Expression platform

To examine the possible effects of coexpression, the dose-dependent fluorescence response of one construct in a single plasmid was compared to that of two constructs in a single plasmid. The purpose of the comparison relates to the design of our potassium, phosphate and nitrate (KPN) sensor, which aims to combine three constructs characterized by different outputs within a single plasmid.

The construction of a double construct araBADp-lacZp allowed for a comparison between GFPmut3b and mRFP measurements while changing concentrations of the individual inducers. Graphs of changes in fluorescence according to different inducer concentrations for both single and double constructs could be analysed and compared.

The construction of a double construct araBADp-GFPmut3b-lacZp-mRFP allowed for a comparison between GFPmut3b and mRFP measurements while changing concentrations of inducers. Dose response curve for both single and double constructs could then be analysed and compared.

Constructs

Constructs were built by digestion and ligation method.

Experiments performed

For all experiments, the bacteria were first grown overnight, then a 25-fold dilution was carried out using M9 minimal medium with specific inducers and concentrations. After that, they were transferred into a 96-well deep well plate for overnight induction. The samples were then further diluted ten-fold the next day. The cells were allowed to grow from lag phase to log phase for several hours. Ultimately, the result was gathered using a EnVision® Multilabel Reader. Data collected was plotted as graphs for further analysis.

Please visit Signal Co-expression Experiment Protocol for more details.

Results

After obtaining characterization data for the araBADp-GFPmut3b single construct, the dose response curve was compared with that expressed in a double construct (with lacZp-mRFP).

From Figure 5a, it can be observed that in a double construct, the maximum expression of the reporter decreases for the L-Arabinose inducible araBADp system. Furthermore, from Figure 5b, it is shown that when the coexpressed system is under full induction, the curve for the double construct araBADp shifts to the right compared to both the single construct and double construct without induction, indicating a decrease in sensitivity.

In order to further investigate the effect of induction level of the coexpressed lacZp-mRFP construct on the GFPmut3b expression of the double construct araBADp system, experiments were carried out varying the IPTG induction concentrations.

Figure 6 exhibits that varying the induction level of the coexpressed lacZp-mRFP construct seems to have an effect on the expression level of araBADp-GFPmut3b in a double construct. At a higher IPTG concentration, the maximum expression of GFPmut3b decreases. However, no trend can be observed for the horizontal shift of the curve, except at the highest IPTG concentration (2² mM) wherethe graph shifts to the right. No significant shift was observed at lower IPTG concentrations.

Conclusion

By using the araBADp-GFPmut3b and lacZp-mRFP inducible systems, the possible difference of dose dependent fluorescence expression in a double construct and single construct was investigated. In a double construct, the maximal expression was lower than the single construct induced araBADp-GFPmut3b, whether or not the coexpressed construct was induced. We conjecture that it is possibly due to the limited cellular resources (e.g. amino acids, enzymes required for gene translation/transcription etc.) and the synthesized proteins (e.g. lacI, mRFP) that could increase the cell load and affect its growth. However, since the investigation was done focusing only on one combination of inducible constructs(araBADp-GFPmut3b and lacZp-mRFP) it may not be used to generalize the effect of expressing a charactered construct in a double construct, including that for the KPN construct. Thus, further improvements should be made.

Further Improvements

Since characterization data from one combination of double construct cannot be applied to all double constructs in general, characterisztion of more combinations of inducible reporter systems (eg. luxRp-GFPmut3-lacZp-mRFP, tetOp-GFPmut3b- lacZp-mRFP) is required.

Charaterize araBADp-GFPmut3b lacZp-mRFP with a third construct (i.e. tetOp and luxRp) This will give a more reliable comparison for a triple construct KPN promoter system, as opposed to a double system.

Coexpress potassium with phosphate and nitrate sensors with their respective inducers.

References

Ang, J., Harris, E., Hussey, B. J., Kil, R., & McMillen, D. R. (2013). Tuning response curves for synthetic biology. ACS synthetic biology, 2(10), 547-567.

Wang, B., Barahona, M., & Buck, M. (2013). A modular cell-based biosensor using engineered genetic logic circuits to detect and integrate multiple environmental signals. Biosensors and Bioelectronics, 40(1), 368-376.

Wang, B., Barahona, M., & Buck, M. (2015). Amplification of small molecule-inducible gene expression via tuning of intracellular receptor densities. Nucleic acids research, gku1388.