IGEM:Melbourne/2008/Concepts/Toggle switch

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Overall System

A Toggle Switch
A Toggle Switch


  • 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

Initial State
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

Switched On
Switched 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

Switched Off
Switched 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.

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