Figure 4

Protein noise is minimized at intermediate stress levels. Protein CV is shown for moderate (a) and low protein copy numbers (b) as a function of the stress level α. In (a) we compare the LNA estimates with those of the IOS method for moderate copy numbers using parameter set (i) in Table 1. Both methods predict that the CV reaches a minimum below the crossover point where the protein expression is activated. The prediction is qualitatively confirmed by the SSA. Notice that the IOS method predicts larger values for the minimum CV and larger values of α at which it is attained; this is also observed using the SSA. In (b) we compare the CV of protein concentration obtained using the LNA and IOS methods for low copy numbers using parameter set (ii) in Table 1. The LNA shows the same dependence as in (a) with increased noise levels. In contrast, the IOS method predicts a monotonic increase in the noise levels as the stress level is increased. The predictions of the IOS method are qualitatively confirmed by stochastic simulations.