Effect of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone on the secondary electron acceptor B of photosystem II

Measurements of chlorophyll fluorescence have been used to monitor electron transport from the primary electron acceptor of photosystem II, Q, to the secondary acceptor, B, in chloroplasts in either the presence or the absence of the plastoquinone analog 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). Electron transport is markedly slower from Q^? to either B or B^? in the presence of DBMIB. Binary oscillations in the rate of reoxidation of Q^? (equivalent to the reactions Q^?B → QB^? and Q^?B^? → QB^2?) after each of a series of flashes were of a phase opposite to those observed in the absence of DBMIB (J. M. Bowes, and A. R. Crofts, (1980) Biochim. Biophys. Acta590, 573–584). The results confirm that inhibition of electron transport by DBMIB in chloroplasts is not restricted to an inhibition of electron transfer from the plastoquinone pool, but that there is also a specific interaction between the reduced form of the inhibitor and the secondary electron acceptor B. Models are discussed to account for the mechanism of this interaction.     

Effect of inhibitors,redox state and isoprenoid chain length on the affinity of ubiquinone for the secondary acceptor binding site in the reaction centers of photosynthetic bacteria

Quinone and inhibitor binding to Rhodopseudomonas sphaeroides (R-26 and GA) reaction centers were studied using spectroscopic methods and by direct adsorption of reaction centers onto anion exchange filters in the presence of ¹⁴C-labelled quinone or inhibitor. These measurements show that as secondary acceptor, Q_B, ubiquinone (UQ) is tightly bound in the semiquinone form and loosely bound in the quinone and quinol forms. The quinol is probably more loosely bound than the quinone. o-Phenanthroline and terbutryn, a triazine inhibitor, compete with UQ and with each other for binding to the reaction center. Inhibition by o-phenanthroline of electron transfer from the primary to the secondary quinone acceptor (Q_A to Q_B) occurs via displacement of UQ from the Q_B binding site. Displacement of UQ by terbutryn is apparently accessory to the inhibition of electron transfer. Terbutryn binding is lowered by reduction of Q_B to Q^?_B but is practically unaffected by reduction of Q_A to Q^?_A in the absence of Q_B. UQ-9 and UQ-10 have a 5- to 6-fold higher binding affinity to the Q_B site than does UQ-1, indicating that the long isoprenoid chain facilitates the binding to the Q_B site.     

Oxidation-reduction properties of the electron acceptors of Photosystem II I. Redox titration of the flash-induced carotenoid band shift,of C550 and of the variable fluorescence yield in spinach chloroplasts

Redox titration of the electrochromic carotenoid band shift, detected at 50 μs after a saturating actinic flash, in spinach chloroplasts, shows that only one electron acceptor in Photosystem II participates in a transmembrane primary electron transfer. This species, the primary quinone acceptor, Q, shows only one midpoint potential (E_m,7.5) of approx. 0 V and is undoubtedly equivalent to the fluorescence quencher, Q_H. A second titration wave is observed at low potential ($\text{E}{}_{\mathrm{<mtext>m</mtext><mtext>,7.5</mtext>}}\text{? ? 240}\text{mV}$) and at greater than 3 ms after a saturating actinic flash. This wave has an action spectrum different from that of Photosystem II centers containing Q and could arise from a secondary but not primary electron transfer. A low-potential fluorescence quencher is observed in chloroplasts which largely disappears in a single saturating flash at ? 185 mV and which does not participate in a transmembrane electron transfer. This low-potential quencher (probably equivalent to fluorescence quencher, Q_L) and Q are altogether different species. Redox titration of C550 shows that if electron acceptor Q_β is indeed characterized by an E_m,7 of + 120 mV, then this acceptor does not give rise to a C550 signal upon reduction and does not participate in a transmembrane electron transfer. This titration also shows that C550 is not associated with Q_L.     

