In the absence of arabinose, what happens to transcription from PBAD?

In the absence of arabinose, what happens to transcription from PBAD?

• A. PBAD is constitutively active
• B. PBAD is repressed and transcription is not activated
• C. RFP is still produced
• D. AraC degrades PBAD

Answer: (B)

Arabinose-dependent control of transcription is managed by the AraC regulator. In the absence of arabinose, AraC binds to two sites near PBAD and forms a DNA loop that blocks RNA polymerase from initiating transcription. So transcription from PBAD is not activated, and genes like RFP aren’t expressed. When arabinose is present, it binds AraC, triggering a change that relieves this repression and allows RNA polymerase to access PBAD, turning transcription ON and producing the reporter protein. The idea that PBAD is always active is incorrect, and the notion that AraC degrades the promoter isn’t how this system works.

Arabinose-dependent control of transcription is managed by the AraC regulator. In the absence of arabinose, AraC binds to two sites near PBAD and forms a DNA loop that blocks RNA polymerase from initiating transcription. So transcription from PBAD is not activated, and genes like RFP aren’t expressed. When arabinose is present, it binds AraC, triggering a change that relieves this repression and allows RNA polymerase to access PBAD, turning transcription ON and producing the reporter protein. The idea that PBAD is always active is incorrect, and the notion that AraC degrades the promoter isn’t how this system works.