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Free keywords:
BLUF domain; AppA; Blue-light photoreceptor; Rhodobacter sphaeroides; Absorption spectroscopy; Fluorescence spectroscopy; Fluorescence up-conversion; Photo-cycle; Flavoprotein; FAD; FMN; Riboflavin
Abstract:
The BLUF domain of the transcriptional anti-repressor protein AppA from the non-sulfur anoxyphototrophic purple bacterium Rhodobacter sphaeroides was characterized by absorption and emission spectroscopy. The BLUF domain constructs AppA148 (consisting of amino-acid residues 1–148) and AppA126 (amino-acid residues 1–126) are investigated. The cofactor of the investigated domains is found to consist of a mixture of the flavins riboflavin, FMN, and FAD. The dark-adapted domains exist in two different active receptor conformations (receptor states) with different sub-nanosecond fluorescence lifetimes (BLUFr,f and BLUFr,sl) and a small non-interacting conformation (BLUFnc). The active receptor conformations are transformed to putative signalling states (BLUFs,f and BLUFs,sl) of low fluorescence efficiency and picosecond fluorescence lifetime by blue-light excitation (light-adapted domains). In the dark at room temperature both signalling states recover back to the initial receptor states with a time constant of about 17 min. A quantum yield of signalling state formation of about 25% was determined by intensity dependent transmission measurements. A photo-cycle scheme is presented including photo-induced charge transfer complex formation, charge recombination, and protein binding pocket reorganisation.