Inactivation of photosynthetic oxygen evolution by UV-B irradiation: A thermoluminescence study

The influence of UV-B irradiation on photosynthetic oxygen evolution by isolated spinach thylakoids has been investigated using thermoluminescence measurements. The thermoluminescence bands arising from the S₂Q_B ^- (B band) and S₂Q_A ^– (Q band) charge recombination disappeared with increasing UV-B irradiation time. In contrast, the C band at 50°C, arising from the recombination of Q_A ^- with an accessory donor of Photosystem II, was transiently enhanced by the UV-B irradiation. The efficiency of DCMU to block Q_A to Q_B electron transfer decreased after irradiation as detected by the incomplete suppression of the B band by DCMU. The flash-induced oscillatory pattern of the B band was modified in the UV-B irradiated samples, indicating a decrease in the number of centers with reduced Q_B. Based on the results of this study, UV-B irradiation is suggested to damage both the donor and acceptor sides of Photosystem II. The damage of the water-oxidizing complex does not affect a specific S-state transition. Instead, charge stabilization is enhanced on an accessory donor. The acceptor-side modifications decrease the affinity of DCMU binding. This effect is assumed to reflect a structural change in the Q_B/DCMU binding site. The preferential loss of dark stable Q_B ^- may be related to the same structural change or could be caused by the specific destruction of reduced quinones by the UV-B light.Abbreviations Chl chlorophyll - DCMU 3-(3,4,-dichlorophenyl)-1,1-dimethylurea - PS II Photosystem II - Q_A first quinone electron acceptor of PS II - Q_B second quinone electron acceptor of PS II - Tyr-D accessory electron donor of PS II - S₀-S₄ charge storage states of the water-oxidizing complex     

Effect of different methods of Ca<Superscript>2+</Superscript> extraction from PSII oxygen-evolving complex on the Q<Subscript>A</Subscript><Superscript>−</Superscript> oxidation kinetics

Lumenal extrinsic proteins PsbO, PsbP, and PsbQ of photosystem II (PSII) protect the catalytic cluster Mn₄CaO₅ of oxygen-evolving complex (OEC) from the bulk solution and from soluble compounds in the surrounding medium. Extraction of PsbP and PsbQ proteins by NaCl-washing together with chelator EGTA is followed also by the depletion of Ca²⁺ cation from OEC. In this study, the effects of PsbP and PsbQ proteins, as well as Ca²⁺ extraction from OEC on the kinetics of the reduced primary electron acceptor (Q_A ^?) oxidation, have been studied by fluorescence decay kinetics measurements in PSII membrane fragments. We found that in addition to the impairment of OEC, removal of PsbP and PsbQ significantly slows the rate of electron transfer from Q_A ^? to the secondary quinone acceptor Q_B. Electron transfer from Q_A ^? to Q_B in photosystem II membranes with an occupied Q_B site was slowed down by a factor of 8. However, addition of EGTA or CaCl₂ to NaCl-washed PSII did not change the kinetics of fluorescence decay. Moreover, the kinetics of Q_A ^? oxidation by Q_B in Ca-depleted PSII membranes obtained by treatment with citrate buffer at pH 3.0 (such treatment keeps all extrinsic proteins in PSII but extracts Ca²⁺ from OEC) was not changed. The results obtained indicate that the effect of NaCl-washing on the Q_A ^? to Q_B electron transport is due to PsbP and PsbQ extrinsic proteins extraction, but not due to Ca²⁺ depletion.     

Impairment of photosystem 2 activity at the level of secondary quinone electron acceptor in chloroplasts treated with cobalt, nickel and zinc ions

The toxicity of heavy metals on photosystem 2 photochemistry, was investigated by monitoring Hill activity, fluorescence, and thermoluminescence properties of photosystem 2 (PS 2) in pea (Pisum sativum L. cv. Bombay) chloroplasts. In Co²⁺-, Ni²⁺- or Zn²⁺-treated chloroplasts 2,6-dichlorophenolindophenol-Hill activity was markedly inhibited. Addition of hydroxylamine which donates electrons close to PS 2 reaction center did not restore the PS 2 activity. Co²⁺-, Ni²⁺ or Zn²⁺ also inhibited PS 2 activity supported by hydroxylamine in tris (hydroxymethyl)aminomethane (Tris)-inactivated chloroplasts. These observations were confirmed by fluorescence transient measurements. This implies that the metal ions inhibit either the reaction center or the components of PS 2 acceptor side. Flash-induced thermoluminescence studies revealed that the S₂Q^?_A charge recombination was insensitive to metal ion addition. The S₂Q^?_B charge recombination, however, was inhibited with increase in the level of Co²⁺, Ni²⁺ or Zn²⁺. The observed sensitivity of S₂^?_B charge recombination in comparison to the stability of S₂Q^?_A recombination suggests that the metal ions inhibit at the level of secondary quinone electron acceptor. Q_B. We suggest that Co²⁺, Ni²⁺ or Zn²⁺ do not block the electron flow between the primary and secondary quinone electron acceptor, but possibly, directly modify Q_B site, leading to the loss of PS 2 activity.     

