Part:BBa_K3128001:Experience

Team Grenoble Alpes 2019

Applications of BBa_K3128001

More information : Grenoble-Alpes 2019

Our detection system is based on the use of a BACTH. The point is to allow the induction of the gene only when the two sub-parts of Adenylate Cyclase (AC) are physically close, which only occurs when the target is present in the sample.
The re-formation of AC then enables cAMP production, which will activate a CAP dependent promotor allowing the transcription of the following gene.
For this we needed to use an AC deficient bacteria strain (BTH101) that that can’t produce endogenic cAMP to prevent any transcription from CAP dependant promoter such as lactose promoter.
For the choice of the promoter, we decide to use the lactose promoter (a CAP dependent promoter) and we have demonstrated its repression in the absence of cAMP (in the AC deficient bacterial strain), thus preventing any transcription of the following gene : the reporter (see table and figure 1). To resume, the gene has to be expressed/overexpressed only when cAMP is produced and there needs to have a clear difference when the sub-parts are brought together by the target or when the target is not here and the sub-parts remain free.

To prove that the reporter gene efficiently works in our system different conditions were tested. First the leak of our reporter when there is no cAMP was measured by transforming the plasmid containing the BioBrick PLac_NanoLuc in BTH101.

Then the free sub-parts condition was tested by co-transforming two plasmids in BTH101: pUT18 containing the AC sub-part T18 and pKT25_NLuc containing both the AC sub-part T25 and the BioBrick PLac_NanoLuc.

At last the target detection condition was tested. Leucine-Zipper (LZ) were used to simulate the presence of the target and the physical connexion between both sub-parts. LZ have the capacity to form homodimer and so were added at the end of both sub-parts making them able to stick to each other thus restoring the AC activity.
Two plasmids were co-transformed in BTH101: pUT18-LZ containing the AC sub-part T18 fused with a LZ and pKT25-LZ_NLuc containing both the AC sub-part T25 fused with a LZ and the BioBrick PLac_NanoLuc.

If there is a notable difference of luminescence between the free sub-parts and the target detection then it will mean that our reporter gene could work in our system.
It will also show that the on/off switch of the transcription depending on cAMP is working.

The assay

Bacterial culture were induced with 0.5mM of IPTG at Optic Density 0.6.
The subtract for Nano Luciferase (furimazine) was added as follow : for 50uL of bacterial culture in a well, 49uL of NanoGlo Assay Buffer and 1uL of NanoGlo Assay Substrat were added.
All the measures are expressed in Relative Luminescence Units (RLU) in a NUNC 96 wells plate.
Two different bacterial cultures (sample) were assessed each time in duplicate (except for the 24 hours condition).
Blank was done with non-transformed BTH101 (RLU = 300) and subtracted to each the measurements.

Results

The second well for sample 2 was removed because the assay substrate wasn’t added.

Conclusion

Figure 4 : Measurement of the Nano Luciferase assays of the 3 conditions.

Those measurements highlight two major things with our reporter Biobrick:
First, there is a luminescence gap between the assay without endogenous adenylate cyclase (grey) and with the free sub-parts (orange) which means that there is a leak of our reporter protein due to random occurrences between both T18 and T25.
However there is a significant* difference between the assay with or without LZ which means that, when the target is present and brings the two parts together, the gene (here Nano Luciferase) is overexpressed.
With this BioBrick it is then possible to see the difference between both condition: if the two sub-parts are close and if they are free and so tell if the theoretical target is present or not.
In realist conditions the difference will probably not be as flagrant as here because the LZ system is more efficient to bring the parts together than the assay done with the target and aptamer (see the full system).
* A T test was done for the values of time above 90 min and led to a p-value below 0.05.

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