Figure 1.

(a) Schematic of three processes (P_i) in series. For the tryptophan system, P₁, P₂, P₃, represent transcriptional ‘process’, translational ‘process’ and tryptophan synthesis, respectively. (b) Schematic of the multiple feedback loop strategy for three-processes-in-series system. For the tryptophan system, C₁ represents genetic regulation, C₂ represents transcriptional attenuation and C₃ represents enzyme inhibition. (c) Schematic of the single feedback loop strategy. In the tryptophan system, such an architecture would arise only if the genetic regulation was operational.