Enzymatic function of cytochrome b559 in photosystem II

The synchronicity of bloom-forming cyanobacteria transcription patterns and hydrogen peroxide dynamics

Hydrogen peroxide is a reactive oxygen species (ROS) naturally occurring at low levels in aquatic environments and production varies widely across different ecosystems. Oxygenic photosynthesis generates hydrogen peroxide as a byproduct, of which some portion can be released to ambient water. However, few studies have examined hydrogen peroxide dynamics in relation to cyanobacterial harmful algal blooms (cHABs). A year-long investigation of algal succession and hydrogen peroxide dynamics was conducted at the Caloosahatchee River, Florida, USA. We aimed to identify potential biological mechanisms responsible for elevated hydrogen peroxide production during cHAB events through the exploration of the freshwater microbial metatranscriptome. Hydrogen peroxide concentrations were elevated from February to September of 2021 when cyanobacteria were active and abundant. We observed one Microcystis cHAB event in spring and one in winter. Both had distinct nutrient uptake and cyanotoxin gene expression patterns. While meaningful levels of microcystin were only detected during periods of elevated hydrogen peroxide, cyanopeptolin was by far the most expressed cyanotoxin during the spring bloom when hydrogen peroxide was at its yearly maxima. Gene expressions of five microbial enzymes (Rubisco, superoxide dismutase, cytochrome b559, pyruvate oxidase, and NADH dehydrogenase) positively correlated to hydrogen peroxide concentrations. Additionally, there was higher nitrogen-fixing gene (nifDKH) expression by filamentous cyanobacteria after the spring bloom but no secondary bloom formation occurred. Overall, elevated environmental hydrogen peroxide concentrations were linked to cyanobacterial dominance and greater expression of specific enzymes in the photosynthesis of cyanobacteria. This implicates cyanobacterial photosynthesis and growth results in increased hydrogen peroxide generation as reflected in measured environmental concentrations.

Rajput V, Kumar Singh R, Guo Y, Kosolapov A, Usova E, Varduny T, Chalenko E, Yadronova O, Dmitriev P, Zaruba T. Chlorophyll fluorometry in evaluating photosynthetic performance: key limitations, possibilities, perspectives and alternatives

Non-destructive methods for the assessment of photosynthetic parameters of plants are widely applied to evaluate rapidly the photosynthetic performance, plant health, and shifts in plant productivity induced by environmental and cultivation conditions. Most of these methods are based on measurements of chlorophyll fluorescence kinetics, particularly on pulse modulation (PAM) fluorometry. In this paper, fluorescence methods are critically discussed in regard to some their possibilities and limitations inherent to vascular plants and microalgae. Attention is paid to the potential errors related to the underestimation of thylakoidal cyclic electron transport and anoxygenic photosynthesis. PAM-methods are also observed considering the color-addressed measurements. Photoacoustic methods are discussed as an alternative and supplement to fluorometry. Novel Fourier modifications of PAM-fluorometry and photoacoustics are noted as tools allowing simultaneous application of a dual or multi frequency measuring light for one sample.

A comparative study on photosynthetic characteristics and flavonoid metabolism between Camellia petelotii (Merr.) Sealy and Camellia impressinervis Chang &Liang

Camellia petelotii (Merr.) Sealy and Camellia impressinervis Chang & Liang belong to the golden subgroup of Camellia (Theaceae). This subgroup contains the yellow-flowering species of the genus, which have high medicinal and ornamental value and a narrow geographical distribution. These species differ in their tolerance to high light intensity. This study aimed to explore the differences in their light-stress responses and light damage repair processes, and the effect of these networks on secondary metabolite synthesis. Two-year-old plants of both species grown at 300 µmol·m^-2·s^-1 photosynthetically active radiation (PAR) were shifted to 700 µmol·m^-2·s^-1 PAR for 5 days shifting back to 300 µmol·m^-2·s^-1 PAR for recovery for 5 days. Leaf samples were collected at the start of the experiment and 2 days after each shift. Data analysis included measuring photosynthetic indicators, differential transcriptome expression, and quantifying plant hormones, pigments, and flavonoids. Camellia impressinervis showed a weak ability to recover from photodamage that occurred at 700 µmol·m^-2·s^-1 compared with C. petelotii. Photodamage led to decreased photosynthesis, as shown by repressed transcript abundance for photosystem II genes psbA, B, C, O, and Q, photosystem I genes psaB, D, E, H, and N, electron transfer genes petE and F, and ATP synthesis genes ATPF1A and ATPF1B. High-light stress caused more severe damage to C. impressinervis, which showed a stronger response to reactive oxygen species than C. petelotii. In addition, high-light stress promoted the growth and development of high zeatin signalling and increased transcript abundance of adenylate dimethylallyl transferase (IPT) and histidine-containing phosphotransferase (AHP). The identification of transcriptional differences in the regulatory networks that respond to high-light stress and activate recovery of light damage in these two rare species adds to the resources available to conserve them and improve their value through molecular breeding.

Photosynthetic modulation during the diurnal cycle in a unicellular diazotrophic cyanobacterium grown under nitrogen-replete and nitrogen-fixing conditions

Cyanobacteria are the only oxygenic photosynthetic organisms that can fix nitrogen. In diazotrophic cyanobacteria, the regulation of photosynthesis during the diurnal cycle is hypothesized to be linked with nitrogen fixation and involve the D1 protein isoform PsbA4. The amount of bioavailable nitrogen has a major impact on productivity in aqueous environments. In contrast to low- or nitrogen-fixing (-N) conditions, little data on photosynthetic regulation under nitrogen-replete (+ N) conditions are available. We compared the regulation of photosynthesis under -N and + N conditions during the diurnal cycle in wild type and a psbA4 deletion strain of the unicellular diazotrophic cyanobacterium Cyanothece sp. ATCC 51142. We observed common changes to light harvesting and photosynthetic electron transport during the dark in + N and -N conditions and found that these modifications occur in both diazotrophic and non-diazotrophic cyanobacteria. Nitrogen availability increased PSII titer when cells transitioned from dark to light and promoted growth. Under -N conditions, deletion of PsbA4 modified charge recombination in dark and regulation of PSII titer during dark to light transition. We conclude that darkness impacts the acceptor-side modifications to PSII and photosynthetic electron transport in cyanobacteria independently of the nitrogen-fixing status and the presence of PsbA4.

