(A) Currents recorded in single oocytes injected with water, AMT1;3, AmTrac, or AmTrac-LS, and perfused with NH4Cl at the indicated concentrations. (B) Kinetics of NH4+-induced currents of AMT1;3, AmTrac, and AmTrac-LS. The Kms were 55 ± 15 μM, 51 ± 24 μM, and 57 ± 19 μM, respectively. The data were fitted to Michaelis–Menten kinetics. Oocytes were clamped at −120 mV (independent data from three different oocytes recorded from three different frogs). (C) Titration of the fluorescence response of AmTrac in yeast (blue, left y-axis) and of ammonium uptake of AMT1;3 in plants (red, right y-axis; Yuan et al., 2007). Data are normalized to water-treated controls (0) (mean ± SE; n = 3). (D) Response of a single yeast cell expressing AmTrac to pulses of NH4Cl at the indicated concentrations (blue frames). (E) Substrate specificity. Yeast cells expressing the sensor were treated with the indicated salts at 1 mM concentration. Data are normalized to water-treated control (0) (mean ± SE; n = 3). Only data for the ammonium treatments were significantly different from control (SNK test: *p<0.01).