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| - Light-Induced Structural Changes in a Putative Blue-Light Receptor with a NovelFAD Binding Fold Sensor of Blue-Light Using FAD (BLUF); Slr1694 ofSynechocystis sp. PCC6803
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| - The sensor of blue-light using FAD (BLUF) domain is the flavin-binding fold categorized toa new class of blue-light sensing domain found in AppA from Rhodobacter sphaeroides and PAC fromEuglena gracilis, but little is known concerning the mechanism of blue-light perception. An open readingframe slr1694 in a cyanobacterium Synechocystis sp. PCC6803 encodes a protein possessing the BLUFdomain. Here, a full-length Slr1694 protein retaining FAD was expressed and purified and found to bepresent as an oligomeric form (trimer or tetramer). Using the purified Slr1694, spectroscopic propertiesof Slr1694 were characterized. Slr1694 was found to show the same red-shift of flavin absorption andquenching of flavin fluorescence by illumination as those of AppA. These changes reversed in the darkalthough the rate of dark state regeneration was much faster in Slr1694 than AppA, indicating that Slr1694is a blue-light receptor based on BLUF with the similar photocycle to that of AppA. The dark decay inD2O was nearly four times slower than in H2O. Light-induced Fourier transform infrared (FTIR) differencespectroscopy was applied to examine the light-induced structure change of a chromophore and apo-proteinwith deuteration and universal 13C and 15N isotope labeling. The FTIR results indicate that light excitationinduced distinct changes in the amide I modes of peptide backbone but relatively limited changes inflavin chromophore. Light excitation predominantly weakened the C(4)O and C(2)O bonding andstrengthened the N1C10a and/or C4aN5 bonding, indicating formational changes of the isoalloxazinering II and III of FAD but little formational change in the isoalloxazine ring I. The photocycle of theBLUF is unique in the sense that light excitation leads to the structural rearrangements of the proteinmoieties coupled with a minimum formational change of the chromophore.
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