Team:TU Eindhoven/Project/Expression System
Project - Expression System
The Expression System
The system we have designed relies on the presence of two Clickable Outer Membrane Proteins within the cells. To obtain cells which are functionalized with these proteins, an expression system is needed capable of producing two proteins which carry unnatural amino acids. In addition to co-expression of two proteins, we thus need to gear up our bacteria with the tools to incorporate unnatural amino acids within their proteins. An overview of the expression system we used to obtain these bacteria is presented below.
Our cells will be co-transformed with two plasmids to enable the expression of our COMBs. The first plasmid is the pETDuet-1 vector, a vector which has been optimized for expression of two proteins. The second plasmid is the pEVOL-pAzF vector which contains pAzF aminoacyl-tRNA synthetase, which is necessary for the incorporation of the unnatural amino acid. Read more about the vectors here:
Unnatural amino acid incorporation
To enable the post-translational click reaction of aptamers to proteins, the outer membrane proteins have to be functionalized with azide groups. These azide groups can be incorporated into the outer membrane proteins by incorporating the unnatural amino acid p-Azido-phenylalanine. This incorporation is enabled through amber codon suppression, expression of the pAzF aminoacyl-tRNA synthetase and addition of pAzF to the medium. Read more on unnatural amino acid expression here:
Unnatural amino acid incorporation
Unnatural Amino Acid Expression
DNA is best known for its function as carrier of hereditary information: DNA carries the biological information that is carried over to the next generation each time cells divide into daughter cells. This biological information is merely composed of four base pairs, being guanine, cytosine, adenine and tyrosine. The sequence of the base pairs is translated into an amino acid code in the form of triplets. These triplets are recognized by tRNA's, which carry the correct to be incorporated amino acid, which is added onto the growing chain (see Figure 2).
The genetic code includes three distinct stop codons. One of these stop codons is known as the amber stop codon. The amber stop codon (TAG) is frequently targeted for the incorporation of unnatural amino acids as this is the least abundant stop codon in most organisms, including E. coli.
The Escherichia coli genome has 4,290 open reading frames of which only 326 end with the amber stop codon . Therefore, amber tRNA suppressors have been used preferentially: hijacking the amber stop codon is expected to cause the least harm to E. coli cells. This does, however, not mean that amber stop codon suppression does not interfere with E. coli's natural function. Research has shown that hijacking the genetic code of E. coli does result in phenotype changes: Herring & Blattner found that even though amber stop codon suppression does not lead to a stress response in E. coli, it does lead to transcriptional changes.In our project, we make use of this amber stop codon to enable click chemistry on E. coli's outer membrane.
To click the aptamers to our membrane proteins post-translationally, we use the SPAAC click chemistry which was introduced to iGEM by Eindhoven 2014 (see Figure 5). Highlights of this click chemistry reaction are that it is bio-orthogonal, non-toxic and occurs very rapidly, both in vivo as well as in vitro.
As shown in Figure 1, our system relies on the presence of two vectors within the host cell. The first of these vectors is pEVOL-pAzF. The second one is the pETDuet-1 expression vector.
pEVOL-pAzF is a small vector which has been designed and optimized for the incorporation of unnatural amino acids into proteins in E. coli. The coding sequence of pEVOL-pAzF encodes tRNA synthetases, which can translate the amber stop codon sequence into the incorporation of the unnatural amino acid. Optimization of the vector has enabled higher yields of mutant proteins in comparison to previous vectors: pEVOL-pAzF showed roughly 250% greater yields in comparison with vectors previously used for the incorporation of unnatural amino acids . One of the first amino acids which has been incorporated into proteins using the relatively novel pEVOL vector was pAzF and we will use this exact vector to construct our mutant protein in vivo.
Features of the pEVOL-pAzF vector:
- The pEVOL-pAzF expression vector features the p15A origin of replication which makes the pEVOL-pAzF plasmid compatible
Plasmid compatibility is generally defined as the failure of two coresident plasmids to be stably inherited in the absence of external selection . The cause of the failure to be stably co-inherited lies in the fact that the origins of replication are too analogous. In that case, the bacteria cannot distinguish between the plasmids and can eventually lose either one of the plasmids as the amount of both plasmids is limited by a single copy number.with many other frequently used plasmids.
- The chloramphenicol resistance gene
- The tRNA synthetase is under the control of the arabinose-inducible AraBAD promotor
The designed system relies on the two membrane proteins which come into close proximity as a result of ligand binding. Often, when such protein assemblies are to be obtained, one can isolate endogenous complexes and reconstitute those components in vitro to analyze whether the assembly takes place . As we have designed the system to be used in vivo, however, we rely on the co-expression of all components within the same host cell.
Generally, this heterologous expression can be reached into two different ways, firstly by transforming multiple constructs and secondly by introducing a single plasmid carrying multiple genes in E. coli. As our device already featured two plasmids, we devised to use a plasmid which could co-express multiple genes, preferably in an equimolar ratio. Since the pETDuet-1TM Expression System from Novagen has been developed for this particular purpose, we have chosen to use this system as our vector of choice.
Features of the pETDuet-1 expression vector:
- The pET-Duet1 vector features two multiple cloning sites, each carrying a dozen cloning sites. This enables insertion of multiple fragments.
- Each of the multiple cloning sites contains its own T7 lac promotor and a ribosome binding site. A single terminator is located after MCS2, such that transcription yields two different mRNAs.
The pET-Duet1 vector not only carries multiple genes but is bicistronic. Therefore, it allows simultaneous expression of two proteins separately from the same RNA transcript. Hence, both the MCS1 insert and MCS2 insert are expressed from the longer mRNA strand. Only the MCS2 insert is expressed from the shorter mRNA strand.
Usually, bicistronic vectors containing two target genes under the control of a single promotor preceding the two genes, show strongly reduced expression of the gene located more distant from the promotor site . The second promotor which initiates the translation of the second mRNA aims to correct for this reduced expression.