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From 2011.igem.org

The balance of Wu xing

        In Wu xing, the promoting and inhibiting relationship between the five elements finally build a balance between each other. This is the essence of Wu xing theory.




The promoting relationship
        The “火”（red）generates the“土” （orange）， then “土”generates“金”（grey），“金”generates “水”（blue），“水”generates “木”（green），“木”generates“火”.
The inhibiting relationship
        The “水”inhibit the“火”，“木”inhibit “土”，“火”inhibit“金”，“土”inhibit “水”，“金” inhibit “木”.

The balance of our system

         We use device 1 to represent “火”，device 2 to represent “土”，device 3 to represent “金”，device 4 to represent “水”，device 5 to represent “木”.
The promoting relationship
        When enough 3-O-C6-HSL is added into the medium. DeviceⅠ(火) is activated, producing leuB, which make E.coli with device I be able to survive in the medium without leucine, lasI, which produces 3-O-C12-HSL, and EYFP（engineered yellow fluorescent protein）
        When the 3-O-C12-HSL accumulates to the threshold concentration, it will bind with lasR produced by deviceⅡ(土) and activate the deviceⅡ, expressing the leuB, ECFP（engineered cyan fluorescent protein）and cinI. The deviceⅡcan grow on the medium without leucine again and produce 3-OH-C14:1-HSL, activating the device Ⅲ (金). Similarly, the device Ⅳ (水) and device Ⅴ(木) will be activated one by one.
        Finally, the device Ⅴ(木) is activated and produce 3-O-C6-HSL, which will activate the deviceⅠ(火) and form a closed loop.
The inhibiting relationship
        When IPTG is added into the medium, the inhibiting circuit in deviceⅠwill be activated.The rhiR is expressed, accept the inhibiting AHL signal C6-HSL, C7-HSL, C8-HSL ( produced by rhiI ) from device Ⅳ, then activate the expression of tetR, which will inhibit the ptetR in promoting circuit in device I.
        The AHLs can diffuse freely between these bacteria , while the inhibiting interaction is only limited within the cell, as the tetR is a macromolecule, it will be difficult for tetR to go through the bacteria.
        Similarly, the deviceⅡwill accept the inhibiting AHL signal 3-O-C6-HSL (produced by luxI )from device Ⅴ.
        The device Ⅲ will accept the inhibiting AHL signal 3-O-C12-HSL (produced by lasI )from deviceⅠ.
        The device Ⅳ will accept the inhibiting AHL signal 3-OH-C14:1-HSL (produced by cinI )from deviceⅡ.
        The device Ⅴ will accept the inhibiting AHL signal C12-HSL、3-Oxo-C14-HSL、C16:1-HSL、3-Oxo-C16:1-HSL、C18-HSL (produced by sinI )from device Ⅲ.

The final results we want to achieve

        When the five bacteria is added into the medium without leucine, and any kind of AHLs is added. The promoting cycle will start working. The engineered E.coli will get the ability to survive in the medium without leucine one by one, they promote each other, more and more leuB and fluorescent protein will be expressed, this will become a run-away model.
        When this promoting model run amok, we can restrain it by adding IPTG, this will relieve the inhibition of lacI, and activate the restraining circuit. Thus, the over-expression of leuB and fluorescent protein will be inhibited, the growth of bacteria can be controlled.
        When the inhibition is too strong, we only need to add anhydrote-tetracycline to relieve the inhibition of tetR.
        By adding IPTG and aTc can we control the promoting cycle and restraining cycle, making it a balanced system.