Initial Characterization of the Primary Photochemistry of AppA, a Blue-light–using Flavin Adenine Dinucleotide–domain Containing Transcriptional Antirepressor Protein from Rhodobacter sphaeroides: A Key Role for Reversible Intramolecular Proton Transfer f

The flavin adenine dinucleotide (FAD)–containing photo-receptor protein AppA (in which the FAD is bound to a novel so-called BLUF domain) from the purple nonsulfur bacterium Rhodobacter sphaeroides was previously shown to be photo-active by the formation of a slightly redshifted long-lived intermediate that is thought to be the signaling state. In this study, we provide further characterization of the primary photochemistry of this photoreceptor protein using UV–Vis and Fourier-transform infrared spectroscopy, pH measure-ments and site-directed mutagenesis. Available evidence indicates that the FAD chromophore of AppA may be protonated in the receptor state, and that it becomes exposed to solvent in the signaling state. Furthermore, experimental data lead to the suggestion that intramolecular proton transfer (that may involve [anionic] Tyr-17) forms the basis for the stabilization of the signaling state