K⁺ uptake by E. coli cells containing KtrB. The strain LB2003/pEL901 was grown and induced for ktrB expression with 0.02% L-arabinose. For the K⁺ uptake experiment, cells were suspended to 1 mg dry weight (dw)/ml of medium containing 200 mM HEPES-triethanolamin, pH 7.5, 0.2% glycerol, and 0.02% L-arabinose. The suspension was shaken at room temperature. (a) After 10 min, KCl was added at the following concentrations: 0.5 mM, 1 mM, 2 mM, and 5 mM. For each data point, a 1.0-ml sample was taken from the suspension and centrifuged through silicone oil, and the cellular K⁺ content was determined by flame photometry. (b) Triplicates of panel a were used to determine K_m and V_max values applying a Lineweaver–Burk plot. (c) K⁺ uptake using 2 mM KCl in the presence of the following NaCl concentrations was examined: 0 mM, 0.05 mM, 0.5 mM, and 5 mM. (d) K⁺ uptake using 2 mM KCl in the presence of competitive ion concentrations was examined using 50 mM NaCl or RbCl; 50 mM LiCl was used as a noncompetitive ion.