Figure 6.

Schematic view of phytochrome and CBCR photochemistry. In phytochromes and CBCRs examined to date, the common element is photoisomerization of the bilin chromophore about the 15/16 double bond. In the cyanobacterial phytochrome Cph1 (red), the (15Z) P_r form converts to a (15E) lumi-R photoproduct which is then thermally converted to the (15E) P_fr form in a process requiring the 12-propionate side chain.[63] P_fr is converted to the (15Z) lumi-R photoproduct, which is then thermally converted to P_r to complete the photocycle. In the B/G photocycle of CBCR Tlr0924 (blue), we propose formation of a second covalent linkage between Cys499 and C10 of the chromophore in the (15Z) state.[46] Such a linkage at C10 would generate a species similar to bilirubin IXα or phycocyanorubin,[29, 36] explaining the blue absorbance of P_b^S. The (15Z) P_b^S form can be photoconverted to a (15E) P_b ^L photoproduct which is in thermal equilibrium with the P_g form.[46] P_g is generated from P_b^L by cleavage of the second linkage, generating a free thiol side chain at Cys499. Photoisomerization of P_g generates an as-yet unidentified (15Z) photoproduct which regenerates P_b^S by reformation of the second linkage at C10. This B/G cycle can occur in either PCB or PVB chromophores, because it does not involve the 4/5 bond which varies between these two bilins. Thermal relaxation of the (15E) form to the (15Z) form (dark reversion) is known in plant phytochrome, Cph1, and some BphPs. Thermal conversion of (15Z) to (15E) is also known in a subset of BphP proteins termed bathyphytochromes, including PaBphP (purple).