K⁺ uptake by E. coli cells containing KtrAB. The strain LB2003/pKT84 was grown and induced for ktrAB 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, 2 mM KCl was added in the presence of the following NaCl concentrations: 0 mM, 0.05 mM, 0.5 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 and c) K⁺ uptake experiments at different KCl concentrations in the presence of different NaCl concentrations were performed in triplicate. Subsequently, K_m and V_max dependent on different NaCl concentration were determined (n = 3, ±SD).