Part:BBa_K4387000

Nitric Oxide Sensing Promoter pNorVβ

Usage and Biology

The inducible pNorVβ is an optimized nitric oxide sensitive promoter controlling the NorRVW operon in E. coli. The corresponding integration host factor IHF2 binding site was removed from the promoter pNorV to exhibit good sensitivity and dosage response at a low range of inducer DETA/NO, the used nitric oxide source in our experiments, activating the downstream genes' transcription [1]. In our constructs below, this promoter was coupled to a superfolder GFP and the transcriptional regulator NorR which creates a feedback loop to finetune the amount of NorR to the number of free NO molecules. It is also an improved part of the 2016 ETH iGEM team pNorV promoter.

This promoter was tested in the bacterial strain E.coli Nissle 1917.

Characterization and Measurements

Measuring parts with different approaches and comparing them to provide a more insightful and multilayered characterization is essential in Synthetic Biology. Here, we focused on two methods:

(i) time-lapse plate reader assays to measure the sensitivity of our circuit to NorR in a dynamic manner and under different concentrations of inducer; and

(ii) endpoint flow cytometry assays to measure the behavior of our circuits at the single-cell scale.

With the first assay, we uncovered essential kinetic information about the circuits on the populational level (every measurement is an average of the individual expression patterns in the sample). With the second assay, we delved deeper into the cell populations to characterize other essential properties of our system, such as expression noise and dose-dependent responses to different inducer concentrations.

To make our experiments reproducible, during plate reader assays (PHERAstar FSX - λEx: 485 nm, λEm: 530 nm), we measured each sample for 16 hours at 37°C and constant orbital shaking, using three biological replicates (three individual colonies per circuit) and three technical replicates (three wells per biological replicate).

We performed the data analysis as follows:

1. Subtracted blanks from raw data
2. Normalize GFP by OD600
3. Calculate technical means of GFP/OD600 normalized data
4. Calculate the biological means of GFP/OD600 normalized data
5. Calculate the biological standard deviation of GFP/OD600 normalized data
6. When necessary, perform imputation. Usually, the first normalized measurements are noisy and unreliable as OD600 values can be very low and significantly impact normalization. Thus, when individual normalized values are extremely high or low (sometimes negative due to blank correction), imputation was used following a na_kalman() function from the ImputeTS R package.
7. Additional transformations, such as log transformations.

Hence, our plots show the averages and standard deviations for the biological replicates for each sample for each time point.

For the flow cytometry experiment, cell cultures were grown overnight in LB medium supplemented with antibiotic, diluted in 2mL of M9 (supplemented with glucose, cas amino acids and an antibiotic) in a 1:10 ratio (v/v), induced with different NO concentrations and grown for 7 hours in a shaker (37C, 220 RPM). Samples were then chilled in ice to halt cell growth and diluted in 1mL of cold PBS (1:500 v/v ratio). A total of 100,000 cells per sample was measured in a BD FACSCanto II flow cytometer (FSC: 625V, SSC: 420V, FITC: 650V, Event threshold: FSC & SSC > 200, Channel: FITC (λEx 488 nm / λEm. 530/30 nm, High flow rate: ~ 10,000 events/s).

We performed all analyses using in-house R scripts.

Nitric Oxide Sensing Genetic Circuit With the ETH promoter pNorV

This part consists of the ETH promoter pNorV, a superfolder GFP preceded by one strong ribosomal binding site (BBa_B0034), the NorR regulator, and a double forward terminator. We chose a high-copy backbone from Twist Bioscience for this part. We wanted to compare this ETH NorV promoter to the pNorVβ promoter and see which one was better suited for sensing nitric oxide at lower concentration ranges. According to figure__, when tested at different concentration levels, the pNorVβ had higher responses to DETA/NO induction than the NorV promoter of the ETH 2016 team.

Nitric Oxide Sensing Genetic Circuit With One Ribosomal Binding Site

This part consists of the inducible pNorVβ promoter, a superfolder GFP preceded by one strong ribosomal binding site (BBa_B0034), the NorR regulator, and a double forward terminator. We chose a high-copy backbone from Twist Bioscience for this part. Due to the competitive binding of the activated and inactivated NorR on the promoter, we decided on this construct with a positive feedback loop that adjusted the levels of NorR based on the amount of nitric oxide present [1]. The presence of nitric oxide would activate pNorVβ to induce GFP and NorR expression. Thereby, we ensure that high amounts of NorR will be produced only when NO is present.

Nitric Oxide Sensing Genetic Circuit With Two Ribosomal Binding Sites

In the frame of our project, we wanted to improve the construct BBa_K4387005 by adding one more ribosomal binding site to see if we could achieve a higher GFP response.

