A single 8,5'-cyclo-2'-deoxyadenosine lesion in a TATA box prevents binding of the TATA binding protein and strongly reduces transcription in vivo.

8,5'-Cyclo-2'-deoxypurine (cPu) lesions result from the action of the hydroxyl radical on DNA. These lesions represent a unique class of oxidative DNA lesions in that they are repaired by the nucleotide excision repair (NER) pathway but not by base excision repair (BER) or direct repair. Previous work has shown that cyclopurines can block mammalian DNA and RNA polymerases.