Thermoluminescence and Delayed Luminescence Investigation of the Green Alga,Chlamydobotrys stellata

Thermoluminescence and delayed luminescence investigations of the autotrophically and photoheterotrophically cultivated green alga, Chlamydobotrys stellata, demonstrated that both the thermoluminescence and delayed luminescence yields are much lower in the photoheterotophic algae than in the autotrophic ones due to an efficient luminescence quenching of unknown mechanism. The relative contributions of the so called Q (S₂Q^?_A charge recombination) and B (S₂Q^?_B and S₃Q^?_B charge recombinations) thermoluminescence bands to the glow curve as well as the Q_A(S₂Q^?_B charge recombination) and Q_B (S₂Q^?_B and S₃Q^?_B charge recombinations) delayed luminescence components to the delayed luminescence decay of autotrophically and photoheterotrophically cultivated Chl. stellata were compared using a computer assisted curve resolution method. It was found that, while in the autotrophic cells the area of the B band was considerably larger than of the Q band, in photoheterotrophic cells the Q band was more effectively charged than the B band. In the delayed luminescence decay curves measured in the seconds to minutes time region the amplitude of the Q_A component relative to that of the Q_B component was larger in the photoheterotrophic cells than in the autotrophic ones. These observations demonstrate that, after light-induced charge separation in the photosystem II reaction centers of autotrophic cells, electrons are “quasipermanently” stored mainly in the secondary quinone acceptor pool, Q_B but in the nonquenched photosystem II reaction centers of photoheterotrophic cells the main reservoir of electrons is the primary quinone acceptor, Q_A. This behaviour indicates an inhibition of electron transport in the photoheterotrophic alga at the level of the secondary quinone acceptor, Q_B.     

Excitation trapping and charge separation in Photosystem II in the presence of an electrical field

To investigate the effects of a membrane potential on excitation trapping and charge separation in Photosystem II we have studied the chlorophyll fluorescence yield in osmotically swollen chloroplasts subjected to electrical field pulses. Significant effects were observed only in those membrane regions where a large membrane potential opposing the photochemical charge separation was built up. When the fluorescence yield was low, close to F₀, a much higher yield, up to F_max, was observed during the presence of the membrane potential. This is explained by an inhibition by the electrical field of electron transfer to the quinone acceptor Q, resulting in a decreased trapping of excitations. A field pulse applied when the fluorescence yield was high, Q and the donor side being in the reduced state, had the opposite effect: the fluorescence was quenched nearly to F₀. This field-induced fluorescence quenching is ascribed to reversed electron transfer from Q^? to the intermediate acceptor, pheophytin. Its field strength dependence suggests that the midpoint potential difference between pheophytin and Q is at most about 300 mV. Even then it must be assumed that electron transfer between pheophytin and Q spans 90% of the potential difference across the membrane.     

Resistance of reaction centers from Rhodobacter sphaeroides against UV-B radiation. Effects on protein structure and electron transport

