DNA photolyases use light energy to repair DNA that comprises ultraviolet-induced lesions such as the cis-syn cyclobutane pyrimidine dimers (CPDs). Here we report the crystal structure of a DNA photolyase bound to duplex DNA that is bent by 50 degrees and comprises a synthetic CPD lesion. This CPD lesion is flipped into the active site and split there into two thymines by synchrotron radiation at 100 K. Although photolyases catalyze blue light-driven CPD cleavage only above 200 K, this structure apparently mimics a structural substate during light-driven DNA repair in which back-flipping of the thymines into duplex DNA has not yet taken place.

The flavin heterocycle is the key component in the important and ubiquitous FMN and FAD coenzymes. As such, the flavin is able to catalyse oxygen-transfer reactions in many monooxygenases, as well as one-and two-electron transfer processes. We recently inserted the flavin heterocycle into oligonucleotides with the future goal of creating flavindependent ribozymes with redox-catalytic properties. We showed that a flavin inside duplex DNA is able to exchange electrons with its environment. These electron-exchange capabilities strongly modulate the fluorescence properties of the flavin heterocycle in DNA. Herein we report a detailed study of how DNA single and double strands change the

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