IGEM:Melbourne/2008/Concepts/Toggle switch

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Overall System
Components: 
 * IPTG (Isopropyl β-D-1-thiogalactopyranoside) - small organic compound that stops the LacI protein from functioning
 * LacI - protein that interacts with the Ptrc-2 promoter to repress transcription of downstream genes
 * CI - protein that interacts with the PL promoter to repress downstream genes
 * GFP (Green Fluorescent Protein) - Protein that fluoresces green (fairly self explanatory), used to indicate states of ON (green) and OFF (no green)

Initial State

 * Consider an initial state where neither of the switch mechanisms is active
 * The promoter PL is active, therefore the lacI gene is expressed
 * This produces the LacI protein which represses the promoter Ptrc-2
 * As this promoter is repressed, the cI & gfp genes are not transcribed
 * No CI protein means that the PL promoter is not repressed and can keep on making the LacI protein
 * No GFP means that there is no green signal (OFF)

Switch On

 * Now switch the circuit on by adding IPTG
 * IPTG stops the LacI protein from functioning, therefore the Ptrc-2 promoter is no longer repressed
 * The cI & gfp genes are now able to be transcribed, producing the CI and GFP proteins
 * GFP gives us a green signal (ON)
 * The CI protein represses the PL promoter, stopping the production of any more LacI protein
 * This system is now stable in the ON state and IPTG can be removed without switching it off

Switch Off

 * To switch it off, raise the temperature to 42°C
 * This degrades the CI protein so that it cannot repress the PL promoter
 * This means that the PL promoter can initiate expression of the lacI gene, producing the LacI protein
 * The LacI protein represses the Ptrc-2 promoter, which stops production of the GFP and CI proteins
 * No GFP means no green signal (OFF)
 * No CI means no repression of LacI production
 * Again, this system is stable in the OFF state. If you were to lower the temperature the diagram would return to that shown as the 'Initial State' and would remain OFF.