Figure 4.
LacIWF outperforms WT at 1 mM IPTG. Cells bearing the WT repressor (dark blue/blue/cyan) or the W220F mutant (dark red/red/brown) were incubated in the absence (open symbols) of inducer or in the presence of either 1 mM or 0.01 mM IPTG (dark blue/dark red and blue/red, respectively). After overnight incubation (panel A), the GFP production levels of WT and mutant are indistinguishable by flow cytometry (but WT shows a marked decrease in fluorescence at the highest IPTG concentration). Although the final expression levels are similar, at early time points (panel B) and with 0.01 mM IPTG, LacIWF shows a lag in induction relative to the WT. Background expression levels are consistently lower for the mutant, which shows no leakiness even before induction (note the different fluorescence values for blue and red lines in the grey-shaded area of panel B).