Visualization of a multi-turnover Cas9 after product release
While the most widely used CRISPR-Cas enzyme is the S. pyogenes Cas9 endonuclease (Cas9), it exhibits single-turnover enzyme kinetics which leads to long residence times on product DNA. This blocks access to DNA repair machinery and acts as a major bottleneck during CRISPR-Cas9 gene editing. Although Cas9 can eventually be forcibly removed by extrinsic factors (translocating polymerases, helicases, chromatin modifying complexes, etc), the mechanisms contributing to Cas9 dissociation following cleavage remain poorly understood. Interestingly, its been shown that Cas9 can be more easily dislodged when complexes collide with the PAM-distal region of the Cas9 complex or when the strength of Cas9 interactions in this region are weakened. Here, we employ truncated guide RNAs as a strategy to weaken PAM-distal nucleic acid interactions and still support Cas9 activity. We find that guide truncation promotes much faster Cas9 turnover and used this to capture previously uncharacterized Cas9 reaction states. Kinetics-guided cryo-EM enabled us to enrich for rare, transient states that are often difficult to capture in standard workflows. From a single dataset, we examine the entire conformational landscape of a multi-turnover Cas9, including the first detailed snapshots of Cas9 dissociating from product DNA. We discovered that while the PAM-distal product dissociates from Cas9 following cleavage, tight binding of the PAM-proximal product directly inhibits re-binding of new targets. Our work provides direct evidence as to why Cas9 acts as a single-turnover enzyme and will guide future Cas9 engineering efforts.