SciCombinator

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Concept: Plastocyanin

167

Based on previous comparative genomic analyses, a set of nearly 600 polypeptides, of which ~300 have unknown physiological function, was identified that is present in green algae and flowering and nonflowering plants, but not present (or highly diverged) in non-photosynthetic organisms. The gene encoding one of these GreenCut proteins, CPLD38, is in the same operon as ndhL in most cyanobacteria; NdhL is part of a complex essential for cyanobacterial respiration. A cpld38 mutant of Chlamydomonas reinhardtii did not grow on minimal medium, was high light sensitive under photoheterotrophic conditions, had lower accumulation of photosynthetic complexes, reduced photosynthetic electron flow to P700+, and reduced photochemical efficiency of photosystem II; these phenotypes were rescued by a wild-type copy of CPLD38. Biophysical and biochemical analyses demonstrated that cytochrome b6f function was severely compromised, and levels of transcripts and polypeptide subunits associated with the cytochrome b6f complex were also significantly lower in the mutant; the subunits of the cytochrome b6f complex turned over much more rapidly in mutant than in wild-type cells. Interestingly, PTOX2 and NDA2, two major proteins involved in chlororespiration, were more than 5-fold higher in mutant relative to wild-type cells, suggesting a shift from photosynthesis toward chlororespiratory metabolism in mutant cells, which is supported by experiments that quantify the reduction state of the plastoquinone pool. These findings support the hypothesis that CPLD38 impacts the stability of the cytochrome b6f complex and may play a key role in balancing redox inputs to the quinone pool from photosynthesis and chlororespiration.

Concepts: Protein, Carbon dioxide, Plastocyanin, Cyanobacteria, Plastoquinone, Gene, Photosystem, Photosynthesis

166

Iron is an essential component in many protein complexes involved in photosynthesis, but environmental iron availability is often low as oxidized forms of iron are insoluble in water. To adjust to low environmental iron levels, cyanobacteria undergo numerous changes to balance their iron budget and mitigate the physiological effects of iron depletion. We investigated changes in key protein abundances and photophysiological parameters in the model cyanobacteria Synechococcus PCC 7942 and Synechocystis PCC 6803 over a 120 hour time course of iron deprivation. The iron stress induced protein (IsiA) accumulated to high levels within 48 h of the onset of iron deprivation, reaching a molar ratio of ∼42 IsiA : Photosystem I in Synechococcus PCC 7942 and ∼12 IsiA : Photosystem I in Synechocystis PCC 6803. Concomitantly the iron-rich complexes Cytochrome b6f and Photosystem I declined in abundance, leading to a decrease in the Photosystem I : Photosystem II ratio. Chlorophyll fluorescence analyses showed a drop in electron transport per Photosystem II in Synechococcus, but not in Synechocystis after iron depletion. We found no evidence that the accumulated IsiA contributes to light capture by Photosystem II complexes.

Concepts: Iron, Light reactions, Plastocyanin, Carbon dioxide, Oxygen, Cyanobacteria, Photosystem, Photosynthesis

29

The machinery that conducts the light-driven reactions of oxygenic photosynthesis is hosted within specialized paired membranes called thylakoids. In higher plants, the thylakoids are segregated into two morphological and functional domains called grana and stroma lamellae. A large fraction of the luminal volume of the granal thylakoids is occupied by the oxygen-evolving complex of photosystem II. Electron microscopy data we obtained on dark- and light-adapted Arabidopsis thylakoids indicate that the granal thylakoid lumen significantly expands in the light. Models generated for the organization of the oxygen-evolving complex within the granal lumen predict that the light-induced expansion greatly alleviates restrictions imposed on protein diffusion in this compartment in the dark. Experiments monitoring the redox kinetics of the luminal electron carrier plastocyanin support this prediction. The impact of the increase in protein mobility within the granal luminal compartment in the light on photosynthetic electron transport rates and processes associated with the repair of photodamaged photosystem II complexes is discussed.

