Photoreceptor proteins are light sensing biomolecules used by bacteria, funghi and plants to react to light stimuli. To date, a range of light-signalling proteins and their function are known. However, the mechanisms of signal propagation from the light sensitive chromophore to the signalling cascade in the cell are still elusive.Protein crystallography is an established method to gain structural information on static protein conformations. Only recently, time-resolved X-ray scattering methods for proteins in aqueous solution have been developed to study conformational changes that proteins undergo while functioning. Though, time-resolved X-ray solution scattering acts on proteins in their natural environment, the method lacks information content. To date, the method cannot be fully exploited because the scattering signal is averaged over all orientations that proteins adopt in solution.I propose to establish a new and promising anisotropic X-ray scattering technique which should double the information content compared to the standard method. I plan to achieve this by arresting the protein rotation in hydrogels and by excitation with polarized light. Furthermore, I will develop indispensable analysis tools to derive structural changes from the anisotropic X-ray scattering.With this new technique, I propose to elucidate the signal transduction sequence in the BLUF-domain containing photoreceptor proteins BlrP1 and AppA, covering all intermediates in the ns to ms time-scale. If successful, this project will uncover the sequence and nature of structural events that lead to signal transduction from the chromophores to the output domains of these proteins.Photoreceptors are not only interesting as easily excitable targets for the development of time-resolved methods, they also reveal nature's highly optimized strategies to transform light signals into cellular response, ultimately providing guidelines to sensing applications.