Simulation of the dependence of the Agm ²⁺ -efflux rate from bacteria on the Arg ⁺ concentration in the medium. The previously observed initial rate of Agm²⁺ extrusion at pH 2.5 (mean ± SEM; closed green squares) is normalized relative to the observed maximum and replotted here against the external Arg⁺ concentration on a logarithmic scale (Iyer et al., 2003). The simulations using the model (with kinetic parameters detemined in the polarization study at the corresponding pH) shown in Fig. S6 itself or with its apo path for conformational transitions eliminated, i.e., a uniporter or an exchanger model, are shown as closed blue or red circles (see supplemental text at the end of the PDF). The red curve is a fit of a Michaelis–Menten equation to the data simulated with the exchanger model, whereas the blue curve is a fit of a modified Michaelis–Menten equation to the data simulated with the uniporter model. The modification allows the rate to approach a nonzero minimum value as the external Arg⁺ concentration approaches zero.