Studies on the structural and functional organization of system II of photosynthesis. The use of trypsin as a structurally selective inhibitor at the outer surface of the thylakoid membrane

Herbicide resistance in Chlamydomonas reinhardtii results from a mutation in the chloroplast gene for the 32-kilodalton protein of photosystem II

We report the isolation and characterization of a uniparental mutant of Chlamydomonas reinhardtii that is resistant to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine). Such herbicides inhibit photosynthesis by preventing transfer of electrons in photosystem II from the primary stable electron acceptor Q to the secondary stable electron acceptor complex B, which is thought to contain a protein of 32 kDa and a bound quinone. It has been proposed that herbicide binding to the 32-kDa protein alters the B complex so that electron transfer from Q is prohibited. Both whole and broken-cell preparations of the mutant alga show a resistance to the effects of herbicide on electron transfer from Q to B, as measured by fluorescence-induction kinetics. In the absence of herbicide, mutant cells exhibit a slower rate of Q to B electron transfer than do wild-type cells. The 32-kDa protein from wild-type cells, but not mutant cells, binds azido[(14)C]atrazine at 0.1 muM. We have isolated psbA, the chloroplast gene for the 32-kDa protein, from both wild-type and herbicide-resistant algae and sequenced the coding regions of the gene that are contained in five exons. The only difference between the exon nucleotide sequences of the wild-type and mutant psbA is a single T-A to G-C transversion. This mutation results in a predicted amino acid change of serine in the wild-type protein to alanine in the mutant. We suggest that this alteration in the 32-kDa protein is the molecular basis for herbicide resistance in the C. reinhardtii mutant.

Regulation of protein metabolism: Coupling of photosynthetic electron transport to in vivo degradation of the rapidly metabolized 32-kilodalton protein of the chloroplast membranes

In Spirodela oligorrhiza, mature chloroplasts copiously synthesize and degrade a 32-kilodalton membrane protein. The rates of synthesis and degradation are controlled by light intensity, the protein being unstable in the light and stable in the dark. Light-driven synthesis, but not degradation, is dependent on ATP. Degradation is blocked by herbicides inhibiting photosystem II electron transport, such as diuron and atrazine. Thus, both anabolism and catabolism of the 32-kilodalton protein are photoregulated, with degradation coupled to electron transport rather than phosphorylation.

Photoaffinity labeling of an herbicide receptor protein in chloroplast membranes

2-Azido-4-ethylamino-6-isopropylamino-s-triazine (azido-atrazine) inhibits photosynthetic electron transport at a site identical to that affected by atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine). The latter is a well-characterized inhibitor of photosystem II reactions. Azido-atrazine was used as a photoaffinity label to identify the herbicide receptor protein; UV irradiation of chloroplast thylakoids in the presence of azido[(14)C]atrazine resulted in the covalent attachment of radioactive inhibitor to thylakoid membranes isolated from pea seedlings and from a triazine-susceptible biotype of the weed Amaranthus hybridus. No covalent binding of azido-atrazine was observed for thylakoid membranes isolated from a naturally occurring triazine-resistant biotype of A. hybridus. Analysis of thylakoid polypeptides from both the susceptible and resistant A. hybridus biotypes by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, followed by fluorography to locate (14)C label, demonstrated specific association of the azido[(14)C]atrazine with polypeptides of the 34- to 32-kilodalton size class in susceptible but not in resistant membranes.

