Expression vectors for bacterial overproduction In a protein overexpression plasmid designed for bacteria, the promoter driving the cloned gene is typically:
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AA bacterial promoter (often strong and regulatable, e.g., T7, tac, trc, lac, araBAD)
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BExpressed both in bacteria and mammalian cells without change
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CNot a regulated promoter (constitutive only)
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DThe natural (native) promoter of the gene being cloned in every case
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EA eukaryotic TATA promoter used directly in <em>E. coli</em>
Answer
Correct Answer: A bacterial promoter (often strong and regulatable, e.g., T7, tac, trc, lac, araBAD)
Explanation
Introduction / Context:Expression vectors used to overproduce proteins in bacteria rely on powerful, well-characterized bacterial promoters to achieve high transcription levels and tight control. Selecting the correct promoter type is critical for yield, solubility, and host viability.
Given Data / Assumptions:
- Goal: overproduce a heterologous protein in a bacterial host.
- Promoters must be recognized by bacterial RNA polymerase (or phage polymerases expressed in bacteria).
- Tight regulation is often needed to avoid toxicity.
Concept / Approach:Common bacterial expression systems use strong promoters such as T7 (driven by T7 RNA polymerase in hosts like BL21(DE3)), tac/trc (hybrids of trp and lac), lac, or arabinose-inducible araBAD. These are specifically optimized for bacterial machinery and frequently inducible for control. Using a gene’s native promoter is uncommon for overexpression because it may be weak or poorly regulated in the host. Cross-kingdom promoter compatibility is generally poor without engineered hybrid systems.
Step-by-Step Solution:Identify the host: bacteria require bacterial or phage-derived promoters recognized in bacteria.Recognize the need for strength and regulation.Select the option describing a bacterial (often inducible) promoter as standard.
Verification / Alternative check:Widely used systems (e.g., T7/lac operon control in BL21(DE3)) exemplify this approach and are standard in molecular cloning labs and industry.
Why Other Options Are Wrong:
- Dual expression in bacteria and mammalian cells with the same promoter is atypical.
- “Not regulated” is inaccurate; most systems are inducible.
- Native promoters often underperform and are less controllable.
- Eukaryotic TATA promoters are not directly functional in bacteria without extensive engineering.
Common Pitfalls:Forgetting the role of host-specific polymerases; assuming a native promoter is sufficient for high-level expression.
Final Answer:A bacterial promoter (often strong and regulatable, e.g., T7, tac, trc, lac, araBAD).