Expression platform

To examine the possible effects of parallel sensing, 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 (NPK) sensor, which aims to combine three constructs characterized by different outputs within a single plasmid.

As the substitute of the KPN inducible promoters, well-characterised promoters P_BAD and P_lac were used as their biological mechanisms are better understood.

The construction of a double construct P_BAD - GFPmut3b - P_lac - 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 Parallel Sensor Experiment Protocol for more details.

Results

After obtaining characterization data for the P_BAD - GFPmut3b single construct, the dose response curve was compared with that expressed in a double construct (with P_lac - 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 P_BAD system. Furthermore, from Figure 5b, it is shown that when the co-expressed system is under full induction, the curve for the double construct P_BAD 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 co-expressed P_lac - mRFP construct on the GFPmut3b expression of the double construct P_BAD system, experiments were carried out using varying IPTG concentrations.

Figure 6 exhibits that varying the induction level of the co-expressed P_lac - mRFP construct seems to have an effect on the expression level of P_BAD - 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) where the graph shifts to the right. No significant shift was observed at lower IPTG concentrations.

Conclusion

By using the P_BAD - GFPmut3b and P_lac - 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 P_BAD - GFPmut3b, whether or not the co-expressed 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(P_BAD - GFPmut3b and P_lac - mRFP), the data obtained cannot be assumed to apply to all instances of expressing a characterized construct in a double construct, including that for the NPK 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, characterization of more combinations of inducible reporter systems (eg. P_luxR - GFPmut3 - P_lac - mRFP, P_tetO - GFPmut3b - P_lac - mRFP) is required.

Charaterize P_BAD - GFPmut3b P_lac - mRFP with a third construct (i.e. P_tetO and P_luxR) This will give a more reliable comparison for a triple construct NPK promoter system, as opposed to a double system.

Co-express potassium, phosphate and nitrate sensors and 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.