Arabinose utilization operon

Control by looping. The arabinose utilization operon of E. coli, araBAD, encodes proteins needed for uptake of arabinose and conversion to D-xylulose 5-P. The repressor AraC in the absence of arabinose binds at operator 1 (Oj) to prevent further synthesis of repressor (autorepression) and also at the aral region to repress transcription of operon araBAD. The operator 2 (02) site, which is 211 bp upstream from aral, is also needed for this repression.145-14713 A loop is apparently formed by repressor binding (Fig. 28-7). Binding of arabinose to the repressor converts it into an activator, which stimulates initiation of transcription at the BAD promoter. Further stimulation is provided by the CAP-cAMP complex, which binds next at aral. 

The arabinose (ara) operon in E. coli utilizes some of the same regulatory principles as the lac operon, but three additional mechanisms are also found. First, in the ara operon, the expression of a transcription factor gene is autoregulated by it s own gene product (AraC controls araC expression). Second, a transcription factor can function as a repressor or an activator in an effector-dependent manner (arabinose changes AraC from a repressor into an activator). Third, transcription factors often function together to effect a change in transcriptional initiation (AraC and CAP must both be bound to fully induce the ara operon). 

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