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Cryptochromes are evolutionarily conserved blue-light absorbing flavoproteins which participate in many important cellular processes including in entrainment of the circadian clock in plants,  Drosophila  and humans.  Drosophila melanogaster  cryptochrome (DmCry) absorbs light through a flavin (FAD) cofactor that undergoes photoreduction to the anionic radical (FAD •- ) redox state both  in vitro  and  in vivo . However, recent efforts to link this photoconversion to the initiation of a biological response have remained controversial. Here, we show by kinetic modeling of the DmCry photocycle that the fluence dependence, quantum yield, and half-life of flavin redox state interconversion are consistent with the anionic radical (FAD •- ) as the signaling state  in vivo . We show by fluorescence detection techniques that illumination of purified DmCry results in enzymatic conversion of molecular oxygen (O 2 ) to reactive oxygen species (ROS). We extend these observations in living cells to demonstrate transient formation of superoxide (O 2  •- ), and accumulation of hydrogen peroxide (H 2 O 2 ) in the nucleus of insect cell cultures upon DmCry illumination. These results define the kinetic parameters of the  Drosophila  cryptochrome photocycle and support light-driven electron transfer to the flavin in DmCry signaling. They furthermore raise the intriguing possibility that light-dependent formation of ROS as a byproduct of the cryptochrome photocycle may contribute to its signaling role.

Blue-light induced accumulation of reactive oxygen species is a consequence of the Drosophila cryptochrome photocycle