Concerning the enigmatic cytochrome b-559 of oxygenic photosynthesis

Although there is an extensive literature on the properties and possible electron transfer pathways of cytochrome b-559, which is a prominent subunit of the multi-subunit photosystem II complex which functions in oxygenic photosynthesis, there is presently no consensus on the function of b-559 in the photosynthetic electron transport chain. The inability in earlier times to define a redox-linked function of this cytochrome was, to a large extent, a consequence of an absence of biochemical and structure information to complement an extensive array of spectrophotometric studies of the cytochrome in situ. Based on the location of hetero-dimeric b-559 in the photosystem II reaction center complex, derived from crystal crystallographic structure analysis, and the absence of a necessary redox function for the cytochrome in PSII, it is proposed that the main function of cytochrome b-559 is linked to its role as a structure component in the PSII reaction center complex. This function resides in the association of b-559 through its heme histidine residues in the trans-membrane domains of the PsbE and PsbF subunits of the PSII reaction center. These subunits, along with PsbJ, are inferred, from the analysis of structure, to define the intra-membrane portal in the PSII reaction center for plastoquinol (PQH₂) export which, through the PSII complex, provides the redox link to the cytochrome b₆f complex in the electron transfer chain.

Designing Artificial Metalloenzymes by Tuning of the Environment beyond the Primary Coordination Sphere

Metalloenzymes catalyze a variety of reactions using a limited number of natural amino acids and metallocofactors. Therefore, the environment beyond the primary coordination sphere must play an important role in both conferring and tuning their phenomenal catalytic properties, enabling active sites with otherwise similar primary coordination environments to perform a diverse array of biological functions. However, since the interactions beyond the primary coordination sphere are numerous and weak, it has been difficult to pinpoint structural features responsible for the tuning of activities of native enzymes. Designing artificial metalloenzymes (ArMs) offers an excellent basis to elucidate the roles of these interactions and to further develop practical biological catalysts. In this review, we highlight how the secondary coordination spheres of ArMs influence metal binding and catalysis, with particular focus on the use of native protein scaffolds as templates for the design of ArMs by either rational design aided by computational modeling, directed evolution, or a combination of both approaches. In describing successes in designing heme, nonheme Fe, and Cu metalloenzymes, heteronuclear metalloenzymes containing heme, and those ArMs containing other metal centers (including those with non-native metal ions and metallocofactors), we have summarized insights gained on how careful controls of the interactions in the secondary coordination sphere, including hydrophobic and hydrogen bonding interactions, allow the generation and tuning of these respective systems to approach, rival, and, in a few cases, exceed those of native enzymes. We have also provided an outlook on the remaining challenges in the field and future directions that will allow for a deeper understanding of the secondary coordination sphere a deeper understanding of the secondary coordintion sphere to be gained, and in turn to guide the design of a broader and more efficient variety of ArMs.

New Structural and Mechanistic Insights Into Functional Roles of Cytochrome b 559 in Photosystem II

Cytochrome (Cyt) b ₅₅₉ is a key component of the photosystem II (PSII) complex for its assembly and proper function. Previous studies have suggested that Cytb ₅₅₉ has functional roles in early assembly of PSII and in secondary electron transfer pathways that protect PSII against photoinhibition. In addition, the Cytb ₅₅₉ in various PSII preparations exhibited multiple different redox potential forms. However, the precise functional roles of Cytb ₅₅₉ in PSII remain unclear. Recent site-directed mutagenesis studies combined with functional genomics and biochemical analysis, as well as high-resolution x-ray crystallography and cryo-electron microscopy studies on native, inactive, and assembly intermediates of PSII have provided important new structural and mechanistic insights into the functional roles of Cytb ₅₅₉. This mini-review gives an overview of new exciting results and their significance for understanding the structural and functional roles of Cytb ₅₅₉ in PSII.

High levels of anoxygenic photosynthesis revealed by dual-frequency Fourier photoacoustics in Ailanthus altissima leaves

In contrast to oxygenic photosynthesis, true anoxygenic photosynthesis is not associated with O2 evolution originated from water photolysis but still converts light energy to that of the phosphoanhydride bonds of ATP. In a narrow sense, anoxygenic photosynthesis is mainly known as to be related to the purple and green sulfur bacteria, but in a broad sense, it also occurs in the vascular plants. The portion of photosynthetic water photolysis that is compensated by the processes of O2 uptake (respiration, photorespiration, Mehler cycle, etc.) may be referred to as 'quasi' anoxygenic photosynthesis. Photoacoustic method allows for the separate detection of photolytic O2 at frequencies of measuring light about 20-40Hz, whereas at 250-400Hz, it detects the photochemical energy storage. We have developed a fast-Fourier transform photoacoustic method enabling measurements of both these signals simultaneously in one sample. This method allows to calculate oxygenic coefficients, which reflect the part of photochemically stored light energy that is used for the water photolysis. We show that the true anoxygenic photosynthesis in Ailanthus altissima Mill. leaves reached very high levels under low light, under moderate light at the beginning of the 1-h period, and at the end of the 40-min period under saturating light.

Modeling the Influence of Temperature, Light Intensity and Oxygen Concentration on Microalgal Growth Rate

Dissolved oxygen plays a key role in microalgal growth at high density. This effect was so far rarely quantified. Here we propose a new model to represent the combined effect of light, oxygen concentration and temperature (LOT-model) on microalgae growth. The LOT-model introduces oxygen concentration in order to represent the oxidative stress affecting the cultures, adding a toxicity term in the expression of the net growth rate. The model was validated with experimental data for several species such as Chlorella minutissima, Chlorella vulgaris, Dunaliella salina, Isochrysis galbana. It successfully predicted experimental records with an average error lower than 5.5%. The model was also validated using dynamical data where oxygen concentration varies. It highlights a strong impact of oxygen concentration on productivity, depending on temperature. The model quantifies the sensitivity to oxidative stress of different species and shows, for example, that Dunaliella salina is much less affected than Chlorella vulgaris by oxidative stress. The modeling approach can support an optimization strategy to improve productivity, especially for managing high oxygen levels.

