Presence Of Dibromothymoquinone Research Articles

NADPH from the oxidative pentose phosphate pathway drives the operation of cyclic electron flow around photosystem I in high-intertidal macroalgae under severe salt stress.

Pyropia yezoensis (Bangiales, Rhodophyta) is a representative species of high-intertidal macroalgae, whose blades can tolerate extreme stresses, such as salt stress and desiccation. In this study, the photosystem (PS) responses of P. yezoensis blades under salt stress were studied. Our results showed that when the effective photochemical quantum yield of PS (Y) II decreased to almost zero under high salt stress, YI still had a relatively high activity rate. PSII was therefore more sensitive to salt stress than PSI. Furthermore, in the presence of 3-(3', 4'-dichlorophenyl)-1,1-dimethylurea (DCMU), YI rose as salinity increased. The YI values for DCMU-treated thalli decreased in the presence of glucose-6-phosphate dehydrogenase (EC 1.1.1.49, G6PDH) inhibitor (glucosamine, Glucm). The YI values were ∼0.09 in the presence of methyl viologen (MV) and almost zero in the presence of dibromothymoquinone (DBMIB). These results demonstrated that under severe salt stress (120‰ salinity) PSI activity was driven from a source other than PSII, and that stromal reductants probably supported the operation of PSI. Under salt stress, the starch content decreased and soluble sugar levels increased. The G6PDH and 6-phosphogluconate dehydrogenase (EC 1.1.1.44) activities increased, but cytosolic glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12) activity decreased. Furthermore, the NADPH content increased, but NADH decreased, which suggested that soluble sugar entered the oxidative pentose phosphate pathway (OPPP). All these results suggested that NADPH from OPPP increases the cyclic electron flow around PSI in high-intertidal macroalgae under severe salt stress.

Transcriptional regulation of the bidirectional hydrogenase in the cyanobacterium Synechocystis 6803

To identify optimal conditions for renewable hydrogen production from sunlight and water we have studied transcriptional changes of the hoxEFUYH genes encoding the bidirectional hydrogenase in the cyanobacterium Synechocystis PCC 6803. Transcript abundance detection by real time polymerase chain reaction was supplemented with variable chlorophyll fluorescence measurements to monitor redox changes of the photosynthetic electron transport chain. Our main observations are: (i) abundance of hox transcripts decreases in the dark and recovers in the light. (ii) Inhibition of the Calvin cycle by glycolaldehyde suppresses hox gene transcription, which can be restored by the addition of electron transport inhibitors 3-(3,4-dichlorophenyl)-1,1-dimethylurea and dibromothymoquinone. (iii) The transcript levels of all hox genes are increased in anoxia, with additional induction of hoxEF in darkness or in the presence of dibromothymoquinone. (iv) Plastoquinone pool redox changes are not correlated with hox transcript level changes. (v) Changes in the transcript levels of lexA and sll0359 genes, encoding putative regulators of hox genes, are only partly correlated with transcript changes of hox genes under different conditions. Our data demonstrate a previously unrecognized light- and oxygen-dependent regulation of hox gene transcription in Synechocystis PCC 6803, which is related to photosynthetic electron transport and to unidentified oxygen and redox sensors. We also conclude that neither LexA nor Sll0359 are likely to be exclusive regulators of hox gene transcription.

Effect of dibromothymoquinone on Chlorophyll a Fluorescence in Chlamydomonas reinhardtii cells incubated in complete or sulfur-depleted medium

The influence of dibromothymoquinone on chlorophyll fluorescence was studied in Chlamydomonas reinhardtii cells using PAM and PEA fluorometers. The reagent affected differently control cells incubated in complete medium and S-starved cells. Thus, the fluorescence yield in the control essentially increased in the presence of dibromothymoquinone, which can be due to the inactivation of light-harvesting complex II protein kinase, followed by the suppression of membrane transition from high-fluorescence state 1 to low-fluorescence state 2. On the contrary, S-starved cells with membranes in state 2 showed a lower fluorescence yield in the presence of dibromothymoquinone than without it. The JIP test of OJIP fluorescence transients suggests that dibromothymoquinone inhibits both light-harvesting complex II kinase and photosynthetic electron transport when added to control, while in starved cells, it acts predominantly as an electron acceptor.

