RECONSTITUTION OF THE ENDOGENOUS PLASTOQUINONE POOL IN PHOTOSYSTEM-II(PS-II) MEMBRANE-FRAGMENTS, INSIDE-OUT-VESICLES, AND PS-II CORE COMPLEXES FROM SPINACH

The possibility of reconstituting a functionally competent endogenous plastoquinone pool in photosystem II (PS II) membrane fragments, insid e-out-vesicles (ISO-vesicles), and PS II core complexes was analyzed b y measuring (i) the characteristic period four oscillation of the oxyg en yield due to excitation of dark-adapted samples with a train of sho rt flashes and (ii) laser flash-induced transients of the relative qua ntum yield of chlorophyll fluorescence, The data obtained revealed tha t (a) an endogenous pool capacity comparable to that of intact thylako ids can be restored in PS II membrane fragments and ISO-vesides by a s onication treatment using native plastoquinone-9 (PQ-9), (b) a more pr onounced oxygen oscillation pattern arises in PS II core complexes aft er application of the same reconstitution procedure, (c) the extent of the endogenous pool restoration at a ratio of 15 quinone molecules pe r PS II in the reconstitution assay strongly depends on the nature of the quinone molecule [maximum effects can be only achieved with PQ-9, while at the same concentration ubiquinone-45 (UQ-9) is almost ineffic ient], and (d) a sonication step is required for stable insertion of P Q-9 into PS II preparations, Measurements of the reconstitution degree as a function of the structure of different quinones with selected pr operties lead to the conclusion that specific binding domains exist in PS II in addition to the Q(B) site. These domains exhibit a surprisin gly high specificity for the type of quinone that can be bound. On the basis of a comparison of the results obtained, the structure of the q uinone head group seems to be more important than the large hydrophobi c side chain and/or the general lipophilicity of the compound.