Practice question

The araBAD operon exemplifies dual positive control integrating substrate availability and catabolite status. In absence of arabinose, AraC represses by looping DNA between araO2 and araI1, occluding RNA polymerase. Arabinose binding converts AraC into an activator that occupies araI1 and araI2, contacting polymerase and inducing bending. Maximal transcription additionally needs glucose starvation signaling. Elevated cAMP binds CAP, which recognizes site upstream of araI, enhancing polymerase alpha-CTD interaction. Both arabinose and CAP-cAMP together relieve looping and stimulate initiation, enabling catabolism only when preferred sugars are depleted.