Spectroscopic and Photochemical Characterization of the Red-Light Sensitive Photosensory Module of Cph2 from Synechocystis PCC 6803

Cyanobacterial phytochromes are a diverse family of light receptors controlling various biological functions including phototaxis. In addition to canonical bona fide phytochromes of the well characterized Cph1/plant-like clade, cyanobacteria also harbor phytochromes that absorb green, violet or blue light. The Synechocystis PCC 6803 Cph2 photoreceptor, a phototaxis inhibitor, is unconventional in bearing two distinct chromophore-binding GAF domains. Whereas the C-terminal GAF domain is most likely involved in blue-light perception, the first two domains correspond to a Cph1-like photosensory module lacking the PAS domain. Biochemical and spectroscopic studies show that this region switches between red (P-r) and far-red (P-fr) absorbing states. Unlike Cph1, the P-fr state of Cph2 decays rapidly in darkness. Mutations close to the PCB chromophore further destabilize the P-fr state without drastically affecting the spectroscopic features such as the quantum efficiency of P-r -> P-fr conversion, fluorescence, or the Resonance-Raman signature of the chromophore. Overall, the PAS-less photosensory module of Cph2 resembles Cph1 including its mode of isomerisation, but the P-fr state is unstable.