Fig. 1.
Domain arrangement and overview of the phytochrome photocycle. a Domain arrangement of different phytochromes. The highly conserved photosensory core module (PCM) is subdivided into PAS, GAF and PHY domains; the C-terminal output modules differ between phytochromes but contain in most cases a histidine kinase (HK) or HK-like module. b Overview of the photocycle of phytochromes. For Pfr and Pr states, the overall folds of representative PCM constructs are depicted in surface representation (Pfr state based on wild-type Agp2-PCM: PDB entry 6G1Y, PAS in grey, GAF in cyan, PHY in black; Pr state based on Agp1-PCMSER13: PDB entry 5HSQ30, PAS in grey, GAF in magenta, PHY in black). The secondary structure changes within the PHY tongue (elongated part of the PHY domain) that occur during the last steps of the Pr/Pfr photo-transformation are highlighted in cartoon representation (Pfr: α-helix, loop and coiled structure - red, Pr: β-sheet and β-hairpin - green). The structure of the biliverdin (BV) chromophore in the Pfr and Pr state is shown above and underneath the photocycle scheme, respectively. The chemical structure of BV is represented by only one of their resonance structures each