Photosynthetic organisms need to sense environmental light effectively in order to regulate cellular processes. AppA (Activation of Photopigment and PUC A protein) and CryB (cryptochrome B) are two flavin (FAD) containing photoreceptors found in the facultatively photosynthetic bacterium Rhodobacter sphaeroides and depending on the oxygen levels can regulate the expression of photosynthetic genes by light. Photoexcitation of AppA and CryB is accompanied by changes on their conformation and their affinity to partner binding proteins to initiate signaling transduction processes. The mechanisms by which AppA and CryB regulate downstream signaling events is not known. Thus there is substantial interest in capturing the conformational landscape that lead to the signaling state and in understanding how absorption of light by the flavin is coupled to these conformational changes. The aim of the project is to provide atomic level characterization of the photoactivation mechanism of AppA and structural information on the photocycle of CryB. To achieve our objectives, we will use a combination of biochemical methods, ultrafast transient absorption spectroscopy, QM/MM (hybrid quantum mechanics/molecular mechanisms) calculations and time-resolved structural methods. The latter include TR-serial crystallography (SX) at synchrotrons (SSX) and X-ray free electron lasers (SFX) and TR-solution scattering (small-angle, SAXS and wide-angle X-ray scattering, WAXS). Ultimately, this project will provide a molecular movie of the AppA that features the structural changes occurring upon blue-light illumination and significant information on the photocycle of CryB. The proposed research lies on the implementation of new instrumentation and novel approaches to advance our knowledge on how photoreceptors function in order to engineer novel systems that use light as a tool to achieve noninvasive control of biological processes with high spatiotemporal resolution.
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