IGEM:Melbourne/2008/Concepts/Toggle switch

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Overall System

A Toggle Switch

Components:

• IPTG (Isopropyl β-D-1-thiogalactopyranoside) - small organic compound that stops the LacI protein from functioning
• LacI - protein that interacts with the P_trc-2 promoter to repress transcription of downstream genes
• CI - protein that interacts with the P_L 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

• Consider an initial state where neither of the switch mechanisms is active
• The promoter P_L is active, therefore the lacI gene is expressed
• This produces the LacI protein which represses the promoter P_trc-2
• As this promoter is repressed, the cI & gfp genes are not transcribed
• No CI protein means that the P_L 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

• Now switch the circuit on by adding IPTG
• IPTG stops the LacI protein from functioning, therefore the P_trc-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 P_L 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

• To switch it off, raise the temperature to 42°C
• This degrades the CI protein so that it cannot repress the P_L promoter
• This means that the P_L promoter can initiate expression of the lacI gene, producing the LacI protein
• The LacI protein represses the P_trc-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.