Concepts: Chloroplast, ATP synthase, Plastocyanin, Chlorophyll, Cyanobacteria, Light-dependent reactions, Thylakoid, Photosynthesis

27

As much as two-thirds of the proton gradient used for transmembrane free energy storage in oxygenic photosynthesis is generated by the cytochrome b(6)f complex. The proton uptake pathway from the electrochemically negative (n) aqueous phase to the n-side quinone binding site of the complex, and a probable route for proton exit to the positive phase resulting from quinol oxidation, are defined in a 2.70-Å crystal structure and in structures with quinone analog inhibitors at 3.07 Å (tridecyl-stigmatellin) and 3.25-Å (2-nonyl-4-hydroxyquinoline N-oxide) resolution. The simplest n-side proton pathway extends from the aqueous phase via Asp20 and Arg207 (cytochrome b(6) subunit) to quinone bound axially to heme c(n). On the positive side, the heme-proximal Glu78 (subunit IV), which accepts protons from plastosemiquinone, defines a route for H(+) transfer to the aqueous phase. These pathways provide a structure-based description of the quinone-mediated proton transfer responsible for generation of the transmembrane electrochemical potential gradient in oxygenic photosynthesis.

Concepts: Redox, Chemistry, Electrochemistry, Cytochrome b6f complex, Cytochrome f, Plastocyanin, Photosynthesis, Photosystem

25

The cytochrome (cyt) b6f complex is involved in the transmembrane redox signaling that triggers state transitions in cyanobacteria and chloroplasts. However, the components and molecular mechanisms are still unclear. In an attempt to solve this long-standing problem, we first focused on the unknown role of a single chlorophyll a (Chla) in cyt b6f with a new approach based on Chla structural properties. Various b6f X-ray crystal structures were analyzed to identify their differences, which correlate with differences in Chla molecular volume. We found that the distance of the Rieske [2Fe-2S] cluster to Chla correlates with the distance between a pair of residues at the Qo-site and the distance between a pair of residues at the opposite membrane side. These correlations were accompanied by the rotation of a key peripheral residue and by changes in the hydrophobic thickness of cyt b6f. Parallel analysis of cyt bc1 crystal structures allowed us to conclude that Chla acts as the crucial redox sensor and transmembrane signal transmitter in b6f for changes in the plastoquinone pool redox state. The hydrophobic mismatch induced by the changed hydrophobic thickness of cyt b6f is the driving force for the structural reorganizations of the photosynthetic apparatus during induction and the progression of state transitions in cyanobacteria and chloroplasts. A mechanism for LHCII kinase activation in chloroplasts is also proposed. Our understanding of the dynamic structural changes in bc-complexes during turnover at the Qo-site and state transitions is augmented by the time-sequence ordering of 56 bc crystal structures.

Concepts: Plastocyanin, Cytochrome b6f complex, Plastoquinone, Cytochrome f, Photosystem, Photosynthesis

5

Photosystem I (PSI) is the dominant photosystem in cyanobacteria and it plays a pivotal role in cyanobacterial metabolism. Despite its biological importance, the native organisation 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 (T.) elongatus, Synechococcus sp. PCC 7002 and Synechocystis sp PCC 6803. Hyperspectral confocal fluorescence microscopy (HCFM) and three-dimensional structured illumination microscopy (3D-SIM) of Synechocystis sp PCC 6803 cells visualise 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 inter-trimer energy transfer the close associations between PSI primarily maximise 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 (PSII) 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.

Concepts: Light-dependent reactions, Plastoquinone, Plastocyanin, Thylakoid, Photosystem, Chlorophyll, Cyanobacteria, Photosynthesis

1

In oxygenic photosynthesis the initial photochemical processes are carried out by photosystem I (PSI) and II (PSII). Although subunit composition varies between cyanobacterial and plastid photosystems, the core structures of PSI and PSII are conserved throughout photosynthetic eukaryotes. So far, the photosynthetic complexes have been characterised in only a small number of organisms. We performed in silico and biochemical studies to explore the organization and evolution of the photosynthetic apparatus in the chromerids Chromera velia and Vitrella brassicaformis, autotrophic relatives of apicomplexans. We catalogued the presence and location of genes coding for conserved subunits of the photosystems as well as cytochrome b6f and ATP synthase in chromerids and other phototrophs and performed a phylogenetic analysis. We then characterised the photosynthetic complexes of Chromera and Vitrella using 2D gels combined with mass-spectrometry and further analysed the purified Chromera PSI. Our data suggest that the photosynthetic apparatus of chromerids underwent unique structural changes. Both photosystems (as well as cytochrome b6f and ATP synthase) lost several canonical subunits, while PSI gained one superoxide dismutase (Vitrella) or two superoxide dismutases and several unknown proteins (Chromera) as new regular subunits. We discuss these results in light of the extraordinarily efficient photosynthetic processes described in Chromera.

