Input–output relationship for Module 1. (a) Block diagram. (b) Nullcline associated to the subsystem composed of ODEs (1)–(4) together with the ODE obtained by summing up Eqs. (5) and (8)–(11). On the x- and y-axis are reported the amount of free SigE and the total concentration of SigE, respectively. The shading removes a region without biological meaning, where free SigE would be larger that total SigE .