Conserved phenylalanine residues are required for blue-shifting of cyanobacteriochrome photoproducts.

Cyanobacteriochromes (CBCRs) are cyanobacterial photosensory proteins distantly related to phytochromes. Both phytochromes and CBCRs reversibly convert between dark-stable and photoproduct states upon photoisomerization of their linear tetrapyrrole (bilin) chromophores. While most phytochromes convert between a red-absorbing dark state and a far-red-absorbing photoproduct, CBCRs exhibit spectral responses spanning the entire near-ultraviolet and visible spectrum. For example, red/green CBCRs such as AnPixJ and NpR6012g4 exhibit a red-absorbing dark state similar to that of phytochrome, but photoconversion yields a green-absorbing photoproduct. "Teal-DXCF" CBCRs convert from blue- or green-absorbing dark states to yield photoproducts with very narrow absorption in the teal region of the spectrum (approximately 500 nm). The recent determination of a crystal structure of AnPixJ in its red-absorbing dark state led to the proposal that movement of a Trp residue (the "lid Trp") upon photoconversion would allow solvation of the photoproduct, thereby producing a large blue-shift. We find that substitution of the lid Trp has little effect on the NpR6012g4 photoproduct. Instead, two Phe residues conserved in red/green and teal-DXCF CBCRs are essential for determining photoproduct absorption in both CBCR groups with no significant influence on the dark-adapted state. We propose that these Phe residues constrain chromophore movement after primary photoisomerization. This work supports a trapped-twist mechanism for generating both red/green and teal-DXCF photoproducts.