The binding change model for the F 1 -ATPase predicts that its rotation is intimately correlated with the changes in the affinities of the three catalytic sites for nucleotides. If so, subtle differences in the nucleotide structure may have pronounced effects on rotation. Here we show by single-molecule imaging that purine nucleotides ATP, GTP, and ITP support rotation but pyrimidine nucleotides UTP and CTP do not, suggesting that the extra ring in purine is indispensable for proper operation of this molecular motor. Although the three purine nucleotides were bound to the enzyme at different rates, all showed similar rotational characteristics: counterclockwise rotation, 120°steps each driven by hydrolysis of one nucleotide molecule, occasional back steps, rotary torque of ϳ40 piconewtons (pN)⅐nm, and mechanical work done in a step of ϳ80 pN⅐nm. These latter characteristics are likely to be determined by the rotational mechanism built in the protein structure, which purine nucleotides can energize. With ATP and GTP, rotation was observed even when the free energy of hydrolysis was ؊80 pN⅐nm/molecule, indicating ϳ100% efficiency. Reconstituted F o F 1 -ATPase actively translocated protons by hydrolyzing ATP, GTP, and ITP, but CTP and UTP were not even hydrolyzed. Isolated F 1 very slowly hydrolyzed UTP (but not CTP), suggesting possible uncoupling from rotation.The F o F 1 ATP synthase is an enzyme that synthesizes ATP from ADP and inorganic phosphate (P i ) using proton flow across a membrane (1). The F o portion of the enzyme resides in the membrane and mediates proton translocation. The F 1 portion, consisting of ␣ 3  3 ␥ 1 ␦ 1 ⑀ 1 subunits, is external to the membrane and catalyzes ATP synthesis. The ATP synthase is a completely reversible molecular machine in that ATP hydrolysis in F 1 can produce a reverse flow of protons through F o . Isolated F 1 only catalyzes ATP hydrolysis and hence is called A crystal structure of F 1 (5) strongly supported the rotation model and has inspired many experiments, which, together, have proved that the ␥ subunit is (part of) the common rotor shaft and that ␣ 3  3 subunits, which surrounded ␥ in the crystal, are the stator in the F 1 motor (6 -8). Single-molecule imaging of F 1 , in particular, has revealed that the ␥ subunit rotates in a unique direction consistent with the crystal structure, that ␥ makes discrete 120°steps, and that the energy conversion efficiency of the F 1 motor driven by ATP hydrolysis can reach ϳ100% (9, 10). The precise mechanism of rotation, however, is not yet clear.In Boyer's model, binding changes play the major role (1). On the three  subunits, each of which hosts a catalytic site, ATP and its hydrolysis products, ADP and P i , are in equilibrium. Thus, in the absence of an external energy supply, the change in free energy associated with ATP hydrolysis should manifest as the higher affinity for ATP than for ADP and P i . During ATP synthesis, the mechanical energy supplied by the ␥ rotation driven by F 0 somehow decreases the affinity for A...