Possible models of the phosphorylation-dependent structural changes of plant cryptochromes in response to blue light. The PHR region is predominantly negatively charged (-), and the carboxy-terminal domain (C) can be made negatively charged by phosphorylation (which requires ATP and releases inorganic phosphate, Pi). In all models, phosphorylation leads to binding of unknown signaling partners (X, Y, Z) and to regulation of plant development. (a) One model is that phosphorylation of the carboxy-terminal domain in response to light is performed by ATP bound to the PHR region; this leads to dissociation of the two domains. (b) A second possibility is that phosphotransfer in response to light involves the interaction of two cryptochromes encoded by the same gene. (c) Alternatively, intermolecular phosphotransfer could involve the interaction of different cryptochromes. All three scenarios may exist in plant cells, and the activity of a cryptochrome may be determined by the kinetics of the different reactions.