Chloroplast membrane alterations in triazine-resistant Amaranthus retroflexus biotypes

The effectiveness of diuron, atrazine, procyazine, and cyanazine were compared in controlling growth of redroot pigweed (Amaranthus retroflexus L.) in hydroponic culture. A very marked differential inhibition response was observed for atrazine between resistant and susceptible biotypes. Procyazine and cyanazine exhibited less dramatic differential responses, whereas diuron was equally effective in controlling growth in both biotypes. Photosystem II activity of chloroplasts from both triazine-resistant and triazine-susceptible biotypes was inhibited by diuron but only the chloroplasts from triazine-susceptible biotypes were inhibited significantly by atrazine. The photochemical activity of chloroplasts from triazine-resistant biotypes was partially resistant to procyazine or cyanazine inhibition. The parallel lack of diuron differential effects, partial procyazine and cyanazine differential response, and very marked atrazine differential response in both whole plant and chloroplast assays indicates that the chloroplast is the site of selective herbicide tolerance in these triazine-resistant redroot pigweed biotypes.Photosystem II photochemical properties were characterized by analysis of chlorophyll fluorescence transients in the presence or absence of herbicides. Data with susceptible chloroplasts indicated that both diuron and atrazine inhibit electron flow very near the primary electron acceptor of photosystem II. Only diuron altered the fluorescence transient in resistant chloroplasts. In untreated preparations there were marked differences in the fast phases of the fluorescence increase in resistant vs. susceptible chloroplasts; these data are interpreted as showing that the resistant plastids have an alteration in the rate of reoxidation of the primary photosystem II electron acceptor. Electrophoretic analysis of chloroplast membrane proteins of the two biotypes showed small changes in the electrophoretic mobilities of two polypeptide species. The data provide evidence for the following herbicide resistance mechanism: genetically controlled modification of the herbicide target site.

Photosystem II: structure and mechanism of the water:plastoquinone oxidoreductase

This mini-review briefly summarizes our current knowledge on the reaction pattern of light-driven water splitting and the structure of Photosystem II that acts as a water:plastoquinone oxidoreductase. The overall process comprises three types of reaction sequences: (a) light-induced charge separation leading to formation of the radical ion pair P680+*QA(-*) ; (b) reduction of plastoquinone to plastoquinol at the QB site via a two-step reaction sequence with QA(-*) as reductant and (c) oxidative water splitting into O2 and four protons at a manganese-containing catalytic site via a four-step sequence driven by P680+* as oxidant and a redox active tyrosine YZ acting as mediator. Based on recent progress in X-ray diffraction crystallographic structure analysis the array of the cofactors within the protein matrix is discussed in relation to the functional pattern. Special emphasis is paid on the structure of the catalytic sites of PQH2 formation (QB-site) and oxidative water splitting (Mn4OxCa cluster). The energetics and kinetics of the reactions taking place at these sites are presented only in a very concise manner with reference to recent up-to-date reviews. It is illustrated that several questions on the mechanism of oxidative water splitting and the structure of the catalytic sites are far from being satisfactorily answered.

Inhibitors in the functional dissection of the photosynthetic electron transport system

The significance of inhibitors and artificial electron acceptor and donor systems as experimental tools for studying the photosynthetic system is described by reviewing early classical articles. The historical development in unravelling the role and sequence of electron carriers and energy conserving sites in the electron transport chain is acknowledged. Emphasis is given to inhibitors of the acceptor side of photosystem II and of the plastoquinol oxidation site in the cytochrome b6/f complex. Their role in regulatory processes under redox control is introduced.

GTP enhances the degradation of the photosystem II D1 protein irrespective of its conformational heterogeneity at the Q(B) site

The light exposure history and/or binding of different herbicides at the Q(B) site may induce heterogeneity of photosystem II acceptor side conformation that affects D1 protein degradation under photoinhibitory conditions. GTP was recently found to stimulate the D1 protein degradation of photoinactivated photosystem II (Spetea, C. , Hundal, T., Lohmann, F., and Andersson, B. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6547-6552). Here we report that GTP enhances the cleavage of the D1 protein D-E loop following exposure of thylakoid membranes to either high light, low light, or repetitive single turnover flashes but not to trypsin. GTP does not stimulate D1 protein degradation in the presence of herbicides known to affect the accessibility of the cleavage site to proteolysis. However, GTP stimulates degradation that can be induced even in darkness in some photosystem II conformers following binding of the PNO8 herbicide (Nakajima, Y., Yoshida, S., Inoue, Y., Yoneyama, K., and Ono, T. (1995) Biochim. Biophys. Acta 1230, 38-44). Both the PNO8- and the light-induced primary cleavage of the D1 protein occur in the grana membrane domains. The subsequent migration of photosytem II containing the D1 protein fragments to the stroma domains for secondary proteolysis is light-activated. We conclude that the GTP effect is not confined to a specific photoinactivation pathway nor to the conformational state of the photosystem II acceptor side. Consequently, GTP does not interact with the site of D1 protein cleavage but rather enhances the activity of the endogenous proteolytic system.

