Figure 1. Selection of Hxk2p variants.

The selection of Hxk2p variants was performed in anaerobic glucose limited chemostat cultivation with feed containing 5 g L⁻¹ glucose and 50 g L⁻¹ xylose. After 96 h of continuous cultivation of TMB3463 transformed with the HXK2-library at D = 0.072 h⁻¹, the dilution rate was increased to D = 0.40 h⁻¹ (indicated as t = 0 h). During the washout the specific growth rate was calculated from the equation d ln(OD) dt ⁻¹ =  μ _max−D. The natural logarithm of OD is shown as red squares (▪). The washout profile displays two growth phases and the switching point occurs when the residual glucose concentration (▴) exceeds 3 g L⁻¹. After 10 h the glucose concentration stabilized at 4 g L⁻¹ which reduced the selection pressure by inhibiting the uptake of xylose and thus slowed down the washout of cells. The measured accumulation of glucose was less than the theoretical (dashed line), calculated according to S =  S _in+(S ₀−S _in)·e^(−D·t), showing that the consumption was not negligible.