as buried in the hydrophobic core of these proteins. The LHC II associated with PS I-LHC I-LHC II membranes, however, exhibited fewer surface-oxidized residues but, rather a large number of oxidative modifications buried in the hydrophobic core regions of both Lhcb1 and Lhcb2, adjacent to the chlorophyll prosthetic groups. These results appear to indicate that ROS, specifically 1O2, can modify the Lhcb proteins associated with both photosystems and that the LHC II associated with PS II membranes represent a different population from the LHC II associated with PS I-LHC I-LHC II membranes.

Room temperature XFEL crystallography reveals asymmetry in the vicinity of the two phylloquinones in photosystem I

Photosystem I (PS I) has a symmetric structure with two highly similar branches of pigments at the center that are involved in electron transfer, but shows very different efficiency along the two branches. We have determined the structure of cyanobacterial PS I at room temperature (RT) using femtosecond X-ray pulses from an X-ray free electron laser (XFEL) that shows a clear expansion of the entire protein complex in the direction of the membrane plane, when compared to previous cryogenic structures. This trend was observed by complementary datasets taken at multiple XFEL beamlines. In the RT structure of PS I, we also observe conformational differences between the two branches in the reaction center around the secondary electron acceptors A1A and A1B. The π-stacked Phe residues are rotated with a more parallel orientation in the A-branch and an almost perpendicular confirmation in the B-branch, and the symmetry breaking PsaB-Trp673 is tilted and further away from A1A. These changes increase the asymmetry between the branches and may provide insights into the preferential directionality of electron transfer.

Structural Analysis of a Polysaccharide Isolated from the Alkali Extract of an Edible Mushroom, Pleurotus sajor-caju, Cultivar Black Japan

A heteropolysaccharide (PS-I) and an insoluble glucan (PS-II) were isolated from the alkaline extract of an edible mushroom Pleurotus sajor-caju, cultivar Black Japan. The polysaccharide (PS-I) was found to consist of D-glucose and D-galactose in a molar ratio of 3:1. Based on total acid hydrolysis, methylation analysis, and NMR experiments (1H, 13C, DQF-COSY, TOCSY, NOESY, ROESY, HMQC, and HMBC), the structure of the repeating unit of the polysaccharide was established.

When Problem Solving Followed by Instruction Works: Evidence for Productive Failure

When learning a new concept, should students engage in problem solving followed by instruction (PS-I) or instruction followed by problem solving (I-PS)? Noting that there is a passionate debate about the design of initial learning, we report evidence from a meta-analysis of 53 studies with 166 comparisons that compared PS-I with I-PS design. Our results showed a significant, moderate effect in favor of PS-I (Hedge’s g 0.36 [95% confidence interval 0.20; 0.51]). The effects were even stronger (Hedge’s g ranging between 0.37 and 0.58) when PS-I was implemented with high fidelity to the principles of Productive Failure (PF), a subset variant of PS-I design. Students’ grade level, intervention time span, and its (quasi-)experimental nature contributed to the efficacy of PS-I over I-PS designs. Contrasting trends were, however, observed for younger age learners (second to fifth graders) and for the learning of domain-general skills, for which effect sizes favored I-PS. Overall, an estimation of true effect sizes after accounting for publication bias suggested a strong effect size favoring PS-I (Hedge’s g 0.87).

From problem-solving to sensemaking: A comparative meta-analysis of preparatory approaches for future learning

Against the backdrop of a growing body of research showing the effectiveness of problem-solving activities followed by instruction (PS-I), we report a meta-analysis of the effectiveness of three broad categories of preparatory activities on future learning from instruction: (a) problem-solving followed by instruction (PS-I), (b) scaffolded problem-solving followed by instruction (+PS-I), or (c) an alternative sensemaking activity followed by instruction (!PS-I)? We examined 118 experimental comparisons spanning 33 articles that compared PS-I with +PS-I and !PS-I designs. Although scaffolding was descriptively associated with a small effect size, there was no significant difference relative to PS-I (Hedge’s g -0.08 [95% CI -0.20, 0.04]). Additionally, PS-I exhibited a non-significant moderate effect (Hedge’s g 0.22 [95% CI -0.06, 0.51]) compared to !PS-I. Bayesian analyses strongly favored the null hypothesis for the comparison of PS-I with +PS-I (suggesting a 99% probability of the difference in effect between these designs being less than 0.2), while it suggested a 40.37% probability of at least a moderate effect favoring PS-I relative to !PS-I. Further, the estimation of true effect sizes after accounting for the publication bias suggested moderate effect sizes in favor of PS-I, when considering both comparison conditions +PS-I (Hedge’s g 0.55) and !PS-I (Hedge’s g 0.64).