Figure 3.

Dynamic simulation of the tryptophan system. (a) Tryptophan network response in the absence of tryptophan in the medium. t_r and t_s represent the rise time and the settling time, respectively, of the network response. x and y represent the amplitudes of the first and second oscillation. The tryptophan concentration rises rapidly to its in vivo steady-state value of 4.2 μM in approximately 4 min. It subsequently settles to its steady-state value in approximately 30 min. The horizontal dotted line represents the ±5% band about the target value within which the response is considered acceptable. (b) The solid curve represents the physiological response of the network indicated by O^* and is reproduced from (a). The dashed curve, indicated by O, represents the response without attenuation and inhibition (i.e. single feedback loop design) and with parameters of genetic regulation altered (η_H1=0.5, K_i,1=8×10⁻⁸ μM) to yield a sub-sensitive regulator.