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Team:Carnegie Mellon/Sensor
From 2014.igem.org
The Sensor
Fluorescent Protein Analysis
From our analysis of possible fluorescent protein reporters, we selected yellow fluorescent protein (YFP) and red fluorescent protein (RFP)
Construction: Overlap PCR
We first synthesized the estrogen sensor by cloning the estrogen responsive intein into the T7 RNA Polymerase. The intein was inserted between the 491 and 492 residues of the T7 RNA Polymerase using overlap PCR. We did this by first using PCR to piece the N-terminus of the T7 RNA Polymerase to the N-terminus of the S. cerevisiae VMA intein. Another PCR reaction was set up to piece together the C-terminus of the intein and the T7 RNA Polymerase. The third reaction pieced together these two parts, along with the estrogen ligand binding domain to produce the intein sensor, which would splice in the presence of estrogen to produce a functional T7 RNA Polymerase. We then checked to see that we made the desired product by running this through a 1 % agarose gel and looking for the specific size bands. In order to create the second plasmid indicating whether the intein was spliced in the presence of estrogen, we ligated the T7 promoter and RFP into the pSB3K3 plasmid. This plasmid, along with the plasmid containing the estrogen sensitive intein, were co-transformed into E. coli MACH cells and Top10 cells to be tested with estrogen
Testing
The cells containing the sensor were grown overnight at 37 °C. They were then diluted ten fold in LB, and grown at 30 °C and treated with 17-β Estradiol from Acros Organics. The E.coli cells containing the sensor were tested with 1, 2 and 10 ul of 10 mg/ml solution of estrogen in ethanol. We grew the cells in the presence of estrogen at 30 °C for 4 hours, taking samples every 30 to 60 minutes. We also tested growth of the cells over time at different temperatures (30 °C and 37 °C).
Results
While we were able to successfully construct our sensor using overlap PCR, and a double transformation into E. coli MACH cells (a W strain) and Top10 cells, we were unable to get a significant fluorescent signal from these cells in the presence of estrogen. Our model also predicted that we would not be able to detect any fluorescent signal. However, we were able to observe growth of the E.coli cells containing the sensor at 30 °C and 37 °C. In addition, we observed significantly higher fluorescence of the YFP and RFP in their respective controls.
A hypothesis for the increased YFP observed in the intein sensors is the stalling of the ribosomes at the intein sequence giving increased access to the RBS of the YFP. This results from these eight, uncommonly used codons in the intein-ligand binding domain, which are AAG, AGA, AUA, CUA, CGA, CGG, CCC and UCG (Source: EMBL). The significant signal from the RFP in the T7 RNAP with YFP added, may be due to mRNA stabilization by additional ribosome binding at the secondary RBS.
Source:
European Molecular Biology Laboratory (EMBL), The 8 least used codons in E. coli, yeast, Drosophila and primates. http://www.embl.de/pepcore/pepcore_services/cloning/choice_expression_systems/codons8/, October 16, 2014
In the following graphs,
- 115 YFP = Plasmid with J23115 promoter and YFP only
- YFP RFP = J23115-YFP plasmid and T7-RFP plasmid. There should be no T7 RNA polymerase.
- T7 RNA Polymerase = J23115-T7RNAP-YFP and T7-RFP plasmids. There is no intein.
- Dead Intein = J23115-T7RNAP with Dead Intein-YFP and T7-RFP plasmids. The dead intein should not be able to splice out to allow T7 RNA polymerase to form.
- Wild Type Intein = J23115-T7RNAP with Wild Type Estrogen Receptor Intein-YFP and T7-RFP plasmids. Ideally the intein would splice in the presence of estrogen.
*The magnitude of fluorescence of T7 RNA polymerase between the red and yellow channels was so large that they are presented on two different graphs.
Sensor in MACH Cells
Sensor in Top10 Cells
Characterization of Sensor System
The sensor final sensor system contains a YFP coding region to report the amount of T7 RNA Polymerase that is produced. The following graphs show the fluorescence of the system and its controls before and after the YFP coding region was added.
