Absence of the PsbZ subunit prevents association of PsbK and Ycf12 with the PSII complex in the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1

Dictyochophyceae Plastid Genomes Reveal Unusual Variability in Their Organization

Dictyochophyceae (silicoflagellates) are unicellular freshwater and marine algae (Heterokontophyta, stramenopiles). Despite their abundance in global oceans and potential ecological significance, discovered in recent years, neither nuclear nor organellar genomes of representatives of this group were sequenced until now. Here, we present the first complete plastid genome sequences of Dictyochophyceae, obtained from four species: Dictyocha speculum, Rhizochromulina marina, Florenciella parvula and Pseudopedinella elastica. Despite their comparable size and genetic content, these four plastid genomes exhibit variability in their organization: plastid genomes of F. parvula and P. elastica possess conventional quadripartite structure with a pair of inverted repeats, R. marina instead possesses two direct repeats with the same orientation and D. speculum possesses no repeats at all. We also observed a number of unusual traits in the plastid genome of D. speculum, including expansion of the intergenic regions, presence of an intron in the otherwise non-intron-bearing psaA gene, and an additional copy of the large subunit of RuBisCO gene (rbcL), the last of which has never been observed in any plastid genome. We conclude that despite noticeable gene content similarities between the plastid genomes of Dictyochophyceae and their relatives (pelagophytes, diatoms), the number of distinctive features observed in this lineage strongly suggests that additional taxa require further investigation.

Photocurrent Generation of Reconstituted Photosystem II on a Self-Assembled Gold Film

Photosystem II (PSII)-modified gold electrodes were prepared by the deposition of PSII reconstituted with platinum nanoparticles (PtNPs) on Au electrodes. PtNPs modified with 1-[15-(3,5,6-trimethyl-1,4-benzoquinone-2-yl)]pentadecyl disulfide ((TMQ(CH₂)₁₅S)₂) were incorporated into the Q_B site of PSII isolated from thermophilic cyanobacterium Thermosynechococcus elongatus. The reconstitution was confirmed by Q_A-reoxidation measurements. PSII reconstituted with PtNPs was deposited and integrated on a Au(111) surface modified with 4,4'-biphenyldithiol. The cross section of the reconstituted PSII film on the Au electrode was investigated by SEM. Absorption spectra showed that the surface coverage of the electrode was about 18 pmol PSII cm^-2. A photocurrent density of 15 nAcm^-2 at E = +0.10 V (vs Ag/AgCl) was observed under 680 nm irradiation. The photoresponse showed good reversibility under alternating light and dark conditions. Clear photoresponses were not observed in the absence of PSII and molecular wire. These results supported the photocurrent originated from PSII and moved to a gold electrode by light irradiation, which also confirmed conjugation with orientation through the molecular wire.

The Use of Advanced Mass Spectrometry to Dissect the Life-Cycle of Photosystem II

Photosystem II (PSII) is a photosynthetic membrane-protein complex that undergoes an intricate, tightly regulated cycle of assembly, damage, and repair. The available crystal structures of cyanobacterial PSII are an essential foundation for understanding PSII function, but nonetheless provide a snapshot only of the active complex. To study aspects of the entire PSII life-cycle, mass spectrometry (MS) has emerged as a powerful tool that can be used in conjunction with biochemical techniques. In this article, we present the MS-based approaches that are used to study PSII composition, dynamics, and structure, and review the information about the PSII life-cycle that has been gained by these methods. This information includes the composition of PSII subcomplexes, discovery of accessory PSII proteins, identification of post-translational modifications and quantification of their changes under various conditions, determination of the binding site of proteins not observed in PSII crystal structures, conformational changes that underlie PSII functions, and identification of water and oxygen channels within PSII. We conclude with an outlook for the opportunity of future MS contributions to PSII research.

Photosystem II, a growing complex: updates on newly discovered components and low molecular mass proteins

Photosystem II is a unique complex capable of absorbing light and splitting water. The complex has been thoroughly studied and to date there are more than 40 proteins identified, which bind to the complex either stably or transiently. Another special feature of this complex is the unusually high content of low molecular mass proteins that represent more than half of the proteins. In this review we summarize the recent findings on the low molecular mass proteins (<15kDa) and present an overview of the newly identified components as well. We have also performed co-expression analysis of the genes encoding PSII proteins to see if the low molecular mass proteins form a specific sub-group within the Photosystem II complex. Interestingly we found that the chloroplast-localized genes encoding PSII proteins display a different response to environmental and stress conditions compared to the nuclear localized genes. This article is part of a Special Issue entitled: Photosystem II.

Recent advances in understanding the assembly and repair of photosystem II

BACKGROUND

Photosystem II (PSII) is the light-driven water:plastoquinone oxidoreductase of oxygenic photosynthesis and is found in the thylakoid membrane of chloroplasts and cyanobacteria. Considerable attention is focused on how PSII is assembled in vivo and how it is repaired following irreversible damage by visible light (so-called photoinhibition). Understanding these processes might lead to the development of plants with improved growth characteristics especially under conditions of abiotic stress.

SCOPE

Here we summarize recent results on the assembly and repair of PSII in cyanobacteria, which are excellent model organisms to study higher plant photosynthesis.

