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 control plasmid encoding BBa_K1045013. Bottom: E. coli transformed with a plasmid harboring the DarR reporter system BBa_K1045017. Cells of both strains were cultured without c-di-AMP and analyzed by fluorescence microscopy. 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.

Experimental details: E. coli cells were grown in LB medium until log phase. A culture aliquot was prepared on slides covered with 1 % agarose (in water) and the cells observed under the fluorescence microscope. For all images, the same exposure time was used. Microscope: Axioskop 40 FL fluorescence microscope; Camera: digital camera AxioCam MRm; Software for image processing: AxioVision Rel version 4.8 (Carl Zeiss, Göttingen, Germany); Objective: Neofluar series objective (×100 primary magnification); Filter set: eGFP HC-Filterset (band-pass [BP] 472/30, FT 495, and long-pass [LP] 520/35; AHF Analysentechnik, Tübingen, Germany) for GFP detection.

Characterization of the Reporter System in a Multi-Well Plate Reader

We furthermore analyzed 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 employed E. coli cells harboring the BBa_K1045013 plasmid. This plasmid carries only the GFP expression unit with a strong promoter and the DarR operator. It does not encode for DarR.

Using the plate reader, we quantified the strength of the DarR construct in E. coli. A dilution series of c-di-AMP (0, 50, 100, 150, 300, 500 and 1000 nmol c-di-AMP) was used to test the reaction of the DarR reporter system to the nucleotide.

Fig. 2 shows the growth curves recorded via the optical density (OD) at the wavelength 600 nm. The GFP fluorescence was measured at 509 nm over the time, as well. Since the fluorescence depends on the growth of the E. coli cells, the GFP fluorescence was normalized to the OD at 600 nm for each time point (Fig. 3). All graphs show the mean value of three technical replicates of one biological replicate. The error bars indicate the standard deviation.

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: The growth of the E. coli cells was measured in a plate reader via the OD at 600 nm. To facilitate the differentiation between the growth phases, the OD at 600 nm is depicted in log scale. Top: E. coli cells carrying the control plasmid BBa_K1045013; Bottom: E. coli cells transformed with the DarR reporter system BBa_K1045017. The cells were cultured with c-di-AMP in different concentrations or without c-di-AMP. Please enlarge the pictures for better reading (click on them).

Fig. 3: The GFP fluorescence measured at 509 nm was normalized to the OD at 600 nm. Top: E. coli cells carrying the control plasmid BBa_K1045013; Bottom: E. coli cells transformed with the DarR reporter system BBa_K1045017. The cells were cultured with c-di-AMP in different concentrations or without c-di-AMP. 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. It seems also possible that the c-di-AMP amounts used might have been too low to see an even stronger repression of gfp expression, since the Kd of DarR was shown to be 2.3 µM (Zhang et al., 2013). In addition, E. coli might be unable to take up c-di-AMP, as the characterization experiments of BBa_K1045002 suggested.

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. For this, a way allowing E. coli to take up c-di-AMP might have to be established first. Alternatively, a c-di-AMP-synthesizing diadenylate cyclase (DAC) could be expressed in E. coli cells harboring the reporter system. Part BBa_K1045003 encodes for a truncated version of the DAC from Listeria monocytogenes, which is active in E. coli. Using this part, c-di-AMP could be generated in vivo.

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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