I'm having problems with protein solubility using a T7 promoter-based vector. What would you recommend to try?

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Answer

- Lower the induction temperature to 30 degrees C, 25 degrees C, or 18 degrees C to help increase solubility and reduce the formation of inclusion bodies. The lower the temperature, the more time needed to do the induction (i.e., 30 degrees C for 3-4 hours, 25 degrees C for 3-5 hours, or 18 degrees C for overnight).
- Grow at a higher temperature (30 degrees C or 37 degrees C) to reach the proper OD, add inducer, then shift to the lower temperature.
- Try different amounts of IPTG (1 mM-0.1 mM IPTG).
- Use a low copy number plasmid.
- Use a less rich medium, such as M9 minimal medium instead of LB.
- If the protein requires a cofactor, such as a metal, add the cofactor to the medium.
- Add glucose to 1%.
- Try the BL21-AI™ strain and use different amounts of arabinose.

Answer Id: E9735

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I am trying to express my protein using a T7 promoter-based vector. What does “leaky expression” mean?

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Leaky expression means there is some basal level expression seen. For example, in all BL21 (DE3) cell lines, there is always some basal level expression of T7 RNA polymerase. This “leaky expression” could lead to reduced growth rates, cell death, or plasmid instability if a toxic gene is cloned downstream of the T7 promoter.

Answer Id: E9722

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How can I make a glycerol stock of my desired construct?

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Answer

Once you have obtained your desired construct, we recommend that you store your clone as a glycerol stock. Please follow these steps to create a glycerol stock:

- Grow 1 to 2 mL of the strain to saturation (12-16 hours; OD600 = 1-2) in LB containing 50-100 μg/mL ampicillin
- Combine 0.85 mL of the culture with 0.15 mL of sterile glycerol
- Mix the solution by vortexing
- Transfer to an appropriate vial for freezing and cap
- Freeze in an ethanol/dry ice bath or liquid nitrogen and then transfer to –80 degrees C for long-term storage.

Answer Id: E9727

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Can I use carbenicillin in place of ampicillin in my transformation/T7 expression experiments?

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Yes; in fact, carbenicillin is generally more stable than ampicillin and may help to increase expression levels by preventing loss of the pET plasmids.

Answer Id: E9704

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I am using a bacterial expression vector containing the T7 promoter. At what temperature should I perform my induction?

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Induction can be performed at a variety of temperatures, ranging from 37 degrees C to 30 degrees C to room temperature. A lower temperature typically requires a longer growth time.

Answer Id: E9699

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I'm getting no expression from my bacterial expression vector, but my cells are growing normally. What should I do?

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Please view the possible causes and solutions to try:

- Frame shifts or a premature stop codon is present in the construct; check the sequence.
- The wrong cell strain was used for expression.
- If using a glycerol stock, the integrity of the plasmid can change because most cell strains for expression are not RecA and EndA-. Use freshly transformed cells.
- The protein is in the insoluble section; check cell lysates and not just the supernatant.
- Rare codons were used in the gene of interest: check the codon usage. (http://nihserver.mbi.ucla.edu/RACC)
- The cells may be kicking out the plasmid during culture: this is more common in plasmids that are ampicillin resistant. Try using carbenicillin instead of ampicillin in the medium; wash and resuspend the overnight culture with LB containing fresh amp/carb before inoculation.

Answer Id: E9736

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I'm trying to express a toxic protein using a T7 promoter-based vector. What competent cell strain would you suggest?

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Answer

The BL21 AI E. coli strain offers the tightest regulation of expression for production of toxic proteins using the T7 promoter. The BL21 AI line uses a completely different mechanism of induction from that of the traditional BL21 (DE3) lines. This cell line utilizes an araBAD promoter cloned upstream of T7 RNA polymerase. This replaces the lacUV5 promoter driving the T7 RNA polymerase gene and all but eliminates the leakiness of the traditional BL21 (DE3) expression systems. This eliminates the need for pLysS and pLysE plasmids. In general, the expression yields from this strain are similar to that of other BL21 strains.

Answer Id: E9723

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What does DE3 mean?

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Answer

The DE3 designation means the strains contain the lambda DE3 lysogen that carries the gene for T7 RNA polymerase under control of the lacUV5 promoter. This promoter is regulated by the endogenous E. coli lacI protein and is induced with IPTG. IPTG is required to induce expression of the T7 RNA polymerase. The DE3 lambda derivative also contains the immunity region of phage 21.

