A prokaryotic protein, YtvA from Bacillus subtilis, was found to possess a light, oxygen, voltage (LOV) domain sharing high homology with the photoactive, flavin mononucleotide (FMN)-binding LOV domains of phototropins (phot), blue-light photoreceptors for phototropism in higher plants. Computer-based three-dimensional modeling suggests that YtvA-LOV binds FMN in a similar pocket as phot-LOVs. Recombinant YtvA indeed exhibits the same spectroscopical features and blue-light-induced photochemistry as phot-LOVs, with the reversible formation of a blue-shifted photoproduct, assigned to an FMN-cysteine thiol adduct (Thio383). By means of laser-flash photolysis and time-resolved optoacoustic experiments, we measured the quantum yield of formation for Thio383, ΦThio = 0.49, and the enthalpy change, ΔHThio = 135 kJ/mol, with respect to the parent state. The formation of Thio383 is accompanied by a considerable volume contraction, ΔVThio = -13.5 ml/mol. Similar to phot-LOVs, Thio383 is formed from the decay of a red-shifted transient species, T650, within 2 μs. In both YtvA and free FMN, this transient has an enthalpy content of ∼200 kJ/mol, and its formation is accompanied by a small contraction, ΔVT ≈ -1.5 ml/mol, supporting the assignment of T650 to the FMN triplet state, as suggested by spectroscopical evidences. These are the first studies indicating that phototropin-related, blue-light receptors may exist also in prokaryotes, besides constituting a steadily growing family in plants.
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