The flavin-binding BLUF do
main of AppA represents a new class of blue light photoreceptorsthat are present in a nu
mber of bacterial and algal species. The dark state X-ray structure of this do
mainwas deter
mined at 2.3 Å resolution. The do
main de
monstrates a new function for the co
mmon ferredoxin-like fold; two long
![](/i<font color=)
mages/gifchars/alpha.gif" BORDER=0>-helices flank the flavin, which is bound with its isoalloxazine ring perpendicular toa five-stranded
![](/i<font color=)
mages/gifchars/beta2.gif" BORDER=0 ALIGN="
middle">-sheet. The hydrogen bond network and the overall protein topology of the BLUF do
main(but not its sequence) bear so
me rese
mblance to LOV do
mains, a subset of PAS do
mains widely involvedin signaling. Nearly all residues conserved in BLUF do
mains surround the flavin chro
mophore,
many ofwhich are involved in an intricate hydrogen bond network. Photoactivation
may induce a rearrange
mentin this network via reorientation of the Gln63 side chain to for
m a new hydrogen bond to the flavin O4position. This shift would also break a hydrogen bond to the Trp104 side chain, which
may be critical ininduction of global structural change in AppA.