The transcriptional antirepressor AppA from the photosynthetic bacterium Rhodobacter sphaeroides senses both the light climate and the intracellular redox state. Under aerobic conditions in the dark, AppA binds to and thereby blocks the function of PpsR, a transcriptional repressor. Absorption of a blue photon dissociates AppA from PpsR and allows the latter to repress photosynthesis gene expression. The N terminus of AppA contains sequence homology to flavin-containing photoreceptors that belong to the BLUF family. Structural and chemical aspects of signal transduction mediated by AppA are still largely unknown. Here we present NMR studies of the N-terminal flavin-binding BLUF domain of AppA. Its solution structure adopts an alpha/beta-sandwich fold with a beta alpha beta beta alpha beta beta topology, which represents a new flavin-binding fold. Considerable disorder is observed for residues near the chromophore due to conformational exchange. This disorder is observed both in the dark and in the light-induced signaling state of AppA. Furthermore, we detect light-induced structural changes in a patch of surface residues that provide a structural link between light absorption and signal-transduction events.