Theophylline Riboswitches

To characterise the theophylline riboswitches (1 and 2), we quantified their activation at different theophylline concentrations (0 mM, 0.5 mM, 1 mM, 1.5 mM and 2 mM) over a period of time using fluorometry. Competent E. coli (strain DH5a) cells were transformed with plasmid vectors containing the riboswitch and were cultured until the mid-log phase of growth. Thereafter, a different concentration of theophylline was added to each culture for induction. The activation of the riboswitch was detected as a fluorescent response as a result of increased translation of the fluorescent fusion protein CheZ-Venus, in the presence of the activator. A Jasco FP-6300 spectrofluorometer was used to excite the cultures at 514 nm and the intensity of the emission peak was detected at 528 nm. Twenty readings were taken every 15 minutes for each culture, for a total period of 135 minutes. An empty vector was used to correct for autofluorescence. Conversely, a construct that comprised of a RBS CheZ Venus (BBa_K537013) was used as a positive control. Fluorescent data for the 3D plots (Fig 1-4) were corrected for autofluorescence by subtracting the fluorescent values of the cultures transformed with the empty vector, from that of the riboswitches.

Both the theophylline riboswitches displayed activation in response to increasing theophylline concentrations and incubation times. Theophylline riboswitch 2 controlling CheZ-Venus expression (BBa_K537012) clearly showed a superior level of activation compared to that theophylline-controlled riboswitch 1 (BBa_K537011). This was expected as it was selected by FACS for its optimised expression platform that allows for minimal leakiness at basal levels, and a strong activation in the presence of theophylline (Lynch and Gallivan, 2009). The activation shown by both the riboswitches regulating CheZ-Venus expression should be able to restore the motility of E. coli cells deficient in CheZ, in the presence of theophylline. More importantly, the theophylline concentration-dependent increase in CheZ-Venus expression alludes to the ability of directed cell movement, that is moving up concentration gradients of theophylline (Topp and Gallivan, 2007). Interestingly, the data suggests that the presence of the CheZ gene, as part of the CheZ-Venus fusion sequence, enhances the activation of the switches when compared to the constructs with venus alone. This may be due to some sort of structural stability conferred by the presence of the CheZ sequence.

Atrazine Riboswitch

Preliminary activation data was obtained for the atrazine riboswitch.Fluorometry was performed on E. coli (DH5a) cells transformed with a construct that contained a Atrazine Riboswitch mRFP(BBa_K537014). Similar to the theophylline riboswitches, different concentrations (0 mM, 0.5 mM, 1 mM, 1.5 mM, 2 mM) of atrazine were added to the cultures for induction. The cultures were excited at 584 nm and detected at 607 nm using a Jasco FP-6300 spectrofluorometer. Readings were taken every 15 minutes for 120 minutes in total. The same empty vector was used as a negative control. The 3D plot data was normalised the same way as for the theophylline riboswitches.

References

Lynch, S. A. & Gallivan, J. P. 2009. A flow cytometry-based screen for synthetic riboswitches. Nucleic Acids Res, 37, 184-92.

Topp, S. & Gallivan, J. P. 2007. Guiding bacteria with small molecules and RNA. J Am Chem Soc, 129, 6807-11.

Epi-fluorescence Wide-Field Microscopy

Fluorescent imaging was performed for a qualitative assay of the engineered bacteria.

200 µl of each bacterial culture sample was imaged using a Zeiss Axioscope (Andor EMCCD Camera, Till Photonics Polychrome V)in microscopic plates.

Widefield fluorescence microscopy was used to assess the expression of the Venus protein. The bacterial samples were excited at 500 nm using the polychrome as an excitation source. The fluorescence was then observed by capturing light in the emission spectrum of Venus (528 nm). Corresponding bright-field images (using white light) were also captured for each experiment. For a protocol of our methods, please click here.

Below are the images that were obtained for the parental strain (BW25113) of the E. coli ΔCheZ- mutant (which served as the negative control) and the ΔCheZ mutants transformed with Riboswitch 1 Venus (BBa_K537009) and Riboswitch 2 Venus (BBa_K537010).

Please note: Bacteria which were transformed with constructs which contained the CheZ gene were not tested in this assay. This experiment served as a means of observing the activation of the theophylline riboswitches, hence only expression of the venus reporter protein was needed for detection of fluorescence. Activation of motility was not assessed in this case – it was examined in the motility assays on semi-solid agar plates. Differences in fluorescence intensities were not calculated from the images below since the fluorometry experiments served as the quantitative approach to measurement of riboswitch activation.

Fluorescent imaging was performed again after the European Jamboree, using a different technique. For a protocol of our "smeared culture" methods, please click here. The aim of this data is to depict the exact same brightfield and fluorescent static fields of view.

