Figure 6.

¹³N-ammonium transport by AtAMT1;1 and AtAMT2 expressed in the yeast. A, Time course of 100 μm ¹³N-ammonium uptake at external pH 6.1 or 7.5. Yeast strain YCW012 was transformed with: pYES3 (●), pAMT2 (○), or pAMT1;1 (▾), and uptake measured at pH 6.1. Uptake was also measured at pH 7.5 for yeast expressing pAMT2 (▿). B, Double reciprocal plots of initial uptake versus ammonium concentration at different external pH. Strain YCW012 was transformed with the following plasmids and assayed at the pH indicated: pAMT1;1 at pH 5.0 (●), pAMT1;1 at pH 6.1 (○), pAMT1;1 at pH 7.5 (▾), pAMT2 at pH 5.0 (□), pAMT2 at pH 6.1 (▪), and pAMT2 at pH 7.5 (▿). The symbols for pAMT1;1 at pH 5.0 and 6.1 partially overlap.