Light-, pH- and uncoupler-dependent association of chloride with chloroplast thylakoids

Abstract

Studies of the association of Cl − with Photosystem (PS) II in CF 1-containing thylakoid membranes revealed that photosynthetically active Cl − is retained in a Cl −-free medium unless it is sufficiently alkaline, uncoupling conditions are established and light is excluded. After treatment under such conditions, electron transport from water became dependent on added Cl − under all conditions. Quantitative measurements of 36Cl − retention in the light revealed that there were about five Cl − anions present in Cl −-sufficient chloroplasts per PS II reaction center, and one-fourth of that in Cl −-deficient samples. Uncouplers representing three different types of uncoupling mechanism were found to be effective mediators of Cl − release from thylakoids. Since the ability to collapse a proton gradient probably is the only property shared by all the tested uncouplers, a proton gradient may be involved in the retention of Cl −. As uncoupler-mediated Cl − release did not depend on preillumination of our samples, a long-lived proton gradient must exist in dark-adapted chloroplasts which may not span the whole thickness of the thylakoid membrane. It is postulated that the Cl − active in PS II reactions resides in a special membrane domain from which protons slowly equilibrate with those in the bulk solutions. Cl − is thought to be released to the bulk phases only when the pH of the membrane domain is raised above a certain threshold by the action of uncouplers. This domain may be identical to the intramembranous compartment which has been postulated to be associated with PS II (Prochaska, L.J. and Dilley, R.A., (1978) Front. Biol. Res. Energ. 1, 265–274).