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Characterization of Synechocystis and plant phytochromes, and the role of phosphorylation in phytochrome signal transduction in plants
Phytochromes are a wavelength-dependent light switch for photomorphogenesis in plants and also in many prokaryotes. To extend the knowledge of phytochromes, two Synechocystis phytochromes, Cph1 and Cph2, and a Type II phytochrome, Arabidopsis phyB were characterized. Synechocystis phytochromes have very similar photochromism to plant phytochromes except for their blue-shifted absorption peaks. However, Cph1 do not show any increase of α-helix content upon light absorption and Cph1 apoproteins exist mainly as monomers, whereas plant phytochromes exist as dimers, suggesting that the mechanisms of photoperception and upstream signaling are different between plant and bacterial phytochromes. In addition, the discovery of Cph2 as a photoconvertible photoreceptor proves that prokaryotes also use multiple phytochromes, similar to plants. ^ Plant phyB has shown a few different characteristics from phyA, including less efficient photoconversion, faster dark reversion, different proteolytic patterns and no α-helix increase during photoconversion. These results suggest that the chromophore apoprotein interactions of phyB are different from those of phyA. These results may also help understand the different roles between phyA and phyB in plants. The dimerization domain assay showed that the PAS-B domain is responsible for phytochrome dimerization, not the PAS-A domain. ^ The role of phytochrome phosphorylation on Ser598 in the hinge region was proven to be a signal switch controlling the interaction between phytochromes and their signal transducer proteins. Since the phosphorylation of this site prevented the interaction of phyA with its signal transducers such as NDPK2, the transgenic plants with Ser598Ala phyA mutant showed hypersensitivity to light and dwarf phenotype. Of three known serine sites of phytochrome phosphorylation, Ser17 was identified as the site for phytochrome autophosphorylation and kinase activity. The domain study for plant phytochrome kinase activity showed that the histidine kinase related domain (HKRD) was not necessary for its kinase activity. Furthermore, deletion of this HKRD increased the kinase activity of phyA. The possible kinase domain was the PRD domain including the PAS-A region and the hinge region of phytochromes. ^
Kim, Jeong-Il, "Characterization of Synechocystis and plant phytochromes, and the role of phosphorylation in phytochrome signal transduction in plants" (2002). ETD collection for University of Nebraska - Lincoln. AAI3074084.