Tribromotoluquinone Induced Modifications of the Oscillation Pattern of Oxygen Evolution and of Herbicide Binding in Thylakoids and PS II Membrane Fragments from Spinach

In the p resen t study the effect o f T B T Q on PS II and its m utual interaction with D C M U was analyzed by m easurem ents of the oxygen yield oscillation p a tte rn and of D C M U binding. It was found: 1)T B TQ in its reduced form is able to induce the reduction of D ox which gives rise to an accelerated decay of S2 and S3 o f the w ateroxidizing com plex. 2 ) Triton X-100 treatm ent used for isolation of PS II m em brane fragm ents does not significantly affect the lateral mobility of p lastoquinone w ithin the m em brane. T B TQ bound to the thy­ lakoid m em brane does not enhance the e lectron pool capacity in PS II m em brane fragm ents. 3) Preincubation of thylakoids with T B T Q dim inishes the blockage of 0 2-evolution by D CM U significantly. In correspondence with previous findings [18, 19] the effect strongly depends on the o rd er of addition of T B T Q and D C M U . 4) Excitation with a single sa tu rating flash causes enhanced D C M U binding in T B T Q p re treated sam ples leading to the inhibition of flash induced oxygen evolution. The ra te of the latter process depends on the D C M U concentra tion . 5) In thylakoids p re treated in the dark with T B T Q the oxygen yield of the 3rd flash slowly declines as a function o f dark incubation time at constant D C M U concentration . Based on the above m entioned findings it is inferred that a m utual in teraction betw een T B T Q and DCM U takes place at the PS II acceptor side. Two alternative m echanism s are discussed: i) T B TQ tightly (covalently?) bound at the Q B-site (or very close to it) is m odified in its function by D C M U via structural effects (allosteric type), o r ii) there occurs a T B T Q /D C M U exchange that is fast in the light and slow in the dark.