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Visualization of a multi-turnover Cas9 after product release
Kaitlyn A Kiernan1,2, David W Taylor3,4,5,6
1Department of Molecular Biosciences, University of Texas at Austin, Austin, TX, USA.
Nature Communications
|July 2, 2025
Summary
The Cas9 enzyme, crucial for CRISPR gene editing, often stays bound to DNA after cutting, hindering repair. This study reveals that the DNA segment near the target site prevents Cas9 from detaching, improving gene editing efficiency.
Area of Science:
- Molecular Biology
- Biochemistry
- Gene Editing Technologies
Background:
- The Streptococcus pyogenes Cas9 (Cas9) enzyme is widely used in CRISPR gene editing.
- Cas9 exhibits single-turnover kinetics, characterized by long residence times on cleaved DNA.
- This prolonged binding impedes DNA repair machinery access, creating a bottleneck in gene editing.
Purpose of the Study:
- To investigate the mechanisms underlying Cas9 dissociation from product DNA.
- To engineer a multi-turnover Cas9 variant with improved dissociation kinetics.
- To understand the structural basis for Cas9's single-turnover behavior.
Main Methods:
- Utilized truncated guide RNAs to weaken PAM-distal interactions.
- Employed kinetics-guided cryo-electron microscopy (cryo-EM) to capture conformational states.
- Analyzed the dissociation of Cas9 from product DNA.
Main Results:
- Demonstrated that truncated guide RNAs promote faster Cas9 enzyme turnover.
- Observed dissociation of the PAM-distal product DNA from Cas9 post-cleavage.
- Identified tight binding of the PAM-proximal product DNA as the primary inhibitor of Cas9 re-binding.
Conclusions:
- Provided direct evidence for the mechanism behind Cas9's single-turnover kinetics.
- The PAM-proximal product interaction is the key factor limiting Cas9 re-engagement.
- Findings will guide the engineering of improved Cas9 variants for enhanced gene editing applications.
