FIG. 2.

Increased K⁺ (Rb⁺) uptake due to QDR2 expression and inhibition of K⁺ (Rb⁺) uptake by quinidine are independent of the Trk1p/Trk2p system. (A) Effect of QDR2 expression on growth curves of the S. cerevisiae DBY746 Δtrk1 Δtrk2 mutant (full line and black symbols) and the corresponding DBY746 Δtrk1 Δtrk2 Δqdr2 mutant (dotted line and white symbols) under potassium limitation. The growth media were prepared using ammonium phosphate medium supplemented with 0.5 mM (• and ○), 5 mM (□ and ▪), and 100 mM (▴ and ▵) of KCl. The cells used as inoculum were exponential-phase cells cultivated in the presence of 100 mM of KCl. (B and C) Effect of QDR2 expression on the time course of Rb⁺ uptake in the DBY746 Δtrk1 Δtrk2 mutant (○ and •) and in the respective Δtrk1 Δtrk2 Δqdr2 mutant (□ and ▪), in the presence or absence of quinidine. At time zero, (B) 50 mM RbCl or (C) 100 mM RbCl was added to the uptake buffer, in the absence (B and C, control) or presence of quinidine (C, 2.1 mM and 4.2 mM). Cell samples were taken during incubation and internal Rb⁺ was determined, as described in Materials and Methods. Data are means ± standard deviations for three independent experiments. Qd, quinidine.