Figure 4.

Regulating bistability of the memory device. (A) The calculated rate of change of the autofeedback activator concentration versus the total activator concentration for the LexA memory device. Two decay rates, calculated using Equation 2 with τ = 90 min (blue line) and 120 min (orange line), predict the effect of decreasing the growth rate on bistability of the autofeedback loop. The production rate versus activator concentration (black line) is as in Figure 3B. (B) DIC and fluorescence images of cells carrying the LexA memory device that were grown in inducer, then to steady state in glucose with a doubling time of 90 or 120 min. (C) The predicted rate of change of autofeedback activator concentration as a function of the total activator concentration for the LexA memory device and the dual autofeedback memory device. The production rates were calculated using Equation 1, the values of n and K for the LexA activator–reporter strain, and either β (black line) or 2*β (orange line) for the LexA autofeedback activator corresponding to one and two copies of the autofeedback gene, respectively. The decay rate (blue line) corresponds to a doubling time of 90 min. (D) The frequency distribution of RFP and YFP intensities from the LexA memory device strain with one and two copies of the autofeedback gene grown in glucose after induction.