Phytochrome: structural basis for its functions.

Phytochrome mediates various physiological as well as developmental responses to light stimuli in plants. Phytochrome is a soluble chromoprotein consisting of the N-terminal photosensory and C-terminal dimerization moieties. Close homologues of plant phytochromes are widely found in prokaryotes. Recently, the crystal structures of the core photosensory module of bacterial phytochromes are resolved. Intriguingly, three sub-domains (PAS, GAF and PHY) in the module are connected by unusual structures named 'light-sensing knot' and 'tongue', which are in tight contact with the chromophore. These findings enable us to review previous data on the structure-function relationships in phytochrome. Consequently, functional importance of these peculiar structures is further highlighted. Thus, the three-dimensional structure provides a framework for understanding how phytochrome processes the light signals.