Questions Regarding the Presumed Role of Thymine Dimer in Photoreactivable U.V. Damage to Dna*

Summary A large volume of compelling evidence has indicated thymine dimer as the principal photoreactivable ultraviolet damage in DNA. Some experiments with the photoreactivating enzyme system clash with this interpretation. Several varieties of thymine dimer containing compound fail to interact with photoreactivating enzyme, while u.v. irradiated polydeoxy GC, a polynucleotide lacking thymine, carries a photoenzymatically reactive product. Photoreactivable u.v. damage does not form in DNA during irradiation below pH 3–3.5, while dimerization of thymine in a simpler compound does. Some evidence, which at first glance seems to support the dimer hypothesis, fails under scrutiny, and may even weigh against it.