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Three psbA genes (psbA 1 , psbA 2 , and psbA 3 ) encoding the D1 subunit of photosystem II (PSII) are present in the thermophilic cyanobacterium Thermosynechococcus elongatus and are expressed differently in response to changes in the growth environment. To clarify the functional differences of the D1 protein expressed from these psbA genes, PSII dimers from two strains, each expressing only one psbA gene (psbA 2 or psbA 3 ), were crystallized, and we analyzed their structures at resolutions comparable to previously studied PsbA1-PSII. Our results showed that the hydrogen bond between pheophytin/D1 (Pheo D1 ) and D1-130 became stronger in PsbA2- and PsbA3-PSII due to change of Gln to Glu, which partially explains the increase in the redox potential of Pheo D1 observed in PsbA3. In PsbA2, one hydrogen bond was lost in Pheo D1 due to the change of D1-Y147F, which may explain the decrease in stability of Pheo D1 in PsbA2. Two water molecules in the Cl-1 channel were lost in PsbA2 due to the change of D1-P173M, leading to the narrowing of the channel, which may explain the lower efficiency of the S-state transition beyond S 2 in PsbA2-PSII. In PsbA3-PSII, a hydrogen bond between D1-Ser270 and a sulfoquinovosyl-diacylglycerol molecule near Q B disappeared due to the change of D1-Ser270 in PsbA1 and PsbA2 to D1-Ala270. This may result in an easier exchange of bound Q B with free plastoquinone, hence an enhancement of oxygen evolution in PsbA3-PSII due to its high Q B exchange efficiency. These results provide a structural basis for further functional examination of the three PsbA variants., Competing Interests: Conflict of interest The authors declare that they have no conflicts of interest with the contents of the article., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

Photosystem II catalyzes light-induced water oxidation leading to the generation of dioxygen indispensable for sustaining aerobic life on Earth. The Photosystem II reaction center is composed of D1 and D2 proteins encoded by psbA and psbD genes, respectively. In cyanobacteria, different psbA genes are present in the genome. The thermophilic cyanobacterium Thermosynechococcus elongatus contains three psbA genes: psbA1 , psbA2 , and psbA3 , and a new c -type heme protein, Tll0287, was found to be expressed in a strain expressing the psbA2 gene only, but the structure and function of Tll0287 are unknown. Here we solved the crystal structure of Tll0287 at a 2.0 Å resolution. The overall structure of Tll0287 was found to be similar to some kinases and sensor proteins with a Per-Arnt-Sim-like domain rather than to other c -type cytochromes. The fifth and sixth axial ligands for the heme were Cys and His, instead of the His/Met or His/His ligand pairs observed for most of the c -type hemes. The redox potential, E ½ , of Tll0287 was -255 ± 20 mV versus normal hydrogen electrode at pH values above 7.5. Below this pH value, the E ½ increased by ≈57 mV/pH unit at 15 °C, suggesting the involvement of a protonatable group with a p K red = 7.2 ± 0.3. Possible functions of Tll0287 as a redox sensor under microaerobic conditions or a cytochrome subunit of an H 2 S-oxidizing system are discussed in view of the environmental conditions in which psbA2 is expressed, as well as phylogenetic analysis, structural, and sequence homologies., (© 2017 by The American Society for Biochemistry and Molecular Biology, Inc.)

Photosynthesis converts light energy into biologically useful chemical energy vital to life on Earth. The initial reaction of photosynthesis takes place in photosystem II (PSII), a 700-kilodalton homodimeric membrane protein complex that catalyses photo-oxidation of water into dioxygen through an S-state cycle of the oxygen evolving complex (OEC). The structure of PSII has been solved by X-ray diffraction (XRD) at 1.9 ångström resolution, which revealed that the OEC is a Mn4CaO5-cluster coordinated by a well defined protein environment. However, extended X-ray absorption fine structure (EXAFS) studies showed that the manganese cations in the OEC are easily reduced by X-ray irradiation, and slight differences were found in the Mn-Mn distances determined by XRD, EXAFS and theoretical studies. Here we report a 'radiation-damage-free' structure of PSII from Thermosynechococcus vulcanus in the S1 state at a resolution of 1.95 ångströms using femtosecond X-ray pulses of the SPring-8 ångström compact free-electron laser (SACLA) and hundreds of large, highly isomorphous PSII crystals. Compared with the structure from XRD, the OEC in the X-ray free electron laser structure has Mn-Mn distances that are shorter by 0.1-0.2 ångströms. The valences of each manganese atom were tentatively assigned as Mn1D(III), Mn2C(IV), Mn3B(IV) and Mn4A(III), based on the average Mn-ligand distances and analysis of the Jahn-Teller axis on Mn(III). One of the oxo-bridged oxygens, O5, has significantly longer distances to Mn than do the other oxo-oxygen atoms, suggesting that O5 is a hydroxide ion instead of a normal oxygen dianion and therefore may serve as one of the substrate oxygen atoms. These findings provide a structural basis for the mechanism of oxygen evolution, and we expect that this structure will provide a blueprint for the design of artificial catalysts for water oxidation.

PsbV2 is a c-type cytochrome present in a very low abundance in the thermophilic cyanobacterium Thermosynechococcus elongatus. We purified this cytochrome and solved its crystal structure at a resolution of 1.5Å. The protein existed as a dimer in the crystal, and has an overall structure similar to other c-type cytochromes like Cytc6 and Cytc550, for example. However, the 5th and 6th heme iron axial ligands were found to be His51 and Cys101, respectively, in contrast to the more common bis-His or His/Met ligands found in most cytochromes. Although a few other c-type cytochromes were suggested to have this axial coordination, this is the first crystal structure reported for a c-type heme with this unusual His/Cys axial coordination. Previous spectroscopic characterizations of PsbV2 are discussed in relation to its structural properties., (Copyright © 2013 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.)