OVERVIEW

Our associative learning circuit requires a sensor that can detect simultaneously occuring signals, in this case nitric oxide and AHL (Figure 1). Such a sensor with these specific inputs has not been described in the literature before. Thus we generated a collection of AND gates responsive to NO and AHL in order to find a design that best fits the requirements of our system.

We also wanted to demonstrate the flexibility of our system and created one more AND gate responsive to lactate and NO. The lactate portion of the AND gate was based on the work done by ETH Zurich 2015 team.

Components of the AHL-based AND gate

NorV promoter (PnorV) is the native promoter controlling the nitric oxide reduction operon (norRVW) in E. Coli. [1]. It's transcriptional regulator, NorR, can bind nitric oxide and activate gene expression. We used this promoter in combination with basal NorR production in E.coli as the Nitric Oxide (NO) sensor of our AND gate. The PnorV promoter can also be found among the biobricks we submitted to the registry Part:BBa_K2116002.

EsaR (Part:BBa_K2116001) is a transcriptional regulator of the Pantoea stewartii quorum sensing system [2]. In the abscence of 3OC6HSL it can bind DNA and inhibit transcription. An esabox is a 18bp sequence where EsaR can bind. Unlike other quorum sensing regulators EsaR acts as a transcriptional repressor and not an activator. We took advantage of this property, and placed esaboxes either;

i) as roadblocks after transcription start site of PnorV, preventing the polymerase from advancing or;

ii) within the PnorV to establish competitive binding between NorR and EsaR or RNA polymerase and EsaR.

Design Considerations

i) Roadblock AND Gates

This collection of AND gates each have either one, two or three esaboxes placed as roadblock. Since more than one EsaR binding close to each other could create steric hindrance, we constructed variants where the spacing between the esaboxes is either 8bp or 15bp.

System Design	Biobrick
	BBa_K2116004
	BBa_K2116005
	BBa_K2116006
	BBa_K2116007
	BBa_K2116008
	BBa_K2116012
	BBa_K2116013
	BBa_K2116014
	BBa_K2116011
	BBa_K2116027

ii) Competitive Binding and/or Roadblock AND Gates

Here our aim was to prevent either NorR or the sigma54 factor of RNA polymerase from binding the promoter. There are 3 NorR binding sites, and one sigma54 binding site on the NorV promoter (Figure 2). We placed esaboxes either right before the sigma54 binding site, as a replacement for part of the sigma54 binding site, or upstream of one of the NorR binding sites. For one of these designs, we also have a variant where an additional esabox is placed as a roadblock. This was designed as a solution to potential problems with leakiness of the AND gate.

System Design	Biobrick	Comments
a second table with the corresponding biobricks

Lactate-based AND gate

Tina.

Model

PnorV is a looping promoter, which allows NorR to come in contact with the RNA polymerase. Transcription is initiated once NO binds and NorR is active. [3] The initial results showed that the AND gate was repressed by EsaR, but could not be fully induced. Our first hypothesis was that looping prevents EsaR from unbinding. However:

Sophie.

Conclusion

After careful evaluation, we picked one variant to be the main AND gate of our system. This AND gate demonstrated the best behaviour within the range of AHL and NO concentrations we wanted to detect.

Based on the model, suggestions of how we can solve the EsaR derepression problem

We have a collection of AND gates characterised experimentally and with a detailed model that could be used by future iGEM teams.

References:

[1] Gardner, A. M. "Regulation Of The Nitric Oxide Reduction Operon (Norrvw) In Escherichia Coli. ROLE OF Norr AND Sigma 54 IN THE NITRIC OXIDE STRESS RESPONSE". Journal of Biological Chemistry 278.12 (2003): 10081-10086.

[2] Minogue, Timothy D. et al. "The Autoregulatory Role Of Esar, A Quorum-Sensing Regulator In Pantoea Stewartii Ssp. Stewartii: Evidence For A Repressor Function". Molecular Microbiology 44.6 (2002): 1625-1635. Web.