Abstact

While the most widely used CRISPR-Cas enzyme is the Cas9 endonuclease from Streptococcus pyogenes (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. Cas9 can eventually be removed from the product by extrinsic factors, such as translocating polymerases, but the mechanisms contributing to Cas9 dissociation following cleavage remain poorly understood. Here, we employ truncated guide RNAs as a strategy to weaken PAM-distal nucleic acid interactions and promote faster enzyme turnover. Using kinetics-guided cryo-EM, we examine the 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.