Isolation and properties of PS II membrane fragments depleted of the non heme iron center

The functional properties and the content of non heme iron and cytochrome b559 were investigated by measuring flash induced transient changes of the relative fluorescence quantum yield and applying Mössbauer spectroscopy. It was found that untreated PS II membrane fragments contain a heterogeneous population of two types of non heme iron centers and about 2 cytochrome b559 per PS II. Twofold treatment of these samples with a recently described 'iron depletion' procedure (MacMillan, F., Lendzian, F., Renger, G. and Lubitz, W. (1995) Biochemistry 34, 3144-3156) leads to a complete loss (below the detection limit of Mössbauer spectroscopy) of the non heme iron center while more than 50% of the PS II complexes retain the functional integrity for light induced formation of the 'stable' radical pair Y(OX)(Z) P680Pheo Q(-.)(A). This sample type deprived of virtually all non heme iron in PS II provides a most suitable material for magnetic resonance studies that require an elimination of the interaction between Fe2+ and nearby radicals.

A possible calcium binding site in D1 protein: A fluorescence and FTIR study of the interaction between lanthanides and a synthetic peptide

A peptide ranging from residue 229 to 240 of the D1 protein of Photosystem (PS) II was synthesized and lanthanides were used as candidates of calcium. Fluorescence and FTIR spectroscopy were used to test the conformational adaptation after lanthanide additions. Fluorescence spectroscopy showed that the synthetic peptide provides lanthanide binding site, and that glutamic acids are involved in lanthanide binding. Resolution enhancement techniques were combined with band curve-fitting procedures to quantitate the FTIR spectral information from the amide 1 bands. The relative areas of these component bands indicate that lanthanide induced a substantial decrease in the amount of unordered structure and turns, while a corresponding increase in the amount of α-helix and 'open loop' was also observed. This indicates that a relatively compact structure of the synthetic peptide is formed if lanthanides are applied. The results may reflect on the physiological and biochemical function of calcium in PS II, including preventing D1 from trypsin digestion.

Effect of trypsin on D1/D 2-cytochrom b 559 Photosystem 2 reaction center complex and reaction center from Rhodopseudomonas viridis

Proteolytic enzyme (trypsin) was used to structurally alter the RCs isolated from plant and bacterium as a way of probing the relation between structure (chromophore-apoprotein interactions) and function (photochemical activity). It was found that neither spectral characteristics (absorption spectrum, the 4th derivative of absorption spectrum) nor photochemical activity (pheophytine photoreduction, P680 photooxidation, etc.) were changed dramatically in D1/D2/cytochrom b 559 PS 2 reaction center complex digested with trypsin. The PS 2 RC treated with trypsin migrates by one green band during electrophoresis with dodecylmaltoside. The peptides with a molecular mass higher than 3-4 kDa were not separated from PS 2 RC. These data indicate that digestion of D1 and D2 proteins does not disturb yet the conformation of peptides or their interactions in so-called 'core' of RC and the native state of pigments. In contrast to that, the RC from Rhodopseudomonas viridis treated with enzyme has changed absorption spectrum and lost photochemical activity. The stability of the bacterial RC increased after exchange of LDAO by dodecylmaltoside.