A trigger mechanism of herbicides to phytoplankton blooms: From the standpoint of hormesis involving cytochrome b559, reactive oxygen species and nitric oxide

The cause of phytoplankton blooms has been extensively discussed and largely attributed to favorable external conditions such as nitrogen/phosphorus resources, pH and temperature. Here from the standpoint of hormesis response, we propose that phytoplankton blooms are initiated by stimulatory effects of low concentrations of herbicides as environmental contaminants spread over estuaries and lakes. The experimental results revealed general stimulations by herbicides on Microcystis aeruginosa and Selenastrum capricornutum, with the maximum stimulation in the 30-60% range, depending on the agent and experiment. In parallel with enhancing stimulation, the ratio of HP (high-potential) form to LP (low-potential) form of cytochrome b₅₅₉ (RHL) was observed decreasing, while intracellular reactive oxygen species (ROS) were observed increasing. We propose that the ROS originated from the thermodynamic transformation of cytochrome b₅₅₉, enhancing the stimulatory response. Furthermore, the results also proved that thermodynamic states of cytochrome b₅₅₉ could be modulated by nitric oxide, thus affecting cellular equilibrium of oxidative stress (OS) and correspondingly causing the inhibitory effect of higher concentrations of herbicides on phytoplankton. This suggests that hormesis substantially derives from equilibrium shifting of OS. Moreover, it is reasonable to infer that phytoplankton blooms would be motivated by herbicides or other environmental pollutants. This study provides a new thought into global phytoplankton blooms from a contaminant perspective.

Hydrogen Peroxide and Superoxide Anion Radical Photoproduction in PSII Preparations at Various Modifications of the Water-Oxidizing Complex

The photoproduction of superoxide anion radical (O₂^−•) and hydrogen peroxide (H₂O₂) in photosystem II (PSII) preparations depending on the damage to the water-oxidizing complex (WOC) was investigated. The light-induced formation of O₂^−• and H₂O₂ in the PSII preparations rose with the increased destruction of the WOC. The photoproduction of superoxide both in the PSII preparations holding intact WOC and the samples with damage to the WOC was approximately two times higher than H₂O₂. The rise of O₂^−• and H₂O₂ photoproduction in the PSII preparations in the course of the disassembly of the WOC correlated with the increase in the fraction of the low-potential (LP) Cyt b₅₅₉. The restoration of electron flow in the Mn-depleted PSII preparations by exogenous electron donors (diphenylcarbazide, Mn²⁺) suppressed the light-induced formation of O₂^−• and H₂O₂. The decrease of O₂^−• and H₂O₂ photoproduction upon the restoration of electron transport in the Mn-depleted PSII preparations could be due to the re-conversion of the LP Cyt b₅₅₉ into higher potential forms. It is supposed that the conversion of the high potential Cyt b₅₅₉ into its LP form upon damage to the WOC leads to the increase of photoproduction of O₂^−• and H₂O₂ in PSII.

Antimycin A inhibits cytochrome b559-mediated cyclic electron flow within photosystem II

The light reactions of photosynthesis are known to comprise both linear and cyclic electron flow in order to convert light energy into chemical energy in the form of NADPH and ATP. Antimycin A (AA) has been proposed as an inhibitor of ferredoxin-dependent cyclic electron flow around photosystem I (CEF-PSI) in photosynthesis research. However, its precise inhibitory mechanism and target site had not been elucidated yet. Here we show that AA inhibits the cyclic (alternative) electron flow via cytochrome b₅₅₉ (Cyt b₅₅₉) within photosystem II (CEF-PSII). When AA was applied to thylakoid membranes isolated from spinach leaves, the high potential form of Cyt b₅₅₉, which was reduced in the dark, was transformed into the lower potential forms and readily oxidized by molecular oxygen. In the absence of AA, the reduced Cyt b₅₅₉ was oxidized by P680⁺ upon light illumination and re-reduced in the dark, mainly by the electron from the Q_B site on the acceptor side of PSII. In contrast, AA suppressed the oxidation of Cyt b₅₅₉ and induced its reduction under the illumination. This inhibition of Cyt b₅₅₉ oxidation by AA enhanced photoinhibition of PSII. Based on the above results, we propose caution regarding the use of AA for evaluating CEF-PSI per se and concurrently propose that AA provides for new insights into, and interpretations of, the physiological importance of Cyt b₅₅₉, rather than that of CEF-PSI in photosynthetic organisms.

Consequences of structural modifications in cytochrome b559 on the electron acceptor side of Photosystem II

Cytb₅₅₉ in Photosystem II is a heterodimeric b-type cytochrome. The subunits, PsbE and PsbF, consist each in a membrane α-helix. Mutants were previously designed and studied in Thermosynechococcus elongatus (Sugiura et al., Biochim Biophys Acta 1847:276-285, 2015) either in which an axial histidine ligand of the haem-iron was substituted for a methionine, the PsbE/H23M mutant in which the haem was lacking, or in which the haem environment was modified, the PsbE/Y19F and PsbE/T26P mutants. All these mutants remained active showing that the haem has no structural role provided that PsbE and PsbF subunits are present. Here, we have carried on the characterization of these mutants. The following results were obtained: (i) the Y19F mutation hardly affect the E_m of Cytb₅₅₉, whereas the T26P mutation converts the haem into a form with a E_m much below 0 mV (so low that it is likely not reducible by Q_B^-) even in an active enzyme; (ii) in the PsbE/H23M mutant, and to a less extent in PsbE/T26P mutant, the electron transfer efficiency from Q_A^- to Q_B is decreased; (iii) the lower E_m of the Q_A/Q_A^- couple in the PsbE/H23M mutant correlates with a higher production of singlet oxygen; (iv) the superoxide and/or hydroperoxide formation was not increased in the PsbE/H23M mutant lacking the haem, whereas it was significantly larger in the PsbE/T26P. These data are discussed in view of the literature to discriminate between structural and redox roles for the haem of Cytb₅₅₉ in the production of reactive oxygen species.

Analysis of the transformation effect in cytochrome b559 of photosystem II in terms of the model of the heme-quinone redox interaction

Transformation of three-component redox pattern of cytochrome (Cyt) b559 in PS II membrane fragments upon various treatments is manifested in decrease of the relative content (R) of the high potential (HP) redox form of Cyt b559 and concomitant increase in the fractions of the two lower potential forms. Redox titration of Cyt b559 in different types of PS II membrane preparations was performed and revealed that (1) alteration of redox titration curve of Cyt b559 upon treatment of a sample is not specific to the type of treatment; (2) each value of R_HP defines the individual shape of the redox titration curve; (3) population of Cyt b559 may exist in several stable forms with multicomponent redox pattern: three types of three-component redox pattern and one type of two-component redox pattern as well as in the form with a single E_m; (4) transformation of Cyt b559 proceeds as successive conversion between the stable forms with multicomponent redox pattern; (5) upon harsh treatments, Cyt b559 abruptly converts into the state with a single E_m which value is intermediate between the E_m values of the two lower potential forms. Analysis of the data using the model of Cyt b559-quinone redox interaction revealed that diminution of R_HP in a range from 80 to 10% reflects a shift in redox equilibrium between the heme group of Cyt b559 and the interacting quinone, due to a gradual decrease of 90 mV in E_m of the heme group at the virtually unchanged E_m of the quinone component.

