G-Quadruplex loops regulate PARP-1 enzymatic activation.

G-Quadruplexes are non-B form DNA structures present at regulatory regions in the genome, such as promoters of proto-oncogenes and telomeres. The prominence in such sites suggests G-quadruplexes serve an important regulatory role in the cell. Indeed, oxidized G-quadruplexes found at regulatory sites are regarded as epigenetic elements and are associated with an interlinking of DNA repair and transcription. PARP-1 binds damaged DNA and non-B form DNA, where it covalently modifies repair enzymes or chromatin-associated proteins respectively with poly(ADP-ribose) (PAR). PAR serves as a signal in regulation of transcription, chromatin remodeling, and DNA repair. PARP-1 is known to bind G-quadruplexes with stimulation of enzymatic activity. We show that PARP-1 binds several G-quadruplex structures with nanomolar affinities, but only a subset promote PARP-1 activity. The G-quadruplex forming sequence found in the proto-oncogene c-KIT promoter stimulates enzymatic activity of PARP-1. The loop-forming characteristics of the c-KIT G-quadruplex sequence regulate PARP-1 catalytic activity, whereas eliminating these loop features reduces PARP-1 activity. Oxidized G-quadruplexes that have been suggested to form unique, looped structures stimulate PARP-1 activity. Our results support a functional interaction between PARP-1 and G-quadruplexes. PARP-1 enzymatic activation by G-quadruplexes is dependent on the loop features and the presence of oxidative damage.