Short Summary
For the application of our best composite part, we decided to nominate our reporter signal enhancing system BBa_K2201373. This part contains a T3 RNA Polymerase with an inverted mRFP under T3 RNA polymerase control for the enhancing of reporter signals. It is an improved reporter and a genetic circuit that could report even weak expression levels. This part was designed based on the model of an amplifier in electrical engineering to intensify an existing input signal and could be used in a broad range of synthetic biology applications. We used this part for our selection system for the incorporation of non-canonical amino acids and demonstrate the advantages of this system in comparison with standard reporter in an integrated modelling and characterizations.Usage and Biology
At the moment, fluorescent proteins with an emission wavelength within the visible spectra are used to report expression of the gene of interest. Therefore, the CDS of the fluorescent protein was placed downstream of the CDS of the target protein without a terminator or promoter in between. The expression level of the target protein is nearly the same as the expression of the fluorescent protein. The fluorescence of the reporter protein indicates if the gene of interest was translated. However, thissystem is limited to strong expression, which generate a sufficienly strong fluorescence signal.
Several applications involve only a weak expression. For our project, we needed a reliable and sensitive reporter to detect the expression of the gene of interest on a selection plasmid. A low expression in this target gene is essential for the selection system. No fluorescence was detectable, when the CDS of mRFP was placed downstream of the gene of interest. Through the function of the gene of interest, we knew it was expressed. To address this reporter challenge we built a genetic circuit following the model of an amplifier used in electrical engineering.
Basic amplifiers were previously submitted to the Registry of biological parts e. g. by iGEM Cambridge 2009. They build a simple circuit using an activator, which increased the transcription of a reporter under control of a second promoter. To explain their system they used the term "Polymerases per second" (PoPs). This unit defined as the flow of RNA polymerase molecules over a promoter region per second. The system developed by Cambridge 2009 (Figure 1) could increase the number of PoPs.
Figure 1: Signal strenthening system of iGEM Cambridge 2009.
Functional Parameters
Figure 2: Construction of the standard mRFP reporter and the genetic circuit for the amplification of mRFP expression(BBa_K2201373). In construct 1 the CDS fot the mRFP transcript is downstream the CDS of the gene of interest. In construct 2 the CDS of the mRFP is downstream a T3 promoter and the CDS coding for the T3 RNA polymerase is downstream the CDS of the gene of interest.
The second system is our genetic circuit consisting of a CDS of the target gene upstream of the CDS for a T3 DNA polymerase encoding sequence. Therefore, the expression of the reporter gene is nearly on the same level as the expression of the target gene. The expressed T3 RNA polymerase transcripts the mRFP under the control of the T3 promoter. To demonstrate the advantages of our improved construct, we modelled the amount of mRFP transcript for both constructs. If we assume the expression of the gene of interest is low and only one E. coli RNA polymerase with a chain elongation rate of 50 nucleotides per second translates the both products, construct 1 produces 1 mRFP transcript in 16 seconds. Construct 2 expresses 1 T3 RNA polymerase transcript every 52 seconds. After translation (with an averange translation rate of 20 amino acids per second ~42 sec), these polymerases transcribe the mRFP transcript with a chain elongation rate of 170 nucleotides per second. Therefore, every T3 RNA polymerase generates one mRFP transcript every 4.7 seconds. The resulting amount of mRFP transcripts is shown in Figure 3. The script for our modelling can be found here.
Figure 3:Modelling on the amount of mRFP transcript transcribed through the two different models. In model 1 the CDS fot the mRFP transcript is downstream the CDS of the gene of interest. In model 2 the CDS of the mRFP is downstream a T3 promoter and the CDS coding for the T3 RNA polymerase is downstream the CDS of the gene of interest.
Figure 4: Two Smear of two clones containing only mRFP under an uninduced T7 promoter (-) and containing the mRFP enhancing system under the same promotor (+), after 12 h of incubation at 37 °C.
Figure 5: Picture of a negative selection round. The clone still containing the positive selection plasmid, thus the mRFP enhancing system, is red.
In addition to our application for this part, there are a lot of potential applications for this signal enhancing part. These are by no way limites to reporter signal enhancements.
References
Davis, R. W. & Hyman, R. W (1971)J. Mol. Biol 62, 287-301.
McGraw, N. J., Bailey, J. N., Cleaves, G. R., Dembinski, D. R., Gocke, C. R., Joliffe, L. K., MacWright, R. S.&McAllister, W. T. (1985) Nucleic Acids Res. 13, 6753–6766.