Light is one of the most important environmental factors affecting growth and survival of plant. Phytochromes, the red and far-red light-sensing photoreceptors that regulate various aspects of plant growth and development, are encoded by a small gene family (phyA-phyE in Arabidopsis and phyA-phyC in rice). All phytochromes exist in two photo-interconvertible forms, an inactive Pr form and an active Pfr form, exchangeable from one form into another upon absorption of light. The phytochrome molecule is divided into two major functional domains. The N-terminal domain with a covalently linked linear tetrapyrrole chromophore is sufficient for light absorption and photoreversibility. The C-terminal domain, which is important for regulatory activity of the protein including dimerization, translocation and downstream signaling, contains two PAS domains and one histidine-kinase-like domain (HKLD) with Ser/Thr kinase activity. The N- and C-terminal domains are connected by a proteolytically-sensitive hinge region that contains a phosphorylable serine in a Pfr preferential manner. Phytochrome dimerization is essential for its full activity. Previous studies using size-exclusion chromatography experiments and lambda repressor-based in vivo assays suggest that two regions, residues 599-683 in oat phyA corresponding to 601-685 in rice phyA within the hinge-PAS 1 region and/or a large portion of HKLD, are capable of mediating dimerization. However, the exact dimerization site has not been identified yet. In addition, majority of loss-of-function missense mutations are observed within the hinge-PAS 1 segment. To gain insight into the role of the hinge-PAS 1 segment (S701-H740) in the rice phyA function, we have determined its three-dimensional solution structure by NMR spectroscopy.
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