Here, we describe studies of the photoreaction of full-length aureochrome 1 by CGP-20712A methanesulfonate salt time-resolved UV/Vis spectroscopy. The kinetics of the decay of the triplet state as well as of the DAF-FM DA adduct state were determined. The full-length aureochrome 1 shows a longer life time of the signaling state compared to the isolated LOV domain. Furthermore, light-induced FTIR difference spectroscopy was used to elucidate the influence of binding the target DNA to aureochrome 1 on the photocycle intermediate LOV390.We were able to demonstrate interactions of arginine and lysine side chains of aureochrome 1 with the phosphate group of the DNA backbone. Based on homology we propose formation of a Y shaped DNA-aureochrome 1 complex, where the DNA is bound in between the two basic regions of the bZIP domain in the major grove. Structural changes occurring upon conversion of the dark state to the blue-shifted intermediate state were monitored by vibrational spectroscopy. As the lifetime of the LOV390 intermediate is very long, the vibrational changes can be recorded under photo-stationary conditions without the need for time-resolved experimentation. Positive bands in the light-dark FTIR difference spectra correspond to the long-lived adduct intermediate LOV390 and negative bands to vibrations of darkstate LOV447. Most of the difference bands are due to vibrations of the chromophore because FMN undergoes the largest dipolar changes of the holoprotein. Band assignment is facilitated by the comparison to other LOV domain proteins. For this purpose, FTIR differences of full-length YtvA recorded under identical conditions are included in Figure 4. YtvA from Bacillus subtilis comprises a LOV domain and a downstream STAS domain, which is involved in general stress response of this bacterium. The sequence identity between both LOV domain is 52%. As expected, the two spectra share similarities but detailed inspection reveals crucial differences. Particularly differing features are observed in the amide I and amide II regions indicating changes in secondary structure. The negative band at 1676 cm21 of dark-state YtvA is absent in the spectrum of aureochrome 1 and the intensity of the positive peak at 1684 cm21 is decreased. In contrast, the negative band at 1641 cm21, which is assigned to the n vibration of ring I of the isoalloxazine moiety, is more intense than in the YtvA spectrum.