Plastoquinol generates and scavenges reactive oxygen species in organic solvent: Potential relevance for thylakoids

The present work reports reactions of plastoquinol (PQH₂-9) and plastoquinone (PQ-9) in organic solvents and summarizes the literature to understand similar reactions in thylakoids. In thylakoids, PQH₂-9 is oxidized by the cytochrome b₆/f complex (Cyt b₆/f) but some PQH₂-9 is also oxidized by reactions in which oxygen acts as an electron acceptor and is converted to reactive oxygen species (ROS). Furthermore, PQH₂-9 reacts with ROS. Light enhances oxygen-dependent oxidation of PQH₂-9. We examined the oxidation of PQH₂-9 via dismutation of PQH₂-9 and PQ-9 and scavenging of the superoxide anion radical (O₂^-) and hydrogen peroxide (H₂O₂) by PQH₂-9. Oxidation of PQH₂-9 via dismutation to semiquinone was slow and independent of pH in organic solvents and in solvent/buffer systems, suggesting that intramembraneous oxidation of PQH₂-9 in darkness mainly proceeds via reactions catalyzed by the plastid terminal oxidase and cytochrome b₅₅₉. In the light, oxidation of PQH₂-9 by singlet oxygen and by O₂^- formed in PSI contribute significantly. In addition, Cyt b_6/f forms H₂O₂ with a PQH₂-9 dependent mechanism. Measurements of the reaction of O₂^- with PQH₂-9 and PQ-9 in acetonitrile showed that O₂^- oxidizes PQH₂-9, forming PQ-9 and several PQ-9-derived products. The rate constant of the reaction between PQH₂-9 and O₂^- was found to be 10⁴ M^-1 s^-1. H₂O₂ was found to oxidize PQH₂-9 to PQ-9, but failed to oxidize all PQH₂-9, suggesting that the oxidation of PQH₂-9 by H₂O₂ proceeds via deprotonation mechanisms producing PQH^--9, PQ^2--9 and the protonated hydrogen peroxide cation, H₃O₂⁺.

Quantitative Proteomics Shows Extensive Remodeling Induced by Nitrogen Limitation in Prochlorococcusmarinus SS120

Prochlorococcus requires the capability to accommodate to environmental changes in order to proliferate in oligotrophic oceans, in particular regarding nitrogen availability. A precise knowledge of the composition and changes in the proteome can yield fundamental insights into such a response. Here we report a detailed proteome analysis of the important model cyanobacterium Prochlorococcus marinus SS120 after treatment with azaserine, an inhibitor of ferredoxin-dependent glutamate synthase (GOGAT), to simulate extreme nitrogen starvation. In total, 1,072 proteins, corresponding to 57% of the theoretical proteome, were identified-the maximum proteome coverage obtained for any Prochlorococcus strain thus far. Spectral intensity, calibrated quantification by the Hi3 method, was obtained for 1,007 proteins. Statistically significant changes (P value of <0.05) were observed for 408 proteins, with the majority of proteins (92.4%) downregulated after 8 h of treatment. There was a strong decrease in ribosomal proteins upon azaserine addition, while many transporters were increased. The regulatory proteins P_II and PipX were decreased, and the global nitrogen regulator NtcA was upregulated. Furthermore, our data for Prochlorococcus indicate that NtcA also participates in the regulation of photosynthesis. Prochlorococcus responds to the lack of nitrogen by slowing down translation, while inducing photosynthetic cyclic electron flow and biosynthesis of proteins involved in nitrogen uptake and assimilation. IMPORTANCEProchlorococcus is the most abundant photosynthetic organism on Earth, contributing significantly to global primary production and playing a prominent role in biogeochemical cycles. Here we study the effects of extreme nitrogen limitation, a feature of the oligotrophic oceans inhabited by this organism. Quantitative proteomics allowed an accurate quantification of the Prochlorococcus proteome, finding three main responses to nitrogen limitation: upregulation of nitrogen assimilation-related proteins, including transporters; downregulation of ribosome proteins; and induction of the photosystem II cyclic electron flow. This suggests that nitrogen limitation affects a range of metabolic processes far wider than initially believed, with the ultimate goal of saving nitrogen and maximizing the nitrogen uptake and assimilation capabilities of the cell.

Close Relationships Between the PSII Repair Cycle and Thylakoid Membrane Dynamics

In chloroplasts, a three-dimensional network of thylakoid membranes is formed by stacked grana and interconnecting stroma thylakoids. The grana are crowded with photosynthetic proteins, where PSII-light harvesting complex II (LHCII) supercomplexes often show semi-crystalline arrays for efficient energy trapping, transfer and use. Although light is essential for photosynthesis, PSII is damaged by reactive oxygen species that are generated from primary photochemical reactions when plants are exposed to excess light. Because PSII complexes are embedded in the lipid bilayers of thylakoid membranes, their functions are affected by the conditions of the lipids. Electron paramagnetic resonance (EPR) spin trapping measurements showed that singlet oxygen was formed through peroxidation of thylakoid lipids, suggesting that lipid peroxidation can damage proteins, including the D1 protein. After photodamage, PSII is restored by a specific repair system in thylakoid membranes. In the PSII repair cycle, phosphorylation and dephosphorylation of the PSII proteins control the timing of PSII disassembly and subsequent degradation of the D1 protein. Under light stress, stacked grana turn into unstacked thylakoids with bent grana margins. These structural changes may be closely linked to the mechanisms of the PSII repair cycle because PSII can move more easily from the grana core to the stroma thylakoids through an expanded stromal gap between each thylakoid. Thus, plants modulate the structure of thylakoid membranes under high light to carry out efficient PSII repair. This review focuses on the behavior of the PSII complex and the active role of structural changes to thylakoid membranes under light stress.

The PsbY protein of Arabidopsis Photosystem II is important for the redox control of cytochrome b559

Photosystem II is a protein complex embedded in the thylakoid membrane of photosynthetic organisms and performs the light driven water oxidation into electrons and molecular oxygen that initiate the photosynthetic process. This important complex is composed of more than two dozen of intrinsic and peripheral subunits, of those half are low molecular mass proteins. PsbY is one of those low molecular mass proteins; this 4.7-4.9kDa intrinsic protein seems not to bind any cofactors. Based on structural data from cyanobacterial and red algal Photosystem II PsbY is located closely or in direct contact with cytochrome b559. Cytb559 consists of two protein subunits (PsbE and PsbF) ligating a heme-group in-between them. While the exact function of this component in Photosystem II has not yet been clarified, a crucial role for assembly and photo-protection in prokaryotic complexes has been suggested. One unique feature of Cytb559 is its redox-heterogeneity, forming high, medium and low potential, however, neither origin nor mechanism are known. To reveal the function of PsbY within Photosystem II of Arabidopsis we have analysed PsbY knock-out plants and compared them to wild type and to complemented mutant lines. We show that in the absence of PsbY protein Cytb559 is only present in its oxidized, low potential form and plants depleted of PsbY were found to be more susceptible to photoinhibition.

