Figure 1.

The expression of lactose permease in E. coli. (A) The repressor LacI and permease LacY form a positive feedback loop. Expression of permease increases the intracellular concentration of the inducer, TMG, which causes dissociation of LacI from the promoter, leading to even more expression of permeases. Cells with a sufficient number of permeases will quickly reach a state of full induction, while cells with too few permeases will stay uninduced. (B) After 24 hours in M9 media containing 30 µM TMG, strain SX700 expressing a LacY-YFP fusion exhibits all-or-none fluorescence in a fluorescence-phase contrast overlay. Fluorescence imaging with high sensitivity reveals single molecules of permease in the uninduced cells zoomed in on the red box from B. (C) After one day of continuous growth in media containing 0 to 50 µM TMG, the resulting bimodal fluorescence distributions show that a fraction of the population exists either in an uninduced or induced state, with the relative fractions depending on the TMG concentration. (D) The distributions of LacY-YFP molecules in the uninduced fraction of the bimodal population at different TMG concentrations, measured with single-molecule sensitivity, indicate that one permease molecule is not enough to induce the lac operon, as previously hypothesized (12). Over 100 cells were analyzed at each concentration. Error bars are standard errors determined by bootstrapping.