Concepts: Algae, Organism, Plastocyanin, Photosystem I, Cyanobacteria, Plant, Photosystem, Photosynthesis

1

Plastoquinone (PLQ) acts as an electron carrier between photosystem II (PSII) and the cytochrome b6f complex. To understand how PLQ enters and leaves PSII, here we show results of coarse grained molecular dynamics simulations of PSII embedded in the thylakoid membrane, covering a total simulation time of more than 0.5 ms. The long time scale allows the observation of many spontaneous entries of PLQ into PSII, and the unbinding of plastoquinol (PLQol) from the complex. In addition to the two known channels, we observe a third channel for PLQ/PLQol diffusion between the thylakoid membrane and the PLQ binding sites. Our simulations point to a promiscuous diffusion mechanism in which all three channels function as entry and exit channels. The exchange cavity serves as a PLQ reservoir. Our simulations provide a direct view on the exchange of electron carriers, a key step of the photosynthesis machinery.

Concepts: Cytochrome f, Plastocyanin, Photosystem II, Light reactions, Photosynthesis, Cytochrome b6f complex, Photosystem, Plastoquinone

1

In photosynthetic organisms, photons are captured by light-harvesting antenna complexes, and energy is transferred to reaction centers where photochemical reactions take place. We describe here the isolation and characterization of a fully functional megacomplex composed of a phycobilisome antenna complex and photosystems I and II from the cyanobacterium Synechocystis PCC 6803. A combination of in vivo protein cross-linking, mass spectrometry, and time-resolved spectroscopy indicates that the megacomplex is organized to facilitate energy transfer but not intercomplex electron transfer, which requires diffusible intermediates and the cytochrome b6f complex. The organization provides a basis for understanding how phycobilisomes transfer excitation energy to reaction centers and how the energy balance of two photosystems is achieved, allowing the organism to adapt to varying ecophysiological conditions.

Concepts: Cytochrome b6f complex, Plastocyanin, Cytochrome f, Cell, Electromagnetic radiation, Cyanobacteria, Photosystem, Photosynthesis

0

The GreenCut encompasses a suite of nucleus-encoded proteins with orthologs among green lineage organisms (plants, green algae), but that are absent or poorly conserved in non-photosynthetic/heterotrophic organisms. In Chlamydomonas reinhardtii, CPLD49 (Conserved in Plant Lineage and Diatoms49) is an uncharacterized GreenCut protein that is critical for maintaining normal photosynthetic function. We demonstrate that a cpld49 mutant has impaired photoautotrophic growth under high light conditions. The mutant exhibits a nearly 90% reduction in the level of the cytochrome b6f complex (Cytb6f), which impacts linear and cyclic electron transport, but does not compromise the ability of the strain to perform state transitions. Furthermore, CPLD49 strongly associates with thylakoid membranes where it may be part of a membrane protein complex with another GreenCut protein, CPLD38; a mutant null for CPLD38 also impacts Cytb6f complex accumulation. We investigated several potential functions of CPLD49, with some suggested by protein homology. Our findings are congruent with the hypothesis that CPLD38 and CPLD49 are part of a novel thylakoid membrane complex that primarily modulates accumulation, but also impacts the activity of the Cytb6f complex. Based on motifs of CPLD49 and the activities of other CPLD49-like proteins, we suggest a role for this putative dehydrogenase in the synthesis of a lipophilic thylakoid membrane molecule that influences the assembly and activity of Cytb6f. This article is protected by copyright. All rights reserved.

Concepts: Photosystem, DNA, Cytochrome b6f complex, Cytochrome f, Plastoquinone, Photosynthesis, Plastocyanin