Changes in the antenna size of Photosystem I and Photosystem II in Synechococcus sp. strain PCC 7942 grown in the presence of SANDOZ 9785 — a Photosystem II inhibitor

SANDOZ 9785, also known as BASF 13.338, is a pyridazinone derivative that inhibits Photosystem II (PS II) activity leading to an imbalance in the rate of electron transport through the photosystems. Synechococcus sp. strain PCC 7942 cells grown in the presence of sublethal concentration of SANDOZ 9785 (SAN 9785) for 48 hours exhibited a 20% decrease in Chl a per cell. However, no changes were observed in the content of phycocyanin per cell, the size of the phycobilisomes or in the PS II:PS I ratio. From an estimate of PS II electron transport rate under varying light intensities and spectral qualities and analysis of room temperature Chl a fluorescence induction, it was deduced that growth of Synechococcus PCC 7942 in the presence of SAN 9785 leads to a redistribution of excitation energy in favour of PS II. Though the redistribution appears to be primarily caused by changes affecting the Chl a antenna of PS II, the extent of energetic coupling between phycobilisomes and PS II is also enhanced in SAN 9785 grown Synechococcus PCC 7942 cells. There was a reduction in the effective size of PS I antenna based on measurement of P700 photooxidation kinetics. These results indicate that when PS II is partially inhibited, the structure of photosynthetic apparatus alters to redistribute the excitation energy in favour of PS II so that the efficiency of utilization of light energy by the two photosystems is optimized. Our results suggest that under the conditions used, drastic structural changes are not essential for redistribution of excitation energy between the photosystems.Abbreviations APC Allophycocyanin - Chl a chlorophyll a - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophyenyl)-1,1-dimethyl urea - DCIP 2,6-dichlorophenolindophenol - F_o fluorescence when all the reaction centres are open - f_m fluorescence yield when all the reaction centres are closed - F_v variable chlorophyll fluorescence - HEPES N-2-Hydroxyethylpiperazine-N-2-ethanesulphonic Acid - I₅₀ concentration that causes 50% inhibition in activity - MV methyl viologen - pBQ para benzoquinone - PBS phycobilisome - PC phycocyanin - PS I, PS II Photosystem I, Photosystem II - P700 reaction centre Chl a of PS I - SAN 9785 SANDOZ 9785 i.e. 4-chloro-5-dimethylamino-2-phenyl-3 (2H) pyridazinone, also known as BASF 13.338     

Some properties of Photosystem II preparations from the cyanobacterium Synechococcus sp. — presence of an l-amino acid oxidase in Photosystem II complexes from Synechococcus sp.

A highly active O₂-evolving Photosystem II complex has been purified from the cyanobacterium Synechococcus sp., and this complex has been compared with the Photosystem II complex previously isolated from this cyanobacterium (Ohno, T., Satoh, K. and Katoh, S. (1986) Biochim. Biophys. Acta 852, 1–8). Further treatment of the O₂-evolving complex with the detergent sodium taurodesoxycholate resulted in a complex which consisted mainly of the 47 and 40 kDa peptides and which had lost the O₂-evolving activity, but which could still reduce 2,6-dichlorophenolindophenol with 1,5-diphenylcarbazide. Previously, we have shown that a flavoprotein of 49 kDa which has an l-amino acid oxidase activity under certain conditions, is a component of highly active Photosystem II preparations from the cyanobacterium Anacystis nidulans (Pistorius, E.K. and Gau, A.E. (1986) FEBS Lett. 206, 243–248). Based on immunological studies with the antiserum raised against the l-amino acid oxidase protein from A. nidulans, we show that a protein which cross-reacts with this antiserum is present in the highly purified Photosystem II preparations from Synechococcus sp. Moreover, an l-amino acid oxidase activity could also be detected in Photosystem II preparations from Synechococcus sp. The enzyme preferentially oxidizes basic l-amino acids as l-arginine, l-ornithine, 2,3-diamino propionic acid and l-citrulline. In contrast to the enzyme from A. nidulansl-lysine is not oxidized. The here shown presence of an l-amino acid oxidase protein in Photosystem II preparations from Synechococcus sp. is an additional support of our hypothesis that a flavoprotein is a functional component of the water-oxidizing enzyme complex.     

