Plant Cell Physiol. 32(8): 1165-1171 (1991) JSPP © 1991 Artificial Quinones Replace the Function of Quinone Electron Acceptor in the Isolated Dl-D2-Cytochrome b S59 Photosystem II Reaction Center Complex Hiroyuki Nakane 1 , Masayo Iwaki 2 , Kimiyuki Satoh 1 , and Shigeru Itoh 2 1 Department of Biology, Faculty of Science, Okayama University, Okayama, 700 Japan 2 Division of Bioenergetics, National Institute for Basic Biology, Nishigonaka, Okazaki, 444 Japan Various benzo- and naphthoquinone derivatives were introduced into the purified photo- system II Dl-D2-cytochrome b 559 reaction center complex, which lacks the intrinsic plasto- quinone electron acceptors. Effects of these quinones on the electron transfer reactions in nanoseconds to milliseconds time range were studied at room and cryogenic temperatures. 1) The addition of quinones to the purified photosystem II reaction center complex suppressed the nanosecond charge recombination between oxidized reaction center chlorophyll a (P680 + ) and re- duced pheophytin a (Ph~), and stabilized P680 + up to millisecond time range at 280 K and at 77 K. 2) In the reaction center complex supplemented with dibromothymoquinone (DBMIB), P68O was almost fully oxidized and cytochrome b 559 was partially reduced by flash excitation. A semi- quinone-like signal with a peak around 320 nm was also induced but the shift of pheophytin absorption band (C55O) was not observed. 3) Halogenated quinones, especially DBMIB, were better electron acceptors than unsubstituted or methylated quinones. 4) The affinities of quinones to the reaction center complex were weakly dependent on their molecular structure. Key words: Electron transfer — Photosynthesis — Photosystem II center. Quinone — Reaction Quinones function as electron acceptors in the RC in photosynthetic organisms (see review by Andreasson and Vangard 1988). PS II RC of oxygenic photosynthetic or- ganisms contains two PQ-9 molecules which interact with a non-heme iron and function as Q A and Q B . The Q A plastoquinone undergoes one-electron reduction and the Q B , two-electron reduction. The difference in the func- tional behavior of Q A and Q B quinones seems to originate from differences in the quinone binding niche in the RC proteins as shown by X-ray crystallographic studies of RC complexes from purple photosynthetic bacteria (Deisen- hofer et al. 1986). Purified PS II RC (Dl-D2-cytochrome b S59 ) complex, first isolated by Nanba and Satoh (1987), lacks plasto- quinone and thus the photo-separated charges are not Abbreviations: DBMIB, dibromothymoquinone (2,5-di- bromo-3-methyl-6-isopropyl-l,4-benzoquinone); ti/2, half decay time; P680, photosystem II reaction center chlorophyll a; Ph, pheophytin a (primary electron acceptor); PQ, plastoquinone; PS, photosystem; Q A and Q B , the secondary and tertiary electron acceptor quinones, respectively; RC, reaction center. stabilized (Danielius et al. 1987). Using this type of material, Chapman et al. (1988) showed that decylplasto- quinone elicits the reduction of cytochrome b }S9 with diphenylcarbazide as an electron donor. Gounaris et al. (1988) later showed that PQ-9, at rather high concentra- tions, can also support similar reactions under continuous illumination. An artificial electron acceptor, silicomolyb- date, also efficiently oxidizes Ph~ at room temperature (Barber et al. 1987, Takahashi et al. 1988) as well as at 77 K (Takahashi et al. 1988, Nugent et al. 1989). Mathis et al. (1989) and Satoh et al. (1990) recently showed that DBMIB suppresses the charge recombination presumably acting as an electron acceptor to Ph~ and mediates reduction of cyto- chrome b ii9 even at -29 C C. In the presence of DBMIB, P680 + was estimated to be reduced either by reduced cyto- chrome b 559 with a t 1/2 of 2 ms (Satoh et al. 1990) or by en- dogenous electron donor Z (tyrosine) with a t\ n of 5 jxs (Mathis et al. 1989). DBMIB also quenches the long lived chlorophyll fluorescence (Booth et al. 1990) which seems to be emitted from the excited state of P680 (P680*) produced by the charge recombination between P680 + and Ph~ (Mimuro et al. 1988). 1165