CRISPR-Cas systems provide adaptive immunity in prokaryotes by targeting and degrading invasive genetic elements. Among them, the Type I-F2 system represents the most compact Type I CRISPR-Cas variant, distinguished by the complete absence of both large (Cas8) and small (Cas11) subunits. In other Type I systems, Cas8 is essential for protospacer adjacent motif (PAM) recognition and for triggering Cas3 recruitment, while Cas11 stabilizes the Cascade backbone and guides the nontarget DNA strand during R-loop formation. To elucidate how I-F2 executes interference in their absence, we determined the cryo-electron microscopy structure of the I-F2 Cascade bound to target DNA and Cas3. Our structure reveals that Cas5 alone mediates PAM sensing, while Cas7 subunits directly recruit Cas3, which adopts a helicase-loaded conformation compatible with DNA engagement. We show how the helicase and C-terminal domains of Cas3 capture the displaced nontarget strand to initiate directional unwinding and degradation. These findings uncover key mechanistic adaptations that enable efficient interference without canonical large and small subunits and emphasize the mechanistic diversity among closely related Type I systems, including I-E, I-F1, and I-F2. These insights provide a structural basis for engineering the hypercompact I-F2 system for genome editing and biotechnological applications.
Perry et al. (Tue,) studied this question.