Functional characterisation of a purified homogeneous Photosystem II core complex with high oxygen evolution capacity from spinach

The functional properties of a purified homogeneous spinach PS II-core complex with high oxygen evolution capacity (Haag et al. 1990a) were investigated in detail by measuring thermoluminescence and oscillation patterns of flash induced oxygen evolution and fluorescence quantum yield changes. The following results were obtained: a) Depending on the illumination conditions the PS II-core complexes exhibit several thermoluminescence bands corresponding to the A band, Q band and Zv band in PS II membrane fragments. The lifetime of the Q band (Tmax=10°C) was determined to be 8s at T=10°C. No B band corresponding to S2QB (-) or S3QB (-) recombination could be detected. b) The flash induced transient fluorescence quantum yield changes exhibit a multiphasi relaxation kinetics shich reflect the reoxidation of Q A (-) . In control samples without exogenous acceptors this process is markedly slower than in PS II membrane fragments. The reaction becomes significantly retarded by addition of 10 μM DCMU. After dark incubation in the presence of K3[Fe(CN)6 c) Excitation of dark-adapted samples with a train of short saturating flashes gives rise to a typical pattern dominated by a high O2 yield due to the third flash and a highly damped period four oscillation. The decay of redox states S2 and S3 are dominated by short life times of 4.3 s and 1.5 s, respectively, at 20°C. The results of the present study reveal that in purified homogeneous PS II-core complexes with high oxygen evolution isolated from higher plants by β-dodecylmaltoside solubilization the thermodynamic properties and the kinetic parameters of the redox groups leading to electron transfer from water to QA are well preserved. The most obvious phenomenon is a severe modification of the QB binding site. The implications of this finding are discussed.

Thermoluminescence and flash-induced oxygen yield in herbicide resistant mutants of the D1 protein in Synechococcus PCC7942

Several strains of Synechococcus PCC7942 carrying point mutations in the gene psbA were studied by thermoluminescence and polarographic measurement of flash-induced oxygen yield. The following results were obtained: (a) Replacement of Ser-264 in D1 by Ala (mutant Di1) or Gly (mutant G264) resulting in DCMU and atrazine resistance leads to a downshift of the thermoluminescence (TL) B-band peak temperature from 40 degrees C in wild-type thylakoids to about 30 degrees C. In dark adapted samples of both mutants the TL and oxygen yield pattern induced by a train of single turnover flashes were strongly damped indicative of a high miss factor. (b) In contrast to Ser-264 mutants, replacement of Phe-255 in D1 by Tyr (mutant Tyr5) induced strong resistance to atrazine but not to DCMU and did not affect the peak termperature of the B-band and the flash-induced TL and oxygen yield patterns. In this respect mutant Tyr5 resembles the wild type. (c) No significant differences have been found between strains with single site mutations in psbAI and normal psbAII/psbAIII genes, and strains with same mutations in psbAI but additional deletion of psbAII and psbAIII. Obviously in strains were psbAI is present, PS II complexes containing gene products of psbAII and psbAIII are not assembled in detectable amounts. (d) Strains with double mutations at positions 264 and 255 display a downshift of the B-band peak temperature. Their oscillatory patterns of B-band intensity and oxygen yield are highly damped. This behaviour is similar to strains D1 and G264 which are modified at position 264 only. We extend reports on additivity of mutation effects on herbicide binding to binding of QB. (e) Mutations at the QB site not only influence the binding of QB and herbicides but also change the thermoluminescence quantum yield and the lifetimes of the redox states S2 and S3 of the water oxidase. This finding might indicate long ranging effects on Photosystem II exerted by structural modifications of the QB site. From these data we conclude that Ser-264 is essential for binding of atrazine, DCMU and QB, whereas Phe-255 is involved in atrazine binding and its substitution by Tyr does not markedly affect QB or DCMU binding in Synechococcus PCC7942.

