Figure 3.

Activity of rtTA_{G19F E37L} variants with all possible amino acids at position 56. (A) The activity of rtTA was measured in HeLa X1/6 cells, see Fig. 2 for details. All variants contain the G19F, E37L, F86Y and A209T mutations in combination with different amino acids at position 56. The wild-type rtTA (wt) with only the F86Y and A209T mutations was included as a control, of which the activity at 1000 ng/ml dox was arbitrarily set at 100%. Average values of two transfections are shown with the error bars indicating the standard deviations. (B) Codon table of rtTA_{G19F E37L} variants with all possible amino acids at position 56. The corresponding codons of inactive variants are marked in black, dox-dependent variants are in green, and variants that are active without dox are in red. See the text for details.