Part:BBa_K1045000:Experience

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Applications of BBa_K1045000

We used this Biobrick in our DarR reporter system (BBa_K1045017). When characterizing this system in E. coli, we noticed that the DarR operator sequence as it is in BBa_K1045000 seems to be strongly bound by DarR (BBa_K1045001) even in the absence of c-di-AMP.

Microscope data

For characterization, E. coli BL21 was transformed either with BBa_K1045017 or with BBa_K1045013 as a control. Both strains were grown in the abscence of c-di-AMP and subjected to fluorescence microscopy.

In BBa_K1045013, gfp is placed downstream of a strong promoter and the DarR operator. This vector does not encode for DarR. The strong fluorescence of the cells transformed with BBa_K1045013 (Fig. 1 top) indicated that GFP was expressed. However, when transformed with BBa_K1045017 (Fig. 1 bottom), the cells showed almost no fluorescence. In contrast to BBa_K1045013, BBa_K1045017 encodes for DarR. The low fluorescence suggested that DarR was expressed and active as a repressor down-regulating gfp transcription. Hence, DarR seems to act as a strong repressor in E. coli even in the absence of cyclic di-AMP.

Fig. 1.: Top: E. coli transformed with a plasmid encoding BBa_K1045013 shows a strong green fluorescence under the fluorescence microscope. Bottom: E. coli transformed with a plasmid harboring the DarR reporter system barely shows fluorescence. Both pictures represent merges of a bright field image and a GFP fluorescence image. The exposure time used to record GFP fluorescence was in both cases 2 seconds.

Plate reader data

We furthermore produced quantitative data characterizing the growth and the fluorescence over time of the BL21 E. colis we transformed with the DarR reporter system construct BBa_K1045017. As a control, we used E. coli cells harboring the BBa_K1045013 plasmid. 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 done. 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 used to test the reaction of the DarR reporter system 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. 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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