The role of plastoquinone and β-carotene in the primary reaction of plant Photosystem II

Extraction of Triton Photosystem II chloroplast fragments with 0.2% methanol in hexane for 3 h results in the removal of 90 to 95% of the plastoquinone in the original preparation. The extracted fragments (chlorophyll : plastoquinone ratio, 900 : 1) showed no P-680 photooxidation at 15 K after a single laser flash. The extracted fragments also showed no light-induced C-550 absorbance change at 77 K. Reconstitution of the primary reaction of Photosystem II, as evidenced by restoration of low-temperature photooxidation of P-680, could be obtained by the addition of plastoquinone A but not by the addition of β-carotene. The addition of β-carotene plus plastoquinone A restored the C-550 absorbance change. These results indicate that plastoquinone functions as the primary electron acceptor of Photosystem II and that β-carotene does not play a direct role in the primary photochemistry but is required for the C-550 absorbance change.     

The Antenna System of Photosystem II From Thermosynechococcus elongatus at 3.2 Å Resolution

The content and type of cofactors harboured in the Photosystem II core complex (PS IIcc) of the cyanobacterium Thermosynechococcus elongatus has been determined by biochemical and spectroscopic methods. 17 ± 1 chlorophyll a per pheophytin a and 0.25 β-carotene per chlorophyll a have been found in re-dissolved crystals of dimeric PS IIcc. The X-ray crystal structure of PS IIcc from Thermosynechococcus elongatus at 3.2 Å resolution clearly shows chlorophyll a molecules arranged in two layers close to the cytoplasmic and lumenal sides of the thylakoid membrane. Each of the cytoplasmic layers contains 9 chlorophyll a, whose positions and orientations are related by a local twofold rotation pseudo-C2 axis passing through the non-haem Fe²⁺. These chlorophyll a are arranged comparably to those in the antenna domains of PsaA and PsaB of cyanobacterial Photosystem I affirming an evolutionary relation. The chlorophyll a in the lumenal layer are less well conserved between Photosystems I and II and even between CP43 and CP47 with 4 chlorophyll a in the former and 7 in the latter.     

Quality control of Photosystem II under light stress – turnover of aggregates of the D1 protein in vivo

When photodamaged under excessive light, the D1 protein is digested and removed from Photosystem (PS) II to facilitate turnover of the protein. In vitro studies have shown that part of the photodamaged D1 protein forms aggregates with surrounding polypeptides before being digested by a protease(s) in the stroma [Yamamoto Y (2001) Plant Cell Physiol 42: 121–128]. The aim of this study was to examine whether light-induced aggregation of the D1 protein also occurs in vivo. The following results were obtained: (1) PS II activity in spinach leaves was significantly inhibited by weak illumination (light intensity, 20–100 μE m⁻² s⁻¹), as monitored by chlorophyll fluorescence Fv/Fm, when the leaves were kept at higher temperatures (35–40 °C); (2) aggregation of the D1 protein, as well as cleavage of the protein, was detected in thylakoids isolated from spinach leaves that had been subjected to heat/light stress; (3) aggregates of the D1 protein disappeared after incubation of the leaves at 25 °C in the dark or under illumination with weak light. Since it is dependent on the presence of oxygen, aggregation of the D1 protein is probably induced by reactive oxygen species produced in thylakoids upon illumination at elevated temperatures. Consistent with this notion, singlet oxygen production in thylakoid samples under illumination was shown to be stimulated significantly at higher temperatures.     

A thermoluminescence study of Photosystem II back electron transfer reactions in rice leaves – effects of salt stress

The recombination reactions of Photosystem II have been investigated in vivo in rice leaves by using the thermoluminescence (TL) emission technique. Excitation of dark-adapted leaf segments at 0 °C with different number of single turn-over flashes induced the appearance of complex TL glow curves. The mathematical analysis of these curves showed the existence of four TL components: B1-band (temperature maximum, t_max, at 24 °C, originating from S₃Q_B⁻ recombination), B2-band (t_max at 35 °C, from S₂Q_B⁻), AG-band (t_max at 46 °C) and C-band (t_max at 55 °C, from Tyr_D⁺Q_A⁻). Their contributions to the total TL signal were different depending on the number of flashes given. AG-band seems to reflect a special electron transfer from some unknown stroma donor to PS II. Q-band (t_max at 19 °C), originating from S₂Q_A⁻ recombination, was recorded after flashing samples incubated in the presence of DCMU. The recombination halftimes (t_1/2) at 20 °C of S₂Q_A⁻, S₃Q_B⁻, S₂Q_B⁻ and Tyr_D⁺Q_A⁻ were, respectively, 0.8 s, 48 s, 74 s and about 1 h. A sharp AG-band (t_max at 50 °C and t_1/2 of 210 s) could be also observed after illumination of leaves with far-red light and after a dark incubation period of whole plants. Incubation of leaf segments with 0.5 M NaCl abolished the inductions of AG-band by darkness and far-red illumination, significantly decreased Q-band intensity, whereas induced a strong increase in C-band intensity. The possible inhibition of S₂/S₃ formation and quinone oxidation by saline stress are discussed.     

