Structural intermediate in the photocycle of a BLUF (sensor of blue light using FAD) protein Slr1694 in a cyanobacterium Synechocystis sp PCC6803

Slr1694 in Synechocystis sp. PCC6803 is a family of blue-light photoreceptors based on flavin adenine dinucleotide (FAD) called BLUF (sensor of blue light using FAD) proteins, which include AppA from Rhodobacter sphaeroides and PAC from Euglena gracilis. Illumination of dark-state Slr1694 at 15 degreesC reversibly induced a signaling light state characterized by the red shift in the UV-visible spectrum and by the light-induced Fourier transform infrared (FTIR) difference spectrum for structural changes of a bound flavin and apo protein. Illumination at the medium-low temperature (-35 degreesC) led to the red shift in the UV-visible spectrum despite some small difference in the light-induced changes. In contrast, the -35 degreesC illumination resulted in a completely different light-induced FTIR spectrum, in which almost all of the bands were suppressed with the exception of the bands for the change of C4=O bonding of the FAD isoalloxazine ring. The C4=O bands were induced at -35 degreesC with almost the same intensity, but the band frequency for the light state was upshifted by 6 cm(-1). The changes in frequency of the light-state C4=0 band and in amplitude of other bands showed the same temperature dependence with a half-change temperature at approximately -20 degreesC. It was indicated that the light-induced structural changes of apo protein and FAD were inhibited at low temperature with the exception of the change in hydrogen bonding to the C4=O group. The light-induced formation of the FTIR bands was similarly inhibited by sample dehydration. We discussed the possibility that this constrained light state is a trapped intermediate state in the photocycle of Slr1694.