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Cyanobacteriochrome is a group of photoreceptors that are distinct relatives of phytochromes but are found only in cyanobacteria. A putative cyanobacteriochrome, CcaS, is known to chromatically regulate the expression of the phycobilisome linker gene (cpcG2) in Synechocystis sp. PCC 6803.We find that CcaS-CcaR-CpcG2 system is merely the only well studied green light sensing system.
Green Light Sensing System
Previous studies have showed the reversible photoconversion between a green-absorbing form (Pg, λmax = 535 nm) and a red-absorbing form (Pr, λmax = 672 nm) of CcaS[1]. Autophosphorylation activity of the histidine kinase domain in the C-terminal region of CcaS is up-regulated by preirradiation with green light. Similarly, phosphotransfer to the cognate response regulator, CcaR, is higher in Pr than in Pg. In cyanobacteria,CcaS phosphorylates CcaR under green light and induces expression of cpcG2, leading to accumulation of CpcG2-phycobilisome as a chromatic acclimation system. The chromophore needed in green light sensing system is phycocyanobilin(PCB),same as red sensing system, which is definitely a good news.
Fig.1Schematic diagram of green light
Downstream Regulation
Certain sgRNA is under control of the green light sensing system. sgRNA targeting RFP is cloned and the base pairing region was easily replaced by reverse PCR to target luciferase and FebI.
But there is a problem we can’t ignore:sgRNA transcripts must be strictly precise to function normally.Redundant deoxyribonucleoside in 5’-terminal of sgRNA may cause inactivation. Considering that we don’t know transcription initiation site of CpcG2 and, more importantly,in order to be consistent with other light sensing systems, we did some modification to CcaR-responding promoter.
Previous studies have shown the G-box of CpcG2 which phosphorylated CcaR recognize and bind to.[2]Six base pairs in the G-box are identical to -35 site of J23107(A constitutive promoter). So we overlap G-box to BBa_J23107. CcaR will bind to G-box thus prevent RNA polymerase from binding to the promoter. This modification(BBa_K1023009) allows down-regulation effect under green light.
Sensor Design
CcaR and CcaS was cloned from cyanobacteria and inserted into constitutive opron we have constructed respectively. Then, the entire opron was cloned and inserted in pCDFDuet-1. sgRNA was inserted in opposite direction to avoid leaking expression from upstream terminator.
Reference
HIROSE, Y., NARIKAWA, R., KATAYAMA, M. & IKEUCHI, M. 2010. Cyanobacteriochrome CcaS regulates phycoerythrin accumulation in Nostoc punctiforme, a group II chromatic adapter. Proceedings of the National Academy of Sciences, 107, 8854-8859.
HIROSE, Y., SHIMADA, T., NARIKAWA, R., KATAYAMA, M. & IKEUCHI, M. 2008. Cyanobacteriochrome CcaS is the green light receptor that induces the expression of phycobilisome linker protein. Proceedings of the National Academy of Sciences, 105, 9528-9533.
TABOR, J. J., LEVSKAYA, A. & VOIGT, C. A. 2011. Multichromatic control of gene expression in escherichia coli. Journal of Molecular Biology, 405, 315-324.
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