ATP synthase (proton-translocating ATPase, H + -ATPase, F, F, ATPase) is a component of the cytoplasmic membrane of eubacteria, the thylakoid membrane of chloroplasts and the inner membrane of mitochondria. The Escherichia coli enzyme the simplest so far defined, is a complex of eight different polypeptides; five of them, M , f i , 7, 6 and E , constitute the extrinsic membrane F, domain of the enzyme, and the remainder, a, b and c, make up the membrane sector, F,,. Their stoichiometries are cc,&y, E , a , bzc,(k,z (Fillingame, 198 I ) . Gene and protein studies, described below, of the chloroplast enzyme and the related complex in the cyanobacterium Synechococcus 6301, have shown that homologues of these eight bacterial proteins are present as they are also in the bovine mitochondria1 enzyme. The latter is considerably more complex and has at least five supernumerary protein components. Thus, these experiments establish a structural unity for the enzymes in eubacteria, chloroplasts and mitochondria which itself suggests that the enzymes function in a basically similar manner.