INVESTIGATIONS ON THE SIZE AND SPECIFICITY OF THE PLASTOQUINONE POOL OF PHOTOSYSTEM-2 MEMBRANE-FRAGMENTS

The quinone acceptor pool capacity of photosystem 2 (PS2) membrane fra gments is diminished compared to that of intact thylakoids. The extent of quinone pool reconstitution was highly specific for the type of qu inone molecules used. The far most efficient component was the natural plastoquinone-9 (PQ-9) that exhibited significant reconstitution effe cts at very low ratios of PQ-9 per PS2. In marked contrast, the widely used exogenous acceptor phenyl-p-benzoquinone (Ph-p-BQ) did not incre ase the pool capacity even at a molar Ph-p-BQ:PS2 ratio of 150:1. The samples reconstituted with PQ-9 provided a most suitable material for all experiments performed in the absence of exogenous artificial accep ters in order to avoid interference by disturbing artefacts. The inher ent potential of PQ-9 reconstituted PS2 membrane fragments was clearly demonstrated by monitoring the characteristic oscillation pattern of flash-induced oxygen evolution. These measurements permit for the firs t time unambiguous determination of the population of superreduced red ox states of the water oxidising complex (WOC) in PS2 membrane fragmen ts. A selective population of up to 90 % can be achieved for redox sta te S-2 by suitable treatment of PQ-9 reconstituted samples with NH2OH.