Inhibition of electron transport and damage to the protein subunits by ultraviolet-B (UV-B, 280–320 nm) radiation have been studied in isolated reaction centers of the non-sulfur purple bacterium Rhodobacter sphaeroides R26. UV-B irradiation results in the inhibition of charge separation as detected by the loss of the initial amplitude of absorbance change at 430 nm reflecting the formation of the P⁺(Q_AQ_B)^– state. In addition to this effect, the charge recombination accelerates and the damping of the semiquinone oscillation increases in the UV-B irradiated reaction centers. A further effect of UV-B is a 2 fold increase in the half- inhibitory concentration of o-phenanthroline. Some damage to the protein subunits of the RC is also observed as a consequence of UV-B irradiation. This effect is manifested as loss of the L, M and H subunits on Coomassie stained gels, but not accompanied with specific degradation products. The damaging effects of UV-B radiation enhanced in reaction centers where the quinone was semireduced (Q_B ^–) during UV-B irradiation, but decreased in reaction centers which lacked quinone at the Q_B binding site. In comparison with Photosystem II of green plant photosynthesis, the bacterial reaction center shows about 40 times lower sensitivity to UV-B radiation concerning the activity loss and 10 times lower sensitivity concerning the extent of reaction center protein damage. It is concluded that the main effect of UV-B radiation in the purple bacterial reaction center occurs at the Q_AQ_B quinone acceptor complex by decreasing the binding affinity of Q_B and shifting the electron equilibration from Q_AQ_B ^– to Q_A ^–Q_B. The inhibitory effect is likely to be caused by modification of the protein environment around the Q_B binding pocket and mediated by the semiquinone form of Q_B. The UV-resistance of the bacterial reaction center compared to Photosystem II indicates that either the Q_AQ_B acceptor complex, which is present in both types of reaction centers with similar structure and function, is much less susceptible to UV damage in purple bacteria, or, more likely, that Photosystem II contains UV-B targets which are more sensitive than its quinone complex.Abbreviations Bchl bacteriochlorophyll - P Bchl dimer - Q_A primary quinone electron acceptor - Q_B secondary quinone electron acceptor - RC reaction center - UV-B ultraviolet-B     

Site of salicylaldoxime interaction with photosystem II

The reversible inhibition of Photosystem II by salicylaldoxime was studied in spinach D-10 particles by fluorescence, optical absorption, and electron spin resonance spectroscopy. In the presence of 15 mM salicylaldoxime, the initial fluorescence yield was raised to the level of the maximum fluorescence, indicating efficient charge recombination between reduced pheophytin (Ph) and P680⁺. In agreement with the rapid (ns) backreaction expected between Ph^– and P680⁺, the optical absorption transient at 820 mm was not observed. When the particles were washed free of salicylaldoxime, the optical absorption transient resulting from the rereduction of P680⁺ was restored to the µs timescale. These results, along with the previously observed inhibition of electron transport reactions and diminution of the 515-nm absorption change in chloroplasts [Golbeck, J.H. (1980) Arch Biochem Biophys 202, 458–466], are consistent with a site of inhibition between Ph and QA in Photosystem II. ESR Signal IIf and Signal Its were abolished in the presence of 25 mM salicylaldoxime, but both signals could be recovered by washing the D-10 particles free of the inhibitor. The loss of Signal Ilf is most likely a consequence of the inhibition between Ph and Q_A; the rapid charge recombination between Ph^– and P680⁺ would preclude electron transfer from an electron donor on the oxidizing side of Photosystem II. The loss of Signal Its may be due to a change in the environment of the donor complex such that the semiquinone radical giving rise to Signal Its interacts with a nearby reductant.Abbreviations D₁ electron donor to P680⁺ in oxygen-inhibited chloroplasts - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F₀ prompt chlorophyll a fluorescence yield - F_i initial chlorophyll a fluorescence yield - F_max maximum chlorophyll a fluorescence yield - F_var variable chlorophyll a fluorescence yield - FWHM full width at half maximum - Mes 2-(N-morpholino) ethanesulfonic acid - P680 reaction center chlorophyll a of photosystem II - Ph pheophytin intermediate electron acceptor - Q_A primary quinone electron acceptor - Q_B secondary quinone electron acceptor - Tris tris(hydroxymethyl)aminomethane - Z electron donor to P680⁺     

Multiple sites of retardation of electron transfer in Photosystem II after hydrolysis of phosphatidylglycerol

