Wednesday, August 20, 2014

207 - Transformation of Azotobacter vinelandii with plasmid DNA

One cool thing about Azotobacter vinelandii is that it is naturally competent—that is, in certain conditions it takes up DNA from its environment and sometimes incorporates it into its genome. This is a useful characteristic for a species to have when studied in the lab, because it means a researcher can modify its genes and such to see what they do.

But besides taking up straight pieces of DNA and incorporating them (recombination), it'd also be useful if A. vinelandii could take up and maintain plasmids, which are small circular pieces of DNA, usually with only a few genes on them (for example, a gene of interest and an antibiotic resistance gene as a selectable marker). Plasmids are useful for studying overexpression or complementation of genes, for example, when it's not necessary to incorporate anything into the genome.

So Glick, Brooks, and Pasternak attempted to transform A. vinelandii with several broad-host-range plasmids:

  • pRK2501 (IncP-1 group, tetracycline and kanamycin resistance) 
  • RSF1010 (IncQ group, sulfonamide and streptomycin resistance)
  • pGSS15 (IncQ group, tetracycline and ampicillin resistance)

They selected for transformants using kanamycin, streptomycin, and tetracycline, respectively.

To transform A. vinelandii, it's necessary to use Transformation (TF) medium:
  • 1.9718 g/L MgSO4
  • 0.0136 g/L CaSO4
  • 1.1 g/L ammonium acetate
  • 10 g/L glucose
  • 0.25 g/L KH2PO4
  • 0.55 g/L K2HPO4
  • For solid medium, 18 g/L agar
This low-iron, nitrogen-containing medium, developed by Page and von Tigerstrom, induces competence in A. vinelandii.

So Glick et al. picked a colony into TF medium, grew at 30ºC to an optical density (620nm) of less than 0.2, then transferred to fresh TF broth and grew some more. They tested transformation at a variety of optical densities at this point to see which is best, standardizing the density of cells transformed to 1.6 x 108 cells per mL with TF broth.

50 µL of cells mixed with 300 µL fresh TF and 50 µL DNA (~22 µg/mL) sat at 30ºC for 30 minutes. These were spun down and resuspended in 400 µL fresh TF and incubated for another hour.

Then the cells were plated onto regular A. vinelandii agar plates, with or without antibiotics, and grown for 3 days at 30ºC. Plates without antibiotics revealed numbers of viable cells after transformation, and plates with antibiotics (compared to those without) indicated frequency of transformation.

From the results, it seemed like cells grown up to optical densities between 0 and 1 (which took 2-24 hours) could be transformed at very similar efficiencies; maybe a slight negative slope, but hardly noticeable. After the first 5 hours of growth, the culture should take on a yellow-green color as it becomes iron-limited.

Interestingly, transformed colonies on plates without antibiotics could be distinguished from non-transformed colonies; the transformed ones grew a lot bigger and more gooey.

The authors also found that, not surprisingly, the more DNA added to the transformation mix (from 0.1 up to 51 µg), the higher the frequency of transformation. At 51, 44% of the viable cells were transformed, which is not bad.

Also useful to note is that even without antibiotic pressure, transformed cells kept their plasmid around for at least 10 generations (not sure if that means 10 cell divisions or 10 transfers from one culture to another); and that the plasmid remained separate from the genome, rather than integrating or recombining or anything.

So this is useful for those who want to introduce genes into A. vinelandii and do some genetic modification; it's not required to integrate anything into the genome to express new proteins.

Citation: Glick, B. R., Brooks, H. E. & Pasternak, J. J. Transformation of Azotobacter vinelandii with plasmid DNA. J. Bacteriol. 162, 276–279 (1985).

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