Evidence for a close spatial location of the binding sites for CO 2 and for Photosystem II inhibitors

Abstract

1. 1. CO 2-depletion of thylakoid membranes results in a decrease of binding affinity of the Photosystem II (PS II) inhibitor atrazine. The inhibitory efficiency of atrazine, expressed as I 50-concentration (50% inhibition) of 2,6-dichlorophenolindophenol reduction, is the same in CO 2-depleted as well as in control thylakoids. This shows that CO 2-depletion results in a complete inactivation of a part of the total number of electron transport chains. 2. 2. A major site of action of CO 2, which had previously been located between the two electron acceptor quinone molecule B (or R) and plastoquinone, appears to be in the same membrane protein which binds the Photosystem II inhibitor atrazine as suggested by the following observations: (a) CO 2-depletion results in a shift of the binding constant ( k b ) of [ 14C]atrazine to thylakoid mebranes indicating a decreased affinity of atrazine to the membrane; (b) trypsin treatment, which is known to modify the Photosystem II complex at the level of B, strongly diminishes CO 2 stimulation of electron transport reactions in CO 2-depleted membranes; and (c) thylakoids from atrazine-resistant plants, which contain a Photosystem II complex modified at the inhibitor binding site, show an altered CO 2-stimulation of electron flow. 3. 3. CO 2-depletion does not produce structural changes in enzyme complexes involved in Photosystem II function of thylakoid membranes, as shown by freeze-fracture studies using electron microscopy.