In many bacterial expression vectors, what is the role of the T7 promoter and lac operator?

In many bacterial expression vectors, what is the role of the T7 promoter and lac operator?

• A. Lac operator sequences control plasmid copy number.
• B. T7 promoter is a transcription terminator; lac operator enables constitutive expression.
• C. T7 promoter drives high-level transcription by T7 RNA polymerase; lac operator provides inducible control to regulate expression.
• D. T7 promoter recruits RNA polymerase II to transcribe; lac operator acts as a terminator.

Answer: (C)

High-level, controllable expression in bacteria often relies on pairing a promoter that is driven by a specific, highly active RNA polymerase with a regulatory element that can be turned on or off. The T7 promoter fits that first part because it is specifically recognized by T7 RNA polymerase, which is highly processive and generates a strong burst of transcription. In many expression systems, the host provides T7 RNA polymerase only when you want expression, typically under control of an inducible promoter regulated by LacI. The lac operator adds the inducible control. It is a DNA sequence that binds the LacI repressor; in the absence of inducer, LacI blocks transcription, keeping expression off. When you add an inducer like IPTG, LacI is inactivated, allowing transcription to proceed. In practice, IPTG induction leads to production of T7 RNAP, which then drives high-level transcription from the T7 promoter on the plasmid, yielding strong expression of the target gene. This combination is not about controlling copy number or acting as a terminator, and it is not about RNA polymerase II—it's specifically about using a strong T7-driven transcription with inducible, LacI-mediated control.

High-level, controllable expression in bacteria often relies on pairing a promoter that is driven by a specific, highly active RNA polymerase with a regulatory element that can be turned on or off. The T7 promoter fits that first part because it is specifically recognized by T7 RNA polymerase, which is highly processive and generates a strong burst of transcription. In many expression systems, the host provides T7 RNA polymerase only when you want expression, typically under control of an inducible promoter regulated by LacI.

The lac operator adds the inducible control. It is a DNA sequence that binds the LacI repressor; in the absence of inducer, LacI blocks transcription, keeping expression off. When you add an inducer like IPTG, LacI is inactivated, allowing transcription to proceed. In practice, IPTG induction leads to production of T7 RNAP, which then drives high-level transcription from the T7 promoter on the plasmid, yielding strong expression of the target gene. This combination is not about controlling copy number or acting as a terminator, and it is not about RNA polymerase II—it's specifically about using a strong T7-driven transcription with inducible, LacI-mediated control.