Fig 3.

Limitation of exponential growth (batch cultures) by low extracellular K⁺ in strains deleted of all three K⁺ transporter genes and then transformed with HA-tagged TRK1 and/or HAK1. Measured dry weights were grouped and averaged for similar plots. Under all conditions, growth became stably exponential for at least 8 h irrespective of the initial extracellular K⁺ concentration. Smooth curves are least-square-fitted exponentials; specific rate constants (top to bottom) were 0.274, 0.259, 0.182, and ∼0.08 h⁻¹, corresponding to doubling times of 2.53, 2.68, 3.81, and >8 h, respectively. Batch cultures of 150 ml were inoculated with 2 × 10⁶ conidia/ml in K⁺-free Vogel's salts plus 2% sucrose plus KCl as indicated, shaken at 25°C, filter harvested (40-ml aliquots) at 1.5- or 2-h intervals, and rinsed 3× in distilled water. Dry weights and cell potassium were measured as described in Materials and Methods. Experiments were run in duplicate and repeated 2 to 4 times for each strain and [K⁺]_o. The scatter of measured dry weights (±1 standard deviation [SD]) was ∼15% of the mean value at most points. The initial growth of such cultures, following harvesting and rinsing of conidia, is always slow and created a lag phase (∼4 h) prior to the plotted zero time. For simplicity, only the K⁺ transporter genes are indicated here. Table 1 lists full genotypes and strain names.