Metabolic kill switch
KillerRed and KillerOrange are fluorescent proteins which, when irradiated with green and blue light respectively, generate reactive oxygen species (ROS).
KillerRed has been shown to effectively kill cells when exposed to green light (540–580 nm) and is much less effective under blue light(460–490 nm) (Bulina, 2006). KillerOrange is a mutant of KillerRed that is excited from 420-530nm that has been shown to work alongside KillerRed (Sarkisyan,2015). Our project has improved the characterisation of a KillerRed codon optimised for E. coli. We are also characterising KillerOrange in the same way. We aim to include both KillerRed and KillerOrange in the same system so as to be a more phototoxic. High levels of ROS lyse the cell but can also damage the DNA, this is an attractive prospect when developing a kill switch to reduce the risk of horizontal gene transfer (HGT).
Method
Preliminary experiments were performed to calibrate the tecan reader. An OD of 0.4 on the cuvette reader was used as the optimum level of growth to induce protein production, this corresponded to 0.26 on the tecan reader.
5ml overnights of the transformed E. coli with the KillerOrange and KillerRed kill switches were used to inoculate five 250ml erlenmeyer flasks covered in tin foil containing 50ml of LB 35µg/ml Chloramphenicol. The five flasks were inoculated with the following conditions.
- KillerOrange induced
- KillerOrange not induced
- KillerRed induced
- KillerRed not induced
- pSB1C3 RFP
The optical density of each culture was measured every 1.5 hours until it had reached 0.26 on the tecan plate reader, then 100µl of 0.1M IPTG was added to induce the protein production in the desired cultures. The cultures are then incubated at 37℃ and 220rpm overnight. A serial dilution was then performed to make 10-3,10-4 and 10-5 dilutions of the cultures. 4.5ml of each dilution factor was placed in 10ml falcon tubes one set were placed on their side label down in the light box, the other set were covered in tin foil and also placed in the light box. The temperature was taken periodically inside the box using a thermocouple. After 6hrs of irradiation, spread plates were performed for all the samples. CFU’s were then counted and the dark and light conditions were compared.
Results
Enzymatic
Lysozyme is a common enzyme used in laboratories and the gallus gallus form is basis of our enzymatic kill switch. It is bacteriolytic when transported into the periplasm of gram negative bacteria, hydrolysing the glycosidic bond connecting N-acetylmuramic acid and N-acetylglucosamine. Under the control of a T7 promoter we can induce lysis of the cell by adding IPTG. Simply lysing the cell does not prevent HGT it may even exacerbate the problem. We have tested the liklihood of HGT by lysing a culture of cells producing RFP, incubating the lysate in a 90 degree water bath to inactivate the lysozyme. Then growing competent E. coli with the lysate will show if the competent cells take up the RFP gene.
Lysozyme EnzChek assaying kit is used to measure lysozyme activity in solution where an increase in fluorescence is proportional to lysozyme activity.
Method
Results
DNA degradation
DNase 1 is an endonuclease that non-specifically cleaves DNA. We are creating a kill switch with DNase 1 to address the foremost problem associated with Biosafety - lateral gene transfer. The DNase 1 kill switch, on induction, will degrade DNA and kill the cell.
Another biosafety ‘kill switch’ we aimed to test was DNase I. We received a G-block of DNase I DNA and cloned it using the MoClo method. Each time the product from the PCR reaction was transformed we encountered issues. Many times no colonies would form, and in cases when colonies were produced DNase was not present. This could be because colonies containing DNase I has their DNA immediately destroyed. However production of Lysozyme was successful so we continue work with this as a kill switch instead.
Continuous culture
Before starting the project we spoke to Prof. Robert Beardmore EPSRC Leadership Fellow in the Mathematical Biosciences at Exeter University. Much of his research has been into antibiotic resistance. We talked about how high selection pressure is applied by prolonged use of antibiotics and how kill switches may be analogous to this. It is clear that cells which develop a mutation that inactivates a toxic protein would be strongly selected for. The length of time for this to happen was estimated to be quite short and if this was the case could have strong implications for the effectiveness of a kill switch over time. To test this we decided to use the ministat to perform a continuous culture. The ministat was developed in the Dunham lab at the University of Washington (Miller et al), each ministat chamber is fed from its own media container via a peristaltic pump and the culture volume is set by the height of the effluent needle in the chamber. Air is bubbled through flasks of water to hydrate it and then used to agitate the culture. We inoculated chambers with freshly transformed E. coli (BL21 DE3) and took samples to test if the kill switch was still viable.
Our kill switches were all under the control of the T7 promoter, which we knew to be leaky. It was highlighted to us by Markus Gershater, chief scientific officer at Synthace Ltd that any leakiness in a system would likely produce sub lethal doses of the toxic proteins accelerating selection for mutants without the kill switch.
References
Bulina, M. E. et al., 2006. A genetically encoded photosensitizer. Nature Biotechnology.24(1).
Sarkisyan, K. S. et al, 2015. KillerOrange, a Genetically Encoded Photosensitizer Activated by Blue and Green Light. PLoS ONE.10(12)
Miller, A. W. et al, 2013. Design and Use of Multiplexed Chemostat Arrays. Journal of Visualised Experiments. (72).
