The flavin-adenine-dinucteotide-binding BLUF domain constitutes a new class of blue-light receptors, and the N-terminal domain of AppA is a representative of this family. AppA functions as a transcriptional antirepressor, controlling the photosynthesis gene expression in the purple bacterium Rhodobacter sphaeroides. Upon light absorption, AppA undergoes a photocycle with a signaling state, which exhibits an approximately 10 nm red shift in the UV-vis absorption spectrum. We have characterized light-dependent changes in the active site of an AppA BLUF domain by Raman spectroscopy. The present study has found that altered chromophore- protein interactions, including a hydrogen bond at the C4 = O position and structural changes around the N10-ribityl side chain, are key events in this activation process. These structural alterations are proposed to be responsible for the transmission of the light signal in the BLUF domain. This is the first report on a signaling-state Raman spectrum of a blue-light photoreceptor with a flavin chromophore.