Nathan Nelson

Plant photosystem I (PSI) consists of at least 13 nuclear-encoded and 4 chloroplast-encoded subunits that together act as a sunlight-driven oxidoreductase. Here we report the structure of a PSI assembly intermediate that we isolated from greening oat seedlings. The assembly intermediate shows an absence of at least eight subunits, including PsaF and LHCI, and lacks photoreduction activity. The data show that PsaF is a regulatory checkpoint that promotes the assembly of LHCI, effectively coupling biogenesis to function.

Photosynthesis, as the primary source of energy for all life forms, plays a crucial role in maintaining the global balance of energy, entropy, and enthalpy in living organisms. Among its various building blocks, photosystem I (PSI) is responsible for light-driven electron transfer, crucial for generating cellular reducing power. PSI acts as a light-driven plastocyanin-ferredoxin oxidoreductase and is situated in the thylakoid membranes of cyanobacteria and the chloroplasts of eukaryotic photosynthetic organisms. Comprehending the structure and function of the photosynthetic machinery is essential for understanding its mode of action. New insights are offered into the structure and function of PSI and its associated light-harvesting proteins, with a specific focus on the remarkable structural conservation of the core complex and high plasticity of the peripheral light-harvesting complexes.

Chlamydomonas reinhardtii Photosystem II (PSII) is a dimer consisting of at least 13 nuclear-encoded and four chloroplast-encoded protein subunits that collectively function as a sunlight-driven oxidoreductase. In this study, we present the inaugural structure of a green alga PSII assembly intermediate (pre-PSII-int). This intermediate was isolated from chloroplast membranes of the temperature-sensitive mutant TSP4, cultivated for 14 hours at a non-permissive temperature. The assembly state comprises a monomer containing subunits A, B, C, D, E, F, H, I, K, and two novel assembly factors, Psb1 and Psb2. Psb1 is identified as a novel transmembrane helix located adjacent to PsbE and PsbF (cytochrome b559). The absence of PsbJ, typically found in mature PSII close to this position, indicates that Psb1 functions as an assembly factor. Psb2 is an eukaryotic homolog of the cyanobacterial assembly factor Psb27. The presence of iron, coupled with the absence of Q_A, Q_B, and the manganese cluster, implies a protective mechanism against photodamage and provides insights into the intricate assembly process.

Photosynthesis stands as a unique biological phenomenon that can be comprehensively explored across a wide spectrum, from femtoseconds to eons. Across each timespan, a delicate interplay exists between coupling and inherent deviations that are essential for sustaining the overall efficiency of the system. Both quantum mechanics and thermodynamics act as guiding principles for the diverse processes occurring from femtoseconds to eons. Processes such as excitation energy transfer and the accumulation of oxygen in the atmosphere, along with the proliferation of organic matter on the Earth’s surface, are all governed by the coupling–slip principle. This article will delve into select time points along this expansive scale. It will highlight the interconnections between photosynthesis, the global population, disorder, and the issue of global warming.

Green alga Chlorella ohadii is known for its ability to carry out photosynthesis under harsh conditions. Using cryogenic electron microscopy (cryoEM), we obtained a high-resolution structure of PSII at 2.72 Å. This structure revealed 64 subunits, which encompassed 386 chlorophylls, 86 carotenoids, four plastoquinones, and several structural lipids. At the luminal side of PSII, a unique subunit arrangement was observed to protect the oxygen-evolving complex. This arrangement involved PsbO (OEE1), PsbP (OEE2), PsbB, and PsbU (a homolog of plant OEE3). PsbU interacted with PsbO, PsbC, and PsbP, thereby stabilizing the shield of the oxygen-evolving complex. Significant changes were also observed at the stromal electron acceptor side. PsbY, identified as a transmembrane helix, was situated alongside PsbF and PsbE, which enclosed cytochrome b559. Supported by the adjacent C-terminal helix of Psb10, these four transmembrane helices formed a bundle that shielded cytochrome b559 from the surrounding solvent. Moreover, the bulk of Psb10 formed a protective cap, which safeguarded the quinone site and likely contributed to the stacking of PSII complexes. Based on our findings, we propose a protective mechanism that prevents Q_B (plastoquinone B) from becoming fully reduced. This mechanism offers insights into the regulation of electron transfer within PSII.

