Absorption and emission spectroscopic characterisation of the LOV2-domain of phot from Chlamydomonas reinhardtii fused to a maltose binding protein

The absorption and emission behaviour of flavin mononucleotide (FMN) in the wild-type light, oxygen and voltage-sensitive (LOV) domain LOV2 of the photoreceptor phot from the green alga Chlamydomonas reinhardtii is studied at pH 8. Actually a LOV2-MBP-fusion protein (MBP = maltose binding protein) expressed in an Escherichia coli strain is investigated. For fresh samples stored in the dark an initial fluorescence quantum yield of phi(F) = 0.08 +/- 0.01 is determined. Blue-light photo-excitation generates a non-fluorescent intermediate photoproduct (flavin-C(4a)-cysteinyl adduct with absorption peak at 390 nm). In the aqueous solutions studied approximately seven percent of the FMN molecules are not bound to the protein (free FMN in oxidized form) and about seven percent of the non-covalently bound FMN are not convertible to an adduct. Approximately two thirds of the intermediate photoproduct recovers with a time constant of 41 +/- 1 s, while approximately one third recovers with a time constant of about 7 min. The photo-adduct formation is thought to proceed via singlet excited-state electron transfer and triplet formation. (C) 2004 Elsevier B.V. All rights reserved.