The effect of norflurazon on protein composition and chlorophyll organization in pigment–protein complex of Photosystem II

The pyridazinone-type herbicide norflurazon SAN 9789 inhibiting the biosynthesis of long-chain carotenoids results in significant decrease in PS II core complexes and content of light-harvesting complex (LHC) polypeptides in the 29.5–21 kDa region. The Chl a forms at 668, 676, and 690 nm that belong to LHC and antenna part of PS I disappear completely after treatment. The intensity of the Chl b form at 648 nm is sharply decreased in treated seedlings grown under 30 or 100 lx light intensity. The bands of carotenoid absorption at 421, 448 (Chl a), 452, 480, 492, 496 (β-carotene), and 508 nm also disappear. The band shift from 740 to 720 nm and decrease in its intensity relative to the 687 nm emission peak in the low-temperature fluorescence spectrum (77 K) suggests a disturbance of energy transfer from LHC to the Chla form at 710–712 nm.     

Excitation energy transfer in aggregates of Photosystem I and Photosystem II of the cyanobacterium Synechocystis sp. PCC 6803: Can assembly of the pigment-protein complexes control the extent of spillover?

The fluorescence profile of Photosystem I/Photosystem II mixtures in different solvent systems shows that both non-hydrophobic and hydrophobic interactions govern their association and control energy transfer from Photosystem II to Photosystem I. The non-hydrophobic interactions lead to a highly efficient excitation energy transfer from Photosystem II to Photosystem I. In view of this, we propose that similar non-hydrophobic interactions, between the Photosystem II and Photosystem I peripheral proteins, also play a significant role in their association in thylakoids that control state transitions in cyanobacteria. This revised version was published online in June 2006 with corrections to the Cover Date.     

Development of Photosystem II in dark grown Chlamydomonas reinhardtii. A light-dependent conversion of PS IIβ, QB-nonreducing centers to the PS IIα, QB-reducing form

The green alga Chlamydomonas reinhardtii is a facultative heterotroph and, when cultured in the presence of acetate, will synthesize chlorophyll (Chl) and photosystem (PS) components in the dark. Analysis of the thylakoid membrane composition and function in dark grown C. reinhardtii revealed that photochemically competent PS II complexes were synthesized and assembled in the thylakoid membrane. These PS II centers were impaired in the electron-transport reaction from the primary-quinone electron acceptor, Q_A, to the secondary-quinone electron acceptor, Q_B (Q_B-nonreducing centers). Both complements of the PS II Chl a–b light harvesting antenna (LHC II-inner and LHC II-peripheral) were synthesized and assembled in the thylakoid membrane of dark grown C. reinhardtii cells. However, the LHC II-peripheral was energetically uncoupled from the PS II reaction center. Thus, PS II units in dark grown cells had a -type Chl antenna size with only 130 Chl (a and b) molecules (by definition, PS II units lack LHC II-peripheral). Illumination of dark grown C. reinhardtii caused pronounced changes in the organization and function of PS II. With a half-time of about 30 min, PS II centers were converted froma Q_B-nonreducing form in the dark, to a Q_B-reducing form in the light. Concomitant with this change, PS II units were energetically coupled with the LHC II-peripheral complement in the thylakoid membrane and were converted to a PS II form. The functional antenna of the latter contained more than 250 Chl(a+b) molecules. The results are discussed in terms of a light-dependent activation of the Q_A-Q_B electron-transfer reaction which is followed by association of the PS II unit with a LHC II-peripheral antenna and by inclusion of the mature form of PS II (PS II) in the membrane of the grana partition region.Abbreviations Chl chlorophyll - PS photosystem - Q_A primary quinone electron acceptor of PS II - Q_B secondary quinone electron acceptor of PS II - LHC light harvesting complex - F₀ non-variable fluorescence yield - F_plf intermediate fluorescence yield plateau leyel - F_max maximum fluorescence yield - F_i initial fluorescence yield increase from F₀ to F_pl (F_pl–F₀) - F_v total variable fluorescence yield (F_m–F0) - DCMU dichlorophenyl-dimethylurea     

Identification of a 32–34-kilodalton polypeptide as a herbicide receptor protein in Photosystem II

