FIGURE 3.

Growth of AmtB twin-histidine variant strains at low NH₃. Strain growth in low NH₃ medium (0.5 mm NH₄Cl, 0.1% glucose (pH 5.5)) containing 21 mm K⁺ (A) or 0.1 mm K⁺ (B) is shown. The absence of AmtB-mediated conductance of ammonium under these culture conditions results in a pronounced growth defect (3, 37). Strain genotypes were wild-type (○), amtB-null (□), H168D (▴), H318D (●), and H168D,H318E (■). Strain doubling times (in minutes) derived from these growth curves are as follows: 48 and 47 for wild-type at K⁺ concentrations of 21 mm and 0.1 mm, respectively; 320 and 210 for amtB-null at K⁺ concentrations of 21 mm and 0.1 mm, respectively; 55 and 53 for H168D at K⁺ concentrations of 21 mm and 0.1 mm, respectively; 55 and 55 for H318D at K⁺ concentrations of 21 mm and 0.1 mm, respectively; 280 and 200 for H168D,H318E at K⁺ concentrations of 21 mm and 0.1 mm, respectively. The data, each from single experiments, are representative of findings made in at least three independent trials.