Summary: Performing photosynthesis in the desert is a challenging task since it requires a fast adaptation to extreme illumination and temperature changes. To understand adaptive mechanisms, we purified Photosystem II (PSII) from Chlorella ohadii , a green alga from the desert soil surface, and identified structural elements that might enable the photosystem functioning under harsh conditions. The 2.72 Å cryogenic electron-microscopy (cryoEM) structure of PSII exhibited 64 subunits, encompassing 386 chlorophylls, 86 carotenoids, four plastoquinones, and several structural lipids. At the luminal side of PSII, the oxygen evolving complex was protected by a unique subunit arrangement - PsbO (OEE1), PsbP (OEE2), CP47, and PsbU (plant OEE3 homolog). PsbU interacted with PsbO, CP43, and PsbP, thus stabilising the oxygen evolving shield. Substantial changes were observed on the stromal electron acceptor side - PsbY was identified as a transmembrane helix situated alongside PsbF and PsbE enclosing cytochrome b559, supported by the adjacent C-terminal helix of Psb10. These four transmembrane helices bundled jointly, shielding cytochrome b559 from the solvent. The bulk of Psb10 formed a cap protecting the quinone site and probably contributed to the PSII stacking. So far, the C. ohadii PSII structure is the most complete description of the complex, suggesting numerous future experiments. A protective mechanism that prevented Q _B from rendering itself fully reduced is proposed.

The unicellular green alga, Chlamydomonas reinhardtii, has been widely used as a model system to study photosynthesis. Its possibility to generate and analyze specific mutants has made it an excellent tool for mechanistic and biogenesis studies. Using negative selection of ultraviolet (UV) irradiation–mutated cells, we isolated a mutant (TSP9) with a single amino acid mutation in the Rieske protein of the cytochrome b6f complex. The W143R mutation in the petC gene resulted in total loss of cytochrome b6f complex function at the non-permissive temperature of 37 °C and recovery at the permissive temperature of 25 °C. We then isolated photosystem I (PSI) and photosystem II (PSII) supercomplexes from cells grown at the non-permissive temperature and determined the PSI structure with high-resolution cryogenic electron microscopy. There were several structural alterations compared with the structures obtained from wild-type cells. Our structural data suggest that the mutant responded by excluding the Lhca2, Lhca9, PsaL, and PsaH subunits. This structural alteration prevents state two transition, where LHCII migrates from PSII to bind to the PSI complex. We propose this as a possible response mechanism triggered by the TSP9 phenotype at the non-permissive temperature.

Photosystem II (PSII) generates an oxidant whose redox potential is high enough to enable water oxidation , a substrate so abundant that it assures a practically unlimited electron source for life on earth . Our knowledge on the mechanism of water photooxidation was greatly advanced by high-resolution structures of prokaryotic PSII . Here, we show high-resolution cryogenic electron microscopy (cryo-EM) structures of eukaryotic PSII from the green alga Dunaliella salina at two distinct conformations. The conformers are also present in stacked PSII, exhibiting flexibility that may be relevant to the grana formation in chloroplasts of the green lineage. CP29, one of PSII associated light-harvesting antennae, plays a major role in distinguishing the two conformations of the supercomplex. We also show that the stacked PSII dimer, a form suggested to support the organisation of thylakoid membranes , can appear in many different orientations providing a flexible stacking mechanism for the arrangement of grana stacks in thylakoids. Our findings provide a structural basis for the heterogenous nature of the eukaryotic PSII on multiple levels.

Plant Photosystem I (PSI) consists of at least 13 nuclear-encoded and four chloroplast-encoded protein subunits, acting as a sunlight-driven oxidoreductase. Here, we report the first structure of PSI assembly intermediate isolated from greening oat seedlings. The assembly state displays absence of at least eight subunits and lacks the photoreduction activity, which is attributed to an elimination of the plastocyanin (Pc) binding site, due to a missing subunit PsaF. The data shows that PsaF is a regulatory checkpoint that promotes the assembly, effectively coupling biogenesis to function.