Production of Reactive Oxygen Species by Photosystem II as a Response to Light and Temperature Stress

The effect of various abiotic stresses on photosynthetic apparatus is inevitably associated with formation of harmful reactive oxygen species (ROS). In this review, recent progress on ROS production by photosystem II (PSII) as a response to high light and high temperature is overviewed. Under high light, ROS production is unavoidably associated with energy transfer and electron transport in PSII. Singlet oxygen is produced by the energy transfer form triplet chlorophyll to molecular oxygen formed by the intersystem crossing from singlet chlorophyll in the PSII antennae complex or the recombination of the charge separated radical pair in the PSII reaction center. Apart to triplet chlorophyll, triplet carbonyl formed by lipid peroxidation transfers energy to molecular oxygen forming singlet oxygen. On the PSII electron acceptor side, electron leakage to molecular oxygen forms superoxide anion radical which dismutes to hydrogen peroxide which is reduced by the non-heme iron to hydroxyl radical. On the PSII electron donor side, incomplete water oxidation forms hydrogen peroxide which is reduced by manganese to hydroxyl radical. Under high temperature, dark production of singlet oxygen results from lipid peroxidation initiated by lipoxygenase, whereas incomplete water oxidation forms hydrogen peroxide which is reduced by manganese to hydroxyl radical. The understanding of molecular basis for ROS production by PSII provides new insight into how plants survive under adverse environmental conditions.

Production of Reactive Oxygen Species by Photosystem II as a Response to Light and Temperature Stress

The effect of various abiotic stresses on photosynthetic apparatus is inevitably associated with formation of harmful reactive oxygen species (ROS). In this review, recent progress on ROS production by photosystem II (PSII) as a response to high light and high temperature is overviewed. Under high light, ROS production is unavoidably associated with energy transfer and electron transport in PSII. Singlet oxygen is produced by the energy transfer form triplet chlorophyll to molecular oxygen formed by the intersystem crossing from singlet chlorophyll in the PSII antennae complex or the recombination of the charge separated radical pair in the PSII reaction center. Apart to triplet chlorophyll, triplet carbonyl formed by lipid peroxidation transfers energy to molecular oxygen forming singlet oxygen. On the PSII electron acceptor side, electron leakage to molecular oxygen forms superoxide anion radical which dismutes to hydrogen peroxide which is reduced by the non-heme iron to hydroxyl radical. On the PSII electron donor side, incomplete water oxidation forms hydrogen peroxide which is reduced by manganese to hydroxyl radical. Under high temperature, dark production of singlet oxygen results from lipid peroxidation initiated by lipoxygenase, whereas incomplete water oxidation forms hydrogen peroxide which is reduced by manganese to hydroxyl radical. The understanding of molecular basis for ROS production by PSII provides new insight into how plants survive under adverse environmental conditions.

Analysis of photosystem II biogenesis in cyanobacteria

Photosystem II (PSII), a large multisubunit membrane protein complex found in the thylakoid membranes of cyanobacteria, algae and plants, catalyzes light-driven oxygen evolution from water and reduction of plastoquinone. Biogenesis of PSII requires coordinated assembly of at least 20 protein subunits, as well as incorporation of various organic and inorganic cofactors. The stepwise assembly process is facilitated by numerous protein factors that have been identified in recent years. Further analysis of this process requires the development or refinement of specific methods for the identification of novel assembly factors and, in particular, elucidation of the unique role of each. Here we summarize current knowledge of PSII biogenesis in cyanobacteria, focusing primarily on the impact of methodological advances and innovations. This article is part of a Special Issue entitled Organization and dynamics of bioenergetic systems in bacteria, edited by Conrad Mullineaux.

Detection of hydrogen peroxide in Photosystem II (PSII) using catalytic amperometric biosensor

Hydrogen peroxide (H2O2) is known to be generated in Photosystem II (PSII) via enzymatic and non-enzymatic pathways. Detection of H2O2 by different spectroscopic techniques has been explored, however its sensitive detection has always been a challenge in photosynthetic research. During the recent past, fluorescence probes such as Amplex Red (AR) has been used but is known to either lack specificity or limitation with respect to the minimum detection limit of H2O2. We have employed an electrochemical biosensor for real time monitoring of H2O2 generation at the level of sub-cellular organelles. The electrochemical biosensor comprises of counter electrode and working electrodes. The counter electrode is a platinum plate, while the working electrode is a mediator based catalytic amperometric biosensor device developed by the coating of a carbon electrode with osmium-horseradish peroxidase which acts as H2O2 detection sensor. In the current study, generation and kinetic behavior of H2O2 in PSII membranes have been studied under light illumination. Electrochemical detection of H2O2 using the catalytic amperometric biosensor device is claimed to serve as a promising technique for detection of H2O2 in photosynthetic cells and subcellular structures including PSII or thylakoid membranes. It can also provide a precise information on qualitative determination of H2O2 and thus can be widely used in photosynthetic research.

Characterization of the structural changes and photochemical activity of photosystem I under Al(3+) effect

The photochemical activity of photosystem I (PSI) as affected by Al(3+) was investigated in thylakoid membranes and PSI submembrane fractions isolated from spinach. Biophysical and biochemical techniques such as oxygen uptake, light induced absorbance changes at 820nm, chlorophyll fluorescence emission, SDS-polyacrylamide gel electrophoresis, and FTIR spectroscopy have been used to analyze the sites and action modes of this cation on the PSI complex. Our results showed that Al(3+) above 3mM induces changes in the redox state of P700 reflected by an increase of P700 photooxidation phase and a delay of the slower rate of P700 re-reduction which reveals that Al(3+) exerted an inhibitory action at the donor side of PSI especially at plastocyanin (PC). Furthermore, results of P700 photooxidation monitored in the presence of DCMU with or without MV suggested that the same range of Al(3+) concentrations impairs the photochemical reaction centers (RC) of PSI, as shown by the decline in the amount of active population of P700, and disrupts the charge separation between P700 and the primary electron acceptor A0 leading to the inhibition of electron transfer at the acceptor side of PSI. These inhibitory actions were also accompanied by an impairment of the energy transfer from light harvesting complex (LHCI) to RC of PSI, following the disconnection of LHCI antenna as illustrated by an enhancement of chlorophyll fluorescence emission spectra at low temperature (77K). The above results coincided with FTIR measurements that indicated a conformational change of the protein secondary structures in PSI complex where 25% of α-helix was converted into β-sheet, β-antiparallel and turn structures. These structural changes in PSI complex proteins are closely related with the alteration photochemical activity of PSI including the inhibition of the electron transport through both acceptor and donor sides of PSI.

