The L -arabinose operon , also called the ara or araBAD operon , is an operon that encodes enzymes needed for the catabolism of arabinose in Escherichia coli . It has both positive and negative regulation and is activated allosterically .
The structural genes , which encode enzymes for arabinose catabolism, are araB , araA , and araD (collectively known as araBAD ). The regulator gene is araC . The genes araBAD and araC are transcribed in opposite directions. 
- araA encodes L-arabinose isomerase , which catalyzes isomerization between L-arabinose and L-ribulose .
- araB encodes ribulokinase , which catalyzes phosphorylation of ( L / D )- ribulose to form ( L / D )- ribulose 5-phosphate .
- araD encodes L-ribulose-5-phosphate 4-epimerase , which catalyzes epimerization between L - ribulose 5-phosphate and D - xylulose 5-phosphate .
D -xylulose 5-phosphate and D -ribulose-5-phosphate are metabolites in the pentose phosphate pathway .
The operators are araI and araO 2 . The operators lie between the AraC .
AraI lies between the structural genes and the operator. The araI 1 and araI 2 are DNA-binding sites that, when occupied by AraC, induce expression.
(Distances not to scale)
The ara operon is regulated by the AraC protein . If arabinose is absent, the dimer AraC protein represses the structural gene by binding to araI 1 and araO 2 and the DNA forms a loop. The loop prevents RNA polymerase from binding to the promoter of the ara operon, thereby blocking transcription .
When arabinose is present, arabinose binds AraC and prevents AraC from interacting. This breaks the DNA loop. The two AraC-arabinose complexes bind to the araI 1 and araI 2 sites which promotes transcription. When arabinose is present, AraC acts as an activator.
|L - arabinose||AraA||isomerase||yes||L -ribulose|
|L - ribulose||AraB||ribulokinase||no||L -ribulose-phosphate|
|L -ribulose-phosphate||AraD||epimerase||yes||D -xylulose-phosphate|
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: CRP (formerly known as CAP) + cyclic AMP
So the activation depends on the presence of arabinose and cAMP.
- The lac operon is another operon in E. coli that has been a focus for research in gene expression.