Effect of cadmium on ferredoxin:NADP + oxidoreductase activity

Ferredoxin:NADP + oxidoreductase (FNR) was treated with cadmium and after that its diaphorase reaction in the presence of dibromothymoquinone (DBMIB) or ferricyanide (FeCy, K 3Fe(CN) 6) was examined. CdSO 4 (5 mM) caused 50% inhibition after half hour incubation. At least two components were distinguishable in the time-course inhibition, suggesting that more than one amino acid residues were engaged in reaction with the metal ion. The Lineweaver–Burk plots indicate that Cd 2+ is an uncompetitive inhibitor for DBMIB reduction but exerts non-competitive inhibition for the NADPH oxidation. The FeCy reduction did not follow Michaelis–Menten kinetics. Zn 2+ diminished inhibitory effect of Cd 2+ on the DBMIB reduction but enhanced inhibition of the FeCy reduction. Incubation with additional chelator (β-mercaptoethanol, or histidine) abolished inhibitory effect of Cd 2+ on the FeCy reduction but not on the DBMIB reduction. The mode of Cd 2+ action on the diaphorase activity of FNR in the presence of DBMIB or FeCy is briefly discussed with the special reference to the implication of two distinct sites at the FNR molecule, which might be involved in the reduction of various non-physiological substrates.

Effects of Amino and Thiol Group Reagents on the Ferredoxin:NADP<sup>+</sup> Oxidoreductase Catalysed Reduction of Dibromothymoquinone

Effects of selective reagents of amino groups (fluorescamine, Fc) and thiol [5,5′-dithio-bis(2-nitrobenzoic) acid, DTNB] groups on the diaphorase activity of spinach ferredoxin:NADP+ oxidoreductase (FNR, E.C 1.18.1.2) in the presence of dibromothymoquinone (DBMIB) as an electron acceptor were studied. The incubation of FNR with 250 μM Fc in the time range from 0 to 120 min led to the gradual decrease of FNR activity according to biphasic kinetics. At the initial phase the activity (defined as the rate of NADPH oxidation) decreased about 4-time faster than at the subsequent second slower phase. Incubation of FNR simultaneously with Fc and DBMIB for more than 20 min caused restoration of the activity to about 80 % of the control. The inhibitory effect of Fc on the FNR-catalysed DBMIB reduction had non-competitive character. Incubation of FNR with DTNB led also to a gradual decrease of the enzyme activity, which reached about 45 % of the control after 2 h of incubation. Thus neither amino nor thiol groups in the FNR molecule are involved directly in the DBMIB reduction. However, the presence of DBMIB in the incubation medium influenced the inhibitory pattern of Fc and DTNB, and this suggests that DBMIB modified the conformational state of the FNR molecule.

Three Substrate Binding Sites on Spinach Ferredoxin:NADP<sup>+</sup> Oxidoreductase. Studies with Selectively Acting Inhibitors

The effects of phenylmercuric acetate (PMA) and apoferredoxin (apoFd) on the diaphorase activity of spinach ferredoxin:NADP+ oxidoreductase (FNR) in the presence of dibromothymoquinone (DBMIB) or cytochrome c (Cyt c) were studied. PMA inhibited effectively (I50 = < 5 μM) ferredoxin-dependent Cyt c reduction but did not affect evidently the enzyme activity in the presence of DBMIB as an electron acceptor. ApoFd caused also inhibition of Cyt c reduction but slightly stimulated, like ferredoxin, DBMIB reduction. We confirm a hypothesis according to which three binding sites for substrates [NADP(H), Fd-Cyt c, quinone/dichlorophenol indophenol] occur within the molecule of isolated FNR.

