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Team:UCSF/Project/Circuit/Data
From 2013.igem.org
Promoter Selection and Testing:
In order to create synthetic circuit sensitive to differing levels of inducer, we first had to select promoters with favorable characteristics we could exploit to make them concentration-dependent -- namely a low basal level and a wide dynamic range. We chose four well-known promoters as possible candidates for our circuit: pLAC (lactose), pTet (tetracycline), pBAD (arabinose), and PprpB (proprionate). We constructed separate plasmids containing each of the promoters individually driving the expression of GFP, and using this system we characterized these promoters by measuring the GFP expression over time as a function of different inducer levels. We determined the basal level, highest induction level, and inducer range. Because of the low basal expression and the varying responsiveness at different inducer concentraions, we selected pLAC & pTET as the promoters for the synthetic circuit.
Altering Promoter Sensitivity:
Our next goal after our promoter assays was to create engineered versions of our promoters responsive to high and low levels of inducer, either by modifying activation or repression of the specific promoter. Based on previous workin the literature characterizing the pLAC promoter, we chose to change both the number and orientation of repressor binding sites in pLAC. To create our "low" inducer promoter, we shifted the O1 operator site further downstream of our promoter sequence and removed another operator site from pLAC to relieve repression. To create our "high" promoter, we took the "low" promoter and added the O3 operator site further upstream near the start of the promoter sequence. The O3 operator site will then form a loop with our O1 operator site, thus creating a physical barrier to prevent transcription, reducing the promoter activity at low levels of inducer.
Our next goal after our promoter assays was to create engineered versions of our promoters responsive to high and low levels of inducer, either by modifying activation or repression of the specific promoter. Based on previous workin the literature characterizing the pLAC promoter, we chose to change both the number and orientation of repressor binding sites in pLAC. To create our "low" inducer promoter, we shifted the O1 operator site further downstream of our promoter sequence and removed another operator site from pLAC to relieve repression. To create our "high" promoter, we took the "low" promoter and added the O3 operator site further upstream near the start of the promoter sequence. The O3 operator site will then form a loop with our O1 operator site, thus creating a physical barrier to prevent transcription, reducing the promoter activity at low levels of inducer.
From Oehler et al., 2006.
Using a similar GFP reporter system as for the first promoter tests, we were able to show that our engineered pLAC promoters are now concentration sensors and can be used to create a circuit that responds differently to varying inducer levels. We plan to implement a similar strategy for the pTET promoter design to implement it in a parallel circuit.
PROMOTER DATA
The essential benefit of using the CRISPR system for the circuit design is that it has a very specific target -- a guideRNA contains nucleotides which are complementary to a unique DNA sequence. As a result, we can use a vast number of individual gRNAs to repress several genes, giving us the ability to design circuits to make numerous and more complex decisions.
Our proof-of-concept design is, at a low level of inducer, to express GFP simultaneously with a gRNA to repress the RFP. When the inducer level increases, the "high" sensing promoter turns on expression of RFP and a gRNA to repress GFP, making a switch-like decision based on the input. We were able to construct an early design of the circuit in the pCDF plasmid that contains both fluorescent proteins and gRNAs. We initially put both components of the circuit under individual promoters, pLAC and pTET, as a control to test whether or not we could obtain expression of both products and see repression from the gRNAs. Since this circuit relies on CRISPR repression, we had to also construct a plasmid (pACYC) containing the catalytically dead dCas9 protein as well as the Csy4 protein to cleave the RNA products (see Design page or Parts Registry for more information). We are in the final stages of cloning the dCas9 plasmid and we hope to test the circuit before the Jamboree.
