1 GENETIC TOGGLE SWITCH IN E COLI COLLINS

1 GENETIC TOGGLE SWITCH IN E. COLI COLLINS LAB (2000) Allen Lin

Main Idea 2 Bistable switch Repressor inhibits transcription, inducer promotes transcription (by inhibiting repressor)

Toggle Model 3 u = [repressor 1] v = [repressor 2] alpha 1 = effect rate of synthesis of repressor 1 alpha 2 = 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

Plasmid 4 p. TAK class Lac repressor (Lac. I) with Ptrc-2 promoter Temp-sensitive Lambda repressor (c. Its) with PLs 1 con promoter Switched by IPTG or thermal pluse p. IKE class Lac repressor (Lac. I) with Ptrc-2 promoter Tet repressor (tet. R) with PLtet. O-1 promoter Switched by IPTG or a. Tc Transcriptional efficiencies: PLs 1 con > Ptrc-2 > PLtet. O-1 Rates of synthesis of repressors changed by modifying RBS (RBS 1 in particular)

GFP 5 Gpmut 3 gene downstream of Ptrc-promoter Transcription of Ptrc-2 >> GFP expressed (high) Transcription of P 1 >> GFP not expressed (low)

Bistability in 5 plasmids 6 5 bistable toggle plasmids p. IKE differ from expected Tet repressor (in p. IKE) weaker than lambda repressor (in p. TAK). Ptet. O-1 promoter in p. IKE 105 not reduced enough

Long term bistability 7 p. TAK 117

Threshold 8 After 17 -h induction p. TAK 117 (red) exhibits quasi -jump p. TAK 102 (blue) control exhibits sigmoidal curve 3 a and 3 b high and low of bimodally distributed curves Figure b - w/o gene expression variability

Blur at Threshold 9

Switching Time 10 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 tempsensitive lambda repressor Faster to switch to low state than to high state