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Membrane protein damage and repair: Selective loss of a quinone-protein function in chloroplast membranes.

A loss of electron transport capacity in chloroplast membranes was induced by high-light intensities (photoinhibition). The primary site of inhibition was at the reducing side of photosystem II (PSII) with little damage to the oxidizing side or to the reaction center core of PSII. Addition of herbicides (atrazine or diuron) partially protected the membrane from photoinhibition; these compounds displace the bound plastoquinone (designated as Q(B)), which functions as the secondary electron acceptor on the reducing side of PSII. Loss of function of the 32-kilodalton Q(B) apoprotein was demonstrated by a loss of binding sites for [(14)C]atrazine. We suggest that quinone anions, which may interact with molecular oxygen to produce an oxygen radical, selectively damage the apoprotein of the secondary acceptor of PSII, thus rendering it inactive and thereby blocking photosynthetic electron flow under conditions of high photon flux densities.