Crystal structures of the Synechocystis photoreceptor Slr1694 reveal distinct structural states related to signaling

Crystal structures of the Synechocystis BLUF phototaxis photoreceptor Slr1694 have been determined in two crystal forms, a monoclinic form at 1.8 angstrom resolution and an orthorhombic form at 2.1 angstrom resolution. In both forms, the photoreceptor is comprised of two pentamer rings stacked face to face. Twenty total subunits in the two asymmetric units of these crystal forms display three distinct tertiary structures that differ in the length of the fifth beta-strand and in the orientation of Trp91, a conserved Trp residue near the FMN chromophore. Fluorescence spectroscopic analysis on Slr1694 in solution is consistent with motion of Trp91 from a hydrophobic environment in the dark state to a more hydrophilic environment in the light-excited state. Mutational analysis indicates that movement of Trp91 is dependent on the occupancy of the hydrophobic Trp binding pocket with a nearby Met. These different tertiary structures may be associated with absorption changes in the blue region of the spectrum.