Figure 8: Theophylline-mediated activation of gene expression using riboswitches type 1 and 2: Riboswitch 1 CheZ Venus(BBa_K537011), Riboswitch 2 CheZ Venus (BBa_K537012), Riboswitch 1 Venus (BBa_K537009) and Riboswitch 2 Venus (BBa_K537013) (Click to enlarge)

Semi-solid agar plates containing different concentrations of theophylline (0mM, 0.5mM, 1.0mM and 1.5mM) were made. Riboswitch 1 CheZ Venus(BBa_K537011), Riboswitch 2 CheZ Venus (BBa_K537012), Riboswitch 1 Venus (BBa_K537009) and Riboswitch 2 Venus (BBa_K537013) were transformed into ΔCheZ mutant E.coli and grown to mid-log phase (OD600 = 0.55). 1.5 µl of culture was plated into the centre of the plates containing the various concentrations of theophylline and incubated at 37°C for 24 hours. This experiment was performed in triplicate. The protocol for this experiment can be found here.

Discussion and conclusion

The construct Riboswitch 1 CheZ Venus transformed into the ΔCheZ bacteria are non-motile at a concentration of 0 mM theophylline, since in the absence of the activator, the riboswitch is sequestering the RBS and thus inhibiting the translation of CheZ (Topps and Gallivan, 2007). On the semi-solid agar plates containing theophylline, the motility of E.coli is evident, as the interaction of theophylline with the riboswitch enables the exposure of the RBS and the translation of the downstream CheZ, allowing for bacterial motility. The distance travelled by the bacteria transformed with the Riboswitch 1 CheZ Venus construct, from the start position (mm) in the presence of different concentrations of theophylline, is not significantly different (P=0.005). This indicates that the CheZ expression at 0.5 mM-1.5 mM is sufficient to allow the bacteria to move a distance of 5 mm. The construct Riboswitch 2 CheZ Venus transformed into the CheZ deletion mutants are slightly motile in the absence of theophylline, travelling 2mm over a 24 hour period indicating that this riboswitch is slightly leaky. Upon addition of 0.5 mM-1.5 mM theophylline, the bacteria were able to travel 20mm over a 24 hour period.

The distance travelled from the start position by the CheZ deletion mutants containing the Riboswitch 2 CheZ Venus construct is significantly higher than that those with the Riboswitch 1 CheZ Venus construct. This can be explained by the high fold-increase in CheZ protein expression by this riboswitch, as shown by fluorometry (46-fold higher compared to 8-fold higher). Theophylline riboswitch 1 has a lower fold increase in CheZ expression from 0 mM to 1.5 mM theophylline, compared to the increase shown by the theophylline riboswitch 2. Therefore, in order to achieve maximum motility under the control of theophylline, at concentrations of between 0.5 mM and 1.5 mM, theophylline riboswitch 2 should be used.

References

Both the Riboswitch 1 Venus and Riboswitch 2 Venus constructs transformed into the ΔCheZ mutant E.coli are non-motile, each having travelled a distance of 0mm at all concentrations of theophylline (See A and B). This indicates that in the absence of the CheZ protein, the bacteria are non-motile (Topps and Gallivan, 2007).

In order to determine whether we had successfully engineered the E. coli CheZ deletion mutants to chemotactically migrate towards theophylline, we performed a capillary assay. Bacteria transformed with Riboswitch 1 Venus (BBa_K537009), Riboswitch 2 Venus (BBa_K537010), RBS CheZ Venus (BBa_K537013), Riboswitch 1 CheZ Venus (BBa_K537011) and Riboswitch 2 CheZ Venus (BBa_K537012) were grown to mid-log phase. Activation of chemotaxis was achieved by incubating the cultures with 1 mM theophylline prior to the assay. A 50 µl drop of each culture was then placed onto separate sterile surfaces and two capillary tubes were placed into each drop. The first capillary tube was a control, containing only PBS and 20 µM EDTA. The second tube contained PBS, 20 µM and 2mM theophylline. The two capillary tubes were suspended into each drop of culture for 30 minutes. A dilution series of the solutions in the tubes were plated and colonies counted after 12 hours. Further details of this protocol can be found here.

Discussion and conclusion

Cells transformed with Riboswitch 1 Venus, Riboswitch 2 Venus and RBS CheZ Venus all displayed an index of approximately 100, indicating the same number of bacteria moved towards the control (0mM theophylline) and the experiment (2mM theophylline) capillary tube. This proves that none of these constructs induce chemotactic behaviour. In terms of both Riboswitch 1 Venus and Riboswitch 2 Venus, the lack of CheZ results in the inability of CheY dephosphorylation and the swimming motion observed in chemotaxis, which is a key driver in directed movement towards an attractant (Topps and Gallivan, 2007). Although the RBS CheZ Venus construct constitutively expresses CheZ, and bacterial swimming is achieved through CheY dephosphorylation, the expression levels remain constant regardless of the presence or absence of theophylline. Random movement will occur as a result.

The Riboswitch 1 CheZ Venus and Riboswitch 2 CheZ Venus transformants, with chemotaxis indices of 230 and 175 respectively, both indicated a significantly higher number of bacteria that travelled to the 2mM theophylline solution in the capillary tube compared to that of the control.

References