Phosphatidylglycerol (PG), containing the unique fatty acid Δ3, trans-16:1-hexadecenoic acid, is a minor but ubiquitous lipid component of thylakoid membranes of chloroplasts and cyanobacteria. We investigated its role in electron transfers and structural organization of Photosystem II (PSII) by treating Arabidopsis thaliana thylakoids with phospholipase A₂ to decrease the PG content. Phospholipase A₂ treatment of thylakoids (a) inhibited electron transfer from the primary quinone acceptor Q_A to the secondary quinone acceptor Q_B, (b) retarded electron transfer from the manganese cluster to the redox-active tyrosine Z, (c) decreased the extent of flash-induced oxidation of tyrosine Z and dark-stable tyrosine D in parallel, and (d) inhibited PSII reaction centres such that electron flow to silicomolybdate in continuous light was inhibited. In addition, phospholipase A₂ treatment of thylakoids caused the partial dissociation of (a) PSII supercomplexes into PSII dimers that do not have the complete light-harvesting complex of PSII (LHCII); (b) PSII dimers into monomers; and (c) trimers of LHCII into monomers. Thus, removal of PG by phospholipase A₂ brings about profound structural changes in PSII, leading to inhibition/retardation of electron transfer on the donor side, in the reaction centre, and on the acceptor side. Our results broaden the simple view of the predominant effect being on the Q_B-binding site.     

Long-range interactions of <Emphasis Type="Italic">Chara</Emphasis> chloroplasts are sensitive to plasma-membrane H<Superscript>+</Superscript> flows and comprise separate photo- and dark-operated pathways

Local illumination of the characean internode with a 30-s pulse of white light was found to induce the delayed transient increase of modulated chlorophyll fluorescence in shaded cell parts, provided the analyzed region is located downstream in the cytoplasmic flow at millimeter distances from the light spot. The fluorescence response to photostimulation of a remote cell region indicates that the metabolites produced by source chloroplasts in an illuminated region are carried downstream with the cytoplasmic flow, thus ensuring long-distance communications between anchored plastids in giant internodal cells. The properties of individual stages of metabolite signaling are not yet well known. We show here that the export of assimilates and/or reducing equivalents from the source chloroplasts into the flowing cytoplasm is largely insensitive to the direction of plasma-membrane H⁺ flows, whereas the events in sink regions where these metabolites are delivered to the acceptor chloroplasts under dim light are controlled by H⁺ fluxes across the plasma membrane. The fluorescence response to local illumination of remote cell regions was best pronounced under weak background light and was also observed in a modified form within 1–2 min after the transfer of cell to darkness. The fluorescence transients in darkened cells were suppressed by antimycin A, an inhibitor of electron transfer from ferredoxin to plastoquinone, whereas the fluorescence response under background light was insensitive to this inhibitor. We conclude that the accumulation of reduced metabolites in the stroma leads to the reduction of photosystem II primary quinone acceptor (Q_A) via two separate (photochemical and non-photochemical) pathways.     

Changes in the redox state of the secondary acceptor of Photosystem II associated with light-induced thylakoid protein phosphorylation

Thylakoid membrane protein phosphorylation affects photochemical reactions of Photosystem II. Incubation of thylakoids in the light with ATP leads to: (1) an increase in the amplitude of three components (4–6, 25–45 and 280–300 μs) of delayed light emission after a single flash without any change in their kinetics; (2) a reduction of the flash-dependent binary oscillations of chlorophyll a fluorescence yield associated with electron transfer from the primary quinone acceptor, Q, to the secondary quinone acceptor, B; (3) an increase in the $\text{B}{}^{\mathrm{?}}\text{B}$ ratio resulting from an increase in stability of the semiquinone anion during dark adaptation; and (4) no change in the redox state of the plastoquinone pool as determined by flash-induced photooxidation of the Photosystem I reaction center, P-700. All the above observations are reversible upon dephosphorylation of the thylakoid membranes. These data are explained by a protein phosphorylation-induced stabilization of the bound semiquinone anion, B^?. It is proposed that this increased stability may be due to an alteration in the accessibility of an endogenous reductant to B, or to an increase in dissipative cycling of charge around Photosystem II.     

