Nature 509, 245–248 (2014)

Phytochromes are red light–sensing kinases found primarily as dimers in plants and bacteria and are composed of a sensory domain (chromophore) consisting of PAS, GAF and PHY domains and a regulatory domain, which transmits the light signal output to downstream regulators. Exposure of phytochromes to red light produces a shift from a relaxed conformation (Pr) to an illuminated conformation (Pfr). Recent structural studies have identified a tongue-like loop within the PHY domain that stabilizes the chromophore, creates contacts with the PAS and GAF domains and ensures the transition from Pf to Pfr. However, the detailed structural changes between Pf and Pfr remain unclear. Takala et al. exposed the chromophore domain from D. radiodurans to laser flashes of red light and detected immediate nanometer-scale structural changes by time-resolved solution X-ray scattering and X-ray crystallography. Light exposure caused the dimer to change from a closed to an open Y-shaped conformation. This change was primarily driven by transition of the PHY tongue secondary structure from a β-sheet to an α-helix. These new structural data provide new insight on how phytochrome proteins can transmit their signal to downstream mediators upon exposure to red light.