Figure 2.

Activation dynamics for the positive autoregulation model described in Box 2. The model reflects the presence of an inducible promoter, which gives rise to a positive feedback loop, and a constitutive promoter (Fig. 1G). The assignment K = ∞ in the model renders the inducible promoter constitutively active, whereas the assignment k₂ = 0 makes the inducible promoter inactive; both of these assignments result in constitutive synthesis of the regulator (no feedback). Regardless of the presence of feedback, the initial (pre-activation) state of the system is its steady state under non-activating conditions. This was implemented in the simulations by solving the algebraic equations for the steady state of the model under non-inducing conditions. These equations had a unique real solution which was used to define the state of the system before and at the time of activation. When the system is activated (in our example, at 0 minutes), it experiences an instantaneous 5-fold increase in the regulator phosphorylation rate (k_a). The post-activation temporal dynamics was simulated by numerically solving the differential equations given in Box 2.