-!- The overall repair reaction consists of two distinct steps, one of which is light-independent and the other one light-dependent. -!- In the initial light-independent step, a 6-iminium ion is thought to be generated via proton transfer induced by two histidines highly conserved among the (6-4) photolyases. -!- This intermediate spontaneously rearranges to form an oxetane intermediate by intramolecular nucleophilic attack. -!- In the subsequent light-driven reaction, one electron is believed to be transferred from the fully reduced FAD cofactor (FADH(-)) to the oxetane intermediate thus forming a neutral FADH radical and an anionic oxetane radical, which spontaneously fractures. -!- The excess electron is then back-transferred to the flavin radical restoring the fully reduced flavin cofactor and a pair of pyrimidine bases.