Fig. 9. Proposed XccBphP photoconversion model.

The Pr and Pfr structures are schematically represented on the left and right rounded panels, and the hypothetical steps involved in the photoconversion are depicted in between. Domains are colored according to Fig. 3. The main series of events comprises (i) BV ring D isomerization, (ii) reorganization of the networks within and around the chromophore-binding pocket, (iii) β-sheet/α-helix tongue transition, (iv) PHY reorientation, with the concomitant changes in the GAF-PHY and PHY-OM helical linkers, (v) dimer dissociation, (vi) PHY-OM helical linker rectification/break with the reorientation of the PAS9 domain, and (vii) parallel/antiparallel dimer inversion assembly. In the Pr-to-Pfr photoconversion, ring D isomerizes to the ZZEssa configuration followed by an α-helix conformational transition of the tongue. As a result, a pull movement of the PHY domain along with a straightening of the GAF-PHY and PHY-OM helical linkers triggers the detachment of the intertwined OMs (PAS9), generating a Y-shaped dimer. The loss of dimeric interactions dissociates the dimer, and the PHY-OM helical linker suffers a break, generating C-shaped monomers. These interact through a dimerization surface generated mainly by the tongue in the α-helix conformation, forming an antiparallel dimer. The Pfr-to-Pr photoconversion starts by ring D isomerization to the ZZZssa configuration, followed by a β-sheet transition of the tongue, disrupting the dimeric interface and pushing the PHY domain. As a result, a bending of the helical linkers occurs (Kinks 1 and 2) and the PHY-OM helical linker suffers a rectification, reconstituting the helical spine dimeric interface and the Pr dimer.