Tristable

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Tri-stable Toggle Switch

The purpose of the Tri-stable Toggle Switch is to produce three distinct, continuous, and stable outputs in response to three distinct inputs. These three inputs are three separate chemicals which will each induce one state of the switch.
The Tri-stable Toggle Switch Architecture
In order to achieve this goal, we are constructing three constructs, each of which consists of a repressible, constitutively-on promoter attached to two repressors. Specifically, our three constructs are pBAD->LacI->TetR, pLacI->AraC->TetR, and pTet->AraC->LacI, where [http://en.wikipedia.org/wiki/Bcl-2-associated_death_promoter AraC] represses pBAD, [http://en.wikipedia.org/wiki/Lac_repressor LacI] represses pLac and [http://en.wikipedia.org/wiki/Tetracycline_controlled_transcriptional_activation TetR] represses pTet. Each of the three repressors are inactivated by one of three chemicals, the three inducer chemicals mentioned earlier. These three([http://en.wikipedia.org/wiki/Arabinose arabinose], [http://en.wikipedia.org/wiki/IPTG IPTG] (Isopropyl β-D-1-thiogalactopyranoside) and [http://en.wikipedia.org/wiki/Tetracycline Tetracycline], respectively), cause conformational changes in their respective repressor proteins which leads to gene expression. For example, in the presence of arabinose, AraC cannot repress pBAD so LacI and TetR are produced which in turn repress pTet and pLac.



AraC/BAD

The gene AraC one of several genes (AraA, AraB, AraD, etc) originally for the metabolism of arabinose.[http://www.mun.ca/biochem/courses/3107/Topics/Ara_operon.html]
Dimer structure with arabinose on the left (yellow)
The left image shows the araC dimer repressing transcription, while the right conformation enables transcription
The protein forms a dimer in with and without arabinose but the structural change activates or represses the pBAD ([http://en.wikipedia.org/wiki/Bcl-2-associated_death_promoter Bcl-2-associated death promoter], an apoptotic regulator in humans).




LacI

In nature, LacI represses pLac which promotes LacYZA genes that metabolize lactose, thus LacI represses pLac except in the presence of lactose (or lactose mimics, eg IPTG). File:Lac




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