The activation requirements and thermodynamic characteristics of ATP synthase from the alkalophilic cyanobacterium Spirulina platensis were studied in coupled membrane vesicles. Activation by methanol increased the V,,, while the K, for MgATP was unaffected (0.7 mM). We propose that in Sp. platensis, as in chloroplasts, the activating effect of methanol is based on perturbation of the y-E subunit interaction. Light-driven ATP synthesis by membrane vesicles of Sp. platensis was stimulated by dithiotreitol. The characteristics of the activation of the ATP synthase by the proton electrochemical potential difference ( A f i H + ) were analyzed on the basis of the uncoupled rates of ATP hydrolysis as a function of a previously applied proton gradient. Two values of AjiH+, at which 50% of the enzyme is active, were found; 13-14 kJ.mol-' for untreated membrane vesicles, and 4-8 kJ. mol-' for light-treated and dithiotreitol-treated membrane vesicles. These values are lower than the corresponding values for the oxidized and reduced forms, respectively, of the chloroplast enzyme. Although no bulk proton gradient could be observed, membrane vesicles of Sp. platensis were able to maintain an equilibrium phosphate potential (AG,) of 40-43.5 kJ.mol comparable to values found for Synechococcus 6716 and Anabaena 7120 membrane vesicles. Acidhase-transition experiments showed that the thermodynamic threshold, A,iiH+, for ATP synthesis, catalyzed by light-treated and dithiotreitol-treated Spirulina membrane vesicles, was less than 5 kJ. mol-' .The activation characteristics and the low thermodynamic threshold allow ATP synthesis to occur at low AjiH+ values. The findings are discussed, both with respect to differences and similarities with the enzymes from chloroplasts and other cyanobacteria, and with respect to the alkalophilic properties of Sp. platensis.The ATP synthase of cyanobacteria resembles that of chloroplasts in that the enzyme requires activation of synthesis or hydrolysis of ATP. All thylakoid membrane ATP synthases studied can be activated by the proton electrochemical potential differenct (A&+). In addition, the chloroplast ATP synthase is regulated by the redox state of the enzyme [l]. The magnitude of A,iiH+ required for activation of the ATP synthase is lowered upon reduction. Reduction of the enzyme involves the cleavage of a disulfide group in the y subunit and a concomitant displacement of the E subunit [2, 31. In vivo, the enzyme is reduced by thioredoxin which, in turn, is reduced by the photosynthetic electron flow from photosystem 1 through ferredoxin and a ferredoxin:thiore- Other in-vitro methods which induce activity of the chloroplast enzyme include treatment by heat, methanol, trypsin, detergents [8] and sulfite [9].It was shown earlier that ATP hydrolysis, catalyzed by the hydrophilic part of the ATP synthase (F,)
