Part:BBa_K1045017:Experience

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Applications of BBa_K1045017

The DarR Reporter System

Microscope Data

As described on our [http://2013.igem.org/Team:Goettingen/Project Wiki], we designed a c-di-AMP-sensing screening system for the Gram-negative bacterium Escherichia coli. Using this system, we can screen for substances that compete with c-di-AMP for binding to essential target proteins. Promising substances that bind these proteins might serve as a starting point for developing novel antibiotics that kill multi-resistant pathogenic bacteria. Since c-di-AMP is not present in E. coli and thus not needed for growth, we can sort out those substances that simply kill the bacteria and do not bind to c-di-AMP-binding proteins!

To characterize the DarR reporter system, the E. coli strain BL21 was transformed either with BBa_K1045017 or with BBa_K1045013 as a control. In BBa_K1045013, gfp is placed downstream of a strong promoter and the DarR operator. This vector does not encode DarR. The strong fluorescence signal of cells transformed with BBa_K1045013 indicated that GFP was produced. However, when transformed with BBa_K1045017 (Fig. 1), the bacteria showed almost no fluorescence signal. In contrast to BBa_K1045013, BBa_K1045017 encodes the repressor DarR. The low fluorescence signal suggests that DarR was synthesized and fully active as a repressor that prevents gfp transcription by binding to the DarR operator. Hence, DarR seems to act as a strong repressor in E. coli even in the absence of c-di-AMP.

Fig. 1.: Top: E. coli cells that harbor plasmid encoding BBa_K1045013 show a strong green fluorescence signal when analyzed with a fluorescence microscope. Bottom: E. coli harboring the plasmid that contains the complete DarR reporter system BBa_K1045017 do not emit a green fluorescence signal. Both pictures represent merges of a bright field image and a GFP channel image. The exposure time used to record GFP fluorescence was in both cases 2 seconds.

Characterization of the reporter system in a multi-well plate reader

We furthermore produced quantitative data characterizing the growth and the fluorescence over time of the BL21 E. colis we transformed with this construct. The following graphs show the results of the plate reader experiments performed to quantify the strength of the DarR construct in E. coli. Shown are growth curves measured at the wavelength 600 nm for the cell density (Fig. 2) and 509 nm for the GFP (Fig. 3), which is encoded in the construct. For each measurement, three technical and two biological replicates were set up. The graphs show the mean value of the technical replicates and one of the biological replicates. As written in the legend, a dilution series of c-di-AMP was set up to test the DarR reporter reaction to the nucleotide. Experimental setup: total time 21 h; 15 min measurement interval; 37°C, medium shaking; 96-well titer plate; Synergy Mx Monochromator-Based Multi-Mode Microplate Reader; Gen5 V2.01

Fig. 2: Top: Growth curve of the cells with the DarR construct; Bottom: Growth curve of the GFP Control (Cells transformed with the reporter system, but without the repressor DarR).Please enlarge the pictures for better reading. (click on them)

Fig. 2: Top: Fluorescence curve of the cells with the riboswitch construct; Bottom: Fluorescence curve of the GFP Control. Please enlarge the pictures for better reading (click on them).

As in the microscope experiments described above, DarR prevented expression of the reporter, even without c-di-AMP. It was observed that the presence of c-di-AMP, regardless of the concentration used, had no effect on the gfp expression. This data indicated a high-affinity binding of DarR to its operator in E. coli in the abscence of c-di-AMP.

In conclusion, the experiments showed that the cells can grow with the construct, and that DarR is highly active as a repressor. In the future, mutagenesis of the operator sequence or the binding motive in the protein might lower the strength of the repressor. This could make it possible to control DarR binding to the operator via different c-di-AMP concentrations. Intermediate GFP expression leves for BBa_K1045017 would allow compound screening to find novel antibiotics directed against c-di-AMP. In contrast, regarding the current binding strenght of DarR BBa_K1045001 to the operator BBa_K1045000, these two biobricks could serve as an "inverter". Controlled by an inducable promoter, DarR would stop the transcription of a gene connected to the DarR operator sequence only upon induction of DarR expression.

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