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In the course of evolution plants have evolved complex mechanisms to ensure their survival as sessile organisms. Especially light plays a key role for autotrophic organisms because it is not only essential for photosynthesis, but also provides informations about the environment and the day/night rhythm. To sense these informations, plants have developed different photoreceptors, among them phytochromes are the best characterized ones. The absorption of red light results in a reversible transformation of the ground state Pr (Red) into the plant-active form Pfr (Far-red). For plants, only a crystal structure of phytochrome B in its Pr state exists.
This work provides a detailed analysis of the conformational changes of the photosensory module of Arabidopsis thaliana (At) PhyB between the Pr and the Pfr form by hydrogen-deuterium exchange (HDX) measurements. Furthermore, an influence of the N-terminal extension (NTE) on the chromophore could be detected. For the first time, a packing model for the NTE of PhyB was developed by means of the HDX of a deletion variant without NTE.
Furthermore, the structure of a phosphatase, PAPP5, interacting with phytochromes was solved and the interaction with the NTE of PhyB could be demonstrated. The structure of PAPP5 showed autoinhibition by the N-terminal TPR domain and a C-terminal motif. The responsible residues could be identified in the TPR domain and in the C-terminal motif. CD-spectroscopic measurements and hydrogen deuterium exchange experiments provided data on the development of a model of PAPP5-fatty acid activation. Another aspect of this work was to develop a hybrid system that allows easier access biophysical studies on plant phytochromes. For this purpose, the PAS and GAF domains of the cyanobacterial phytochrome SynCph1 were fused with that of the PHY domain from AtPhyB. Although, the hybrid lacked complete photoconversion, it was able to interact with the phosphatase PAPP2c in a light-dependent manner.