Fractional phosphorylation of RsbR and RsbS during stress. Comparison of mean and variance of RsbR (triangles) and RsbS (squares) phosphorylation levels during NaCl or ethanol induction and simulation results (lines). The experimental data were extracted from Kim et al. (2004) . The three different models for simulation are: the product activation model (continuous line), the substrate activation model (dashed line), and the no-cooperativity model (dashed-dotted line). Stress is simulated assuming an increase in the phosphorylation probability of RsbR, kphr, to 1. In the simulation stress is stopped at 5 min by reversion of kphr to the according stress-free value of 0.1.