Experiments started with the construction of devices that contained constitutive
promoters with low (J23117), medium (J23106) and higher (J23101) strength.
Each promoter was coupled to BioBrick I13504, containing a RBS, GFP protein and a double terminator.
The above mentioned BioBrick and the promoters were transformed in E. cloni competent cells. The cells were grown on LB (Sigma-Aldrich) 1.5% agar (VWR Chemicals) plates with chloramphenicol (from Acros Organics) as a selection
marker. As a positive control, cells were also transformed with the pUC19 plasmid and
plated on LB plates containing ampicillin. E. cloni without any
plasmid was also plated as a negative control on LB plates containing chloramphenicol.
Transformation of the BioBricks was performed twice by using chemically competent
cells. The first time, no colonies from any of the four BioBricks were obtained. The
second time, only a few colonies grew. Nevertheless, the positive controls were
correct every time and the transformation efficiency of our E. cloni was previously proven to be very high. Therefore, we switched to electroporation. This technique showed a higher efficiency and enough
colonies grew to perform the measurements.
Thereafter, the BioBrick Assembly Method was used to combine the promoters with GFP. Subsequently, electrocompetent E. cloni cells were transformed, plated on LB agar plates with antibiotic selection markers, illuminated with blue/UV-light to check for the presence of GFP, and thus a functional device. For the fluorescent measurements, liquid cultures (3 mL-LB + Antibiotic) were inoculated in polypropylene round-bottom tubes and incubated for 16 to 18 hours in a shaking incubator (200 rpm) at 37 °C. The fluorescence data from cells grown to an OD of ~0.5 (if the OD was higher, it was brought in the range 0.48-0.52) were measured at 300 nm. Finally, the fluorescence data were collected from the overnight cultures of the constructed devices with excitation and emission wavelengths of 483 nm and 525 nm respectively in a 96-well plate by a Tecan Safire2 monochromator MTP Reader. Besides, the absorbance measurements at 600 nm were repeated in the plate reader to normalize for cell density.
Preparing electrocompetent cells
- - Make a liquid culture of a single colony in 1-3 mL salt free LB
- - Grow 300-400 mL cells (without salt) at 37 °C until the O.D. reaches 0.6
- - Cool down on ice and perform all the steps at 4 °C
- - Spin the cells down in falcon tubes (3500 g, 20 min, 4 °C)
- - Resuspend the cells in 10% glycerol, spin the cells down (5000 g, 10 min, 4 °C). Repeat this step 3 times
- - Resuspend the cells in 10% glycerol to obtain a dense pulp (usually not more than 1.5 mL)
- - Take 50 µL sample and do the electroporation test (without DNA). Pulses should be between 4 and 6 msec. If shorter, wash the cells once again with 30 mL glycerol
- - Aliquot the cells (50 µL), quick-freeze in liquid nitrogen and store at -80 °C
- - Add 1 µl DNA to 50 µl electrocompetent cells in an ice-cold cuvette (1 mm)
- - Electroporate (Eppendorf, 1700 V, 4 msec)
- - Add 950 µl of SOC solution
- - Incubate for one hour at 37 °C
- - Plate out on pre-warmed plates containing the correct selective medium, in this case chlormaphenicol for J23101, J23106 and J23117 and ampicillin for I13504 (37 °C)
BioBrick Assembly Method
- - Digest I13504 (GFP) with XbaI and PstI in Tango buffer
- - Digest the promoters J23101, J23106 and J23117 with PstI in buffer O
- - Load the digested I13504 on a 1.5% agarose gel and visualize it under UV light.
- - Thereafter, perform a gel purification of I13504 (GeneJET Gel Extraction Kit - - ThermoFisher Scientific)
- - PCR purify the promoters J23101, J23106 and J23117
- - Digest the promoters J23101, J23106 and J23117 with FD SpeI in 10x Fast Digest Buffer
- - Ligate every promoter with I13504 using T4 DNA ligase
- - Digest with NcoI (cuts 1x in pSB1C3) and XhoI (cuts 1x in GFP) in Tango buffer
- - Mix gently and spin down
- - Incubate for 2 hours at 37 °C in a heating block
- - Separate the fragments using gel electrophoresis on a 1.5% agarose gel
Our wetlab team worked well together to fulfill this challenge. Vincent Van Deuren and Laurens Vandebroek performed the BioBrick assembly and the transformation experiments. The measurements were recorded by Laetitia Van Wonterghem, Ovia Margaret Thirukkumaran and Laurens Vandebroek. Laura Van Hese, Astrid Deryckere, Ines Cottignie and Vincent Van Deuren carried out the restriction digestion to check for the inserts. Finally, the results were processed by Ovia Margaret Thirukkumaran and Laurens Vandebroek and our wiki-page was filled with provided data by Vincent Van Deuren and Laetitia Van Wonterghem. Our supervisor Katarzyna Malczewska coordinated the overall works and the rest of the team members served with a helping hand whenever needed.
To grow our cells, we made use of a New Brunswick Innova® 43/43R Shaker purchased from Eppendorf. This incubator has a throw of 2.54 cm. Our devices were measured by a Tecan Safire2 monochromator MTP Reader. This machine was last calibrated on the 31th of March in 2015 by Tecan and our measurements took place on the 25th of August in 2015. The cells were excited at 483 nm and the emission was recorded at 525 nm. To capture the light emission, a Quad4 Monochromator was used. The absorbance was measured at 600 nm with a sampling frequency of 0.11 seconds/ sample while the sampling frequency of the fluorescence was 0.15 seconds/sample.
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