Non-photochemical reduction of thylakoid photosynthetic redox carriers in vitro: relevance to cyclic electron flow around photosystem I?

Non-photochemical (dark) increases in chlorophyll a fluorescence yield associated with non-photochemical reduction of redox carriers (Fnpr) have been attributed to the reduction of plastoquinone (PQ) related to cyclic electron flow (CEF) around photosystem I. In vivo, this rise in fluorescence is associated with activity of the chloroplast plastoquinone reductase (plastid

NAD(P)H

plastoquinone oxidoreductase) complex. In contrast, this signal measured in isolated thylakoids has been attributed to the activity of the protein gradient regulation-5 (PGR5)/PGR5-like (PGRL1)-associated CEF pathway. Here, we report a systematic experimentation on the origin of Fnpr in isolated thylakoids. Addition of NADPH and ferredoxin to isolated spinach thylakoids resulted in the reduction of the PQ pool, but neither its kinetics nor its inhibitor sensitivities matched those of Fnpr. Notably, Fnpr was more rapid than PQ reduction, and completely insensitive to inhibitors of the PSII QB site and oxygen evolving complex as well as inhibitors of the cytochrome b6f complex. We thus conclude that Fnpr in isolated thylakoids is not a result of redox equilibrium with bulk PQ. Redox titrations and fluorescence emission spectra imply that Fnpr is dependent on the reduction of a low potential redox component (Em about − 340 mV) within photosystem II (PSII), and is likely related to earlier observations of low potential variants of QA within a subpopulation of PSII that is directly reducible by ferredoxin. The implications of these results for our understanding of CEF and other photosynthetic processes are discussed.

Structure/function/dynamics of photosystem II plastoquinone binding sites

Photosystem II (PSII) continuously attracts the attention of researchers aiming to unravel the riddle of its functioning and efficiency fundamental for all life on Earth. Besides, an increasing number of biotechnological applications have been envisaged exploiting and mimicking the unique properties of this macromolecular pigment-protein complex. The PSII organization and working principles have inspired the design of electrochemical water splitting schemes and charge separating triads in energy storage systems as well as biochips and sensors for environmental, agricultural and industrial screening of toxic compounds. An intriguing opportunity is the development of sensor devices, exploiting native or manipulated PSII complexes or ad hoc synthesized polypeptides mimicking the PSII reaction centre proteins as bio-sensing elements. This review offers a concise overview of the recent improvements in the understanding of structure and function of PSII donor side, with focus on the interactions of the plastoquinone cofactors with the surrounding environment and operational features. Furthermore, studies focused on photosynthetic proteins structure/function/dynamics and computational analyses aimed at rational design of high-quality bio-recognition elements in biosensor devices are discussed.

The Roles of Cytochrome b 559 in Assembly and Photoprotection of Photosystem II Revealed by Site-Directed Mutagenesis Studies

Cytochrome b 559 (Cyt b 559) is one of the essential components of the Photosystem II reaction center (PSII). Despite recent accomplishments in understanding the structure and function of PSII, the exact physiological function of Cyt b 559 remains unclear. Cyt b 559 is not involved in the primary electron transfer pathway in PSII but may participate in secondary electron transfer pathways that protect PSII against photoinhibition. Site-directed mutagenesis studies combined with spectroscopic and functional analysis have been used to characterize Cyt b 559 mutant strains and their mutant PSII complex in higher plants, green algae, and cyanobacteria. These integrated studies have provided important in vivo evidence for possible physiological roles of Cyt b 559 in the assembly and stability of PSII, protecting PSII against photoinhibition, and modulating photosynthetic light harvesting. This mini-review presents an overview of recent important progress in site-directed mutagenesis studies of Cyt b 559 and implications for revealing the physiological functions of Cyt b 559 in PSII.

Evidence for the involvement of loosely bound plastosemiquinones in superoxide anion radical production in photosystem II

Recent evidence has indicated the presence of novel plastoquinone-binding sites, Q_C and Q_D, in photosystem II (PSII). Here, we investigated the potential involvement of loosely bound plastosemiquinones in superoxide anion radical (O₂^•−) formation in spinach PSII membranes using electron paramagnetic resonance (EPR) spin-trapping spectroscopy. Illumination of PSII membranes in the presence of the spin trap EMPO (5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide) resulted in the formation of O₂^•−, which was monitored by the appearance of EMPO-OOH adduct EPR signal. Addition of exogenous short-chain plastoquinone to PSII membranes markedly enhanced the EMPO-OOH adduct EPR signal. Both in the unsupplemented and plastoquinone-supplemented PSII membranes, the EMPO-OOH adduct EPR signal was suppressed by 50% when the urea-type herbicide DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) was bound at the Q_B site. However, the EMPO-OOH adduct EPR signal was enhanced by binding of the phenolic-type herbicide dinoseb (2,4-dinitro-6-sec-butylphenol) at the Q_D site. Both in the unsupplemented and plastoquinone-supplemented PSII membranes, DCMU and dinoseb inhibited photoreduction of the high-potential form of cytochrome b₅₅₉ (cyt b₅₅₉). Based on these results, we propose that O₂^•− is formed via the reduction of molecular oxygen by plastosemiquinones formed through one-electron reduction of plastoquinone at the Q_B site and one-electron oxidation of plastoquinol by cyt b₅₅₉ at the Q_C site. On the contrary, the involvement of a plastosemiquinone formed via the one-electron oxidation of plastoquinol by cyt b₅₅₉ at the Q_D site seems to be ambiguous. In spite of the fact that the existence of Q_C and Q_D sites is not generally accepted yet, the present study provided more spectroscopic data on the potential functional role of these new plastoquinone-binding sites.

