Revision as of 15:57, 12 October 2023

TetR repressor

Introduction

The 2023 UT Austin iGEM Team’s modular microcin expression parts collection includes parts necessary for engineering a bacterial chassis to secrete microcins, a type of small antimicrobial peptide. Our team has specifically designed parts to engineer a modular two-plasmid system that facilitates extracellular secretion of microcins by the chassis. One plasmid contains the microcin with a signal peptide sequence that indicates to the cell that the microcin is to be secreted. The other plasmid (pSK01) is from the literature (Kim et al., 2023) and contains genes for the proteins CvaA and CvaB, which are necessary to secrete small peptides using the E. coli microcin V (MccV) type I secretion system (T1SS) shown in Figure 2 of our Project Description.

Our parts collection includes a a selection of promoter (Type 2), coding sequence (Type 3), and terminator/regulatory gene (Type 4) parts that can be easily assembled to express microcins either constitutively or under inducible control. This allows for the modular engineering of microcin expression plasmids containing various microcins that can undergo extracellular secretion when used in conjunction with the secretion system plasmid pSK01.

Our basic and composite parts follow the Bee Toolkit/Yeast Toolkit standard of Golden Gate assembly (Lee et al., 2015; Leonard et al., 2018). Our assembly method involves the use of BsmBI digestion-ligation to create basic parts which can then be further digested with BsaI and ligated to form composite parts. The BTK/YTK standard includes part type-specific prefix and suffix overhangs generated by BsaI for each part, and these overhangs are NOT included in their sequences in the registry. For reference, our standard’s part type-specific overhangs are listed in Figure 2 on our Parts page.

Categorization

Basic parts

• Promoters (Type 2) – Seven inducible promoters selected due to their relatively high dynamic range (Meyer et al., 2019) and apparent functionality in a variety of Proteobacteria (Schuster & Reisch, 2021), and one constitutive CP25 promoter (Leonard et al., 2018).
• Coding Sequences (Type 3) – Signal peptide + microcin fusion coding sequences, a green fluorescent protein gene, and secretion system genes cvaA and cvaB which are together referred to as CvaAB.
• Terminators/Regulatory Genes (Type 4) – An rpoC terminator plus a collection of seven regulatory genes, each associated with one of our seven inducible promoters.

Composite parts

• Constitutive Microcin Expression Assemblies - Assemblies of microcins (some with immunity proteins) with a constitutive CP25 promoter and rpoC terminator. These function alongside pSK01 in a two-plasmid secretion system, and we use these two-plasmid systems to assess if our novel microcins are effective inhibitors of pathogenic targets.
• Inducible GFP Expression Assemblies – Assemblies of GFP under the control of various inducible promoter systems. These were used to assess the dynamic range of our inducible promoter systems.
• Inducible Microcin Expression Assemblies – Assemblies of select microcins under the control of an inducible promoter system.

Usage and Biology

This is a Type 4 part that includes a terminator upstream of a transcriptional unit that constitutively expresses a regulatory transcription factor. This part can be used in conjunction with the Type 2 P_Tet* promoter part to regulate transcription of a Type 3 (or Type 3p+3q) coding sequence determined by the user. In the absence of anhydrotetracycline (aTc), transcription of the coding sequence is in a net ‘off’ state. When aTc is added, it binds to TetR and removes it from P_Tet* promoter. This turns transcription of the Type 3 coding sequence to the ‘on’ state. The terminator at the 5’ end of this part’s sequence marks the end of the preceding transcriptional unit started with the Type 2 promoter and Type 3 coding sequence parts. The remainder of this part’s sequence is the constitutively active transcriptional unit for TetR.

Composite Parts

• TetR regulated Mcc04 + IP expression plasmid (BBa_K4579056)
• TetR regulated Mcc04 expression plasmid (BBa_K4579057)
• TetR regulated GFP expression plasmid (BBa_K4579061)

Design Notes

When designing our transcriptional regulator parts, we chose to use the YFP expression individual sensor plasmids from the E. coli ‘Marionette’ paper as PCR templates (Meyer et al., 2019) as these contain inducible promoters and their regulatory transcription factors controlling expression of YFP on a standardized backbone. Due to the homology of the backbone between these plasmids, we created universal primers that allowed us to amplify the regulator from any one of the sensor plasmids.

