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We engineered three light-sensing systems wherein transcription from an output promoter is controlled by different light wavelengths.
The three systems are differently designed but they have the same result-----Different-colored lights activate expression of corresponding florescent protein.
Each TCS comprises a light-switchable sensor Histidine Kinase (SK) containing an N-terminal phytochrome-family photosensory domain and a C-terminal bifunctional kinase-phosphatase signalling domain.
Green
In the green light-inducible system, the SK CcaS is produced in a green-absorbing ground state, termed Pg.
In dark, no light activates the whole circuit so there is no florescent protein produced. Absorption of red light switches CcaS Pr back to Pg, which dephosphorylates phospho-CcaR, deactivating transcription.
Absorption of green light flips CcaS to a kinase-active red-absorbing state (Pr) that phosphorylates the response regulator CcaR, which then binds to the cpcG2 promoter and activates the transcription of GFP.
Red
In the red light-inducible system, The SK Cph8 is produced in a kinase-active Pr state .
In dark, Chp8 phosphorylates the response regulator OmpR, activating the transcription from the ompC promoter thus producing more lambda CI. The increase of lambda CI repress its transcription of downstream promoter for RFP.
Red light switches Cph8 Pr to a far red–absorbing state (Pfr), which dephosphorylates phospho-OmpR, deactivating the transcription of OmpR. So the inverter has no effect and RFPs are continuously transcribed.
Blue
In the blue light-inducible system, FixK2 promoter is the downstream signalling promoter of YF1/FixJ blue light-sensing system. The YF1/FixJ system becomes inactive when illuminated by blue otherwise active. An inverter is placed behind FixK2 promoter.
In the absence of blue light, YF1 phosphorylates FixJ that activates FixK2 promoter, allowing the transcription of lambda CI, thus repressing the amilCP output (Blue Chromophore Protein).
In light, however, YF1/FixJ has no effect on FixK2 promoter, which in turn has near-zero activity. The lambda CI repressor concentration drops, eventually amilCP will be produced.