Binding affinity of bicarbonate and formate in herbicide-resistant D1 mutants of Synechococcus sp. PCC 7942

We examined the effects of mutations at amino acid residues S264 and F255 in the D1 protein on the binding affinity of the stimulatory anion bicarbonate and inhibitory anion formate in Photosystem II (PS II) in Synechococcus sp. PCC 7942. Measurements on the rates of oxygen evolution in the wild type and mutant cells in the presence of different concentrations of formate with a fixed bicarbonate concentration and vice versa, analyzed in terms of an equilibrium activator-inhibitor model, led to the conclusion that the equilibrium dissociation constant for bicarbonate is increased in the mutants, while that of the formate remains unchanged (11±0.5 mM). The hierarchy of the equilibrium dissociation constant for bicarbonate (highest to lowest, ±2 μM) was: D1-F255L/S264A (46 μM)>D1-F255Y/ S264A (31 μM)≈D1-S264A (34 μM)≈D1-F255Y (33 μM)>wild type (25 μM). The data suggest the importance of D1-S264 and D1-F255 in the bicarbonate binding niche. A possible involvement of bicarbonate and these two residues in the protonation of QB (-), the reduced secondary plastoquinone of PS II, in the D1 protein is discussed.

Effects of photoinhibition on the PS II acceptor side including the endogenous high spin Fe(2+) in thylakoids, PS II-membrane fragments and PS II core complexes

Effects of photoinhibition at 0 °C on the PS II acceptor side have been analyzed by comparative studies in isolated thylakoids, PS II membrane fragments and PS II core complexes from spinach under conditions where degradation of polypeptide(s) D1(D2) is highly retarded. The following results were obtained by measurements of the transient fluorescence quantum and oxygen yield, respectively, induced by a train of short flashes in dark-adapted samples: (a) in the control the decay of the fluorescence quantum yield is very rapid after the first flash, if the dark incubation was performed in the presence of 300 μM K3[Fe(CN)6]; whereas, a characteristic binary oscillation was observed in the presence of 100 μM phenyl-p-benzoquinone with a very fast relaxation after the even flashes (2nd, 4th. . . ) of the sequence; (b) illumination of the samples in the presence of K3[Fe(CN)6] for only 5 min with white light (180 W m(-2)) largely eliminates the very fast fluorescence decay after the first flash due to QA (-) reoxidation by preoxidized endogenous non-heme Fe(3+), while a smaller effect arises on the relaxation kinetics of the fluorescence transients induced by the subsequent flashes; (c) the extent of the normalized variable fluorescence due to the second (and subsequent) flash(es) declines in all sample types with a biphasic time dependence at longer illumination. The decay times of the fast (6-9 min) and the slow degradation component (60-75 min) are practically independent of the absence or presence of K3[Fe(CN)6] and of anaerobic and aerobic conditions during the photo-inhibitory treatment, while the relative extent of the fast decay component is higher under anaerobic conditions. (d) The relaxation kinetics of the variable fluorescence induced by the second (and subsequent) flash(es) become retarded due to photoinhibition, and (e) the oscillation pattern of the oxygen yield caused by a flash train is not drastically changed due to photoinhibition.Based on these findings, it is concluded that photoinhibition modifies the reaction pattern of the PS II acceptor side prior to protein degradation. The endogenous high spin Fe(2+) located between QA and QB is shown to become highly susceptible to modification by photoinhibition in the presence of K3[Fe(CN)6] (and other exogenous acceptors), while the rate constant of QA (-) reoxidation by QB(QB (-)) and other acceptors (except the special reaction via Fe(3+)) is markedly less affected by a short photoinhibition. The equilibrium constant between QA (-) and QB(QB (-)) is not drastically changed as reflected by the damping parameters of the oscillation pattern of oxygen evolution.

