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From 2012.igem.org

Double Population Toggle Switch Design

Our double population toggle uses many of the same concepts as our single population toggle.

<img src="" width="750">

In order for either population to produce fluorescence proteins, it must be receiving a proper amount of quorum signalling molecules from the other population. When one of the populations is signalled by the other, it begins to produce fluorescence proteins along with the inhibitor proteins LacI or TetR. These proteins in turn inhibit the production of signalling molecules to be sent to the opposite population of cells. In short, either population is either fluorescing or sending a signal to the other population to fluoresce. The mechanism is controlled with the addition of IPTG or ATc in much the same way as the single population toggle.