The ara operon
The Arabinose operon
Structure of the ara operon
The ara operon contains three structural genes: araB, araA, and araD. These genes code for enzymes used in the catabolism of arabinose. These genes are transcribed from the PBAD promoter. The regulatory gene, araC is close upstream and transcribed divergently from Pc The regulatory region has RNA polymerase binding sites for both promotors, three binding sites for AraC dimers (araI, araO1, and araO2) and a site for cAMP-CAP binding.
Absence of arabinose
When arabinose is absent, AraC protein binds to araI and araO2. Since these sites are about 200 base pairs apart, the DNA between them is bent into a closed loop by the AraC interaction. AraC thus acts negatively because RNA polymerase is unable to bind to its site in PBAD
Presence of arabinose
When arabinose is present, it binds to AraC and causes a change in conformation. As a result, the AraC complex is broken down and AraC now facilitates RNA polymerase binding to PBAD. AraC is now acting as a positive regulator. Positive control by cAMP-CAP binding is required for activation of the ara operon, like in the lac operon