Abstract
Slr1694 is a BLUF (sensor of blue-light using FAD) protein and a blue-light photoreceptor in the
cyanobacterium Synechocystis sp. PCC6803. Illumination of Slr1694 induced a signaling
light-state characterized with a red-shift in the UV-visible absorption of flavin, and formation of the
bands from flavin and apo-protein in the light-minus-dark Fourier-transform infrared (FTIR)
difference spectrum. The FTIR bands were classifiable into three groups according to their decay
rates (t1/2 = 1.5, ~2.4 and 4.5 min) and the effects of deuteration. The C4=O stretching bands of a
flavin isoalloxazine ring had the highest decay rate and were most effected by deuteration, which
corresponded to the decay properties of the absorption red-shift. The result indicated that the
hydrogen bonding at C4=O is responsible for the UV-visible red-shift. A photocycle mechanism
involving the proton transfer was proposed.
