The BLUF protein Slrl694 from the cyanobacterium Synechocystis sp. PCC6803 is characterized by absorption and emission spectroscopy. Slrl694 expressed from E. coli which non-covalently binds FAD, FMN, and riboflavin (called Slr1694(I)), and reconstituted Sh-1694 which dominantly contains FAD (called Slr1694(II)) are investigated. The receptor conformation of Slrl694 (dark adapted form Slr1694(r)) is transformed to the putative signalling state (light adapted form Slrl694,) with red-shifted absorption and decreased fluorescence efficiency by blue-light excitation. In the dark at 22 degrees C, the signalling state recovers back to the initial receptor state with a time constants of about 14.2 s for Slr1694(I) and 17 s for Slr1694(II). Quantum yields of signalling state formation of approximately 0.63 +/- 0.07 for both Slr1694(I) and Slr1694(II) 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. (c) 2006 Elsevier B.V. All rights reserved.