Single-stranded genomic DNA can fold into G- quadruplex (G4) structures or form DNA:RNA hybrids (R loops). Recent evidence suggests that such non- canonical DNA structures affect gene expression, DNA methylation, replication fork progression and genome stability. When and how G4 structures form and are resolved remains unclear. Here we report the use of Cleavage Under Targets and Tagmenta- tion (CUT&Tag) for mapping native G4 in mammalian cell lines at high resolution and low background. Mild native conditions used for the procedure retain more G4 structures and provide a higher signal-to-noise ratio than ChIP-based methods. We determine the G4 landscape of mouse embryonic stem cells (ESC), observing widespread G4 formation at active promot- ers, active and poised enhancers. We discover that the presence of G4 motifs and G4 structures dis- tinguishes active and primed enhancers in mouse ESCs. Upon differentiation to neural progenitor cells (NPC), enhancer G4s are lost. Further, performing R- loop CUT&Tag, we demonstrate the genome-wide co- occurrence of single-stranded DNA, G4s and R loops at promoters and enhancers. We confirm that G4 structures exist independent of ongoing transcrip- tion, suggesting an intricate relationship between transcription and non-canonical DNA structures.
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