8-Oxo-7,8-dihydroguanine in the Context of a Gene Promoter G-Quadruplex Is an On Off Switch for Transcription

Interplay between DNA repair of the oxidatively modified base 8-oxo-7,8-dihydroguainne (OG) :and transcriptional activation has been documented in mammalian-genes. Previously, we synthesized OG into the VEGF potential G-quadruplex sequence (PQS) in the coding strand of a luciferase promoter to identify that base excision repair (BER) unmasked the G-quadruplex (G4) fold for gene activation. In the present work, OG was site-specifically synthesized into a luciferase reporter plasmid to follow the time-dependent expression in mammalian cells when OG in the VEGF PQS context Was located in the coding vs template strands of the hiciferase promoter: Removal of OG from the coding strand by OG glycosylaIse-1 (OGG1)-mediated BER upregulated transcription. When OG was in the template strand in the VEGE PQS-context, transcription was downregulated by a BM-independent process. The time course changes in transcription show that. repair in the template strand was more efficient than repair, in the coding strand Promoters were synthesized with an OG:A base pair that requires repair on both strands to yield a canonical G:C base pair. By monitoring the up-Xclown luciferase expression, we followed the timing of repair of an OGA. base pair-occurring on both strands in mammalian cells in which one lesion resides in a Gquadruplex loop and one in, a potential i-motif. Depending on the strand in,Which OG resides, coding vs template, this modification is an up/downregulator of transcription that couples DNA repair with transcriptional regulation.