Part:BBa_K1894001

shuttle plasmid which can replicate in cyanobacteria and E.coli

This part is originally a self-constructed shuttle plasmid that can both replicate in cyanobacteria and E.coli, but we extract its key sequence and link it to pSB1C3 plasmid backbone. The shuttle plasmid we constructed possesses the replication origins of E.coli plasmid and cyanobacterium plasmid, CaMV35S promoter, multiple cloning sites (MCS) and rbcS polyA terminator, Which makes it convenient to insert and express target gene and to screen out the recombinants. Ori site of cyanobacterium plasmid comes from indigenous plasmids pPbs extracted from Plectonema boryanum.

Usage and Biology

After we recover gene segment of GvpA1 from cloning plasmid pET30a(+), we then link GvpA1 gene with shuttle plasmid pPKE2. We introduce shuttle plasmid into our experiments because we need to transform both E.coli and microcystis aeruginosa during experiments. （The plasmid of microcystis aeruginosa itself does not contain selectable marker and cannot replicate in E.coli. Traditional plasmid vector used to transform E.coli cannot transform microcystis either. ）The shuttle plasmid pPKE2 we constructed and used possesses the replication origins of E.coli plasmid and cyanobacterium plasmid, CaMV35S promoter, multiple cloning sites (MCS) and rbcS polyA terminator subcloned from plasmid pKYLX一71.35, which makes it convenient to insert and express target gene and to screen out the recombinants. Ori site of cyanobacterium plasmid comes from indigenous plasmids pPbs extracted from Plectonema boryanum.

Characterization of the part BBa K1894001

The goal of the experiment is to prove that the complete shuttle plasmid we constructed can both replicate in E.coli and cyanobacteria. Therefore, five measures were taken, using methods of transformation, resistance screening and endonuclease identification.

• Use shuttle plasmid to transform E.coli.
• Positive clones were screened by kanamycin and plasmid DNA extracted is identified with restriction endonuclease.
• Conduct basic kanamycin resistance test of microcysis.
• Use shuttle plasmid to transform microcysis.
• Carry out kanamycin resistance screening after the shuttle plasmid was transformed into microcysis.

Considering that the shuttle plasmid contains kanamycin gene, we decided to carry out kanamycin resistance screening. Basic kanamycin resistance test of Microcystis aeruginosa shows that the algae cannot resist concentration of 5μg/mL and above on BG11. Therefore, we choose 10-15μg/mL kanamycin for screening. After shuttle plasmid is transformed into microcysis, transposon screening is carried out by 7-10 days of culturing on BG-11 medium which contains 15µg/mL of kanamycin.

First experiment: Validation of E.coli Transformation

After transformation of E.coli BL21(DE3), we successfully screened out the positive clones and extracted the recombinant plasmid (the shuttle plasmid) containing GvpA1 gene for restriction endonucleases identification.

Figure 1: Electropherogram of plasmid pPKE2 DNA

• A1:products after EcoRI digestion
  
• A2: products after BamHI digestion
  
• M: λDNA marker for Hind III digestion
  
• B1: DNA of recombinant plasmid after EcoRI and SphI digestion
  
• B2: Recombinant plasmid pPKE2
  
• M: λDNA marker for EcRI and HindIII digestion
  

The plasmid should have a length of 9.8kb after ligation with target gene GvpA1, which is verified and showed in A1. Gene segment of GvpA1 and pPKE2 both include BamHI sites and two separated segments with length 2.8kb and 7.0kb each will show up after BamHI digestion， which is verified in A2. When gene segment of GvpA1 is inserted into the plasmid, a new SphI site will show up between original EcRI-SphI segment (2.8kb). Therefore, two separated segments with length 1.3kb and 1,5kb each will show up after EcoRI and SphI digestion, which is verified in B1.

Second experiment: Validation of Microcystis transformation

Fig.2. growth condition of microcystis a week after transformation

Left: control group (no recombinant plasmid introduced)
Right: Microcystis of Recombination group appears on BG11 medium containing kanamycin

