Molecules and Cells

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Fig. 1.

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Fig. 1. Color-tuning mechanisms adopted by cyanobacterial phytochromes (Cphs) and cyanobacteriochromes (CBCRs). Cphs and CBCRs exhibit reversible photoisomerization at C15-C16 (R/FR photocycle). Single-cysteine (Cys) red/green or green/red CBCRs bind to phycocyanobilin (PCB) in the dark state, exhibiting red and green absorption maxima. Upon red or green illumination, green or red light-absorbing forms are formed by lowering effective π-conjugation length via protochromicity of bilin (RcaE) or geometry tilting by the A-ring (SyRGS) or D-ring (NpR3784). A second Cys residue located at the DXCF or CXXR/K motif forms a second thioether linkage at C10, yielding a blue light-absorbing dark state. Some dual-Cys CBCRs isomerize PCB to phycoviolobilin (PVB), forming violet or blue light-absorbing dark state. These adducts are either photolabile or photostable upon photoisomerization, yielding green or blue-absorbing lit states. Such variation is further enhanced by bilin protochromicity or tilted geometry, yielding violet, teal, orange and red -absorbing photoproducts. In dual-Cys cyanobacterial phytochromes, tandem cysteine cyanobacterial phytochrome (TCCP)-specific Cys is responsible for the bilin split, and lit state tuning is related to the protonated lit form with different ionization state of the Cys thiol group. Black double arrows and red forward arrow represent reversible photoisomerization around the C15=C16 double bond of a bilin and second thioether linkage formation with or without bilin isomerization, respectively.
Mol. Cells 2020;43:509~516
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