Diaphorase activity of ferredoxin: NADP oxidoreductase in the presence of dibromothymoquinone

In the presence of dibromothymoquinone (10 μM) NADPH- and ferredoxin:NADP oxidoreductase-dependent oxygen uptake was observed in buffered (40 mM Tris-HCl buffer) medium. The rate of O 2 uptake depended on the pH of the reaction mixture and was about two orders of magnitude faster at pH 8.7 than at pH 6.7. In alkaline medium the rate was about 0.2 μmol O 2 min −1 cm −3 (or 9.3 nmol O 2 min −1 mg −1 FNR) at the time of tracing, whereas at a lower pH (7.7 or 6.7) oxygen consumption was 1.5–2.0-fold faster during the first minute of monitoring than during the second minute. Ferredoxin was not an obligatory component involved in this process. However, at a lower pH (7.7, 6.7) oxygen consumption by the reaction mixture was stimulated 2–8-fold by adding ferredoxin (0.6 μM). The v 2 value was also enhanced by ca 36% in the presence of 0.2 M NaCl, whereas inorganic pyrophosphate caused reduction of v 1 and v 2 to about 62 and 3% respectively, of the control. Under anaerobic conditions the Michaelis constant ( K m) and the pseudo-first-order rate constant ( k) for the FNR-catalysed DBMIB reduction was about 16 μM and 0.34 s −1, respectively. The results indicate that DBMIB can accept electrons from NADPH-ferredoxin: NADP oxidoreductase, and the reduced DBMIB (presumably in the unprotonated form) can be reoxidized by molecular oxygen.

Temperature Dependence of Photosynthesis in Agropyron smithii Rydb.

Protoplasts and intact chloroplasts isolated from Agropyron smithii Rybd. were utilized in an effort to determine the limiting factor(s) for photosynthesis at supraoptimal temperatures. Saturated CO(2)-dependent O(2) evolution had a temperature optimum of 35 degrees C for both protoplasts and intact chloroplasts. A sharp decline in activity was observed as assay temperature was increased above 35 degrees C, and at 45 degrees C only 20% of the maximal rate remained. The temperature optimum for 3-phosphoglycerate reduction by intact chloroplasts was 35 degrees C. Above this temperature, 3-phosphoglycerate reduction was more stable than CO(2)-dependent O(2) evolution. Reduction of nitrite in coupled intact chloroplasts had a temperature optimum of 40 degrees C with only slight variation in activity between 35 degrees C and 45 degrees C. Reduction of nitrite in uncoupled chloroplasts had a temperature optimum of 40 degrees C, but increasing the assay temperature to 45 degrees C resulted in a complete loss of activity. Reduction of p-benzoquinone by protoplasts and intact chloroplasts had a temperature optimum of 32 degrees C when measured in the presence of dibromothymoquinone. This photosystem II activity exhibited a strong inhibition of O(2) evolution as assay temperature increased above the optimum. It is concluded that, below the temperature optimum, ATP and reductant were not limiting photosynthesis in these systems or intact leaves. Above the temperature optimum, photosynthesis in these systems is limited in part by the phosphorylation potential of the stromal compartment and not by the available reductant.

Stimulation of electron transport from photosystem II to photosystem I in spinach chloroplasts.

Electron transport from Photosystem II to Photosystem I of spinach chloroplasts can be stimulated by bicarbonate and various carbonyl or carboxyl compounds. Monovalent or divalent cations, which have hitherto been implicated in the energy distribution between the two photosystems, i.e., "spillover" phenomena at low light intensities, show a similar effect under high light conditions employed in this study. A mechanism for this stimulation of forward electron transport from Photosystem II to Photosystem I could involve inhibition of two types of Photosystem II partial reactions, which may involve cycling of electrons around Photosystem II. One of these is the DCMU-insensitive silicomolybdate reduction, and the other is ferricyanide reduction by Photosystem II at pH 8 in the presence of dibromothymoquinone. Greater stimulation of forward electron transport reactions is observed when both types of PHotosystem II cyclic reactions are inhibited by bicarbonate, carbonyl and carboxyl-type compounds, or by certain mono- or divalent cations.

