The H +/ATP ratio of the ATP synthase from the cyanobacterium Synechococcus 6716 varies with growth temperature and light intensity

Abstract

The proton translocation stoichiometry (H +/ATP ratio) and other bioenergetic features were investigated in membrane vesicles from the moderately thermophile Synechococcus 6716 grown at 38°C and 50°C with saturating light intensity, and at 38°C with limiting light intensity. At 50°C growth is slower but proceeds to a higher cell density than at 38°C. Increasing the growth temperature from 38°C to 50°C resulted in an altered membrane fatty acid composition, with increased length and saturation of the acyl chains. At 38°C and lower light intensity chain length was somewhat decreased and saturation increased to a small extent. Membrane vesicles from cells grown at 50°C performed cyclic photophosphorylation at lower light intensities and lower threshold Δ μ ̄ H + than vesicles from cells grown at 38°C. The 50°C vesicles also displayed a diminished light-induced proton uptake, but ATP synthesis activity and the attained Δ G p remained constant. Moreover, ATP synthesis became more resistant to uncoupling. From acid–base transition induced ATP synthesis experiments the H +/ATP ratios were determined to be 3.9, 3.1 and 3.3 for membrane vesicles from cells grown at 50°C, 38°C and light-limited 38°C, respectively. In vesicles from cells grown at 50°C, ATP hydrolysis is inhibited by a lower valinomycin-induced K +-diffusion potential than in vesicles from cells grown at 38°C. A molecular mechanism to explain changes in H +/ATP as well as the physiological implications are discussed.