Difference between revisions of "Part:BBa K2817000"

Revision as of 16:20, 16 October 2018

PnorV-RBS-amilCP

When nitric oxide is present in the environment, the promoter PnorV will initiate the expression of the blue chromoprotein.

Usage and Biology

Engineered bacteria could be an ideal way to treat IBD. First, we need an appropriate sensor to sense the inflammatory signal and initiate the release of the drug. After comparing the inflammatory sensors used in some iGEM teams, we selected the inflammatory sensor based on nitric oxide molecules used by ShanghaiTechChina_B 2016 team. Nitric oxide is a natural signaling molecule of inflammation, and the concentration of intestinal inflammation in patients with IBD is significantly increased. It has been reported that the average concentration of nitric oxide in the rectum of normal people reaches 60 nM, while in patients with IBD it reaches 5.5 μM, nearly 100 times higher than that of the former. Therefore, the nitric oxide molecule is an ideal input signal for engineered bacteria, allowing the engineered bacteria to function only at the target location, thereby avoiding inappropriate output.

There are some natural nitric oxide sensors in E. coli, in which the enhancer binding protein NorR is specific and can only react with nitric oxide and cannot interact with other nitrogen forms. NorR binds to three conserved sites on the NorV promoter (PnorV) via the C terminal HTH domain. When nitric oxide is absent, the N terminal domain GAF of NorR blocks the central domain AAA+ to inhibit its binding to the transcription factor σ 54, and thereby prevent transcription of the nitric oxide reductase NorV. When nitric oxide binds to the GAF domain of NorR, it will release the AAA+ domain, allowing σ 54 to initiate transcription of NorV.

In our inflammation sensor, we used two parts which are NorR and PnorV. amilCP was selected as our reporter. Although the E. coli host has native NorR expression, increasing its expression facilitates the elimination of stoichiometric imbalances between NorR in the genome and PnorV on the foreign plasmid, avoiding interference with the host's release of nitric oxide. We used the double expression plasmid pCDFDuet-1 to construct our sensor module. Placing NorR under the control of the first T7 promoter, we are able to regulate its expression by IPTG induction. We also replaced the second promoter with PnorV and added the reporter gene downstream which allows us to activate the expression of the reporter by adding nitric oxide.

We transformed the constructed plasmid of NO sensor into DH5α, and cultured at 37 ℃ overnight to dilute to OD = 0.2. After 1.5 h of growth at 37 ℃, the inducer IPTG and SNP aqueous solution were added. After 6 h at 37 ℃, 1 mL of the bacterial solution was centrifuged at 8000 rpm for 1 min (Figure 1). It can be seen that the NO released by the SNP aqueous solution can effectively activate the expression of the reporter gene. But to our surprise, the blue chromoprotein in group 1mM IPTG without SNP was also be activated. We have not yet figured out the reasons behind this phenomenon. But we speculate that it may be caused by leaky expression of promoter PnorV when the NorR overexpress. Due to this, we will use a weak promoter to express NorR in future engineered bacteria to avoid leakage.

Figure 1. Pellets of bacteria transformed with constructed NO sensor plasmid after induction of 6h. From left to right: control, 0.5mM IPTG without SNP, 1mM IPTG without SNP, 0.5mM IPTG with 100μM SNP, 1mM IPTG with 100μM SNP.

Sequence and Features