Testing Result :
Exp 1: Function of transcriptional cascade
Purpose:
Testing the properties of three transcriptional cascade circuit, which composed of different ribosome binding site and duplication of CI coding region respectively.
[Notice]Before reading further details, please notice that we have named these circuits with three symbols:
II*2: pfadBA-RBS34-CI-pfadBA-RBS34-CI-pCI-RBS34-GFP40-term
II 34: pfadBA-RBS34-CI-pCI-RBS34-GFP40-term
II 30: pfadBA-RBS30-CI-pCI-RBS34-GFP40-term
Method:
1. Three E.coli strains is cultured overnight in 200mL LB medium respectively in Erlenmeyer flask, under 37℃ environment.
2. 10mL oleic acid is added to the flasks respectively and mixed by hand-shake for 1 minute.
3. 30ml LB-bacteria mixture is moved from the flasks every 30 minutes. Then the lysis buffer is added to the mixture.
100 c.c Lysis buffer:
Na3PO4．12H2O 1.9006 g
NaCl 0.5844 g
EDTA(0.5M) 20λ
Triton X 100 200λ
Add 1/1000 V β-mercaptoethanol & 120λ protein inhibitor/c.c(Roche)
4. E.coli in LB medium with lysis buffer is destructed by sonicator.
5. Centrifuge by 9000 rpm, 15 minutes.
6. Move the supernatant liquid into eppendorfs.
7. Records the absorbance of green fluorescence protein with fluorophotometer.
Result:
| Hour* |
0 |
0.5 |
1 |
1.5 |
2 |
2.5 |
| II 34 |
137.6 |
435 |
125.5 |
78.91 |
105.5 |
121.5 |
| II 30 |
62.44 |
353.2 |
139.4 |
123.8 |
78.55 |
182.8 |
Hour* : Time after adding oleic acid into the medium.
This is the raw data of Experiment 1. Since bacteria is keep growing in this 2.5 hour-long period, the amount of bacteria in each time point should be taken onto consideration. These data should be adjusted by dividing the theoretical amount of bacteria.
Adjusted value = log(O.D/Nbacteria)
The theoretical amount of bacteria is calculated by this equation, which is developed by Hiroshi Fujikawa in 2004 (A new logistic model for Escherichia coli growth at constant and dynamic temperatures):
dN/dt=rN(1-N/Nmax)(1-Nmin/N)^c
Parameters are proposed in the article:
T (K) 300
r 1.262802
Nmax 7.94E+08
c 0.74
Nmin 999.99
dt 0.01
N0 1000
These are the theoretical amount of E.coli in LB medium and GFP amount’s adjusted value:
| Hour* |
0 |
0.5 |
1 |
1.5 |
2 |
2.5 |
| N bacteria |
6.20E+03
| 1.09E+04
| 1.99E+04
| 3.65E+04
| 6.78E+04
| 1.26E+05
|
| II 34 adjusted |
-1.65E+00 |
-1.40E+00 |
-2.20E+00 |
-2.67E+00 |
-2.81E+00 |
-3.02E+00 |
| II 30 adjusted |
-2.00E+00 |
-1.49E+00 |
-2.15E+00 |
-2.47E+00 |
-2.94E+00 |
-2.84E+00 |
The expression of GFP protein decreases significantly after the bacteria senses fatty acid. At the beginning, bacteria still utilize nutrient component in LB medium (mainly yeast extract) as carbon source. Therefore, the metabolism of fatty acid has not started yet and repression of CI protein keeps going. This is the main reason that causes the increasing of GFP amount at the beginning of the curve. After the carbon containing material in LB medium being exhausted, the fatty acid metabolism started and the CI protein begins to be expressed. As a result, the amount of GFP keep decreases at the later part of the curve.
Although the decreasing rate of II 34 group is slightly greater than II30, the slope of two curves is quite similar with each other, suggesting that using different ribosome binding site in gene circuit seems to have limited influence on changing the decreasing rate of GFP protein in E.coli cells. To confirm the properties of the gene circuits, we need to conduct more experiment and lengthen the observation interval. The exact time point that the expression of GFP protein is completely shut down should be confirmed. In addition, the experiment of pfadBA-RBS34-CI-pfadBA-RBS34-
CI-pCI-RBS34-GFP40-terminator plasmid will be conducted in future.
Exp 2: Function of J23119-RBS34-FadR-terminator-J23119-RBS34-FadL-terminator
Purpose:
Confirm that FadR protein and FadL protein assist the bacterial uptake of fatty acid.
Method:
1. We use modified LB medium for culturing E.coli strain. The amount of yeast extract is decreased.
tryptone 10g
yeast extract 1g
NaCl 10g /1L ddH2O
We added 20λ, 40λ, 60λ, 80λ, 100λ oleic acid in 2mL modified LB medium respectively. This fatty acid – composing broth is used for culturing E.coli.
2. The E.coli strain with FadR/FadL coding gene and the DH5α cell (without FadR/FadL coding gene) is cultured overnight in the modified LB medium, 2 c.c respectively, with different fatty acid ratio.
3. Record the absorbance of the suspension culture by spectrophotometer.
Since the amount of yeast extract is decreased significantly in LB medium, the bacteria is forced to metabolizes oleic acid for carbon source (yeast extract is the main carbon source in origin LB medium). Therefore, the E.coli strain that contain FadR/FadL coding gene will be more adjustable to the critical environment and grow better in the medium. We compared the growing condition of FadR/FadL (+) and FadR/FadL (-) strain in modified medium.
Result:
| % of oleic acid |
plasmid (-) |
plasmid(+) |
| 1% |
2.542
| 2.661
|
| 2% |
2.390 |
2.623 |
| 3% |
2.523 |
2.357 |
| 4% |
2.610 |
2.616 |
| 5% |
2.351 |
2.653 |
Conclusion:
FadR/FadL(+) E.coli strain grows better in the critical environment, compared to the control group. The phenomenon suggests FadR and FadL protein play important roles in the uptake and metabolism of fatty acid in E.coli. Fatty acid sensing system can be designed on the basis of the dynamic interaction between FadR-acyl-coA complex , fatty acid molecules and pfadBA promoter
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