Thus this part consists of the inducible pNorVβ promoter, a superfolder GFP preceded by two strong ribosomal binding sites (BBa_B0029, BBa_B0034), the (NorR regulator), and a double forward terminator. We chose a high-copy backbone from Twist Bioscience for this part. Due to the competitive binding of the activated and inactivated NorR on the promoter, we decided on this construct with a positive feedback loop that adjusted the levels of NorR based on the amount of nitric oxide present [1]. The presence of nitric oxide would activate pNorVβ to induce GFP and NorR expression. Thereby, we ensure that high amounts of NorR will be produced only when NO is present.

According to figure__, we could prove that this construct with two ribosomal binding sites and the codon-optimized NorR had the highest GFP response.

While this part has higher overall GFP expression values, it is also leakier than the constructs with one or three ribosomal binding sites. If high GFP expression is required, but some leakiness does not matter much, we recommend choosing BBa_K4387006. If lower leakiness is essential, but GFP expression does not need to be very high, we recommend using parts BBa_K4387005 or BBa_K4387007 instead.

Nitric Oxide Sensing Genetic Circuit With Three Ribosomal Binding Sites

In the frame of our project, we wanted to further improve the construct BBa_K4387005 by adding two more ribosomal binding sites to see if we could achieve a higher GFP response.

Thus this part consists of the inducible pNorVβ promoter, superfolder GFP preceded by three strong ribosomal binding sites (BBa_K4387020 [2], BBa_B0029, BBa_B0034), the NorR regulator, and a double forward terminator. We chose a high-copy backbone from Twist Bioscience for this part. Due to the competitive binding of the activated and inactivated NorR on the promoter, we decided on this construct with a positive feedback loop that adjusted the levels of NorR based on the amount of nitric oxide present [1]. The presence of nitric oxide would activate pNorVβ to induce GFP and NorR expression. Thereby, we ensure that high amounts of NorR will be produced only when NO is present.

According to figure__, we could prove that the construct with two ribosomal binding sites and the codon-optimized NorR BBa_K4387006 had the highest GFP response.

Nitric Oxide Sensing Genetic Circuit Without the NorR regulator

In the frame of our project, we wanted to improve the sensitivity of our construct BBa_K4387005. For this purpose, we removed the codon-optimized NorR, creating a circuit that would rely on endogenous NorR.

Thus this part consists of the inducible pNorVβ promoter, superfolder GFP preceded by two strong ribosomal binding sites (BBa_B0029, BBa_B0034), and a double forward terminator. We chose a high-copy backbone from Twist Bioscience for this part.

According to figure__, we could prove that the construct with two ribosomal binding sites and the presence of the codon-optimized NorR BBa_K4387006 had the highest GFP response.

Exchanging the sfGFP with a Single Domain Antibody

This composite part contains the nitric oxide sensor pNorVß (BBa_K4387000), followed by 2 different ribosome binding sites. The promoter induces upon NO binding the expression of a monovalent anti-TNFα nanobodoy BBa_K4387996. The additional NorR at the end of the composite part enhances the positive feedback-loop, increasing the response of the promoter to NO. [1] Together with the hemolysin A secretion machinery BBa_K4387987, a complete genetic circuit is obtained that allows secretion of nanobodies [2] or if exchanged, secretion of other proteins of interest.

Sequence and Features

Assembly Compatibility:

• 10
  COMPATIBLE WITH RFC[10]
• 12
  COMPATIBLE WITH RFC[12]
• 21
  COMPATIBLE WITH RFC[21]
• 23
  COMPATIBLE WITH RFC[23]
• 25
  COMPATIBLE WITH RFC[25]
• 1000
  COMPATIBLE WITH RFC[1000]

References

• [1] Xiaoyu J. Chen, Baojun Wang, Ian P. Thompson, and Wei E. Huang et al. Rational Design and Characterization of Nitric Oxide Biosensors in E. coli Nissle 1917 and Mini SimCells ACS Synthetic Biology 2021 10 (10), 2566-2578 DOI: 10.1021/acssynbio.1c00223
• [2] Ayelet Levin-Karp, Uri Barenholz, Tasneem Bareia, Michal Dayagi, Lior Zelcbuch, Niv Antonovsky, Elad Noor, and Ron Milo et al. Quantifying Translational Coupling in E.coli Synthetic Operons Using RBS Modulation and Fluorescent Reporters ACS Synthetic Biology 2013 2 (6), 327-336 DOI: 10.1021/sb400002n