G-quadruplex structures act as a novel recognition motif for the meiosis-specific histone methyltransferase PRDM9

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Abstract The histone methyltransferase PR domain containing protein 9 (PRDM9) is a key determinant of meiotic recombination in humans. It deposits activating histone marks thereby promoting recruitment of the meiotic recombination machinery. It recognizes DNA through a repetitive zinc-finger array that binds specific sequence motifs whose complementary G-rich strands can form DNA secondary structures, particularly G-quadruplexes (G4s). These may present an additional binding platform for PRDM9 and contribute to the formation of a chromatin environment permissive for meiotic recombination. We investigated the relationship between PRDM9 binding sites and G4 motifs using computational analyses of predicted and experimentally validated G4s and found that G4 motifs are among the most prevalent features at PRDM9 binding sites, with the strongest enrichment observed for highly stable G4s, largely independent of loop length. Using electrophoretic mobility shift assays, we further examined whether PRDM9 can bind short, single-stranded G4-forming oligonucleotides in addition to its canonical double-stranded DNA targets. PRDM9 directly bound folded G4 structures, and binding increased with G4 stability. This relationship was observed across different G4 motifs and following stabilization of the same G4 by increasing the potassium concentration or adding a G4-stabilizing ligand. PRDM9 also bound an artificial G4-forming sequence absent from the human genome, which was abolished when mutating the G4 motif to avoid structure formation. Together, these results support a model in which stable G4 structures facilitate PRDM9 recruitment by creating and discrete increased local chromatin accessibility, thereby contributing to the initiation of meiotic recombination. Graphical abstract

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