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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. The role of plastoquinone in photosynthesis has been investigated mainly in petrolether extracted chloroplasts, a procedure first introduced by BISHOP and by direct spectroscopic reading of absorption changes in chloroplasts and algae (see summaries and 1. c. 2 ' 3 ). From these experiments it evolved that plastoquinone functions close to and migh actually be the primary acceptor of photosystem II 4 . We have recently introduced a new inhibitor of photosynthetic reactions in chloroplasts -dibromothymoquinone -which blocks the electron transport chain between the two light reactions 5 . We wish to report on further effects of this inhibitor on different photosynthetic reactions in chloroplasts and on the reversal of the Reprint requests to Prof. Dr. A. TREBST, Ruhr-Univ. Bochum, Abt. f. Biologie, D-4630 Bochum-Querenburg, Postfach 2148. * Abbreviations used: AQ = anthraquinone-2-sulfonate, Chi = Chlorophyll, DAD = diaminodurene, DCPIP = dichlorophenolindophenol, DCMU = dichlorophenyl-dimethyl-urea, MV = methylviologen, PMS = methyl-phenazoniummethosulfate. 1 N. I. BISHOP, Proc. nat. Acad. Sei. USA 44, 501 [1958]. inhibition by plastoquinone. The results support the conclusion that the new inhibitor is a plasto...

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The plastoquinone antagonist dibromothymoquinone interrupts the photosynthetic electron transport chain between photosystem II and I by inhibiting the oxidation of plastohydroquinone by photosystem I. The effect of phenylenediamines on NADP and ferricyanide photoreduction inhibited by dibromothymoquinone is studied in isolated chloroplasts. IV-substituted phenylene­ diamines fully restore NADPH and ATP formation as well as oxygen evolution in DBMIB inhibited NADP reduction, but are practically inactive in stimulating ferricyanide reduction by photo­ system II. On the other hand phenylenediamine and C-substituted phenylenediamines are only weakly active in restoring NADP reduction but are very active in stimulating ferricyanide reduc­ tion. The P/e2 ratio in restoration of NADP reduction by phenylenediamines is close to 1, whereas the P/e2 ratio in ferricyanide reduction is about 0.4 (the ratio is dependent on the DBMIB con­ centration). The reversal of the DBMIB inhibition in NADP reduction by phenylenediamines is attributed to a bypass of the inhibition site: Reduced phenylenediamines are electron donors for photosystem I after and oxidized phenylenediamines are electron acceptors for photosystem II be­ fore the DBMIB inhibition site. To explain the different rates and P/e2 ratios in the various systems it is assumed that all phenylenediamines are reduced at the expense of water oxidation by photo­ system II but via plastoquinone and on the inside of the membrane. Therefore two energy con­ serving steps (i. e. two proton releasing sites on the inside of the membrane) are involved, N-sub­ stituted phenylenediamines like TMPD reduced by photosystem II inside remain inside the mem­ brane and are reoxidized inside by photosystem I. This way they connect photosystem II bade onto photosystem I, electron transport rates are high and the P/e2 ratio is one in the restored NADP photoreduction. Phenylenediamine and C-substituted phenylenediamines on the other hand travel back to the outside through the membrane, and by doing so carry some of the protons released on the inside of the membrane by the energy conserving steps back to the outside. This way they are very active in stimulating the rate of ferricyanide photoreduction but the P/e2 ratio is only about 0.4. These phenylenediamines being removed from the inside are less active in restoring the rate of NADP reduction