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In Vivo Characterization

TetR - RFP system

Description

The first system is the simplest one; it is composed of TetR driven by a constitutive promoter with RFP (red fluorescent protein) under Ptet control. If TetR binds to Ptet, then RFP is repressed. By applying different concentrations of ATC (anhydrotetracycline) to the cells, RFP is expressed - the stronger the TetR mutant binds to Ptet, the higher the ATC concentration needed to have full expression of RFP.

The TetR and RFP genes were put on two different plasmids: pSB3K1 pConst-TetR and J61002 Ptet-RFP. For more details about them, please refer to the Plasmids details page.

Experimental validation - with wild-type TetR

ATC induction

To validate our first readout system, we performed platereader experiments with different concentrations of anhydrotetracycline (ATC). This molecule binds to the TetR dimers and induces conformational changes that make the transcription factor unable to bind to Ptet. As a result, RFP can be expressed.

Gene expression in the cell without ATC:

Gene expression in the cell with ATC:

Here are the results of our ATC induction experiment:

The induction curves show that RFP expression increases with addition of ATC. In the absence of ATC, cells express about 2500 normalized RFUs (relative fluorescence units) when they reach a plateau, whereas at the highest concentrations of ATC we observe 25'000 normalized RFUs. There is a 10x difference between normal medium (without ATC) and addition of ATC, showing that our first readout system is sensitive to ATC concentration. Moreover, expression of RFP in cells without ATC is 10-fold lower than the values obtained for Ptet characterization alone.

We can reasonably assume that this system can be used for in vivo screening: TetR mutants that are not capable of recognizing the consensus Ptet sequence will yield more RFP than other mutants that can still recognize Ptet.

Dose-response

By looking at the dose-response graph, we can see a significant increase between 0 and 200 ng/microL ATC; then the RFU values begin to plateau. The graph shows that the TetR-Ptet interaction has a strong impact on RFP expression. The highest RFUs measured here correspond to the levels of the Ptet-RFP construct alone (in the absence of the the TetR plasmid), showing that we can demonstrate complete TetR suppression in our experiments.

TetR mutant characterization

With the wild-type TetR in our system, RFP is repressed because wt-TetR binds to Ptet. However, mutants can potentially recognize a different promoter sequence and Ptet would be unable to repress RFP expression, as seen in the cartoons:

Gene expression with wild-type TetR:

Gene expression with a TetR mutant not recognizing Ptet:

add platereader results asap