Arabidopsis Phytochrome A Is Modularly Structured to Integrate the Multiple Features That Are Required for a Highly Sensitized Phytochrome

摘要

Phytochrome is a red ()/far-red () light–sensing photoreceptor that regulates various aspects of plant development. Among the members of the phytochrome family, phytochrome A (phyA) exclusively mediates atypical phytochrome responses, such as the high irradiance response (), which is elicited under prolonged . A proteasome-based degradation pathway rapidly eliminates active Pfr (the -absorbing form of phyA) under . To elucidate the structural basis for the phyA-specific properties, we systematically constructed 16 chimeric phytochromes in which each of four parts of the phytochrome molecule, namely, the N-terminal extension plus the Per/Arnt/Sim domain (N-PAS), the cGMP phosphodiesterase/adenyl cyclase/FhlA domain (GAF), the phytochrome domain (PHY), and the entire C-terminal half, was occupied by either the phyA or phytochrome B sequence. These phytochromes were expressed in transgenic Arabidopsis thaliana to examine their physiological activities. Consequently, the phyA N-PAS sequence was shown to be necessary and sufficient to promote nuclear accumulation under , whereas the phyA sequence in PHY was additionally required to exhibit . Furthermore, the phyA sequence in PHY alone substantially increased the light sensitivity to . In addition, the GAF phyA sequence was important for rapid Pfr degradation. In summary, distinct structural modules, each of which confers different properties to phyA, are assembled on the phyA molecule.