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Interaction of benzoquinones with q negative a and q negative b in oxygen evolving photosystem ii particles from the thermophilic cyanobacterium synechococcus elongatus



Interaction of benzoquinones with q negative a and q negative b in oxygen evolving photosystem ii particles from the thermophilic cyanobacterium synechococcus elongatus



Plant and Cell Physiology 31(7): 1039-1048



The interactions of benzoquinones with the reduced forms of the bound plastoquinone acceptors, QA and QB, were studied with oxygen-evolving photosystem II (PS II) particles from the thermophilic cyanobacterium Synechococcus elongatus, which largely lack pool plastoquinone molecules [Takahashi and Katoh (1986) Biochim, Biophys. Acta 845: 183]. Oxygen evolution in the presence of various electron acceptors was determined and flash-induced changes in absorbance in the blue region were analyzed in terms of difference spectra, dependence on the concentration of benzoquinone and on temperature, and sensitivity to 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea (DCMU). The more hydrophobic the quinone molecule, the higher was the rate of oxygen evolution, and the maximum rate of 3,000 .mu.moles O2 .cntdot. (mg chlorophyll)-1 .cntdot. h-1 was recorded in the presence of phenyl- and dichloro-p-benzoquinones. DCMU inhibited oxygen evolution by more than 95%. However, spectrophotometric studies revealed that, even though electrons were transferred to benzoquinones predominantly via QB2-, the direct oxidation of QA- by added benzoquinones occurred in such a way as to indicate that about 40% of PS II reaction centers were not associated with functional QB sites. QB- was very stable in the presence of ferricyanide. However, benzoquinones induced the slow oxidation of QB-. The characteristics of the benzoquinone reduction in the PS II preparation is discussed.

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