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Team:Grenoble/Projet/Design
From 2011.igem.org
Design and principle
Toggle switch explanation
In a few words
Our project is based on a toggle switch system. So we have to explain how it works. Basically it corresponds to an irreversible choice between two pathways depending on the environment. In reality, the reversibility is possible but very difficult to obtain because of the strength of the interactions and the stability of the system (defined by hysteresis * insérer un lien ici*).
Genetically, what is happening ?
Contrary to us, bacteria can’t think or make voluntary choices. So how can they choose a pathway? Through our project, we are creating a genetic system which helps bacteria to take such a decision. For that, we exploit chemical components of the environment which influence the bacterial behaviour. By taking into account two proteins repressing each other’s expression (LacI and XR), we create the fundaments of a toggle system that can switch when using two inducers (IPTG and X) in the medium.
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Suppose that initially the genes in the Toggle Switch are both expressed. |
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If the effect of LacI is dominant through the presence of a high level of IPTG, the bacteria will synthesise more XR and switch off the expression of lacI |
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The bacteria toggle to one side and totally block the expression of lacI. |
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We need to use very high level of the second inducer X to try to switch the toggle back because the active pathway creates an excess of XR, inhibiting lacI expression. |
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The amount of X would have to be so high as to relieve the inhibition of lacI expression and consequently turn off the other pathway. |
Importance of Toggle Switch for 'Le Projet' ?
Importance of Toggle Switch for Le Projet ?
Our Toggle switch is composed by the two repressors (LacI and TetR), also the ways express ether a Quorum Sensing receptor (CinR) or the Quorum Sensing synthesise enzyme (CinI). By this ways the bacterium will engage itself in secretion path or receptor path.
(missing picture)
That’s why we use Toggle Switch to have two different behaviours in one cell type, and simplify the immobilisation on plate. We use cell’s communication to finalise our biosensor so we need a homogeneous repartition of ‘sender cell’ and ‘receptor cell’ to have a good communication and response. So locate specifically (homogeneously) two type of cell, will be very hard work contrary to one ‘differentiable type cell’ that we can locate uniformly on the plate.
Quorum sensing explanation
In a few words
Quorum sensing is an important phase of physiological development of the bacteria. It’s the communication signal between bacteria carried by a signalling molecule. All bacteria can produce or detect the quorum sensing molecule. Quorum sensing involves a secreted molecule which allows the number of bacteria to be known and give a logical response to a high level of population. Frequently the Quorum sensing is the signal to stop the growth, activate the virulence state, or activate luminescence.
The quorum sensing (QS) detection is linked to his concentration in the medium, so when the concentration of bacteria is low few QS molecules diffuse and it does not activate the response. But when the bacteria become too abundant the QS concentration is higher and activates the response.
Beginning of growth phase, cells are few, so the production of QS is very low, no response activity. |
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Middle Growth phase bacteria are many but not enough to activate QS response, the concentration in QS is not high enough. |
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In Stationary phase, bacteria are very concentrated so the production of QS is maximal and the ??? lack of space up the concentration of QS ??? . Both phenomena are sufficient to activate a response. |
Biologically, what is happening ?
The Quorum sensing system is composed of three elements: an enzyme, a secreted molecule and a receptor.
A bacterium synthesises this Quorum sensing enzyme which in turn synthesizes the quorum sensing molecule. This molecule diffuses within and outside the cell (around 1mm/h), and can enter inside the issuing bacterium or any other bacteria in its neighbourhood. The QS molecule interacts with its receptor protein. The complex binds to DNA and activates or represses genes generating the appropriate response.