Electrostatic state of the membrane surface and the reactivity between ferricyanide and electron transport components inside chloroplast membranes

Department of Biology, Faculty of Science, Kyushu University 33, Fukuoka 812, Japan(Received September 5, 1977)The relationship between the electrostatic state of the thylakoid membrane and thephotoreduction rate of ferricyanide were studied in intact and sonically treated chloro-plasts.In sonicated chloroplasts, reduction of ferricyanide by photosystem II requireda high concentration of ferricyanide for full activity, the apparent low affinity of thereduction site for ferricyanide suggests that permeation of ferricyanide to the site is therate-limiting factor. The affinity was increased by addition of salts or a cationicdetergent, but not by anionic or nonionic detergent at neutral pH. It was also increasedby decreasing pH or by adding lipophilic electron mediators such as naphthoquinonesor p-phenylenediamine.In intact chloroplasts similar characteristics of the system II reducing site werealso observed in the presence of dibromothymoquinone, an antagonist of plastoquinone.It is suggested from these results that the site of ferricyanide reduction in system IIexists inside the membrane with negative surface charges, which hinder the access offerricyanide to the site by electrostatic repulsion. The charges were probably screenedby salts or decreased by protonation at low pH. Study with the fluorescent probe,8-anilinonaphthalene-l-suIfonate, in sonicated chloroplasts also showed the pH- orsalt-induced changes and confirmed the interpretation above.From the rate constant for ferricyanide reduction, electrical potential differenceof the membrane surface with respect to the bulk aqueous phase was estimated byassuming potential-induced changes in the ferricyanide concentration at the membranesurface. A potential of —25 30 mV and a surface density of 1 negative charge per12-22 chlorophylls were estimated at pH 7.5 under low salt conditions. This potentialwas reduced by salt addition and changed the sign to positive at low pH (+17 mV atpH 4.9).The effect of the surface potential in photosynthetic energy conversion is discussed.Keywords: Electron transport — Hill activity — Spinach chloroplast — Surfacepotential — System II — Thylakoid membrane.

Kinetics of the photoreduction of cytochrome b-559 by Photosystem II in chloroplasts

The kinetics of the photoreduction of cytochrome b-559 and plastoquinone were measured using well-coupled spinach chloroplasts. High potential (i.e. hydroquinone reducible) cytochrome b-559 was oxidized with low intensity far-red light in the presence of N-methyl phenazonium methosulfate or after preillumination with high intensity light. Using long flashes of red light, the half-reduction time of cytochrome b-559 was found to be 100±10 ms, compared to 6–10 ms for the photoreduction of the plastoquinone pool. Light saturation of the photoreduction of cytochrome b-559 occurred at a light intensity less than one-third of the intensity necessary for the saturation of ferricyanide reduction under identical illumination conditions. The photoreduction of cytochrome b-559 was accelerated in the presence of dibromothymoquinone with a t 1 2 = 25–35 ms . The addition of uncouplers, which caused a stimulatory effect on ferricyanide reduction under the same experimental conditions, resulted in a decrease in the rate of cytochrome b-559 reduction. The relatively slow photoreduction rate of cytochrome b-559 compared to the plastoquinone pool implies that electrons can be transferred efficiently from Photosystem II to plastoquinone without the involvement of cytochrome b-559 as an intermediate. These results indicate that it is unlikely that high potential cytochrome b-559 functions as an obligatory redox component in the main electron transport chain joining the two photosystems.

Influence of unsaturated fatty acids in chloroplasts. Shift of the pH optimum of electron flow and relations to deltapH, thylakoid internal pH and proton uptake.

