Energy coupling for three K+ transport systems of Escherichia coli K-12 was studied by examining effects of selected energy sources and inhibitors in strains with either a wild type or a defective (Ca2+, Mg2+)-stimulated ATPase. This approach allows discrimination between transport systems coupled to the proton motive force from those coupled to the hydrolysis of a high energy phosphate compound (ATP-driven). The three K+ transport systems here studied are: (a) the Kdp system, a repressible high affinity (Km=2 muM) system probably coded for by four linked Kdp genes; (b) the Trka system, a constitutive system with high rate and modest affinity (Km=1.5 mM) defined by mutations in the single trkA gene; and (c) the TrkF system, a nonsaturable system with a low rate of uptake (Rhoads, D.B., Waters, F.B., and Epstein, W. (1976) J. Gen. Physiol. 67, 325-341). Each of these systems has a different mode of energy coupling: (a) the Kdp system is ATP-driven and has a periplasmic protein component; (b) the TrkF system is proton motive force-driven; and (c) the TrkA system is unique among bacterial transport systems described to date in requiring both the proton motive force and ATP for activity. We suggest that this dual requirement represents energy fueling by ATP and regulation by the proton motive force. Absence of ATP-driven systems in membrane vesicles is usually attributed to the requirement of such systems for a periplasmic protein. This cannot explain the failure to demonstrate the TrkA system in vesicles, since this system does not require a periplasmic protein. Our findings indicate that membrane vesicles cannot couple energy to ATP-driven transport systems. Since vesicles can generate a proton motive force, the inability of vesicles to generate ATP or couple ATP to transport (or both) must be invoked to explain the absence of TrkA in vesicles. The TrkF system should function in vesicles, but its very low rate may make it difficult to identify.