Characterization

​​Below is the fluorescence characterization data for the four inducible promoter-regulator systems that we tested in composite parts using a GFP reporter gene (BBa_K4579035.. The table below lists the inducible promoters within our parts collection, their basic part numbers, and their composite part numbers with the GFP reporter. Below that, the first figure shows fluorescence/OD600 data for the characterization of our inducible GFP assemblies in E. coli DH5α. This first experiment was done as a proof of concept to test whether our inducible GFP assemblies were functional. After confirming functionality of the assemblies by measuring levels of fluorescence under induced and non-induced conditions, we moved on to testing the assemblies in Pantoea agglomerans PNG92-11 in order to determine whether these inducible promoter systems are functional in Pantoea. This data confirms the functionality of our inducible promoter systems in Pantoea.

To test each of the inducible promoter systems we incorporated them with GFP into composite parts (BBa_K4579058, BBa_K4579059, BBa_K4579060, BBa_K4579061). The parts were sequence confirmed and subsequently transformed into DH5α. Inducers were added using the following volumes (LacI: 50 µL of 0.1 M IPTG, VanR: 5 µL of 100 mM vanillic acid, CinR: 5 uL of 10 mM OHC14, TetR: 18 µL of a ~54 µM stock). These cultures were then loaded into a clear backed 96 well plate (in triplicate) and into a plate reader. A single fluorescence reading was taken using Ex: 485, Em: 535 (Elston et al., 2023).

In order to test our inducible promoters with their respective regulatory genes in a chassis organism with a secretion system we transformed our inducible GFP expression plasmids into a P. Agglomerans strain with a T1SS. We opted to not use the LacI regulatory system (BBa_K4579058) since it had high levels of expression in the absence of inducer. 5 ml induced and uninduced overnight cultures were created per each strain. Inducers were added using the following volumes (VanR: 5 uL of 100 mM vanillic acid, CinR: 5 uL of 10 mM OHC14, TetR: 18 uL of a ~54 uM stock). These cultures were then loaded into a clear backed 96 well plate (in triplicate) and into a plate reader. A single fluorescence reading was taken using Ex: 485, Em: 535 (Elston et al., 2023).

Source

This transcriptional regulator part was amplified from pAJM.011 using PCR before being integrated into a basic part plasmid for use in our assemblies. pAJM.011 contains YFP under inducible control by P_tet* and TetR.

References

1. Cole, T. J., Parker, J. K., Feller, A. L., Wilke, C. O., & Davies, B. W. (2022). Evidence for widespread class II microcins in Enterobacterales Genomes. Applied and Environmental Microbiology, 88(23), e01486-22.
2. Kim, S. Y., Parker, J. K., Gonzalez-Magaldi, M., Telford, M. S., Leahy, D. J., & Davies, B. W. (2023). Export of Diverse and Bioactive Small Proteins through a Type I Secretion System. Applied and Environmental Microbiology, 89(5), e00335-23.
3. Lee, M. E., DeLoache, W. C., Cervantes, B., & Dueber, J. E. (2015). A highly characterized yeast toolkit for modular, multipart assembly. ACS Synthetic Biology, 4(9), 975-986.
4. Leonard, S. P., Perutka, J., Powell, J. E., Geng, P., Richhart, D. D., Byrom, M., Kar, S., Davies, B. W., Ellington, D. E., Moran, N. A., & Barrick, J. E. (2018). Genetic engineering of bee gut microbiome bacteria with a toolkit for modular assembly of broad-host-range plasmids. ACS Synthetic Biology, 7(5), 1279-1290.
5. Meyer, A. J., Segall-Shapiro, T. H., Glassey, E., Zhang, J., & Voigt, C. A. (2019). Escherichia coli “Marionette” strains with 12 highly optimized small-molecule sensors. Nature Chemical Biology, 15(2), 196-204.
6. Schuster, L. A., & Reisch, C. R. (2021). A plasmid toolbox for controlled gene expression across the Proteobacteria. Nucleic Acids Research, 49(12), 7189-7202.

Sequence and Features