Genetic Toggle Switch in E. Coli Collins Lab (2000)

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Genetic Toggle Switch in E. ColiCollins Lab
(2000)

• Allen Lin

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Main Idea

• Bistable switch
• Repressor inhibits transcription, inducer
  promotes transcription (by inhibiting repressor)

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Toggle Model

• u repressor 1
• v repressor 2
• alpha1 effect rate of synthesis of repressor 1
• alpha2 effect rate of synthesis of repressor 2
• beta cooperatively of repression of promoter 2
• gamma cooperatively of repression of promoter 1

• Sigmodal shape from cooperative repression of
  transcription greater than 1
• Rates of synthesis of two repressors needs to be
  balanced

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Plasmid

• pTAK class
• Lac repressor (LacI) with Ptrc-2 promoter
• Temp-sensitive Lambda repressor (cIts) with
  PLs1con promoter
• Switched by IPTG or thermal pluse
• pIKE class
• Lac repressor (LacI) with Ptrc-2 promoter
• Tet repressor (tetR) with PLtetO-1 promoter
• Switched by IPTG or aTc
• Transcriptional efficiencies PLs1con gt Ptrc-2 gt
  PLtetO-1
• Rates of synthesis of repressors changed by
  modifying RBS (RBS1 in particular)

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GFP

• Gpmut3 gene downstream of Ptrc-promoter
• Transcription of Ptrc-2 gtgt GFP expressed (high)
• Transcription of P1 gtgt GFP not expressed (low)

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Bistability in 5 plasmids

• 5 bistable toggle plasmids
• pIKE differ from expected
• Tet repressor (in pIKE) weaker than lambda
  repressor (in pTAK). PtetO-1 promoter in pIKE105
  not reduced enough

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Long term bistability

• pTAK117

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Threshold

• After 17-h induction
• pTAK117 (red) exhibits quasi-jump
• pTAK102 (blue) control exhibits sigmoidal curve
• 3a and 3b high and low of bimodally distributed
  curves
• Figure b - w/o gene expression variability

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Blur at Threshold
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Switching Time

• Switching time depends on rate of elimination of
  repressor protein
• Low to high dilution by cell growth of
  IPTG-bound Lac repressor
• High to low thermal destabilization of
  temp-sensitive lambda repressor
• Faster to switch to low state than to high state