CONCLUSIONS

Assembly of PSII is highly co-ordinated and proceeds through a number of distinct assembly intermediates. Associated with these assembly complexes are proteins that are not found in the final functional PSII complex. Structural information and possible functions are beginning to emerge for several of these 'assembly' factors, notably Ycf48/Hcf136, Psb27 and Psb28. A number of other auxiliary proteins have been identified that appear to have evolved since the divergence of chloroplasts and cyanobacteria. The repair of PSII involves partial disassembly of the damaged complex, the selective replacement of the damaged sub-unit (predominantly the D1 sub-unit) by a newly synthesized copy, and reassembly. It is likely that chlorophyll released during the repair process is temporarily stored by small CAB-like proteins (SCPs). A model is proposed in which damaged D1 is removed in Synechocystis sp. PCC 6803 by a hetero-oligomeric complex composed of two different types of FtsH sub-unit (FtsH2 and FtsH3), with degradation proceeding from the N-terminus of D1 in a highly processive reaction. It is postulated that a similar mechanism of D1 degradation also operates in chloroplasts. Deg proteases are not required for D1 degradation in Synechocystis 6803 but members of this protease family might play a supplementary role in D1 degradation in chloroplasts under extreme conditions.

The PsbK subunit is required for the stable assembly and stability of other small subunits in the PSII complex in the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1

PsbK is a small membrane protein of the PSII core complex and is highly conserved from cyanobacteria to plants. Here, we studied its role in the thermophilic cyanobacterium, Thermosynechococcus elongatus BP-1, by focusing on a psbK disruptant with hexahistidine-tagged CP47. The psbK disruptant showed photoautotrophic growth comparable with that of the wild type under a wide range of light conditions. The mutant PSII complex retained the oxygen-evolving activity with a unique modification of the acceptor Q(B) site. N-terminal sequencing revealed that Ycf12 and PsbZ proteins were lost in the PSII complex prepared from the mutant. Immunoblotting detected reduced accumulation of PsbZ in the mutant thylakoid. These results suggest that PsbK is required not only for association of PsbZ and Ycf12 with the isolated PSII complex but also for the stabilization of PsbZ in the thylakoid membrane.

Is the photosystem II complex a monomer or a dimer?

It is widely believed that the photosystem II (PSII) complex may function as a dimer in the thylakoid membrane. Here, we report experimental conversion from the monomeric PSII to the dimeric form by treatment with high concentrations of n-dodecyl-beta-D-maltopyranoside (DM). The content of the PSII monomer in a PsbTc deletion mutant was much higher than in the wild type when solubilized with low concentrations of DM. However, upon treatment with higher concentrations of DM, the PSII dimer was also recovered in the PsbTc deletion mutant. These results suggest that there are at least two distinct processes of dimerization: (i) PsbTc dependent and (ii) DM induced. We discuss the results with regard to the native assembly form(s) of PSII.

Cyanobacterial photosystem II at 2.9-A resolution and the role of quinones, lipids, channels and chloride

Photosystem II (PSII) is a large homodimeric protein-cofactor complex located in the photosynthetic thylakoid membrane that acts as light-driven water:plastoquinone oxidoreductase. The crystal structure of PSII from Thermosynechococcus elongatus at 2.9-A resolution allowed the unambiguous assignment of all 20 protein subunits and complete modeling of all 35 chlorophyll a molecules and 12 carotenoid molecules, 25 integral lipids and 1 chloride ion per monomer. The presence of a third plastoquinone Q(C) and a second plastoquinone-transfer channel, which were not observed before, suggests mechanisms for plastoquinol-plastoquinone exchange, and we calculated other possible water or dioxygen and proton channels. Putative oxygen positions obtained from a Xenon derivative indicate a role for lipids in oxygen diffusion to the cytoplasmic side of PSII. The chloride position suggests a role in proton-transfer reactions because it is bound through a putative water molecule to the Mn(4)Ca cluster at a distance of 6.5 A and is close to two possible proton channels.

2,2'-beta-hydroxylase (CrtG) is involved in carotenogenesis of both nostoxanthin and 2-hydroxymyxol 2'-fucoside in Thermosynechococcus elongatus strain BP-1

We identified the molecular structures, including the stereochemistry, of all carotenoids in Thermosynechococcus elongatus strain BP-1. The major carotenoid was beta-carotene, and its hydroxyl derivatives of (3R)-beta-cryptoxanthin, (3R,3'R)-zeaxanthin, (2R,3R,3'R)-caloxanthin and (2R,3R,2'R,3'R)-nostoxanthin were also identified. The myxol glycosides were identified as (3R,2'S)-myxol 2'-fucoside and (2R,3R,2'S)-2-hydroxymyxol 2'-fucoside. 2-Hydroxymyxol 2'-fucoside is a novel carotenoid, and similar carotenoids of 4-hydroxymyxol glycosides were previously named aphanizophyll. Ketocarotenoids, such as echinenone and 4-ketomyxol, which are unique carotenoids in cyanobacteria, were absent, and genes coding for both beta-carotene ketolases, crtO and crtW, were absent in the genome. From a homology search, the Tlr1917 amino acid sequence was found to be 41% identical to 2,2'- beta-hydroxylase (CrtG) from Brevundimonas sp. SD212, which produces nostoxanthin from zeaxanthin. In the crtG disruptant mutant, 2-hydroxymyxol 2'-fucoside, caloxanthin and nostoxanthin were absent, and the levels of both myxol 2'-fucoside and zeaxanthin were higher. Therefore, the gene has a CrtG function for both myxol to 2-hydroxymyxol and zeaxanthin to nostoxanthin. This is the first functional identification of CrtG in cyanobacteria. We also investigated the distribution of crtG-like genes, and 2-hydroxymyxol and/or nostoxanthin, in cyanobacteria. Based on the identification of the carotenoids and the completion of the entire nucleotide sequence of the genome in T. elongatus, we propose a biosynthetic pathway of the carotenoids and the corresponding genes and enzymes.