The araBAD operon in E. coli contains three genes which encode proteins involved in the metabolism of L-arabinose into a form useable by the pentose phosphate pathway. The operon can be repressed by the arabinose operon regulatory protein, a protein dimer that is produced in the cell. When arabinose is not present, it binds to the DNA in two specific locations, forming a loop in the DNA and preventing transcription from occurring. Inversely, the operon can be activated by the same arabinose operon regulatory protein in the presence of arabinose. In this case, the arabinose binds to the protein dimer, preventing it from forming the DNA loop and instead binding to the activator binding site upstream of the promoter. This binding helps RNA polymerase to bind and transcribe the operon. Finally, the araBAD operon functions like the lac operon, which only produces its enzymes in the absence of glucose. This is regulated by the cAMP-activated global transcriptional regulator (CRP). In the absence of glucose, cAMP levels build up in the cell, and it can bind to and activate CRP. The presence of both activated CRP and the arabinose operon regulatory protein promote the binding of RNA polymerase. The first gene in the operon, araB, encodes ribulokinase, an enzyme that irreversibly converts L-ribulose to L-ribulose-5-phsophate. The second gene, araA, encodes L-arabinose isomerase, an enzyme that convertsz L-arabinose to L-ribulose. This enzyme is the first step in the metabolism of arabinose. In between araA and araD are several non-coding extragenic sites. Finally, the last gene in the operon, araD, encodes L-ribulose-5-phosphate 4-epimerase, a protein that converts L-ribulose-5-phosphate to D-xylulose-5-phosphate, which then is used in the pentose phosphate pathway.

ara Operon References