Unusual low reactivity of the water oxidase in redox state S3 toward exogenous reductants. Analysis of the NH2OH- and NH2NH2-induced modifications of flash-induced oxygen evolution in isolated spinach thylakoids

The effect of redox-active amines NH2R (R = OH or NH2) on the period-four oscillation pattern of oxygen evolution has been analyzed in isolated spinach thylakoids as a function of the redox state Si (i = 0, ..., 3) of the water oxidase. The following results were obtained: (a) In dark-adapted samples with a highly populated S1 state, NH2R leads via a dark reaction sequence to the formal redox state "S-1"; (b) the reaction mechanism is different between the NH2R species; NH2OH acts as a one-electron donor, whereas NH2NH2 mainly functions as a two-electron donor, regardless of the interacting redox state Si (i = 0, ..., 3). For NH2NH2, the modified oxygen oscillation patterns strictly depend upon the initial ratio [S0(0)]/[S1(0)] before the addition of the reductant; while due to kinetic reasons, for NH2OH this dependence largely disappears after a short transient period. (c) The existence of the recently postulated formal redox state "S-2" is confirmed not only in the presence of NH2NH2 [Renger, G., Messinger, J., & Hanssum, B. (1990) in Current Research in Photosynthesis (Baltscheffsky, M., Ed.) Vol. 1, pp 845-848, Kluwer, Dordrecht] but also in the presence of NH2OH. (d) Activation energies, EA, of 50 kJ/mol were determined for the NH2R-induced reduction processes that alter the oxygen oscillation pattern from dark-adapted thylakoids. (e) Although marked differences exist between NH2OH and NH2NH2 in terms of the reduction mechanism and efficiency (which is about 20-fold in favor of NH2OH), both NH2R species exhibit the same order of rate constants as a function of the redox state Si in the nonperturbed water oxidase: kNH2R(S0) greater than kNH2R(S1) much less than kNH2R(S2) much greater than kNH2R(S3) The large difference between S2 and S3 in their reactivity toward NH2R is interpreted to indicate that a significant change in the electronic configuration and nuclear geometry occurs during the S2----S3 transition that makes the S3 state much less susceptible to NH2R. The implications of these findings are discussed with special emphasis on the possibility of complexed peroxide formation in redox state S3 postulated previously on the basis of theoretical considerations [Renger, G. (1978) in Photosynthetic Water Oxidation (Metzner, H., Ed.) pp 229-248, Academic Press, London].

Evidence for two types of electron transfer processes through Photosystem II

Inhibition of electron flow from H2O to methylviologen by 3-(3'4' dichlorophenyl)-1,1 dimethyl urea (DCMU), yields a biphasic curve - an initial high sensitivity phase and a subsequent low sensitivity phase. The two phases of electron flow have a different pH dependence and differ in the light intensity required for saturation.Preincubation of chloroplasts with ferricyanide causes an inhibition of the high sensitivity phase, but has no effect on the low sensitivity phase. The extent of inhibition increases as the redox potential during preincubation becomes more positive. Tris-treatment, contrary to preincubation with ferricyanide, affects, to a much greater extent, the low sensitivity phase.Trypsin digestion of chloroplasts is known to block electron flow between Q A and Q B, allowing electron flow to ferricyanide, in a DCMU insensitive reaction. We have found that in trypsinated chloroplasts, electron flow becomes progressively inhibited by DCMU with increase in pH, and that DCMU acts as a competitive inhibitor with respect to [H(+)]. The sensitivity to DCMU rises when a more negative redox potential is maintained during trypsin treatment. Under these conditions, only the high sensitivity, but not the low sensitivity phase is inhibited by DCMU.The above results indicate the existence of two types of electron transport chains. One type, in which electron flow is more sensitive to DCMU contains, presumably Fe in a Q A Fe complex and is affected by its oxidation state, i.e., when Fe is reduced, it allows electron flow to Q B in a DCMU sensitive step; and a second type, in which electron transport is less sensitive to DCMU, where Fe is either absent or, if present in its oxidized state, is inaccessible to reducing agents.

