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From 2011.igem.org

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Module 3: Gene Guard

Containment is a serious issue concerning the release of genetically modified organisms (GMOs) into the environment. To prevent horizontal gene transfer of the genes we are expressing in our chassis, we have developed a system based on the genes encoding holin, anti-holin and endolysin. We are engineering anti-holin into the genome of our chassis, where it acts as an anti-toxin, and holin and endolysin on plasmid DNA. In the event of horizontal gene transfer with a soil bacterium, holin and endolysin will be transferred without anti-holin, rendering the recipient cell non-viable and effectively containing the Auxin Xpress and Phyto-Route genes in our chassis.

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Specifications

1. Prevent horizontal gene transfer by making any other cell that is not our own GMO non-viable

Horizontal gene transfer was the main problem that we could think of when it came to releasing GMO's for field tests. This is a common issue even in GM crops where cross-pollination between the GMO and natural organisms is always a worry. In bacteria, genes are usually transferred to other species through conjugation or by the uptake of the genetic material from lysed GMO's. Since the genetic material can still be taken up by other species after the lysis of our GMO, a traditional kill switch where one uses an input to induce lysis is not a viable and safe option. Therefore, we thought that the device must be present within the genetic material itself.

We then looked at past kill switches to research on what parts had been used previously and discovered the T4 Holin-Endolysin system. If we were to add these genes into our plasmid we could cause the immediate lysis of any organism that had taken up our plasmid.

2. Our own GMO must not be harmed by this module

The T4 bacteriophage must be able to delay the lysis of its host before it is able to replicate and assemble many copies of itself. In order to do that the T4 bacteriophage also has a protein called Anti-Holin that binds to the Holin thereby preventing the lysis of the cell. In essence, this system works as a timer giving the phage enough time to replicate.

By using the Anti-Holin

3. Must not be too much of a metabolic burden

4. Must be able to test whether it works

As part of our human practices work, we need to consider what will happen in the event that our modified bacteria are released into the soil. The potential consequences of their release relate to their uncontrolled spread and the possibility that they pass on the auxin genes to naturally occurring soil bacteria.

The auxin compound that we are using is the natural indole-3-acetic acid, which is not used as a herbicide like many other synthetic auxins. However, in high concentrations, indole-3-acetic acid can retard plant growth - as shown in our experiments.

While there are already a few species of bacteria that are able to secrete auxin^[1], it would be careless of us to release our bacteria without giving some thought to a containment device. Initially we looked at designing a killswitch based upon the idea that the bacteria would be destroyed under certain conditions, for example, when they left the designated area. This was discarded, as creating an environment with a signal that the bacteria cannot live without would affect the delicate ecosystem of the soil.

References

[1] http://m.biotecharticles.com/Biology-Article/Natural-Growth-Hormone-IAA-Indole-3-Acetic-Acid-602.html