Linolenic acid (C18:3) is the main endogenous unsaturated fatty acid of thylakoid membrane lipids, and seems in its free form to exert significant effects on the structure and function of photosynthetic membranes. In this investigation the effect of linolenic acid was studied at various pH values on the electron flow rate in isolated spinach chloroplasts and related to deltapH, the proton pump and the pH of the inner thylakoid space (pHi). The deltapH and pHi were estimated from the extent of the fluorescence quenching of 9-aminoacridine. Linolenic acid caused a shift (approximately one unit) of the pH optimum for electron flow toward acidity in the following systems: (a) photosystems II + I (from H2O to NADP+ or to 2,6-dichlorophenolindophenol) coupled or non-coupled; (b) photosystem II (from H2O to 2,6-dichlorophenolindophenol in the presence of dibromothymoquinone). In photosystem I conditions (phenazine methosulphate), the deltapH of the control increased as a function of external pHo with a maximum around pH 8.8. When linolenic acid was added, the deltapH dropped, but its optimum was shifted toward more acidic pHo. The same phenomena were also observed in photosytems II + I (from H2O to ferricyanide) and in photosystem II conditions (from H2O to ferricyanide in the presence of dibromothymoquinone). However, the deltapH was smaller and the sensitivity of the proton gradient toward linolenic acid was eventually higher than for photosystem I electron flow activity. The proton pump which might be considered as a measure of the internal buffering capacity of thylakoids was optimum at pHo, 6.7 in the controls. An addition of linolenic acid diminished the proton pump and shifted its optimum toward higher pHo. As a consequence, pHi increased when pHo was raised. At the optimal pHo 8.6 to 9, pHi were 5 to 5.5. Additions of increasing concentrations of linolenic acid displaced the curves toward higher pHi. A decrease of pHo was therefore required to maintain the pHi in the range of 5-5.5 for maximum electron flow. In conclusion, the electron flow activity seems to be delicately controlled by the proton pump (buffer capacity), deltapH, pHi and pHo. Fatty acids damage the membrane integrity in such a way that the subtile equilibrium between the factors is disturbed.

Properties of photoreductions by photosystem II

Dimethyl-methylenedioxy- p-benzoquinone is found to be an excellent mediator for photoreductions by photosystem II only, i.e. in the presence of dibromothymoquinone (DBMIB) blocking reduction by photosystem I. The new quinone system is highly autoxidizable, thus also supporting pseudocyclic electron flow involving photosystem II only. It is not affected by the uncoupler gramicidin at external pH 8, whereas a phenylenediamine acceptor system is strongly inhibited at this pH. This is taken as further support for the notion that the internal pH affects acceptor systems for photosystem II and that therefore the reduction site of these systems occurs in the internal pH range.

Quenching of chlorophyll fluorescence and primary photochemistry in chloroplasts by dibromothymoquinone

The quenching action of dibromothymoquinone on fluorescence and on primary photochemistry was examined in chloroplasts at −196 °C. Both the initial ( F 0) and final ( F M) levels of fluorescence as well as the fluorescence of variable yield ( F v = F M − F 0) were quenched at −196 °C to a degree which depended on the concentration of dibromothymoquinone added prior to freezing. The initial rate of photoreduction of C-550 at — 196 °C, which was assumed to be proportional to maximum yield for primary photochemistry, ϕ P o , was also decreased in the presence of dibromothymoquinone. Simple theory predicts that the ratio F V F M should equal ϕ P o . Excellent agreement was found in a comparison of relative values of ϕ P o with relative values of F V F M at various degrees of quenching by dibromothymoquinone. These results are taken to indicate that F 0 and F V are the same type of fluorescence, both emanating from the bulk chlorophyll of Photosystem II. Dibromothymoquinone appears to create quenching centers in the bulk chlorophyll of Photosystem II which compete with the reaction centers for excitation energy. The rate constant for the quenching of excitation energy by dibromothymoquinone is directly proportional to the concentration of the quencher. Rate constants for the de-excitation of excited chlorophyll molecules by fluorescence, k F, by nonradiative decay processes, k D, by photochemistry, k P, and by the specific quenching of dibromothymoquinone, k Q, were calculated assuming the absolute yield of fluorescence at F 0 to be either 0.02 or 0.05.

