Direct Demonstration of Binding-Site Competition between Photosystem II Inhibitors at the Q_B Niche of the D1 Protein

Inhibitors of photosystem II function have been shown to block electron flow in vitro by competitively displacing plastoquinone from the Q_B niche on the D1 protein. Few studies have tested this well-accepted concept in vivo and none in higher plants. The D1 protein degradation assay was used to directly demonstrate, in vivo, the displacement of diuron by bromonitrothymol (BNT) at the level of the Q_B niche. We show that diuron blocks D1 degradation less effectively in the presence of BNT, and that this effect of BNT can be nullified by increasing the diuron concentration. These data directly demonstrate binding-site competition at the level of the Q_B niche, under the complex physiological conditions of the intact higher plant.