SHAHAK, Y., Biochemistry Department, Weizmann Institute of Science, Rehovot, Israel

The involvement of Mg2+ ions in the light‐dependent regulation of the chloroplast H+ ‐ATPase was studied in both type C and osmotically shocked type A chloroplasts. The following results were obtained. 1. ATPase activity measured under dark, partially uncoupling conditions, following light activation with dithiothreitol and pyocyanine, was markedly enhanced by the presence of Mg2+ in the activation stage. This Mg2+ effect required concentrations in the millimolar range, was rather slow (time range of minutes), reversible. rather unspecific and did not involve changes in the affinity to dithiothreitol. 2. Dark deactivation of the ATPase in the absence of substrate was accelerated by Mg2+. The dark effect of Mg2+ also required millimolar concentrations, but was fast (time range of seconds), highly specific for Mg2+, and did not involve thiol oxidation. 3. The major effect of the absence of Mg2+ from the light‐activation stage or of its presence in the dark interval between activation and assay was the induction of an ‘abnormal’ sensitivity to uncouplers: after these treatments ATP hydrolysis was not stimulated but rather inhibited by NH4Cl or other uncouplers. 4. The pretreatments in the light without Mg2+ or dark with Mg2+ did not affect the membrane proton permeability, nor the proton pumping coupled to ATPase activity. The results are discussed in terms of Mg2+ ‐dependent regulation of the enzyme complex at the level of subunit interaction and its effect on the affinity to protons. Copyright © 1986, Wiley Blackwell. All rights reserved