An ATP synthase has been isolated from green nonsulfur photosynthetic bacterium Chloroflexus aurantiacus, a representative of a lower branch of eubacteria. The enzyme, reconstituted with the bacterial lipids into proteoliposomes, is shown to catalyze [32P]Pi-ATP exchange (at a rate of 180 nmol [32P]ATP/min/mg). The ATP synthase is composed of nine polypeptide species (60, 50, 33, 19, 16.5, 15.5, 14.5, 13, and 8 kDa as determined by urea-SDS-PAGE). The catalytic part of the ATP synthase (which is detached by chloroform treatment) contains the first four polypeptides. In the intact ATP synthase the 14.5 and 13 kDa polypeptides are connected by disulfide bonds to form a heterodimer of 25 kDa.