Team:Leicester
Leicester iGEM
We will be updating this page as we go.
Introduction
Colonisation
Kill Switch
There are two kill switch systems in the genetically engineered bacteria: One, a kill switch which causes the bacteria to die upon addition of an inducer; the other, a maintenance kill switch whereupon the bacteria die if they leave the gut. This dual containment system allows for the reduction in bacterial escape and horizontal gene transfer as well as the death of the bacteria as a backup option if it goes wrong.
Potentially could use a toxin-antitoxin system but with a modified antitoxin protein which incorporates a nonstandard amino acid that is vital for the toxin function. Therefore upon induction of the nonstandard amino acid the toxin will have the right amino acids needed for its correct synthesis and thus will kill the cell. It is easier and simpler than the altering 22 essential enzymes like Rovner et al, 2015 but more effective than toxin-antitoxin systems. However, this approach will need to add a stop codon for the nonstandard amino acid in a key hydrophobic region (if the nonstandard amino acid is hydrophobic) (Rovner et al, 2015) so that without the non-natural amino acid the protein cannot fold correctly and thus be subject to proteolysis. This results in cell death upon the addition of the non-standard amino acid. However this would require the changing of the stop codon used to another stop codon in all other genes as well as engineering a tRNA synthase that charges the non-natural amino acid to an edited tRNA molecule that is cognate for the stop codon used.
The CCDA/CCDB kill switch is in an operon system with CCDA under the control of a temperature sensitive RBS (Part K115002) whereas the CCDB is under the control of a generic RBS (Part K541503). This means that at under 37OC (i.e. not in the human body) the translation rate of the CCDA will dramatically decrease relative to other genes in the bacteria at the same temperature, whilst CCDB will remain the same in respect to the fundamental decrease in rate due to the lower temperature. Thus there will be a high enough ratio of CCDB (once translated) to kill the cell. Summed up: When the temperature is too cold, the antitoxin (CCDA) doesn’t work, so the toxin (CCDB) kills the cell.
However, CCDB is patented so another toxin/antitoxin system would be preferred such as MazF/MazE. If no antitoxin is available then the antitoxin in the system described above could be replaced with a polymerase gene (e.g. T7 Polymerase) and the desired toxin under the control of a promoter for that polymerase (i.e. a T7 consensus promoter). This would give higher levels of transcription and would be perhaps a more sure system. For our iGEM team however, due to time requirements, the simpler the system the better.
Another Kill Switch which could be used to selectively kill the bacteria would be to use the X and Y expansion of the genetic code by Malyshev, et al 2014. This would use a toxin (Such as MazF toxin) that is dependent on the bases X and Y. This can be done by using long-range PCR for the MazF inside pUC19 but leaving a few bases in between the forward and the reverse primer. Then synthesis and PCR amplify an oligonucleotide which contains the primers and the X (dNaM)/Y (d5SICS) codon (Malyshev, et al 2014) for the non-natural amino acid for that codon (assuming there is a tRNA synthase for this). This can then be inserted into the PCR amplified pUC19 plasmid (with the gaps) through Gibson Assembly to then be transformed (Malyshev, et al 2014) as can be seen in figure 1. Proof will be needed for the incorporation of the X/Y through testing whether the transformed cells only die through addition of the X and/or Y base in a medium lacking these. This would significantly reduce the likelihood of the bacterium kill switch being activated naturally in the human microbiome. However, like through the addition of a nonstandard amino acid via stop codons, a tRNA molecule will be needed that can recognise the synthetic bases as well as a tRNA synthase which can accurately charge the nonstandard amino acid to this tRNA. As such this method can only be used in theory for our iGEM team. This will be a very useful application of synthetic biology once these key tRNA’s and tRNA synthases are ready.