Gene regulation in prokaryotes/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 P_BAD promoter. The regulatory gene, araC is close upstream and transcribed divergently from P_c 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 P_BAD

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 P_BAD. 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