Methods

1)Transformation of E.coli cells

Preparation of chemically competent E.coli cells

• Inoculate 2ml LB broth with an aliquot (about 50μL)of the desired E.coli from the -80℃ freezer stock of cells.
• Incubate for 2h at 37℃.
• Add the 2ml seed cμLture to 250ml LB broth and grow at 37℃, shaking (about 200rpm) until OD600 of 0.3-0.4 (about 5 hours).
• Pre-cool the 50ml polypropylene tube, 80 EP tubes, CaCl2-glycerine (0.1mol/L CaCl2) and CaCl2- MgCl2 (80mmol/L MgCl2, 20mmol/L CaCl2). Set the centrifuge and prepare the ice tray.
• Transfer the bacteria into the 50ml polypropylene tube. Place it on ice for 10 minutes.
• Centrifuge at 4℃, 4100rpm for 10 minutes.
• Discard supernatant, then place the tube upside down to make sure trace liquid medium runs out.
• Add 30ml of pre-cooled CaCl2- MgCl2 per 50ml of initial liquid medium to resuspend bacteria cell pellet.
• Centrifuge at 4℃, 4100rpm for 10 minutes.
• Discard supernatant then place the tube upside down to make sure trace liquid medium runs out.
• Add 2ml of pre-cooled CaCl2 per 50ml of initial liquid medium to resuspend bacteria cell pellet.
• Transfer to EP tubes (50μL every tube) and store at -80℃.
  

2)Transformation of E.coli BL21(DE3)
Thaw competent cells rapidly by immersing frozen tubes in a 37℃ water bath after remove from 70℃ refrigerator. Draw about 50μL of the competent cells in a clean tube and add 5μL recombined plasmid, throw on ice for about 30 min. then put them in 42℃ for 90 second, immediately turn on ice for 2min. add 900μL LB medium and incubate in a roller drum at 37℃ fro 1hours. Took 100μL LB medium contain ampicillin, upside down when dried, put in 37℃ incubator over night. Validation by agarose electrophoresis after the plasmid DNA was extracted from transforming Escherichia coli.

3) Determination of the basic resistance of kanamycin

• Kanamycin was prepared in water at 0µg/mL、5µg/mL、10µg/mL、15µg/mL、20µg/mL and stored at -20℃ in 10-µL aliquots. An aliquot was thawed as needed and used once without refreezing.
• Equivalently draw the cyanobacterium at logarithmic growth stage and inoculate in BG-11 liquid medium containing kanamycin with all above concentration respectively, the cyanobacterium solution was cultured under the optimum conditions for one week and observation of growth status was carried out.
• It is determined that the cell of cyanobacterium cannot resist certain concentration of kanamycin if no algae community were found after one week of culture. Basic kanamycin resistance test of Microcystis aeruginosa shows that the algae cannot resist concentration of 5μg/mL and above on BG11. Therefore, we choose 10-15μg/mL kanamycin for screening after shuttle plasmid pPKE2 containing modified GvpA1 gene is transformed into microcysitis aeruginosa.

4) Transformation of microcysis

• Microcysis (FACHB-854) was cultured in BG-11 medium until OD730 reached 0.25-0.35 which is measured with a spectrophotometer.
• The cells were prepared for transformation by washing once in 10 mM NaCl and resuspending in BG-11 at 5 x 108 cells per ml (the cells were concentrated 10 times)
• Aliquots of cells (300 or 400 µL) were dispensed to glass test tubes or microcentrifuge tubes, and recombined plasmid DNA in TE buffer (10 mM Tris, 1 mM EDTA, pH 7.6) was added to a final concentration of 2 to 3 µg/ml. Plasmid DNA was stored as a 20 µg/ml solution, so that no more than 50µL was added to cells.
• The transformation mixtures were incubated for various times at 28 to 30°C in a constant-temperature chamber under standard conditions. The key parameter was the time of incubation of the cells with the donor DNA.

5) Microcysis transposon screening

• The cμLture mixture was coating on Millipore membrane (Φ9cm, pore diameter 0.45µm) covered on BG-11 medium and mircrocysis was cultured under standard condition with 20 hours.
• Transfer the membrane in BG-11 solid medium which containing 15µg/mL of kanamycin, This plasmid confers kanamycin resistance to transformed recipient cells. Single colony was found after 7-10 days of culture, select the growth colony and transfer to BG-11 liquid medium(containing 10-15µg/mL of kanamycin ), shake frequently and culture another 7 days.
• The cyanobacteria of best growth were selected and expansively cultured step by step (always containing 15µg/mL of kanamycin).

Contribution

(iGEM17_Nanjing_NFLS, 2017.10.20)
We verified the part BBa_K1894001. Electrophoresis was carried out after cut with the restriction endonuclease, expected band was obtained. The obtained shuttle plasmid contain gene GvpA1 was cloned and transfected. Microcystis aeruginosa from TaiHu Lake were collected and treated, then cultured in laboratory as receptor cells, and transfected with the plasmid mentioned above. After cultured several days, results showed that many of the microcystis were sunk as expected, it can be seen in the figure below. This suggested that gene GvpA1 modified in the plasmid were expressed and the functions were preliminarily demonstrated. We plan to extracted the protein and verified with west blotting in further.

Figure: Modified gene in shuttle plasmid transfer to Microcystis aeruginosa collected from TaiHu Lake

Assembly Compatibility:

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