Figure 3.

Present-day model for regulation of the ara operon. (A) In the absence of arabinose, the AraC N-terminal dimerization domain is constrained and favors binding of AraC to distant sites, forming a DNA loop between the araI1 and araO2 sites. (B) In the presence of arabinose, a reorientation of the dimerization domain favors AraC binding to two adjacent half sites at araI where it stimulates transcription from the araBAD promoter (pBAD). CAP (catabolite activator protein) bound to cAMP binds adjacent to AraC and likely makes separate contact with RNA polymerase (RNAP) to stimulate transcription. AraO1 is an operator where AraC binds to repress its own transcription from the araC promoter (pC). Figure reprinted from Schleif (2010).