L-arabinose operon

The L-arabinose operon of the model bacterium Escherichia coli has been a focus for research in molecular biology for over 40 years, and has been investigated extensively at the genetic, biochemical, physiological, and biophysical levels. It is controlled by a dual positive and negative system. There are 3 structural genes: araB, araA, and araD. They encode the metabolic enzymes for breaking down the monosaccharide sugar arabinose into D-xylulose-5-phosphate, which is then metabolised via the pentose phosphate pathway. The initiator region, containing an operator site as well as a promoter, is called araI (the last letter of araI is an uppercase letter " i "). Near this site lies the araC gene, which encodes an activator protein. The AraC protein binds to initiator region araI.

Activation

If arabinose is present, it builds a complex: AraC + arabinose

This complex is needed for RNA polymerase to bind to the promoter and transcribe the ara operon. Also for activation the binding of another structure to araI is needed: CAP + cyclic AMP

So the activation depends on the presence of arabinose and cAMP.

Repression

If arabinose is absent, the AraC protein assumes a different conformation. In this conformation the AraC protein binds as well to araI as to ara0 region. Now the DNA is forming a loop. This prevents transcription of the ara operon.

Sequence of the Operon:

araC : araO : araI : araB : araA : araD