Clara cell damage and inhibition of pulmonary mixed-function oxidase activity by naphthalene

The photosystem II electron acceptor 3,6-dichloro-2,5-dimethoxy-p-benzoquinone [DCDMQ] is suggested to replace the second quinone-type two electron acceptor B (or R); the DCDMQ Hill reaction is sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea, but is insensitive to dry heptane extraction of thylakoids and other photosystem II inhibitors. Addition of HCO₃^? to CO₂-depleted thylakoids in silicomolybdate, DCDMQ, diaminodurene and ferricyanide Hill reactions brought about 1,3,10 and 10 fold increase in the electron transport rates; these data confirm that HCO₃^? affects both Q^? to B and B^2? to PQ reactions.     

Theoretical study of the binding site and mode of action for photosystem II herbicides

Several features of a proteinaceous binding site and a molecular mode of action are proposed for photosystem II (PS II) herbicides based upon a variety of experimental and theoretical evidence. Experimental studies have established that PS II herbicides bind non-covalently to a 32 kdalton protein in the PS II complex and inhibit electron transfer between the first quinone (Q) and the second quinone (B) on the reducing side of PS II. The herbicides each contain hydrophobic components as well as a flat polar component with a dipole moment in the range of 3–5 Debyes. The primary function of the hydrophobic components is to increase the lipid solubility of the entire herbicide molecule; the secondary function of the hydrophobic components is to fit the hydrophobic surface of the herbicide binding site. It is proposed that the flat polar component binds electrostatically at a highly polar protein site, probably a protein salt bridge or the terminus of a protein alpha helix. Further, it is proposed that the PS II herbicides shift the equilibrium Q^?Bz?QB^? to the left (i) by reducing the magnitude of an anion-stabilizing electric field across the B-binding site, or (ii) by inhibiting the conformational relaxation or protonation of the PS II protein in response to reduction of B to B^?, or (iii) by displacing the quinone head of B from its binding site. Ab initio molecular quantum mechanical calculations have been carried out to investigate the electrostatic interactions in specific herbicide-binding site models.     

Oxidation-reduction properties of the electron acceptors of Photosystem II. II. Redox titration at various pH values of the flash-induced formation of C550 in Chlamydomonas Photosystem II particles lacking the secondary quinone electron acceptor

Redox titrations of the flash-induced formation of C550 (a linear indicator of Q^?) were performed between pH 5.9 and 8.3 in Chlamydomonas Photosystem II particles lacking the secondary electron acceptor, B. One-third of the reaction centers show a pH-dependent midpoint potential (E_m,7.5) = ? 30 mV) for redox couple $\text{Q}\text{Q}{}^{\mathrm{?}}$, which varies by ?60 mV/pH unit. Two-thirds of the centers show a pH-independent midpoint potential (E_mm = + 10 mV) for this couple. The elevated pH-independent E_m suggests that in the latter centers the environment of Q has been modified such as to stabilize the semiquinone anion, Q^?. The midpoint potentials of the centers having a pH-dependent E_m are within 20 mV of those observed in chloroplasts having a secondary electron acceptor. It appears therefore that the secondary electron acceptor exerts little influence on the E_m of $\text{Q}\text{Q}{}^{\mathrm{?}}$. An EPR signal at g 1.82 has recently been attributed to a semiquinone-iron complex which comprises Q^?. The similar redox behavior reported here for C550 and reported by others (Evans, M.C.W., Nugent, J.H.A., Tilling, L.A. and Atkinson, Y.E. (1982) FEBS Lett. 145, 176–178) for the g 1.82 signal in similar Photosystem II particles confirm the assignment of this EPR signal to Q^?. At below ?200 mV, illumination of the Photosystem II particles produces an accumulation of reduced pheophytin (Ph^?). At ?420 mV Ph^? appears with a quantum yield of 0.006–0.01 which in this material implies a lifetime of 30–100 ns for the radical pair P-680⁺Ph^?.     

Kinetics of the oxidation—reduction reactions of the photosystem II quinone acceptor complex,and the pathway for deactivation

When the photosystem II quinone acceptor complex has been singly reduced to the state Q_AQ^?_B, there is a 22 s half-time back-reaction of Q^?_B with an oxidized photosystem II donor (S₂), directly measured here for the first time. From the back-reaction kinetics with and without inhibitors, kinetic and equilibrium parameters have been estimated. We suggest that the state Q_AQ^?_B of the complex is formed by a second-order reaction of vacant reaction centers in the state Q^?_A with plastoquinone from the pool, and discuss the physico-chemical parameters involved.     

