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, inside-out-vesicles (ISO-vesicles), and PS II core complexes was analyzed by measuring (i) the characteristic period four oscillation of the oxygen yield due to excitation of dark-adapted samples with a train of short flashes and (ii) laser flash-induced transients of the relative quantum yield of chlorophyll fluorescence, The data obtained revealed that (a) an endogenous pool capacity comparable to that of intact thylakoids can be restored in PS II membrane fragments and ISO-vesides by a sonication treatment using native plastoquinone-9 (PQ-9), (b) a more pronounced oxygen oscillation pattern arises in PS II core complexes after application of the same reconstitution procedure, (c) the extent of the endogenous pool restoration at a ratio of 15 quinone molecules per 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 inefficient], and (d) a sonication step is required for stable insertion of PQ-9 into PS II preparations, Measurements of the reconstitution degree as a function of the structure of different quinones with selected properties lead to the conclusion that specific binding domains exist in PS II in addition to the Q(B) site. These domains exhibit a surprisingly 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 quinone head group seems to be more important than the large hydrophobic side chain and/or the general lipophilicity of the compound.