Inhibition by plastoquinone analogues of ferricyanide reduction by a Photosystem II chloroplast preparation

Abstract

There is wide agreement that inhibition of plastoquinone function by plastoquinone analogues, of which the best known is dibromothymoquinone (DBMIB), inhibits electron transport from Photosystem II (PS II) to Photosystem I (PS I), but does not inhibit electron transport that is dependent on PS II alone. In contrast, we have recently presented evidence in support of a hypothesis for a hitherto unrecognized additional role of plastoquinone in the conductance of protons derived from the photooxidation of water by PS II (Arnon, D.I. and Tang, G.M.-S. (1985) Biochim. Biophys. Acta 809, 167–172). Since the transport of water-derived electrons and protons is coupled, this hypothesis predicts that photosynthetic electron transport by PS II alone would still be sensitive to DBMIB inhibition. We now report that this prediction has been verified by obtaining inhibition by low concentrations of DBMIB of electron transport from water to ferricyanide in a PS II preparation depleted of plastocyanin and PS I. The preparation consisted of inside-out vesicles isolated by the aqueous polymer two-phase partition method. The pattern of DBMIB inhibition in the PS II preparation was the same as that in unfractionated chloroplasts: inhibition of ferricyanide reduction was relieved by the addition of catalytic amounts of one of several lipophilic acceptors that appear to bypass the site of DBMIB inhibition at the Rieske FeS center of the cytochrome f b 6 complex. The bearing of these findings on the role of plastoquinones in the conductance of protons released by the photooxidation of water is discussed.