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| - Structural Intermediate in the Photocycle of a BLUF (Sensor of Blue Light UsingFAD) Protein Slr1694 in a Cyanobacterium Synechocystis sp. PCC6803
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| - Slr1694 in Synechocystis sp. PCC6803 is a family of blue-light photoreceptors based on flavinadenine dinucleotide (FAD) called BLUF (sensor of blue light using FAD) proteins, which include AppAfrom Rhodobacter sphaeroides and PAC from Euglena gracilis. Illumination of dark-state Slr1694 at 15°C reversibly induced a signaling light state characterized by the red shift in the UV−visible spectrumand by the light-induced Fourier transform infrared (FTIR) difference spectrum for structural changes ofa bound flavin and apo protein. Illumination at the medium-low temperature (−35 °C) led to the red shiftin the UV−visible spectrum despite some small difference in the light-induced changes. In contrast, the−35 °C illumination resulted in a completely different light-induced FTIR spectrum, in which almost allof the bands were suppressed with the exception of the bands for the change of C4O bonding of theFAD isoalloxazine ring. The C4O bands were induced at −35 °C with almost the same intensity, butthe band frequency for the light state was upshifted by 6 cm-1. The changes in frequency of the light-state C4O band and in amplitude of other bands showed the same temperature dependence with a half-change temperature at approximately −20 °C. It was indicated that the light-induced structural changesof apo protein and FAD were inhibited at low temperature with the exception of the change in hydrogenbonding to the C4O group. The light-induced formation of the FTIR bands was similarly inhibited bysample dehydration. We discussed the possibility that this constrained light state is a trapped intermediatestate in the photocycle of Slr1694.
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