Dual control of the ara operon. (a) In the presence of arabinose, the AraC protein binds to the araI region and, when bound to cAMP, the CAP protein binds to a site adjacent to araI. This binding stimulates the transcription of the araB, araA, and araD genes. (b) In the absence of arabinose, the AraC protein binds to both the araI and araO regions, forming a DNA loop. This binding prevents transcription of the ara operon.
From: Dual positive and negative control: the arabinose operon