Properties of photoreductions by photosystem II in isolated chloroplasts. III. The effect of uncouplers on phenylenediamine shuttles across the membrane in the presence of dibromothymoquinone

In the presence of the plastoquinone antagonist dibromothymoquinone the photoreduction of ferricyanide by isolated chloroplast membranes is attributed to Photosystem II. The reaction is stimulated by the addition of phenylenediamine or C-substituted phenylenediamines (which may form a diimine on oxidation) but not of N-substituted phenylenediamines (which form a stable radical on oxidation). Phenylenediamines also restore NADP reduction (and O 2 evolution) in 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone (DBMIB)-treated chloroplasts. In this bypassing of the inhibition site, N-substituted phenylenediamines are very effective, whereas p-phenylenediamine and C-substituted phenylenediamines are inefficient. Uncouplers exhibit a surprising effect on these systems. Even under coupling conditions uncouplers inhibit electron flow to ferricyanide mediated by phenylenediamine in the pH range 7.3–8.0, whereas the restoration of the NADP system is stimulated. For the interpretation of the results the side of the membrane involved is considered. It is proposed that in ferricyanide reduction by Photosystem II, a phenylenediimine/diamine shuttle operates which moves reducing equivalents from the inside to the outside across the membrane. This shuttle requires a pH gradient across the membrane because of different optimal ratios of diimine/diamine inside and outside. This pH difference is abolished by the uncoupler, accounting for the observed inhibition. The restoration of electron flow from water to NADP in DBMIB-treated chloroplasts is assumed to be a bypass of the inhibition site inside the membrane via a phenylenediamine. Because the imine/amine ratio brought about by the pH gradient is not favorable for the inside oxidation an uncoupler stimulates NADP reduction even under coupling conditions. Also in photoreductions by Photosystem I, for example NADP reduction at the expense of P-phenylenediamine/ascorbate, a shuttle of reducing equivalents across the membrane occurs but this time from outside to inside.

Studies on the energy coupling sites of photophosphorylation. III. The different effects of methylamine and ADP plus phosphate on electron transport through coupling sites I and II in isolated chloroplasts

1. 1. The reduction of lipophilic (Class III) oxidants such as oxidized p-phenylenediamine consists of two components. One component requires both Photosystem II and Photosystem I and includes both sites of energy coupling associated with noncyclic electron transport. The second component requires only Photosystem II and includes only the site of energy coupling located before plastoquinone (Site II). When oxidized p-phenylenediamine is being reduced by both pathways, the overall rate of electron transport is stimulated by the addition of ADP plus phosphate or the uncoupler methylamine. However, if the Photosystem I component of oxidized p-phenylenediamine reduction is eliminated by a low concentration of the plastoquinone-antagonist dibromothymoquinone, the stimulation of electron transport by ADP plus phosphate or methylamine is also abolished, although the remaining Photosystem II-dependent electron transport remains firmly coupled to phosphorylation ( via coupling Site II). These results indicate that coupling Site II, unlike the well-known rate-limiting coupling site between plastoquinone and cytochrome f (Site I), does not exert any control over the rate of associated electron transport. 2. 2. When substituted p-benzoquinones ( e.g. 2,5-dimethyl- p-benzoquinone) or quinonediimides ( e.g. p-phenylenediimine) are used as Class III acceptors in conjunction with dibromothymoquinone, a small but significant stimultation of electron transport by ADP plus phosphate is observed. However, it can be shown that this stimulation does not arise from coupling Site II but rather is due to a low rate of electron flux through coupling Site I even in the presence of dibromothymoquinone. Apparently the p-benzoquinones can catalyze an electron “bypass” around the dibromothymoquinone-induced block at plastoquinone, possibly by substituting partially for the natural electron carrier. If this bypass electron flow is blocked at plastocyanin by KCN treatment, the stimulation of electron transport by ADP plus phosphate is eliminated, although a high rate of phosphorylation (from Site II only) remains. 3. 3. These results provide strong evidence that a profound difference exists between the two sites of energy coupling associated with non-cyclic electron transport in isolated chloroplasts. That is, the rate of electron flow through coupling Site I, which is the rate-determining step in the Hill reaction, is strictly regulated by phosphorylating conditions, whereas the rate of electron flux through coupling Site II is independent of phosphorylating conditions. 4. 4. A model is presented which accounts for the lack of control over electron transport exhibited by coupling Site II. It is postulated that Site II is coupled to an essentially irreversible electron transport step, so that conditions which affect the phosphorylation reaction would have no effect on the rate of electron transport through the coupling site. Two essentially irreversible reactions, closely associated with Photosystem II—the water-splitting reaction and the System II photoact itself—are discussed as possible locations for coupling Site II.

