The site of inhibition of the chloroplast electron‐transport system by 2,3‐dithiopropan‐1‐ol (BAL)

BAL (2,3‐dithiopropan‐1‐ol) treatment of chloroplasts has previously been reported to induce a block in electron transport from water to NADP⁺ at a site preceding plastocyanin [Belkin et al. (1980) Biochim. Biophys. Acta 766, 563–569]. In the present work the block was further characterized. The following properties of BAL treatment are described. a) Inhibition of electron transport from water to lipophilic acceptors but not to silicomolybdate. b) Inhibition of the slow, sigmoidal phase of chlorophyll a fluorescence induction. c) Inability of N, N, N′,N′,‐tetramethyl‐p‐phenylenediamine to bypass the inhibition of NADP⁺ photoreduction with water as the electron donor. d) Inhibition of electron transport from externally added quinols to NADP⁺ e) Inhibition of cytochrome f reduction by photosystem II, but not its oxidation by photosystem I. f) Inhibition of cytochrome b₆ turnover and cytochrome f rereduction after single‐turnover flash illumination under cyclic electron‐flow conditions. The BAL‐induced block is therefore located between the secondary quinone acceptor (Q_B) and the cytochrome b₆f complex. It was further found that (a) the isolated cytochrome complex is not inhibited after BAL treatment; (b) BAL‐reacted plastoquinone‐1 inhibits electron transport in chloroplasts; (c) BAL does not inhibit electron transport in chromatophores of Rhodospirilum rubrum or Rhodopseudomonas capsulata. It is suggested that the inhibition of electron transport in chloroplasts results from specific reaction of BAL with the endogenous plastoquinone.