Part:BBa_K3905005
gen2 210-3p Toehold Switch
This Toehold Switch upregulates the translation of a reporter protein in the CDS downstream in the presence of miRNA-210-3p- an miRNA upregulated in preeclampsia patients. We characterised this switch to show it can discriminate between homologs and in a 4:1 ratio of concentrations, corresponding to the fourfold increase in concentration indicative of the condition.
Condrat, C.E. et al., 2020. miRNAs as Biomarkers in Disease: Latest Findings Regarding Their Role in Diagnosis and Prognosis. Cells, 9(2), p.276. Available at: http://dx.doi.org/10.3390/cells9020276. [Accessed 19 October 2021].
Functionality
Toehold switches contain multiple parts, namely: the trigger binding site, the ribosome binding site (RBS), the seven amino acid long linker sequence and the protein coding sequence.
The trigger binding site on each toehold switch is the reverse complement to the trigger RNA strand. As the miRNAs are both 22 nucleotides long, the trigger site is also 22 nucleotides long. The trigger binding site makes up the lower stem in the ‘off’ state as well as part of the toehold base. We were advised to ensure the trigger site was >10 nucleotides long by Dr Wooli.
When few microRNAs are present, the ribosome binding site of an mRNA coding for luciferase is folded up, so the ribosome cannot unzip the long stem and move down the strand to the start codon to initiate translation.
However, when a microRNA binds to the trigger site, the switch unfolds, decreasing the length of the stem, and exposing the start codon, so translation of luciferase - a reporter protein - can begin.
This was designed using a python script taken from the NUPACK API, there is more information about this here: https://2021.igem.org/Team:City_of_London_UK/Software .
Characterisation
Methodology
For this 210-3p gen2 switch, we tested, at a constant DNA concentration, the target miRNA, miR-210-3p at 9M and 2.25M to replicate the roughly 4-fold increase of expression levels in patients with preeclampsia, as well as a homolog for the miRNA which was miR-6867-5p, this was determined to be the miRNA that has the highest probability of activating the switch aside from the target miRNA itself by our software tool (see software page https://2021.igem.org/Team:City_of_London_UK/Software). We then incubated the DNA, miRNAs with a plasmid containing a gene coding for T7 polymerase (to promote transcription of our toehold switch mRNA) as well as a TXTL cell-free master mix for 1 hour, and then pipetted the contents of each tube into 8 wells of a 96 well plate, and took four measurements after D-luciferin in a buffer was injected, at different times.
Results
The results show a brief, high peak in luminescence for a high concentration of miR-210-3p. This is because, during the incubation time, more luciferase is produced due to an increased rate of translation, so there is a higher concentration of luciferase, so more luciferase-luciferin complexes form at the beginning, converting luciferin to oxyluciferin, producing luminescence.
The first bar chart graph shows the luminescence when measurement was taken at 124 seconds, and the bottom graph shows % increase in luminescence compared to the negative control (no miRNAs added). This shows an 82% increase in expression for the target miRNA at 9M concentration, which drops to just 16% for a 2.25M concentration, showing how our toehold switch can clearly discriminate between these percentage differences in miRNA concentrations, and these concentrations are based on what we’d expect in patients with and without the condition at ten weeks. These diagrams show how the toehold switch can discriminate clearly between miR-210-3p at 9M and 2.25M.
Conclusion
The gen2 210-3p toehold switch causes a noticeable increase in translation of luciferin with increased miRNA concentration, and, with software, they would be able to binarily discriminate between levels of miRNA at concentrations indicative of the condition, given they are above 2.25M in ‘normal’ patients.
Sequence and Features
- 10COMPATIBLE WITH RFC[10]
- 12COMPATIBLE WITH RFC[12]
- 21COMPATIBLE WITH RFC[21]
- 23COMPATIBLE WITH RFC[23]
- 25COMPATIBLE WITH RFC[25]
- 1000COMPATIBLE WITH RFC[1000]
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