Abstract
The flavoprotein AppA is a blue-light photoreceptor that functions as an anti-repressor of photosynthesis gene expression in the purple bacterium, Rhodobacter sphaeroides. Heterologous expression studies show that FAD binds to a 156 amino acid N-terminal domain of AppA. A pulse of white light causes FAD absorption to be red shifted and the absorbance shift was spontaneously restored over 30 min periods, as observed in the full-length wild-type AppA. Site-directed replacement of Tyr21 with Leu or Phe abolished the photochemical reaction of the FAD-binding domain implicating involvement of Tyr21 in the photocycle. Gel filtration analysis also indicated that the size of this domain was altered upon photo-excitation. These results indicate that the N-terminal flavin-binding domain of AppA is itself photoactive.
