What happens when bacteria with the pARA-R plasmid are not given arabinose?

What happens when bacteria with the pARA-R plasmid are not given arabinose?

• A. The rfp gene cannot be expressed unless arabinose; only in presence of arabinose will AraC turn on promoter.
• B. The rfp gene is expressed constitutively.
• C. AraC activates promoter without arabinose.
• D. The rfp gene is overexpressed.

Answer: (A)

This question tests how the arabinose-inducible system controlled by AraC regulates transcription. Without arabinose, AraC binds to the promoter in a way that blocks RNA polymerase, keeping the rfp gene from being transcribed. In other words, the reporter cannot be expressed unless arabinose is present. When arabinose is added, AraC binds the sugar, changes shape, and activates transcription from the promoter, allowing rfp expression. So, in the absence of arabinose, the system stays off, and only with arabinose does expression occur.

This question tests how the arabinose-inducible system controlled by AraC regulates transcription. Without arabinose, AraC binds to the promoter in a way that blocks RNA polymerase, keeping the rfp gene from being transcribed. In other words, the reporter cannot be expressed unless arabinose is present. When arabinose is added, AraC binds the sugar, changes shape, and activates transcription from the promoter, allowing rfp expression. So, in the absence of arabinose, the system stays off, and only with arabinose does expression occur.