Structural basis for Cas9-directed spacer acquisition in type II-A CRISPR-Cas systems

CRISPR-Cas systems confer prokaryotic immunity by integrating foreign DNA (prespacers) into host arrays. Type II-A systems employ Cas9 for protospacer-adjacent motif (PAM) recognition and coordinate with Csn2 and the Cas1-Cas2 integrase during spacer acquisition, yet their structural basis remains unresolved. Here, we report cryo-electron microscopy (cryo-EM) structures of the Enterococcus faecalis Cas9-Csn2-Cas1-Cas2 supercomplex in apo and DNA-bound states. The apo state (Cas9₂-Csn2₈-Cas1₈-Cas2₄) is a resting complex, while DNA binding forms a prespacer-catching complex threading DNA through Csn2's channel, enabling Cas9 to interrogate the PAM sequence while sliding along the DNA. Cas9 and Csn2 jointly define a 30-bp DNA segment matching the prespacer length. Cas9 dissociation triggers structural reconfiguration of the Csn2-Cas1-Cas2 assembly. This exposes the PAM-proximal DNA, allowing Cas1-Cas2 to bind the exposed site for subsequent prespacer processing and directional integration. These findings reveal how Cas9, Csn2, and Cas1-Cas2 couple PAM recognition with prespacer selection, ensuring fidelity during adaptation.