Involved in repair of UV radiation-induced DNA damage. Catalyzes the photoreactivation of pyrimidine [6-4] pyrimidone photoproduct (6-4 products) (By similarity). (6-4) photoproduct (in DNA) = 2 pyrimidine residues (in DNA). Binds 1 FAD per subunit. Belongs to the DNA photolyase class-1 family.