The QB site modulates the conformation of the photosystem II reaction center polypeptides

The sensitivity of the D-1 and D-2 polypeptide subunits of photosystem II towards trypsin treatment of the thylakoid membrane has been probed with specific antibodies. As long known, electron flow from water to ferricyanide becomes inhibitor insensitive after this trypsin treatment. We show that under these conditions the D-2 polypeptide is cut by trypsin at arg 234. Also the D-1 polypeptide is cut, probably at arg 238. When short time trypsination of the membrane is done in the presence of inhibitors, electron flow also becomes inhibitor insensitive and the D-2 polypeptide is still cut, but the D-1 polypeptide is cut only under certain conditions. A protection of the D-1 polypeptide is possible with inhibitors of photosystem II of the DCMU/triazine-type and with an artificial acceptor quinone, but not with inhibitors of the phenol-type. In hexane extracted membranes plastoquinone has been removed from the QB site. Both the D-1 and D-2 polypeptides are more trypsin sensitive in such preparations. The D-1, but not the D-2 polypeptide is protected when plastoquinone has been readded to the membrane before the trypsin digestion.The results show that plastoquinone, artificial quinones and inhibitors of photosystem II at the QB site, but also carotene to a lesser extent, have an effect on the conformation of both the D-1 and D-2 polypeptide. it is postulated that the amino acid sequence around arginine 238 of the D-1 polypeptide is part of the QB binding niche. Furthermore this sequence is modified or its conformation is changed if the QB site is occupied by either plastoquinone or a DCMU-type inhibitor because under these conditions arginine 238 is less accessible to the trypsin. If the QB site, however, is empty, the amino acid sequence with arg 238 is very trypsin sensitive. This property of modulation or the conformation of the amino acid sequence of the D-1 polypeptide by the state of the QB site is likely to be relevant also for the events in the rapid turnover of the D-1 polypeptide.

Studies on the deconvolution of flash-induced absorption changes into the difference spectra of individual redox steps within the water-oxidizing enzyme system

The possibility to determine the difference spectra Δεi+1jλ of each univalent redox step Si→Si+1(i=0,...3) of the water-oxidizing enzyme system was analyzed by theoretical calculations and by measurements of 320 nm absorption changes induced by a train of saturating laser flashes (FWHM:7 ns) in PS II membrane fragments. It was found: a) Lipophilic quinones complicate the experimental determination of optical changes due the Si-state transitions because they lead to an additional binary oscillation probably caused by a reductant-induced oxidation of the Fe(2+) at the PS II acceptor side. b) In principle, a proper separation can be achieved at sufficiently high K3[Fe(CN)6] concentrations. c) An unequivocal deconvolution into the difference spectra Δεi+1jλ of flash train-induced optical changes which are exclusively due to Si-state transitions is impossible unless the Kok parameters α, β and [Si]0 can be determined by an independent method.Measurements of the oxygen yield induced by a flash train reveals, that in thylakoids and PS II membrane fragments Si is the stable state of dark adapted samples even at alkaline pH (up to pH=9). However, in PS II membrane fragments at pH>7.7 the misses probability α markedly increases, in contrast to the properties of intact thylakoids. Based on these data the possibility is discussed that an equilibrium exists of two types of S2-states with different properties.

Regulation of photosynthetic electron transport by bicarbonate formate and herbicides in isolated broken and intact chloroplasts

In this paper, we have presented a minireview on the interaction of bicarbonate, formate and herbicides with the thylakoid membranes.The regulation of photosynthetic electron transport by bicarbonate, formate and herbicides is described. Bicarbonate, formate, and many herbicides act between the primary quinone electron acceptor QA and the plastoquinone pool. Many herbicides like the ureas, triazines and the phenol-type herbicides act, probably, by the displacement of the secondary quinone electron acceptor QB from its binding site on a QB-binding protein located at the acceptor side of Photosystem II. Formate appears to be an inhibitor of electron transport; this inhibition can be removed by the addition of bicarbonate. There appears to be an interaction of the herbicides with bicarbonate and/or It has been suggested that both the binding of a herbicide and the absence of bicarbonate may cause a conformational alteration of the environment of the QB-binding site. The alteration brought about by a herbicide decreases the affinity for another herbicide or for bicarbonate; the change caused by the absence of bicarbonate decreases the affinity for herbicides. Moreover, this change in conformation causes an inhibition of electron transport. A bicarbonate-effect in isolated intact chloroplasts is demonstrated.