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 present study the effect of TBTQ on PS II and its mutual interaction with DCMU was analyzed by measurements of the oxygen yield oscillation pattern and of DCMU binding. It was found: 1) TBTQ in its reduced form is able to induce the reduction of Dox which gives rise to an accelerated decay of S2 and S3 of the wateroxidizing complex. 2) Triton X-100 treatment used for isolation of PS II membrane fragments does not significantly affect the lateral mobility of plastoquinone within the membrane. TBTQ bound to the thylakoid membrane does not enhance the electron pool capacity in PS II membrane fragments. 3) Preincubation of thylakoids with TBTQ diminishes the blockage of 02-evolution by DCMU significantly. In correspondence with previous findings [18, 19] the effect strongly depends on the order of addition of TBTQ and DCMU. 4) Excitation with a single saturating flash causes enhanced DCMU binding in TBTQ pretreated samples leading to the inhibition of flash induced oxygen evolution. The rate of the latter process depends on the DCMU concentration. 5) In thylakoids pretreated in the dark with TBTQ the oxygen yield of the 3rd flash slowly declines as a function of dark incubation time at constant DCMU concentration. Based on the above mentioned findings it is inferred that a mutual interaction between TBTQ and DCMU takes place at the PS II acceptor side. Two alternative mechanisms are discussed: i) TBTQ tightly (covalently?) bound at the QB-site (or very close to it) is modified in its function by DCMU via structural effects (allosteric type), or ii) there occurs a TBTQ/DCMU exchange that is fast in the light and slow in the dark.