The project aims to study how bacterial DNA organization can influence gene expression. In order to answer this question, we decided to create a new tool based on CRISPR/Cas9 technology to bring together two DNA regions separated in space. This new tool is expected to assess the effect of DNA structure on gene expression. We designed a bring DNA closer tool (BDC tool) and a visualization tool.
In this part, we expose our final tool design and our experimental strategy is available here.
Molecular tools
In order to design our two tools, we need specific molecular tools with specific functions: DNA recognition, DNA binding function, dimerization and a fluorescence system.
DNA recognition and fixation
The two tools have to recognize a specific target sequence on DNA and then bind to it. Several options were possible, such as a protein-DNA or a RNA-DNA recognition. First of all, we thought about using engineered nuclease proteins, such as zing-finger nuclease (ZFN), transcription activator-like effector nuclease (TALEN) or a RNA-guided engineered nuclease (RGEN) for their ability to target a specific sequence thanks to a specific proteic or nucleic sequence (for TALEN for instance, we speak about TALE code). We chose a based CRISPR/Cas9 recognition system but for the Cas9 protein, we chose an endonuclease deficient Cas9 (dCas9). For more information about CRISPR/Cas9 technology, click here. The CRISPR/Cas9 system is a quite new tool for synthetic biology which has many applications, such as targeting specific DNA sequences. Nevertheless, our system allows a new application for the CRISPR/Cas9 technology. We chose the dCas9 for three reasons:
- simplicity: to target DNA sequence, we only have to design a specific sgRNA, which takes about a week.
- adaptability: the tool would be adaptable to target any sequences in the genome through the sgRNA sequence design.
- specificity: the use of orthologous dCas9s allows a specific one by one sgRNA/dCas9 recognition by a specific PAM sequence for each pair (Ma, Hanhui et al. 2015).
Dimerization system
Our project aims at recognizing specific sequences on the genome and then inducing a spatial proximity. So once we chose a system allowing DNA targeting, we had to choose a system which could dimerize the dCas9/sgRNA, in order to get these two dCas9/sgRNA closer. We chose the induced dimerization FRB/FKBP12 system. In fact, this system allows a dimerization induced by rapamycin. To avoid any problem with rapamycin and E. coli, we decided to used an analog of rapamycin called rapalog (gratefully offered by Takara Clontech). We then used a FRB mutated protein sequence (FRB*) to be recognized by rapalog (Bayle and al, 2006)
Fluorescence system
For the visualization tool, we needed a fluorescence system to asses a spatial proximity due to our system. We chose the tripartite split-GFP, which is divided into three different parts: two twenty amino acids parts corresponding to the 10th and 11th beta-sheets and a third part corresponding to the other beta-sheets of the GFP. This solution avoids poor folding and/or self-assembly background fluorescence. Indeed, the different sub-units do not have specific affinity, it can only assemble and emit fluorescence upon excitation when the three sub-units are spatially close, and it was not the case by using a bipartite GFP. With this system, only two sgRNAs/dCas9s fused to their specific GFP tags are necessary to obtain an accurate fluorescence (Nguyen and al, 2013).
Bring DNA Closer tool construction
We have designed a tool based on the CRISPR/Cas9 system to target a specific DNA sequence. We imagined a system using a dCas9 that dimerizes under an induction signal to bring two DNA sequences closer.
A dCas9 is a protein which recognizes precisely a DNA sequence without nuclease activity (cf Fig1). We chose this system for its high adaptability, as it targets DNA through a sgRNA which could be easily designed. But as we need to target two different sequences we also need to work with dCas9s which will not interfere with each other. Thus we chose two orthologous dCas9s which are respectively expressed by T. denticola (TD) and S. pyogenes (SP). As they come from different organisms, they recognize different sgRNAs thanks to their PAM sequence. We ordered from Addgene the plasmids coding for each one of these dCas9s and their sgRNAs.
Fig 1: Specific DNA sequence recognition by our tool based on sgRNA/dCas9 assembling
To dimerize this two dCas9s, we chose an inducible system using the FRB and FKBP12 proteins (cf Fig2). Originally found in mammals, these two proteins form an heterodimer when rapamycin is added in the middle. It is particularly used in protein interaction studies (Cui et al., 2014). However, rapamycin is toxic for bacteria, but studies showed that a mutated FRB (FRB*) still allows dimerization with a non toxic analog of rapamycin called rapalog. The mutations implied are: T2098L, K2095P, W2101F.(Bayle et al., 2006; Liberles, Diver, Austin, & Schreiber, 1997).
