Which enzyme is required to transcribe from a T7 promoter in common bacterial expression strains?

Which enzyme is required to transcribe from a T7 promoter in common bacterial expression strains?

• A. T7 promoter works with E. coli RNA polymerase.
• B. T7 RNA polymerase is required to transcribe from a T7 promoter.
• C. T7 promoter can be transcribed by any bacterial RNA polymerase.
• D. T7 promoter acts as a replication origin.

Answer: (B)

The key idea is that transcription from a T7 promoter requires a specific enzyme: T7 RNA polymerase. This promoter is tailored for the phage enzyme and is not efficiently recognized by the host’s RNA polymerase. In common bacterial expression strains, you provide T7 RNA polymerase (often from a DE3 lysogen or a plasmid under an inducible promoter) so that, when you induce, the T7 RNAP binds the promoter and drives strong transcription of the downstream gene. Without T7 RNA polymerase, the host RNA polymerase won’t initiate well at the T7 promoter, so expression would be very low or absent. The T7 promoter isn’t a replication origin, so it doesn’t drive DNA replication.

The key idea is that transcription from a T7 promoter requires a specific enzyme: T7 RNA polymerase. This promoter is tailored for the phage enzyme and is not efficiently recognized by the host’s RNA polymerase. In common bacterial expression strains, you provide T7 RNA polymerase (often from a DE3 lysogen or a plasmid under an inducible promoter) so that, when you induce, the T7 RNAP binds the promoter and drives strong transcription of the downstream gene. Without T7 RNA polymerase, the host RNA polymerase won’t initiate well at the T7 promoter, so expression would be very low or absent. The T7 promoter isn’t a replication origin, so it doesn’t drive DNA replication.