The PAS2 domain is required for dimerization of phytochrome A

Phytochromes are plant photoreceptors that regulate the growth and development of plants in response to light. Phytochromes exist as dimers and dimerization is thought to be important for phytochrome function. Phytochromes contain two major domains, the N-terminal domain responsible for chromophore ligation and photosensory specificity and the C-terminal domain responsible for dimerization and regulatory functions. We have investigated the dimerization motifs by means of the yeast two-hybrid assays and size exclusion chromatography using purified recombinant phytochromes. From dimerization analyses using internal deletion mutants, site-specific mutants, and C-terminal fragments of the pea phytochrome A, the primary contact region for dimerization was localized to the region between Val730 and Gly821. Further analysis using purified full-length phytochrome mutants and Per–Arnt–Sim 1 (PAS1) and PAS2 fragments revealed that the PAS2 domain is required for dimerization, but that the PAS1 domain is not.