When selecting for blasticidin-resistant transformants in the X-33 strain using pPIC6/pPIC6α vectors, why do I get large and small colonies on YPD plates containing 300 µg/ml blasticidin?
Generally, large colonies represent transformants containing pPIC6/pPIC6α integrants, while small colonies represent transformants containing pPIC6/pPIC6α non-integrants. These non-integrants have transduced the pPIC6/pPIC6α plasmid, and therefore, exhibit a low level of blasticidin resistance in the initial selection process. Upon subsequent screening, these non-integrant transformants do not retain blasticidin resistance.
When choosing a blasticidin-resistant transformant for your expression studies, we recommend that you pick blasticidin-resistant colonies from the initial transformation plate and streak them on a second YPD plate containing the appropriate concentration of blasticidin. Select transformants that remain blasticidin-resistant for further studies.
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My transformation is not working. Do you have any suggestions?
Here are some suggestinos:
- Make sure that you have harvested cells during log-phase growth (OD <1.0 generally).
- If electroporation is being used, see the electroporator manual for suggested conditions. Vary electroporation parameters if necessary.
- Use more DNA.
- Use freshly made competent cells.
- If the LiCl transformation method is being used, try boiling the carrier DNA.
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My spheroplasting of Pichia worked twice, but hasn't worked since. The OD of the culture simply does not drop.
Here are some things to consider:
- If the OD of cells that are used is too high, they will not spheroplast. Do not overgrow cells.
- Do not use old cells and make sure that they are in log phase of growth.
- Make sure to mix zymolyase well before using. Zymolyase is more of a suspension than a solution.
- Make the PEG solution fresh each time and check the pH.
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Is there a recommended protocol for fermentation using constitutive expression vectors such as pGAPZ?
Use the following high cell density protocol for pGAP clones. Feed carbon until the desired density is reached (300 to 400 g/L wet cell weight (WCW)). If the protein is well-behaved in the fermenter, increase to 300-400 g/L WCW as with methanol inducible clones. These densities can be reached in less than 48 hours of fermentation. We have fermented constitutive expressers on glycerol using these protocols with good results. Some modifications to the Fermentation Basal Salts Medium that you might want to make are:
1) Substitute 2% dextrose for the 4% glycerol in the batch medium.
2) Substitute 40% dextrose for the 50% glycerol in the fed-batch medium.
3) Feed the 40% dextrose at 12 mL/L/hr (Jim Cregg has published data on expression using several carbon sources as substrates; dextrose gave the highest levels of expression).
4) Yeast extract and peptone may be added to the medium for protein stability.
One warning: If you are working with His- strains, they remain His- after transformation with pGAPZ. Fermentation in minimal medium will require addition of histidine to the fermenter.
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Can the methanol and ammonium hydroxide solutions used to prepare Pichia fermentation medium be autoclaved?
No, you cannot autoclave methanol. There are two approaches to this, depending a bit on the size of the bioreactor and the volumes involved. You can either dilute to working concentration and filter-sterilize with a filter suitable for alcohols, or you can just assume that methanol is sterile (it should be) and dilute into sterile water. For the ammonium hydroxide solution, you should also not autoclave it. You can assume the 30% stock solution is sterile (nothing should live in this solution) and dilute into sterile water to the working concentration.
Find additional tips, troubleshooting help, and resources within our Protein Expression Support Center.
