Molecular characterization of DCF cyanobacteriochromes from the cyanobacterium identifies a blue-light power sensor.

Cyanobacteriochromes (CBCRs) are linear tetrapyrrole-binding photoreceptors that sense a wide range of wavelengths from ultraviolet to far-red. The primary photoreaction in these reactions is a / isomerization of the double bond between rings C and D. After this isomerization, various color-tuning events establish distinct spectral properties of the CBCRs. Among the various CBCRs, the DCF CBCR lineage is widely distributed among cyanobacteria. Because the DCF CBCRs from the cyanobacterium vary widely in sequence, we focused on these CBCRs in this study. We identified seven DCF CBCRs in and analyzed them after isolation from that produces phycocyanobilin, a main chromophore for the CBCRs. We found that six of these CBCRs covalently bound a chromophore and exhibited variable properties, including blue/green, blue/teal, green/teal, and blue/orange reversible photoconversions. Notably, one CBCR, AM1_1870g4, displayed unidirectional photoconversion in response to blue-light illumination, with a rapid dark reversion that was temperature-dependent. Furthermore, the photoconversion took place without / isomerization. This observation indicated that AM1_1870g4 likely functions as a blue-light power sensor, whereas typical CBCRs reversibly sense two light qualities. We also found that AM1_1870g4 possesses a GDCF motif in which the Asp residue is swapped with the next Gly residue within the DCF motif. Site-directed mutagenesis revealed that this swap is essential for the light power-sensing function of AM1_1870g4. This is the first report of a blue-light power sensor from the CBCR superfamily and of photoperception without / isomerization among the bilin-based photoreceptors.