Figure 4.

ICF design describes l-arabinose utilization system. (A) The structure of P_BAD promoter in l-arabinose utilization system. In the absence of arabinose, a loop between O₂ and I₂ binding sites is formed through AraC, which prevents RNA polymerase from accessing the promoter. When arabinose is present, the loop is released and AraC binds to I₁ and I₂ sites. This leads to RNA polymerase (RNAP) binding to DNA sites (-35, -10) and the initiation of transcription. (B) A diagram model for AraC and P_BAD promoter showing that the system resembles an ICF network. On the one hand, the arabinose acts as an input to activate the P_BAD by forming arabinose–AraC complex. On the other hand, the free AraC represses the P_BAD promoter and is equal to the total concentration of AraC (AraC_T) minus the arabinose-AraC complex concentration. (C) The measured transfer function of wild-type P_BAD and synthetic P_BAD (P_BADsyn). The synthetic P_BADsyn contains only I₁ and I₂ binding sites without O₂ DNA sites. AraC is expressed by a constitutive promoter, encoded on a medium-copy-number plasmid (MCP). The synthetic P_BADsyn and wild-type P_BAD promoters regulate green fluorescent protein (GFP), encoded on a high-copy-number plasmid (HCP). The dotted lines are Hill function fittings. All experimental data are averaged from three experiments.