The Arabinose Operon

• Structural genes (araB, araA and araD) encode enzymes that break down arabinose.

• The initiator region contains both an operator site (araO), and promoter site (araI).

• AraC gene encodes for an activator protein.

a)  Presence of Arabinose

• AraC protein and the CAP-cAMP complex bind to the araI region at different sites

• Transcription can occur with these proteins in place. 

b) Absence of Arabinose

• AraC binds to both the araI and araC regions to form a DNA loop that prevents transcription

AraC has two conformations.  One acts as an activator, when bound by arabinose, allowing transcription to take place and the other acts as a repressor protein when unbound.