Absorption and emission spectroscopic characterization of blue-light receptor Slr1694 from Synechocystis sp. PCC6803

The BLUF protein Slr1694 from the cyanobacterium Synechocystis sp. PCC6803 is characterized by absorption and emission spectroscopy. Slr1694 expressed from E. coli which non-covalently binds FAD, FMN, and riboflavin (called Slr1694I), and reconstituted Slr1694 which dominantly contains FAD (called Slr1694II) are investigated. The receptor conformation of Slr1694 (dark adapted form Slr1694r) is transformed to the putative signalling state (light adapted form Slr1694s) with red-shifted absorption and decreased fluorescence efficiency by blue-light excitation. In the dark at 22 °C, the signalling state recovers back to the initial receptor state with a time constants of about 14.2 s for Slr1694I and 17 s for Slr1694II. Quantum yields of signalling state formation of approximately 0.63 ± 0.07 for both Slr1694I and Slr1694II were determined by transient transmission measurements and intensity dependent steady-state transmission measurements. Extended blue-light excitation causes some bound flavin conversion to the hydroquinone form and some photo-degradation, both with low quantum efficiency. The flavin-hydroquinone re-oxidizes slowly back (time constant 5–9 min) to the initial flavoquinone form in the dark. A photo-cycle dynamics scheme is presented.