Properties of photoreductions by Photosystem II in isolated chloroplasts. An energy-conserving step in the photoreduction of benzoquinones by Photosystem II in the presence of dibromothymoquinone

Dibromothymoquinone has been used to block photosynthetic electron flow in chloroplasts between Photosystem I and II. The inhibition by dibromothymoquinone of electron flow and of coupled ATP formation in photoreductions by Photosystem I is reversed by the addition of plastoquinone, but not of other substituted benzoquinones. Because dibromothymoquinone does not permit electron flow from water through Photosystem I, Hill reactions in the presence of dibromothymoquinone are attributed to photoreductions by Photosystem II. The photoreduction of p-benzoquinone and 2,6-dimethylbenzoquinone in the presence of dibromothymoquinone is coupled to ATP formation with an ATP e 2 ratio of 0.3. Both photoreductions are stimulated by the addition of uncouplers. Ferricyanide photoreduction in the presence of dibromothymoquinone has a P e 2 ratio of only 0.16 and is not stimulated by an uncoupler. It is concluded that photoreductions by Photosystem II include an energy-conserving step and that lipophilic and hydrophilic acceptors are reduced on different sides of the membrane.

On the Role of Plastoquinone in Photosynthesis

Dibromothymoquinone and its hydroquinone are inhibitors of non cyclic electron flow from water to NADP, anthraquinone or methylviologen. The inhibition is competetively reversed by plastoquinone. It appears that dibromothymoquinone is an antagonist of plastoquinone and that it prevents the enzymic (by the next endogenous carrier of the chloroplast electron transport chain) but not the chemical (by ferricyanide) reoxidation of reduced plastoquinone. This follows from the result that the photoreduction of ferricyanide and DCPIP * is not inhibited by dibromothymoquinone in sonicated chloroplasts and is inhibited in intact chloroplasts to only 60% or 80% respectively. It is concluded that dibromothymoquinone does not inhibit photoreductions by photosystem II. According to their response to dibromothymoquinone, cyclic photophosphorylations can be subdivided in those requiring plastoquinone and those which do not. Menadione catalyzed cyclic photophosphorylation is inhibited by dibromothymoquinone, whereas the PMS catalyzed system is not. The DAD cyclic system is only partly inhibited by dibromothymoquinone. The PMS catalyzed cyclic photophosphorylation in the presence of dibromothymoquinone is antimycin sensitive, which suggests that the PMS system can switch from a plastoquinone dependent system to a plastoquinone independent, but cytochrome b dependent system, which is now antimycin sensitive. Ferredoxin catalyzed cyclic photophosphorylation is inhibited by dibromothymoquinone as well as by antimycin. The data indicate that non cyclic electron flow through both photosystems is obligatory dependent on plastoquinone, whereas cyclic systems do not necessarily include plastoquinone. The relevance of the results to the possibility of different coupling sites in cyclic and non cyclic electron flow systems is discussed.