Arabinose Operon

The arabinose operon features a single protein which acts as both a positive and a negative transcriptional regulator, depending upon the binding of particular ligands (Figure 26.39).

Three structural genes are located in the arabinose operon--araB, araA, and araD. They encode enzymes that convert arabinose to xylulose-5-phosphate.

Regulation - The araC gene encodes a regulatory protein that binds to arabinose, the inducer of the operon. Binding of the AraC - arabinose complex at a site called araI activates transcription of the arabinose operon, but only when the cAMP - CRP complex (see here) is bound at an adjacent site. Thus, whereas the lac operon requires one protein to be bound and one to be dissociated for maximal transcription, the arabinose operon requires two proteins to be bound at adjacent sites.

When arabinose levels are low, the AraC protein acts as a repressor and binds to two operator sites, araO 1 and araO 2, as well as to araI. Binding to araO 1 inhibits transcription of the araC gene itself. Thus, araC is autoregulated at the level of its own transcription. AraC molecules bound to araO 2 and araI interact with each other to form a DNA loop. This structure causes repression of transcription of the araBAD genes.

See also: Lactose Operon Regulation, Transcription Regulation in Phage, Galactose Operon, trp Operon Regulation