Photosystem II particles which retained high rates of herbicide-sensitive activity were used to examine the site(s) of action of various herbicides. A polypeptide of 32–34 kdaltons was identified as the triazine-herbicide binding site based upon: (a) parallel loss of atrazine activity and the polypeptide during either trypsin treatment or selective detergent depletion of protein in the Photosystem II complex, and (b) covalent labeling of the polypeptide by a ¹⁴C-labeled photoaffinity triazine.In Photosystem II particles depleted of the 32–34-kdalton polypeptide, electron transport was still active and was slightly sensitive to DCMU and largely sensitive to dinoseb (urea and nitrophenol herbicides, respectively). On the basis of this result it is proposed that the general herbicide binding site common to atrazine, DCMU and dinoseb is formed by a minimum of two polypeptides which determine affinity and/or mediate herbicide-induced inhibition of electron transport on the acceptor side of Photosystem II.     

Role of the oxidized secondary acceptor Q B of Photosystem II in the delayed ‘afterglow’ chlorophyll luminescence

Leaf discs of dark-adapted tobacco plants were excited by 2 flashes and kept in darkness at 20 °C for various time periods, then thermoluminescence emission was recorded without freezing the sample. The B band at 30 °C decreased with a half-time t_1/2~1 min and the AG band at 45 °C with a t_1/2~5 min. This corresponds to the decay kinetics of S_2/3 in PS II centres in the state S_2/3 Q_B^- (B band) or S_2/3 Q_B. Assuming that the 45 °C band is an ‘afterglow’ emission originating from those centres with an oxidized Q_B on which an electron is back-transferred from stroma reductants through a pathway induced by warming, the theoretical ratio of the B and AG band was compared to that measured experimentally. After 2 or 3 flashes producing mainly S₃, the intensity of AG band encompassed several fold that of the B band, because recombining S₃ recreated S₂ Q_B AG-emitting centres. In order to confirm that the AG band is governed by the heat-induced activation of a dark Q_B-reducing pathway rather than by PS II charge recombination, the AG emission was characterized in triazine-resistant Chenopodium album weed biotypes. In these mutants where the Q_B pocket is altered, the B band is strongly downshifted to 18 °C, compared to 32 °C in the wild type, whereas the AG band is only downshifted by 3 or 4 °C, demonstrating that S_2/3 Q_B^- is not the limiting step of the AG emission.     

Simultaneous detection of spin-coupled and decoupled QA– EPR-signals in Photosystem II complexes isolated with isoelectric focusing

The Photosystem II multisubunit protein complex can be extracted from thylakoid membranes with non-ionic detergents and subjected to various spectroscopical and biochemical investigations. This paper shows that after extraction with dodecyl--D-maltoside, several Photosystem II complexes could be resolved by isoelectric focusing. Structurally, the various Photosystem II complexes differed from each other in polypeptide composition, especially with regard to the chlorophyll a/b-binding proteins, which gave rise to differing isoelectric points. Functionally, the various Photosystem II complexes differed from each other on the acceptor side, as judged by acceptor side-dependent electron transfer and electron paramagnetic resonance (EPR). The Q_A ^- Fe2+-signal (g = 1.84), arising from Q_A ^- spin-coupled to the acceptor-side iron, and a radical signal arising from decoupled Q_A ^- (g = 2.0045) could be detected simultaneously in some of the Photosystem II complexes, and the amount of each of the two signals were inversely related. The results are discussed in relation to previously known heterogeneities in Photosystem II.     

Isolation and Characterization of Photosystem Ⅱ of Porphyra yezoensis Ueda

The thylakoid membranes were isolated and purified from gametophyte of Porphyra yezoensis Ueda (P. yezoensis) by sucrose density gradient ultracentrifugation. After P. yezoensis gametophyte thylakoid membranes were solubilized with SDS, the photosystem Ⅱ (PSⅡ) particles were isolated and purified. The activity of PSⅡ     

Cyanobacterial Photosystem II at 3.2 Å resolution – the plastoquinone binding pockets

Photosystem II from thylakoid membranes of the thermophilic cyanobacterium Thermosynechococcus elongatus was solubilized with n-β-dodecylmaltoside and purified using anion exchange chromatography. Molecular weight, pigment stoichiometry and subunit composition were assayed using various techniques. The holocomplex is dimeric with a molecular mass of 756 ± 18 kDa and functionally fully active. Crystals obtained from these samples showed significantly improved quality leading to a 3D structure at 3.2 Å resolution. Several loop regions of the principal protein subunits are now defined that were not interpretable at lower (3.8 Å) resolution, thus resulting in a more complete model. The head groups of the cofactors of the electron transfer chain and of the antennae have been modeled, coordinating and hydrogen bonding amino acids identified and the nature of the binding pockets derived. The orientations of these cofactors resemble those of the reaction centre from anoxygenic purple bacteria. For the two plastoquinones, electron density was only found for the head group of Q_A and none for Q_B indicating low or even no occupancy of this site in the crystal structure. Both binding pockets and problems related to the Q_B site are discussed here and compared to the situation in the purple bacterial reaction centre.     

