FIGURE 6.

Kinetics of AmtB-mediated K⁺ transport. A, initial (1 min) rates of AmtB-mediated K⁺ uptake were measured in K⁺-depleted kdp trk kup cells. Strain genotypes were wild-type (○), amtB-null (□), H168D (▴), H318D (●), and H168D,H318E (■). The K⁺ uptake activity of the prototrophic E. coli K-12 parental strain NCM3722 (31) is shown (solid red squares). B, initial (1 min) rates of K⁺ uptake were measured in K⁺-depleted kdp trk kup glnK cells expressing AmtB^H318D (●) and AmtB^H168D,H318E (■). Values reported in A and B are means ± S.D. (error bars) for at least three independent experiments. Kinetic constants for K⁺ transport determined from the data shown in A are reported under “Results” as means ± S.D. Half-saturation constant values and maximal transport rates determined from the data shown in B were 26 ± 9.1 mm and 260 ± 8.5 nmol/ml per A₆₀₀ per min for AmtB^H318D and 60 ± 21 mm and 230 ± 85 nmol/ml per A₆₀₀ per min for AmtB^H168D,H318E.