A biobrick coding for FRB with the mutation T2098L was already in the iGEM registry (iGEM Part_ J18926) but was not available. Moreover, it contains only one mutation on the 3 described in the literature. So we decided to work with the fully mutated FRB. The rapalog and the plasmid containing the mutated FRB and FKBP12 were offered to us by Takara Clontech. But as mentioned previously, this system is used in mammal cells, so we decided to optimize the sequences for an expression in E. coli with the Jcat plateforme. So we finally ordered gBlocks coding for the FRB* and FKBP12 optimized sequences and a linker in prevision for the fusion with their respective dCas9s.
Using these two systems (dCas9 recognition and FRB/FKBP12 dimerization) we design our new tool (cf Fig2 and Fig3) based on the two following BioBricks:
Fig 2: Final biobrick designs for the BDC tool and dimerization mecanism
These two biobricks will be assembled in the pSB1C3 plasmid giving us our BDC tool which would work as presented bellow:
Fig 3: BDC tool mecanism
Visualization tool construction
As every system needs an efficient control, we set up a new part of the project in order to assess our Bring DNA Closer tool: the visualization tool.
To build this new tool, two other orthologous dCas9s were used and respectively come from the organisms N. meningitidis and S. thermophilus. These dCas9s will be fused to sub-units of a fluorescent protein (cf Fig5). We also decided to use a dCas9 system for this tool in order to have detection of an accurate and unique sequence in the genome. It will be associated with a new and unreleased in the iGEM competition tripartite split-GFP.
The tripartite split-GFP is composed of two twenty amino-acids long GFP tags (GFP 10 and GFP 11) and a third complementary subsection (GFP 1-9). The tags will be fused to the two dCas9 previously quoted. A functional GFP would be formed when the tools would be close enough to allow the three split-GFP parts reunion and the fluorescence emission. This fluorescence system avoids poor folding and/or self-assembly background fluorescence. With this system, only two sgRNAs associated with their dCas9s fused to their specific GFP tags were necessary instead of nearly 30 with mundane GFP due to background fluorescence.
We designed three biobricks (cf Fig4) to achieve this part of the project:
- BioBrick n°3: dcas9 from N.meningitidis fused to GFP-10 expressed by a constitutive promoter, a RBS and a double terminator
- BioBrick n°4: dcas9 from S.thermophilus fused to GFP-11 expressed by a constitutive promoter, a RBS and a double terminator
- BioBrick n°5: The third part of the GFP 1-9 expressed by a constitutive promoter, a RBS and a double terminator
The biobricks would then be inserted into pSB1C3 by digestion/ligation using the restriction sites EcoRI and PstI.
Fig 4: Biobrick designs for the visualization tool
Then, we considered to establish a composite biobrick composed of the three biobricks in the same pSB1C3 plasmid (cf Fig5). This plasmid would have been build using the iGEM restriction site technique.
Fig 5: Visualization tool mecanism
We chose to express sgRNAs on another plasmid pZA11 (cf Fig6) which is compatible with pSB1C3, as it is resistant to ampicilin and do not have the same replication origin.
Fig 6: Plasmid containing the sgRNAs specific to their cognate dCas9 and that can be adaptable to target any sequences in the genome
References
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Cui, B., Wang, Y., Song, Y., Wang, T., Li, C., Wei, Y., … Shen, X. (2014). Bioluminescence resonance energy transfer system for measuring dynamic protein-protein interactions in bacteria. mBio, 5(3), 1–10.http://doi.org/10.1128/mBio.01050-14
Liberles, S. D., Diver, S. T., Austin, D. J., & Schreiber, S. L. (1997). Inducible gene expression and protein translocation using nontoxic ligands identified by a mammalian three-hybrid screen. Proceedings of the National Academy of Sciences of the United States of America, 94(15), 7825–7830.http://doi.org/10.1073/pnas.94.15.7825
Ma, Hanhui et al. 2015. “Multicolor CRISPR Labeling of Chromosomal Loci in Human Cells.” Proceedings of the National Academy of Sciences of the United States of America.
Nguyen, Hau B et al. 2013. “A New Protein-Protein Interaction Sensor Based on Tripartite Split-GFP Association.” Scientific Reports 10: 1–9.
