What are the advantages and/or disadvantages of using TOP10, DH5α, or other cloning strains versus BL21 Star (DE3) or BL21 (DE3) cells for expression with the pTrc system?
Top10, DH5α, other cloning strains
Advantages:
- Saves time, can go directly from cloning to expression.
- The glycerol stock is more stable because these strains are endA- and recA-.
Disdvantages:
- If GOI is toxic, the cloning step can be difficult.
- These cloning strains are not protease-deficient; therefore, the protein may be degraded.
BL21 Star(DE3) or BL21 (DE3)
Advantages:
- These expression strains are protease-deficient.
- You have to transform the plasmid into an expression strain.
- The glycerol stock may be unstable because these expression strains are not endA- and recA-.
- The (DE3) part is wasted because the promoter does not need T7 RNA polymerase.
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What competent cell strain can I use for expression with my pTrc Expression System?
The pTrc promoter is recognized by E. coli RNA polymerase, not T7 polymerase, and therefore can be expressed in any E. coli strain, not just BL21 strains. Therefore, you can use Top10, DH5α, etc. for expression. However, if your gene is toxic, the cloning step can be difficult as there is leakiness in expression.
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How does glucose repress the pTrc promoter?
A transcriptional activator protein called CAP (catabolite activator protein) normally binds upstream of the trc promoter and activates transcription. This protein requires cAMP to bind the DNA. Adding glucose to the medium can reduce intracellular cAMP levels. Supplementing LB medium and agar plates with glucose will repress basal level transcription from the trc promoter. We recommend that you include 25 mM, 0.5% glucose in the selection medium to ensure stability of your insert.
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What are the benefits of the pTrc expression system?
The pTrc promoter in the pTrc system is a strong hybrid promoter composed of the -35 region of the trp promoter and the -10 region of the lacUV5 promoter/operator. Expression of pTrc is repressed by the LacI protein and induced by IPTG.
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Do I need to include a ribosomal binding site (RBS/Shine Dalgarno sequence) or Kozak sequence when I clone my gene of interest?
ATG is often sufficient for efficient translation initiation although it depends upon the gene of interest. The best advice is to keep the native start site found in the cDNA unless one knows that it is not functionally ideal. If concerned about expression, it is advisable to test two constructs, one with the native start site and the other with a Shine Dalgarno sequence/RBS or consensus Kozak sequence (ACCAUGG), as the case may be. In general, all expression vectors that have an N-terminal fusion will already have a RBS or initiation site for translation.
Find additional tips, troubleshooting help, and resources within our Protein Expression Support Center.