Regulation of the ara operon. (a) When arabinose is absent, AraC forms a dimer in which one monomer binds to araO2 and the other to araI1, preventing RNA polymerase binding and transcription of the operon. (b) Activation of the ara operon occurs when AraC binds arabinose (its small-molecule effector) and CRP-cAMP (formed in the absence of glucose). The AraC dimer changes conformation such that one monomer binds araI1 and the other binds araI2. The interaction with araI2 recruits RNA polymerase to the promoter and activates transcription of the ara operon. The molecular models show the AraC dimerization domain in the (a) absence and (b) presence of arabinose.