Random mutagenesis in the large extrinsic loop E and transmembrane α-helix VI of the CP 47 protein of Photosystem II

The intrinsic chlorophyll-protein CP 47 is a component of Photosystem II which functions in both light-harvesting and oxygen evolution. Using the Escherichia coli mutator strain XL-1 Red, we introduced mutations at 14 sites in the large extrinsic loop E of CP 47 and its adjacent transmembrane -helix VI. Four mutant cell lines were recovered in which the histidyl residues 455H, 466H and 469H were altered. The cell lines H455T, H455Y, H469Y, and the double mutant F432L,H466R exhibited phenotypes that supported the identification of the histidyl residues 455H, 466H and 469H as chlorophyll ligands. Four additional mutant cell lines were recovered which contained mutations at positions 448R in the large extrinsic loop of CP 47. These mutants, R448K, R448Q, R448S, and R448W, exhibited variable phenotypes ranging from moderate alteration of photoautotrophic growth and oxygen evolution rates to a complete inhibition of these parameters. Those mutants exhibiting photoautotrophic growth and oxygen evolution capability under standard conditions were unable to grow photoautotrophically or evolve oxygen when grown at low chloride concentrations. Finally, a mutant cell line exhibiting a substitution at position 342G was recovered. The mutant G342D exhibited moderate alterations of photoautotrophic growth and oxygen evolution. In addition to these alterations, mutants were recovered in which deletions and insertions (leading to frame shifts) and stop codons were introduced. These mutants uniformly lacked the ability to either grow photoautotrophically or evolve oxygen.     

Selective quenching of the fluorescence of core chlorophyll–protein complexes by photochemistry indicates that Photosystem II is partly diffusion limited

The spectral characteristics of fluorescence quenching by open reaction centres in isolated Photosystem II membranes were determined with very high resolution and analysed. Quenching due to photochemistry is maximal near 687 nm, minimal in the chlorophyll b emission interval and displays a distinctive structure around 670 nm. The amplitude of this `quenching hole' is about 0.03 for normalised spectra. On the basis of the absorption spectra of isolated chlorophyll–protein complexes, it is shown that these quenching structures can be exactly described by assuming that photochemistry lowers the fluorescence yield of the reaction centre complex (D1/D2/cytb ₅₅₉) plus CP47, with quenching of the former complex being approximately double that of the latter complex. These data, which qualitatively indicate that there are kinetically limiting processes for primary photochemistry in the antenna, have been analysed by means of several different kinetic models. These models are derived from present structural knowledge of the arrangement of the chlorophyll–protein complexes in Photosystem II and incorporate the reversible charge separation characteristic of the exciton/radical pair equilibration model. In this way it is shown that Photosystem II cannot be considered to be purely trap limited and that exciton migration in the antenna imposes a diffusion limitation of about 30%, irrespective of the structural model assumed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mutation of Residue Arginine18 of Cytochrome b55α-Subunit and its Effects on Photosystem II Activities in Chlamydomonas reinhardtii

It has been known that arginine is used as the basic amino acid in the α-subunit of cytochrome b559 (Cyt b559) except histidine. However, previous studies have focused on the function of histidine in the activities of photosystem (PS) II and there are no reports regarding the structural and/or functional roles of arginine in PSII complexes. In the present study, two arginine18 (R18) mutants of Chlamydomonas reinhardtii were constructed using site-directed mutagenesis, in which R18 was replaced by glutamic acid (E) and glycine (G). The results show that the oxygen evolution of the PSII complex in the R18G and R18E mutants was approximately 60% of wild-type (WT) levels and that, after irradiation at high light intensity, oxygen evolution for the PSII of mutants was reduced to zero compared with 40% in WT cells. The efficiency of light capture by PSII (Fv/Fm) of R18G and R18E mutants was approximately 42%–46% that of WT cells. Furthermore, levels of the α-subunit of Cyt b559 and PsbO proteins were reduced in thylakoid membranes compared with WT. Overall, these data suggest that R18 plays a significant role in helping Cyt b559 maintain the structure of the PSII complex and its activity, although it is not directly bound to the heme group.