Question

At the promoter for the Ara pBAD operon, how does the DNA loop cause repression?

A. by creating torsional strain in the DNA at the promoter which prevents the transition from closed to open promoter complex.

B. by blocking the binding of AraC protein

C. by blocking access to the core promoter by RNA polymerase

D. by causing RNA polymerase to bind to the "dead-end" promoter

Answer 1

DNA looping at the Ara pBAD operon represses gene expression by blocking access to the core promoter for RNA polymerase, preventing transcription initiation. Option C

Step-by-step explanation:

At the promoter for the Ara pBAD operon, repression is achieved through the formation of a DNA loop involving the AraC protein. When the loop is formed, it can prevent the necessary interactions for transcription initiation. The correct answer to this question is C. by blocking access to the core promoter by RNA polymerase.

The looping mechanism can block RNA polymerase from accessing the core promoter region, thus preventing the transition from a closed to open promoter complex and halting the initiation of transcription.

It is analogous to how, in the regulation of the lac and trp operons, active repressor proteins can bind to the operator region and block RNA polymerase from transcribing the downstream genes, thereby controlling gene expression based on the availability of specific molecules like tryptophan.

So Option C is correct.