Part:BBa_K3927002
3C120-CYC-LacO
This part encodes for a truncated CYCp core promoter with three C120 repeats replacing the native upstream activating sequence, and a lacO sequence downstream of the TATA box.
Sequence and Features
- 10INCOMPATIBLE WITH RFC[10]Illegal XbaI site found at 203
- 12INCOMPATIBLE WITH RFC[12]Illegal NheI site found at 185
- 21COMPATIBLE WITH RFC[21]
- 23INCOMPATIBLE WITH RFC[23]Illegal XbaI site found at 203
- 25INCOMPATIBLE WITH RFC[25]Illegal XbaI site found at 203
- 1000COMPATIBLE WITH RFC[1000]
Description
3C120-CYC-LacO is the implementation of an abstracted, hypothetical synthetic promoter developed by the NUS iGEM team 2021 for tight, blue light regulated expression in S.cerevisiae.
Usage
This part is a blue light inducible promoter, therefore requires blue light for this promoter to be induced. Additionally, it requires part BBa_K3927000 / BBa_K3570021 (NLS-VP16-EL222) and part BBa_K3927006 (yeast LacI) to function.
Design
Our team was motivated to develop a new framework for optogenetic promoters, spurred on by the need to increase inducible expression, but confronted by the dilemma that single layer methods, such as adding additional activation motifs or optimizing the TATA box, were usually accompanied by increased leakiness of the promoter. Taking inspiration from the tightly regulated, yet powerful native promoter GAL1p , it was decided that a combination of conditionally activiating and suppression motifs were required to achieve the desired outcome of both high expresssion and low leakiness. The design that was conceptualized included an artificial upsteam activated module, a core promoter and a reprssion module downstream of the core promoter, drawing from the architecture of native yeast promoters[1].
Figure 1: Circuit design for the modular promoter 3C120-CYC-LacO. The abstracted modules include a core promoter, a blue light activated module, and a module for repressing promoter activity
In darkness, the repression module prevents leakiness, and in the presence of blue light, a secondary, trans-regulatory repression module represses the primary repressor module, allowing the blue light activated module to power the core promoter. In this way, highly active activation modules could be coupled to the core promoter without the issue of increasining leakiness(Figure 1). This modularity also meant that activation and repression of the promoter could be controlled separately, allowing for AND gate logic, where represson module could be linked to an alternative response(Figure 2).
Figure 2: 3C120-CYC-LacO can either be used to tighten expression of a single blue light input by linking LacI to a secondary, blue light repressed module, or can be coupled to alternative
To implement this design, the NUS iGEM team 2021 decided to further improve the optogenetic system in part BBa_K3570005 designed by the Toulouse iGEM team 2020. The part depends on the expression of an NLS-VP16-EL222 fusion transcription factos, which dimerizes in blue light and binds to C120 repeats[2], activating a core CYC1 promoter element in close proximity(Figure 1). Thus, our chosen activation module a 3x repeat of the C120 motif. For the primary repression module, a Lac operon sequence was chosen, at it has been demonstrated to be a functional repressor of synthetic promoters in yeast[3]. This was inserted downstream of the TATA box in the core CYC1 promoter element.
Characterization
This part was characterized by placing the part upstream of a mKO reporter gene. The overall performance of the composite part was quantified based on the levels of mKO produced.
A DNA fragment containing the C120-CYC promoter, as well as a constitutive expression cassette for NLS-VP16-EL222 was ordered from IDT, and Gibson assembly was used to assemble it into a plasmid with the mKO orange fluorescent protein terminated by LSC2 terminator, producing the plasmid pC120-mKO-EL-U. The plasmid was then transformed into BY4741 to test for EL222 mediated blue light induction of mKO.
pC120-mKO-EL-U(BY4741) was compared to the wildtype BY4741 as a negative control. Cells were cultured overnight, and the next day two cultures inoculated in 25ml YNB-URA media to OD600~1.2, and then cultured in a shaking incubator at 30 degrees Celsius, 220rpm in either blue light or darkness, with wildtype BY4741 undergoing an identical protocol. After 6 hours, the cells were washed and the level of mKO fluorescence(Ex:515, Em:560) was measured and normalized to the OD of the culture to ascertain the level of mKO expression under the C120-CYC promoter.
Other than the absolute induction fold, it was also important to characterize the effect of dose depended activation as well as the possible metabolic burden that the circuit may impose. As such, the experiment was replicated with the same induction protocol, but instead of an endpoint measurement the OD600 and mKO expression was measured hourly to plot the expression and growth curve in darkness, 100% blue light or using a 50%, half-hour-on half hour-off cycle.
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