Favorite composite part: BBa_K3524005
To build up a regulable genetic circuit, we chose the bio-sensor (nahR gene and Psal promoter) for the detection of salicylic acid we adopted is originally on the chromosome of Pseudomonas Stutzeri. With the appearance of salicylic acid, the protein translated from nahR binds with PsaI promoter and initiates the expression of the enhancer green fluorescent protein (EGFP).
Our team has verified this bio-sensor worked well. The future iGEM teams can exchange the EGFP to other functional proteins to make some significant utiles. For example, our team chooses an essential gene GAPDH, which belongs to Tatumella citrea, which can provide phosphorus for crops to replace the EGFP. As a result, in the presence of salicylic acid, GADPH can be expressed, and T. citrea can survive. Since salicylic acid is a root exudate, this genetic circuit that T. citrea can commensalism with crops.
Results of plasmids construction:
*n represents for pTrc99k-nahR plasmids.
*G represents for pMW119-Psa1-EGFP plasmids.
We then used the successfully constructed plasmids to examine the effectiveness of the bio-sensor system.
Some possible factors that led to a failure:
1．Mistakes in homologous recombination
Unable to recombine the right fragments into the vector.
2．Mistakes in PCR
During the amplification, the fragments may not be correctly amplified by PCR, so some of the recovered fragments are wrong fragments, which will cause problems when used for vector construction.
The pTrc99k-NahR and pMW119-Psa1-EGFP plasmids were co-transformed into Tatumella Citrea and screened by Kanamycin resistance and Ampicillin resistance LB solid plates. The co-transformed strains were inoculated into fresh LB medium, cultured at 37°C until OD600 was approximately equal to 0.6. Different concentrations of salicylic acid (dissolved in absolute ethanol) were added. After 24 hours, observations with a fluorescence microscope were used to show the regulatory effect of salicylic acid on the PsaI promoter and it was indicated by fluorescence intensity.
Figure 1. Microscopic view of fluorescence shown by Tatumella Citrea
1.With different salicylic acid concentrations:
We measured the fluorescence intensity to obtain a quantitative result. The data is plotted as the following: the measured fluorescence intensity is divided by the number of cells (OD600) to obtain a homogenized fluorescence intensity. The fluorescence intensity of all samples added with salicylic acid is subtracted from the fluorescence intensity of the background without salicylic acid. It can be seen that from 1 μM to 100 μM salicylic acid, the fluorescence intensity rises sharply. Under the condition of 1000 μM salicylic acid, the fluorescence intensity decreased, which is presumably due to the influence of salicylic acid solvent (ethanol) on cell growth, or other unknown regulatory effects of salicylic acid and NahR.
Figure 2. Relative fluorescence unit under different Salicylic acid concentration
2.With different hours:
With the same concentration of salicylic acid, we also measured the change of fluorescence intensity over time. It can be seen from figure 3. that within 3 to 22 hours after the addition of salicylic acid, the fluorescence intensity increases successively. But it mostly stops increasing at 24 hours.
Figure 3. Relative fluorescence unit with different time after addition of salicylic acid
The mechanism we created did work out well, and the expressions of EGFP protein was detected. This proves that the potential of the bio-sensor system we made is effective,. The two factors we examined are the most convenient, and available tests we could conduct. For the concentration of salicylic acid, we found out that at 100 μM, it shows the highest fluorescence intensity. This means that the system works the best in that range of salicylic acid concentration. For the time after the addition of salicylic acid, the highest fluorescence unit is shown at 22 hours, the trend is mainly increasing. But after 22 hours, it is decreasing. It is best to leave the mechanism for 15-22 hours after the addition of salicylic acid so it can show the most effective results.
In conclusion, we have succeeded made a new Part: BBa_K3524005 that improves the function of the existing Part: BBa_E0040. At first, the sequence of our part BBa_K3524005 is different from the existing part BBa_E0040, and the blast result is as figure 4. Furthermore, we gave the GFP protein a new function that can be regulated by the presence of salicylic acid.
Figure 4. The blast result of the sequence of GFP in Part: BBa_E0040 and Part:BBa_K3524005.
 Whitelaw, M.a. “Growth Promotion of Plants Inoculated with Phosphate-Solubilizing Fungi.” Advances in Agronomy, 1999, pp. 99–151., doi:10.1016/s0065-2113(08)60948-7.
 Bar-Yosef, B., et al. “Pseudomonas Cepacia-Mediated Rock Phosphate Solubilization in Kaolinite and Montmorillonite Suspensions.” Soil Science Society of America Journal, vol. 63, no. 6, 1999, pp. 1703–1708., doi:10.2136/sssaj1999.6361703x.
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