The structural basis for de novo DNA methylation in chromatin

Abstract

De novo cytosine methylation is essential for mammalian development and is deposited by DNMT3A and DNMT3B. In cells, DNA methylation occurs in the context of chromatin, where nucleosomes are connected by DNA linkers. Here, we report Cryo-EM structures of DNMT3A2/3B3 bound to di-nucleosomes with different linker lengths. We show that DNMT3A2/3B3 preferentially binds di-nucleosomes separated by short DNA linkers by inducing large-scale changes to the di-nucleosome structure, enabling each DNMT3B3 subunit to bind each nucleosome. Linker length and the position of cytosines within the linker control DNA methylation, indicating that a significant fraction of linkers in chromatin are naturally resistant to DNMT3A2/3B3 activity. Finally, DNMT3A2/3B3 scans for H3K36me2-3 modifications, explaining how H3K36 methylation simulates DNMT3A2 activity. Our structure is the first example of a DNA methyltransferase interacting with higher-order nucleosome substrates and provides new insights on how DNA methylation takes place in chromatin.