Reaction mechanisms of DNA photolyase

DNA photolyase uses visible light and a fully reduced flavin cofactor FADH(-) to repair major UV-induced lesions in DNA, the cyclobutane pyrimidine dimers (CPDs). Electron transfer from photoexcited FADH(-) to CPD, splitting of the two intradimer bonds, and back electron transfer to the transiently formed flavin radical FADH degrees occur in overall 1 ns. Whereas the kinetics of FADH degrees was resolved, the DNA-based intermediates escaped unambiguous detection yet. Another light reaction, named photoactivation, reduces catalytically inactive FADH degrees to FADH(-) without implication of DNA. It involves electron hopping along a chain of three tryptophan residues in 30 ps, as elucidated in detail by transient absorption spectroscopy. The same triple tryptophan chain is found in cryptochrome blue-light photoreceptors and may be involved in their primary photoreaction.