Answer Id: E9728

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Can I use one of your mammalian expression vectors with a T7 promoter for expression in E. coli?

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Answer

Transcripts can be made, but there is no ribosome binding site or Shine Dalgarno sequence to initiate translation; therefore, little protein will be produced.

Answer Id: E9702

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What is the difference between the Champion™ pET expression systems and the T7 expression systems?

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Answer

Both the T7 and Champion™ pET expression vectors contain a strong bacteriophage T7 promoter. After induction with IPTG, T7 RNA polymerase will bind the T7 promoter, leading to transcription and translation of your gene of interest. Studies have shown that there is always some basal expression of T7 RNA polymerase from the lacUV5 promoter in lambda DE3 lysogens, even in the absence of inducer (Studier and Moffatt, 1986 [http://www.ncbi.nlm.nih.gov/pubmed/3537305]). In general, this is not a problem, but if the gene of interest is toxic to the E. coli host, basal expression of the gene of interest may lead to plasmid instability and/or cell death. To address this problem, the Champion™ pET vectors have been designed to contain a T7lac promoter to drive expression of the gene of interest. The T7lac promoter consists of a lac operator sequence placed downstream of the T7 promoter. The lac operator serves as a binding site for the lac repressor (encoded by the lacI gene) and functions to further repress T7 RNA polymerase-induced basal transcription of the gene of interest in BL21 Star™ (DE3) cells.

Answer Id: E9705

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What are the advantages and mechanism of the T7 expression system?

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The T7 expression system allows for high-level expression from the strong bacteriophage T7 promoter. The system relies upon the T7 RNA polymerase. While it is not endogenous to bacteria, some strains of E. coli (such as BL21 (DE3) and BL21 (DE3)pLysS) have been engineered to carry the gene encoding for this RNA polymerase in a piece of DNA called the DE3 bacteriophage lambda lysogen. This lambda lysogen contains the lacI gene, the T7 RNA polymerase gene under control of the lacUV5 promoter, and a small portion of the lacZ gene. This lac construct is inserted into the int gene, thus inactivating it. Disruption of the int gene prevents excision of the phage (i.e., lysis) in the absence of helper phage. The lac repressor represses expression of T7 RNA polymerase. Therefore, under normal circumstances in these cells, the lac repressor (the lacI product) binds to the lac operator region between the T7 promoter and the gene encoding for T7 RNA polymerase. This effectively prevents transcription of the T7 RNA polymerase gene. Of course, there is always a small basal level of T7 RNA polymerase present. This is due to the fact that the lac repressor is in equilibrium with the lac operator region, causing the operator site to be occupied most, but not all of the time.

Adding a substance that prevents the lac repressor from binding to the lac operator then induces protein expression. This compound is isopropyl b-D-thiogalactoside (IPTG). IPTG binds to the lac repressor, changing its conformation in such a way that it is no longer able to bind the lac operator. This enables the cells to make T7 RNA polymerase in much more substantial amounts. As the T7 RNA polymerase is specific for the T7 promoter (which is only found in the transformed plasmid), the protein encoded by the plasmid will be overexpressed.

Answer Id: E9700

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My cells are growing very slowly, and I'm not getting any protein expression from my baterial expression system. What can I do to fix this?

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Answer

This typically occurs when your gene of interest is toxic. Try using a tighter regulation system, such as BL21 (DE3) (pLysS) or BL21 (DE3) (pLysE), or BL21(AI).

Answer Id: E9737

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What are BL21 cells derived from?

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Answer

All BL21 cells are derived from strain B834, and are therefore B strains.

Answer Id: E9719

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What is the optimal spacing between the ribosome-binding site (RBS) and the ATG when cloning into a bacterial expression vector?

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Answer

The sequence between the RBS and ATG should be between 8-12 bp and not contain any palindromic sequence.

Answer Id: E9696

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I'm trying to express my protein using a bacterial expression system and am getting inclusion bodies. What should I do?

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Answer

If you are having a solubility issue, try to decrease the temperature or decrease the amount of IPTG used for induction. You can also try a different, more stringent cell strain for expression. Adding 1% glucose to the bacterial culture medium during expression can also help.

Answer Id: E9739

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