Yan, Y.; Zhou, X. E.; Thomas, S. L.; Liu, M.; Lai, G.-Q.; Worden, E. J.; Jones, P. A.; Xu, T.-H.

De novo DNA methylation is a fundamental epigenetic process essential for early development. It is mediated by DNA methyltransferase DNMT3A and DNMT3B, in cooperation with catalytically inactive DNMT3L. Abnormal DNA methylation contributes to genomic instability and oncogenesis, yet the mechanisms governing its establishment and regulation remain unclear. Here, we present high-resolution cryo-EM structures of the nucleosome-bound full-length DNMT3A2-3L and its oligomeric assemblies in the nucleosome-free state. Strikingly, DNMT3L C-terminal \'\'Switching Helix\'\' displayed a distinct conformation with a 180{degrees} rotational difference compared to its DNMT3B3 counterpart in the presence of nucleosome, preventing direct interactions with the nucleosome acidic patch. Instead, DNMT3L ADD domain promotes nucleosome binding, while DNMT3A PWWP domain inhibits it, indicating multi-layer regulation. Additionally, the novel oligomeric arrangement of DNMT3A2-3L in nucleosome-free states highlights its dynamic assembly and potential allosteric regulation. Furthermore, we identified the critical role of DNMT3L as a sensor of histone modifications, guiding the complex\'s chromatin interaction. These findings uncover a previously unknown mechanism by which DNMT3A-3L mediates de novo DNA methylation on chromatin through complex reorganization, histone tail sensing, dynamic DNA search, and nucleosome engagement, providing key insights into epigenetic regulation.