Heterogeneity of system II secondary electron acceptors in Tris-washed chloroplasts

Tris-washed chloroplasts were submitted to saturating short flashes, and then rapidly mixed with dichlorophenyldimethylurea (DCMU). The amount of singly reduced secondary acceptor was estimated from the DCMU-induced increase in fluorescence, caused by the reverse electron flow from secondary to primary acceptor. The back-transfer from the singly reduced secondary acceptor to the primary acceptor Q induced by DCMU addition affects only a part (60%) of the variable fluorescence (ΔF_max). As previously shown, the quenchers involved in this phenomenon, ‘B-type’ quenchers, are different from those controlling the complementary part of the fluorescence, the non-B-type. In this report, we show that at pH 8.5 in the B-type systems, there exist two kinds of secondary electron acceptors: B, a two-electron acceptor, the corresponding Q accounting for 40% of the variable fluorescence; B′, a one-electron acceptor, the corresponding Q accounting for 20% of the variable fluorescence. The lifetimes of B^? and B′^? in the absence of DCMU are 40 and 1 s, respectively. The primary acceptors of the B and B′ systems can be considered as corresponding to the Q₁s defined previously (Joliot, P. and Joliot, A. (1981) in Proceedings of the 5th International Congress on Photosynthesis (Akoynoglou, G., ed.), pp. 885–899, Balaban International Science Services, Philadelphia). The B′ centers seems to be equivalent to the Q_β centers as defined by other workers (Van Gorkom, H.J., Thielen, A.P.G.M. and Gorren, A.C.F. (1982) in The Function of Quinones in Energy Conserving Systems (Trumpower, B.L., ed.), Academic Press, New York, in the press).     

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 Q_A 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 Q_B from its binding site on a Q_B-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 Q_B-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.Paper presented at the FESPP meeting (Strasbourg, 1984)     

Charge accumulation and recombination in Photosystem II studied by thermoluminescence. I. Participation of the primary acceptor Q and secondary acceptor B in the generation of thermoluminescence of chloroplasts

In the glow curves of chloroplasts excited by a series of flashes at +1°C the intensity of the main thermoluminescence band appearing at +30°C (B band; B, secondary acceptor of Photosystem II) exhibits a period-4 oscillation with maxima on the 2nd and 6th flashes indicating the participation of the S₃ state of the water-splitting system in the radiative charge recombination reaction. After long-term dark adaptation of chloroplasts (6 h), when the major part of the secondary acceptor pool (B pool) is oxidized, a period-2 contribution with maxima occurring at uneven flash numbers appears in the oscillation pattern. The B band can even be excited at ?160°C as well as by a single flash in which case the water-splitting system undergoes only one transition (S₁ → S₂). The experimental observations and computer simulation of the oscillatory patterns suggest that the B band originates from charge recombination of the S₂B^? and S₃B^? redox states. The half-time of charge recombination responsible for the B band is 48 s. When a major part of the plastoquinone pool is reduced due to prolonged excitation of the chloroplasts by continuous light, a second band (Q band; Q, primary acceptor of Photosystem II) appears in the glow curve at +10°C which overlaps with the B band. In chloroplasts excited by flashes prior to DCMU addition only the Q band can be observed showing maxima in the oscillation pattern at flash numbers 2, 6 and 10. The Q band can also be induced by flashes after DCMU addition which allows only one transition of the water-splitting system (S₁ → S₂). In the presence of DCMU, electrons accumulate on the primary acceptor Q, thus the Q band can be ascribed to the charge recombination of either the S₂Q^? or S₃Q^? states depending on whether the water-splitting system is in the S₂ or the S₃ state. The half-time of the back reaction of Q^? with the donor side of PS II (S₂ or S₃ states) is 3 s. It was also observed that in a sequence of flashes the peak positions of the Q and B bands do not depend on the advancement of the water-splitting system from the S₂ state to the S₃ state. This result implies that the midpoint potential of the water-splitting system remains unmodified during the S₂ → S₃ transition.