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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.

Design

To create a system to prevent horizontal gene transfer of our genetic constructs, we designed a toxin-antitoxin system. We will be using the T4 Anti-Holin as the anti-toxin and the T4 Holin and endolysin as the toxin.

Holin is a protein that forms pores in cell membranes and anti-holin binds to holin, inhibiting its action. Once pores are formed by holin, endolysin can access the periplasmic space and degrade the cell wall, causing cell lysis^[1]

Our chassis will have the gene for antiholin on its genome, and holin and endolysin on the plasmid with the auxin genes. This would mean that while the auxin plasmid is inside our chassis, antiholin would inhibit the activity of holin and prevent lysis. However, when that same plasmid is passed on to another bacterium in the soil by conjugation, the recipient bacterium will not have antiholin on its genome and so the holin concentration will build up. As the holin builds up inside the cell, it will form pores in the inner membrane that will allow the endolysin to enter the periplasmic space and break down the cell wall, causing the cell to lyse.

So it can be seen that the crucial part of our system is the balance between the levels of holin and antiholin in the cell. This will require careful modelling to influence our design, as the holin level should be sufficiently high as to induce lysis quickly in other cells, but low enough as to be easily controllable inside our own chassis.

The expression levels will be governed by the promoter and RBS combination. In order to make this easier, we fixed the promoter in front of the antiholin as J23100, and then the Salis Lab RBS designer was used to generate an RBS sequence that would give the correct level of expression, a value that would be generated using computer modelling.

Because we are limited to using the antiholin gene that is in the cell lysis cassette submitted to the registry by Berkeley 2008, it would be too difficult to replace the RBS that is upstream of the gene. Instead, we calculated its strength using the Salis Lab RBS calculator and used this as a fixed value around which to model the required promoter strength.

As can be seen from the data presented on the modelling page, we were able to calculate the required RBS strength for the antiholin expression and the appropriate promoter for the holin expression. Our RBS was generated by the Salis Lab RBS designer to give a level of expression that is stronger than necessary to ensure that our bacteria are protected. The promoter for the holin was chosen from the Anderson promoter library, using the relative promoter strengths as a guide. We chose J23103 because it is one of the weakest promoters in the Anderson library.

In order to integrate the antiholin gene into the genome, we will use a CRIM plasmid ^[2]. This will however, introduce a kanamycin resistance gene into the genome of our bacteria, which is far from ideal in a project that is designed to be released into the environment. This is not something that is intended as part of the overall design, but is necessary due to the time constraints in the iGEM competition.

References

[1] - 1] Gr ndling et al.. (2001). Holins kill without warning. PNAS. 16