Multi-level effects of the <i>lac</i> operon.

<p><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0042790#s3" target="_blank">Results</a> from a model of the <i>lac</i> operon that considers the states of individual cells as well as the population as a whole. A) Bimodal state distributions including the external inducer concentration and level of <i>lac</i> permease, . Since the model does not explicitly include an indicator, was used as a proxy measure. Low and high external inducer concentrations bias the population toward an uninduced or induced state respectively, and all concentrations see coexistence of states in the form of a bimodal distribution of . Dashed line indicates the overall population induction (average) that would be measured by purely observing at the population level, i.e., not taking into account the bimodal distribution of individual states. B) Effect of growth rate on coexistence of induced and uninduced states within the population. Line color indicates external inducer concentration, , (yellow = 30 M, green = 80 M, blue = 110 M,), solid and dashed lines indicate simulations where induction did and did not inhibit growth respectively. C) Bifurcation diagram showing bistability in the intracellular inducer concentration, . Red line illustrates the equilibrium state of in the deterministic GRN equations for a single cell as a function of external inducer, , computed via numerical continuation (solid and dashed lines indicate stable and unstable equilibrium respectively); blue line illustrates ensemble average concentration in a BSim simulation which incorporates this deterministic GRN and stochastic agent creation and removal in which was slowly varied (dashed lines indicate population minimum and maximum).