---+ FLP Recombination in _E. coli_ Commonly used to eliminate the Kanamycin resistance cassette from _E. coli_ strains from the Keio collection. ---++ Day 1: Transform recombinase plasmid 1 Make [[ProtocolsElectrocompetentCells][electrocompetent cells]] of the desired _E. coli_ strain. 1 Transform with pCP20. This plasmid has a temperature-sensitive origin of replication, confers ampicillin resistance, and encodes the FLP recombinase. 1 Add 500 µl SOC and incubate in a 1.7 ml tube shaking on its side at ≥120 rpm for 1 hr at 30°C to allow antibiotic resistance induction. 1 Plate 20 µl of the culture on an LB + Amp plate, save the rest for additional plating if necessary. 1 Grow overnight at 30°C. Expected results: ---++ Day 2: Induce recombination 1 Pick a single colony from the LB + Amp plate. 1 Inoculate into 5 ml of LB in a test tube. 1 Grow overnight at 43°C to induce FLP recombinase expression and select for loss of pCP20. ---++ Day 3: Plate to get single candidate recombinants 1 Make a 10<sup>6</sup> dilution of the overnight culture via 3 wet DTs (100 µl into 9.9 ml each time). 1 Plate 50 µl of this dilution on LB. This should yield a couple hundred colonies. 1 Grow overnight at 30°C to prevent partial loss of plasmid from colonies founded by cells that did not lose plasmid. Expected results: ~200 colonies on LB plates. ---++ Day 4: Screen for genomic recombination and plasmid loss 1 Patch six individual colonies from this plate onto LB + Kan, LB + Amp, and LB plates. Do the patching by picking a colony and then streaking a small spot on each plate *in this order*. Be sure you patch on LB last. Failure to grow can sometimes occur because no cells were transferred to the later plates. This order ensures that, if you get the desired growth on the last plate and no growth on the LB + Kan and LB + Amp plates, it was not due to "running out" of cells in the later patches. 1 Grow overnight at 37°C for LB and LB + Kan and grow at 30°C for LB + Amp plates. Expected results: Most if not all patches should be completely Kanamycin and Ampicillin sensitive. ---++ Day 5: Grow up successful recombinants 1 Inoculate 5 ml LB from patches on LB plates that score as sensitive to both antibiotics. 1 Grow overnight at 37°C, shaking at 120 rpm. ---++ Day 6: Archive clones 1 Freeze and archive copies of the newly created strain, there the Kan cassette has been eliminated from between the FRT sites. To guard against mutations that may occur during strain construction, it may be wise to archive and test the fitness of several separate colonies (patches). ---++ Sources 1 Datsenko, K.A., and Wanner, B.L. (2000) One-step inactivation of chromosomal genes in Escherichia coli K-12 using PCR products. _Proc. Natl. Acad. Sci. U.S.A._ *97*: 6640-6645. 1 Baba, T., Ara, T., Hasegawa, M., Takai, Y., Okumura, Y., Baba, M., Datsenko, K.A., Tomita, M., Wanner, B.L., and Mori, H. (2006) Construction of Escherichia coli K-12 in-frame, single-gene knockout mutants: the Keio collection. Mol Syst Biol 2: 0008. 1 [[http://ecoli.aist-nara.ac.jp/gb6/Resources/deletion/Del_construct.html][Keio Strain construction details]]
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Topic revision: r9 - 2012-04-10 - SteveSowa