Molecular Basis for Methylation-sensitive Editing by Cas9

However, it has yet to fully leverage the pervasive presence of DNA methylation in genomes4-10. To fill this gap, we report biochemical, structural, and human genome editing characterizations of a methylation-sensitive Cas9 (ThermoCas9).

ThermoCas9 efficiently binds and cleaves DNA upstream of its protospacer adjacent motif (PAM) 5´-NNNNCGA-3´ or 5´-NNNNCCA-3´ in vitro. Methylation of the fifth cytosine in either PAM sequence (5mCpG or 5mCpC), however, significantly inhibits ThermoCas9 activity. Cryogenic electron microscopy structures of ThermoCas9 in pre- and post-cleavage states at 2.8 Å and 2.2 Å resolution, respectively, reveal the molecular basis for the stringent requirement of the unmethylated cytosine in PAM binding and provides guidance for further enzyme engineering.