Influence of thylakoid membrane lipids on the structure and function of the plant photosystem II core complex

MGDG leads to a dimerization of isolated, monomeric PSII core complexes. SQDG and PG induce a detachment of CP43 from the PSII core, thereby disturbing the intrinsic PSII electron transport. The influence of the four thylakoid membrane lipids monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) on the structure and function of isolated monomeric photosystem (PS) II core complexes was investigated. Incubation with the negatively charged lipids SQDG and PG led to a loss of the long-wavelength 77 K fluorescence emission at 693 nm that is associated with the inner antenna proteins. The neutral galactolipids DGDG and MGDG had no or only minor effects on the fluorescence emission spectra of the PSII core complexes, respectively. Pigment analysis, absorption and 77 K fluorescence excitation spectroscopy showed that incubation with SQDG and PG led to an exposure of chlorophyll molecules to the surrounding medium followed by conversion to pheophytin under acidic conditions. Size-exclusion chromatography and polypeptide analysis corroborated the findings of the spectroscopic measurements and pigment analysis. They showed that the negatively charged lipid SQDG led to a dissociation of the inner antenna protein CP43 and the 27- and 25-kDa apoproteins of the light-harvesting complex II, that were also associated with a part of the PSII core complexes used in the present study. Incubation of PSII core complexes with MGDG, on the other hand, induced an almost complete dimerization of the monomeric PSII. Measurements of the fast PSII fluorescence induction demonstrated that MGDG and DGDG only had a minor influence on the reduction kinetics of plastoquinone QA and the artificial PSII electron acceptor 2,5-dimethyl-p-benzoquinone (DMBQ). SQDG and, to a lesser extent, PG perturbed the intrinsic PSII electron transport significantly.

Structure and function of the hydrophilic Photosystem II assembly proteins: Psb27, Psb28 and Ycf48

Photosystem II (PS II) is a macromolecular complex responsible for light-driven oxidation of water and reduction of plastoquinone as part of the photosynthetic electron transport chain found in thylakoid membranes. Each PS II complex is composed of at least 20 protein subunits and over 80 cofactors. The biogenesis of PS II requires further hydrophilic and membrane-spanning proteins which are not part of the active holoenzyme. Many of these biogenesis proteins make transient interactions with specific PS II assembly intermediates: sometimes these are essential for biogenesis while in other examples they are required for optimizing assembly of the mature complex. In this review the function and structure of the Psb27, Psb28 and Ycf48 hydrophilic assembly factors is discussed by combining structural, biochemical and physiological information. Each of these assembly factors has homologues in all oxygenic photosynthetic organisms. We provide a simple overview for the roles of these protein factors in cyanobacterial PS II assembly emphasizing their participation in both photosystem biogenesis and recovery from photodamage.

The role of the high potential form of the cytochrome b559: Study of Thermosynechococcus elongatus mutants

Cytochrome b559 is an essential component of the photosystem II reaction center in photosynthetic oxygen-evolving organisms, but its function still remains unclear. The use of photosystem II preparations from Thermosynechococcus elongatus of high integrity and activity allowed us to measure for the first time the influence of cytochrome b559 mutations on its midpoint redox potential and on the reduction of the cytochrome b559 by the plastoquinone pool (or QB). In this work, five mutants having a mutation in the α-subunit (I14A, I14S, R18S, I27A and I27T) and one in the β-subunit (F32Y) of cytochrome b559 have been investigated. All the mutations led to a destabilization of the high potential form of the cytochrome b559. The midpoint redox potential of the high potential form was significantly altered in the αR18S and αI27T mutant strains. The αR18S strain also showed a high sensitivity to photoinhibitory illumination and an altered oxidase activity. This was suggested by measurements of light induced oxidation and dark re-reduction of the cytochrome b559 showing that under conditions of a non-functional water oxidation system, once the cytochrome is oxidized by P680(+), the yield of its reduction by QB or the PQ pool was smaller and the kinetic slower in the αR18S mutant than in the wild-type strain. Thus, the extremely positive redox potential of the high potential form of cytochrome b559 could be necessary to ensure efficient oxidation of the PQ pool and to function as an electron reservoir replacing the water oxidation system when it is not operating.

Photosynthesis-related quantities for education and modeling

A quantitative understanding of the photosynthetic machinery depends largely on quantities, such as concentrations, sizes, absorption wavelengths, redox potentials, and rate constants. The present contribution is a collection of numbers and quantities related mainly to photosynthesis in higher plants. All numbers are taken directly from a literature or database source and the corresponding reference is provided. The numerical values, presented in this paper, provide ranges of values, obtained in specific experiments for specific organisms. However, the presented numbers can be useful for understanding the principles of structure and function of photosynthetic machinery and for guidance of future research.

Molecular mechanisms for generating transmembrane proton gradients

Membrane proteins use the energy of light or high energy substrates to build a transmembrane proton gradient through a series of reactions leading to proton release into the lower pH compartment (P-side) and proton uptake from the higher pH compartment (N-side). This review considers how the proton affinity of the substrates, cofactors and amino acids are modified in four proteins to drive proton transfers. Bacterial reaction centers (RCs) and photosystem II (PSII) carry out redox chemistry with the species to be oxidized on the P-side while reduction occurs on the N-side of the membrane. Terminal redox cofactors are used which have pKas that are strongly dependent on their redox state, so that protons are lost on oxidation and gained on reduction. Bacteriorhodopsin is a true proton pump. Light activation triggers trans to cis isomerization of a bound retinal. Strong electrostatic interactions within clusters of amino acids are modified by the conformational changes initiated by retinal motion leading to changes in proton affinity, driving transmembrane proton transfer. Cytochrome c oxidase (CcO) catalyzes the reduction of O2 to water. The protons needed for chemistry are bound from the N-side. The reduction chemistry also drives proton pumping from N- to P-side. Overall, in CcO the uptake of 4 electrons to reduce O2 transports 8 charges across the membrane, with each reduction fully coupled to removal of two protons from the N-side, the delivery of one for chemistry and transport of the other to the P-side.

