Question

Which of the following is true regarding the araBAD operon?

• expression of araC occurs in concert with the expression of araB, araA and araD

• binding of arabinose causes dimerization of AraC allowing it to bind to araO2 and araI1

•   when bound to arabinose, the AraC dimer must also bind to a CAP-cAMP dimer for expression to occur

•   AraC bound to arabinose can bind to CAP regardless of whether or not cAMP is present

Answer 1

When bound to arabinose, the AraC dimer must also bind to a CAP-cAMP dimer for expression to occur.

Arabinose is a five-carbon sugar that is found widely in nature and can serve as a sole carbon source in many bacteria. When arabinose is present in the environment, bacteria take it and on reaching inside, the arabinose interacts directly with AraC, which is bound to the DNA. The interaction causes AraC to change shape, which in turn helps RNA polymerase bind the promoter, and the three genes araB, araA, and araD are transcribed. Transcription is activated at araI, the initiator region, which contains both an operator site and a promoter. The araC gene encodes an activator protein that, when bound to arabinose, activates transcription of the ara operon, perhaps by helping RNA polymerase bind to the promoter, located within in the araI region.This is mediated by CAP–cAMP complex. In the presence of arabinose, both the CAP–cAMP complex and the AraC–arabinose complex must bind to the initiator region in order for RNA polymerase to bind to the promoter and transcribe the ara operon.