Team:UANL Mty-Mexico/Modelling/QS

QS is the ability of cells to sense their own cell density, to communicate with each other through the production of quorum sensing molecules (QSMs), and act like a population instead of individual cells.

The QSMs are small diffusible signaling molecules, which are accumulated in the bacterial habitat and diffuse trough the cell membrane, to form a dimer inside the receptor cell with a quorum sensing protein (QSP). The function of these dimers is to trigger increased expression of specific genes in the bacterium. In our system, the activated gene (TetR) represses another group of genes, thus we say that a cell is “QS-repressed” when is in a QSM-triggered state and we say a cell is “normal” when the cell is not under QSM activation. dfasads asd dfdfgnal (procedent trigger increased expression of specific genes in the bacteriumto reduce nosy signal (procedent da dads asd dfdf

In our system this function relies on the basis of the two quorum-sensing systems of Pseudomonas aeruginosa, the las and the rhl systems.

The las system comprehends the lasR gene that codes for transcriptional activator protein lasI and the Autoinducer Synthase gene, which synthetizes 3-oxo-C12-HSL, the QSM (quorum sensing molecule). These gene products form the dimer LasR/3-oxo-C12-HS -the activated form of LasR- which activates the genes under its regulation.

Briefly, after red-light exposure, the red light-induced system will produce QSMs. This will constitute the red light-induced QS system (rQS). The QSMs will be diffused across the cells membranes to finally reach the receiver cell (in this case, the green light-induced system), and form the LasR/3-oxo-C12-HSL dimer, activating pLasR/PAI for the production of TetR. TetR will then inhibit the production of cI on the receiver cell. The figure 6 is a diagram of the circuit in the receiver strain that responds to the rQS produced in the sender strain.

Figure 6. In red is shown the rQS, produced by the red light-sensing cell; in green, the circuitry of the receiver green light-sensing strain. Note that the Biphasic Switch is turned-off, so that there will be no BFP nor RFP signal.

Figure 6. In red is shown the rQS, produced by the red light-sensing cell; in green, the circuitry of the receiver green light-sensing strain. Note that the Biphasic Switch is turned-off, so that there will be no BFP nor RFP signal.

The rhl system is comprised by the rhlR gene –which codes for the transcription factor- and the rhlI gene -coding for the autoinducer C4-HSL (HHL).

In this case, after green-light induction, the green light-induced system will produce C4-HSL molecules (QSM). This will constitute the green light-induced QS system (gQS). The QSM will diffuse across the cells membranes to finally reach the receiver cell (in this case, the red light-induced system), and form the RhlR/C4-HSL dimer, activating pRhlR/HHL for the production of TetR. The figure 7 is a diagram of the circuit in the receiver cell that responds to the gQS produced in the sender cells.

In this way, the rQS will induce TetR production in the green light-induced system, while the gQS will do the same in the red light-induced system. Note that each of these systems is present in separate bacterial strains. TetR will then repress the cI gene, turning off the Biphasic Switch and its output signals. This mutual repression is used as a way to reduce noisy signals –i.e., the Biphasic Switch outputs- that may hinder the AND-gate output and also to get non-overlapping output signals from each of the systems considered.

Figure 7. In green is shown the gQS, produced by the green light-sensing strain; in red, the circuitry of the receiver red light-sensing strain. Note that the Biphasic Switch is turned-off by TetR, so that there will be no YFP nor GFP signal.

Figure 7. In green is shown the gQS, produced by the green light-sensing strain; in red, the circuitry of the receiver red light-sensing strain. Note that the Biphasic Switch is turned-off by TetR, so that there will be no YFP nor GFP signal.