Spectroscopic and functional characterization of cyanobacterium Synechocystis PCC 6803 mutants on the cytoplasmic-side of cytochrome b559 in photosystem II

We performed spectroscopic and functional characterization on cyanobacterium Synechocystis PCC6803 with mutations of charged residues of the cytoplasmic side of cytochrome (Cyt) b559 in photosystem II (PSII). All of the mutant cells grew photoautotrophically and assembled stable PSII. However, R7Eα, R17Eα and R17Lβ mutant cells grew significantly slower and were more susceptible to photoinhibition than wild-type cells. The adverse effects of the arginine mutations on the activity and the stability of PSII were in the following order (R17Lβ>R7Eα>R17Eα and R17Aα). All these arginine mutants exhibited normal period-four oscillation in oxygen yield. Thermoluminescence characteristics indicated a slight decrease in the stability of the S3QB(-)/S2QB(-) charge pairs in the R7Eα and R17Lβ mutant cells. R7Eα and R17Lβ PSII core complexes contained predominantly the low potential form of Cyt b559. EPR results indicated the displacement of one of the two axial ligands to the heme of Cyt b559 in R7Eα and R17Lβ mutant reaction centers. Our results demonstrate that the electrostatic interactions between these arginine residues and the heme propionates of Cyt b559 are important to the structure and redox properties of Cyt b559. In addition, the blue light-induced nonphotochemical quenching was significantly attenuated and its recovery was accelerated in the R7Lα and R17Lβ mutant cells. Furthermore, ultra performance liquid chromatography-mass spectrometry results showed that the PQ pool was more reduced in the R7Eα and R17Lβ mutant cells than wild-type cells in the dark. Our data support a functional role of Cyt b559 in protection of PSII under photoinhibition conditions in vivo.

Control of STN7 transcript abundance and transient STN7 dimerisation are involved in the regulation of STN7 activity

Reversible phosphorylation of LHCII, the light-harvesting complex of photosystem II, controls its migration between the two photosystems (state transitions), and serves to adapt the photosynthetic machinery of plants and green algae to short-term changes in ambient light conditions. The thylakoid kinase STN7 is required for LHCII phosphorylation and state transitions in vascular plants. Regulation of STN7 levels occurs at the post-translational level, depends on the thylakoid redox state, and might involve reversible autophosphorylation. Here, we have analysed the effects of different light conditions and chemical inhibitors on the abundance of STN7 transcripts and their products. This analysis was performed in wild-type Arabidopsis thaliana plants, in several photosynthetic mutants, and in lines overexpressing STN7 (oeSTN7) or expressing mutant variants of STN7 carrying single or double cysteine-serine exchanges. It was found that accumulation of the STN7 protein is also controlled at the level of transcript abundance. Under certain conditions, exposure to high light or far-red light treatment, the relative decreases in LHCII phosphorylation can be attributed to decreases in STN7 abundance. Nevertheless, inhibitor experiments showed that redox control of LHCII kinase activity persists in oeSTN7 plants. STN7 dimers were found in oeSTN7 plants and in lines with single cysteine-serine exchanges, indicating that dimerisation involves disulphide bridges. We speculate that transient STN7 dimerisation is required for STN7 activity, and that the altered dimerisation behaviour of oeSTN7 plants might be responsible for the unusually high phosphorylation of LHCII in the dark found in this genotype.

Biphasic reduction of cytochrome b559 by plastoquinol in photosystem II membrane fragments: evidence for two types of cytochrome b559/plastoquinone redox equilibria

In photosystem II membrane fragments with oxidized cytochrome (Cyt) b559 reduction of Cyt b559 by plastoquinol formed in the membrane pool under illumination and by exogenous decylplastoquinol added in the dark was studied. Reduction of oxidized Cyt b559 by plastoquinols proceeds biphasically comprising a fast component with a rate constant higher than (10s)(-1), named phase I, followed by a slower dark reaction with a rate constant of (2.7min)(-1) at pH6.5, termed phase II. The extents of both components of Cyt b559 reduction increased with increasing concentrations of the quinols, with that, maximally a half of oxidized Cyt b559 can be photoreduced or chemically reduced in phase I at pH6.5. The photosystem II herbicide dinoseb but not 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) competed with the quinol reductant in phase I. The results reveal that the two components of the Cyt b559 redox reaction reflect two redox equilibria attaining in different time domains. One-electron redox equilibrium between oxidized Cyt b559 and the photosystem II-bound plastoquinol is established in phase I of Cyt b559 reduction. Phase II is attributed to equilibration of Cyt b559 redox forms with the quinone pool. The quinone site involved in phase I of Cyt b559 reduction is considered to be the site regulating the redox potential of Cyt b559 which can accommodate quinone, semiquinone and quinol forms. The properties of this site designated here as QD clearly suggest that it is distinct from the site QC found in the photosystem II crystal structure.

Investigation of the biological properties of (hetero)aromatic thiosemicarbazones

Two series of thiosemicarbazone-based iron chelators (twenty-seven compounds) were designed and synthesized using a microwave-assisted approach. Quinoline and halogenated phenyl were selected as parent scaffolds on the basis of a similarity search. The lipophilicity of the synthesized compounds was measured using HPLC and then calculated. Primary in vitro screening of the synthesized compounds was performed against eight pathogenic fungal strains. Only a few compounds showed moderate activity against fungi, and (E)-2-(quinolin-2-ylvinyl)-N,N-dimethylhydrazine-carbothioamide appeared to be more effective than fluconazole against most of the fungal strains tested. Antiproliferative activity was measured using a human colon cancer cell line (HCT-116). Several of the tested compounds showed submicromolar antiproliferative activity. Compounds were also tested for their activity related to the inhibition of photosynthetic electron transport (PET) in spinach (Spinacia oleracea L.) chloroplasts. The structure-activity relationships are discussed for all of the compounds.

Probing the N-terminal sequence of spinach PsbO: evidence that essential threonine residues bind to different functional sites in eukaryotic photosystem II

The N-terminal ¹E-⁶L domain of the manganese-stabilizing protein (PsbO) from spinach prevents non-specific binding of the subunit to photosystem II (PSII) and deletions of the ¹E-⁷T or ¹E-¹⁵T sequences from the PsbO N-terminus reduce or impair, respectively, functional binding of PsbO to PSII (Popelkova et al., Biochemistry 42:6193-6200, 2003). The work presented here provides deeper insights into the interaction of PsbO with PSII. The data show that a single mutation, ¹⁵T → A in mature PsbO from spinach reduces the stoichiometry of its functional binding from two to one subunit per PSII and decreases reconstitution of activity to about 45 % of the wild-type control. Replacement of the ¹E-⁶L domain with ⁶M in the T15A PsbO mutant has no additional negative effect on recovery of O₂ evolution activity, but it significantly weakens both functional and nonspecific binding of the truncated mutant to PSII. These results suggest that the ¹⁵T side-chain by itself is essential for binding of one of two PsbO subunits to eukaryotic PSII and that specific PSII-binding sites for PsbO are distinguishable; one PSII-binding site does not require PsbO-¹⁵T and probably interacts with the other N-terminal domain of PsbO. Identity of the latter domain is revealed by a requirement for the presence of the ¹E-⁶L sequence that is shown here to be necessary for high-affinity binding of PsbO to PSII. When combined with previous results, the data presented here lead to a more detailed model for PsbO binding in eukaryotic PSII.