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Updated: Apr 20, 2026
Enhanced Genome Editing with Cas9 Ribonucleoprotein in Diverse Cells and Organisms
Published on: May 25, 2018
Landscape of target:guide homology effects on Cas9-mediated cleavage
Becky Xu Hua Fu1, Loren L Hansen2, Karen L Artiles2
1Department of Genetics, Stanford University, Stanford, CA 94305, USA xuhua@stanford.edu.
Abstract:
To study target sequence specificity, selectivity, and reaction kinetics of Streptococcus pyogenes Cas9 activity, we challenged libraries of random variant targets with purified Cas9::guide RNA complexes in vitro. Cleavage kinetics were nonlinear, with a burst of initial activity followed by slower sustained cleavage. Consistent with other recent analyses of Cas9 sequence specificity, we observe considerable (albeit incomplete) impairment of cleavage for targets mutated in the PAM sequence or in 'seed' sequences matching the proximal 8 bp of the guide. A second target region requiring close homology was located at the other end of the guide::target duplex (positions 13-18 relative to the PAM). Sequences flanking the guide+PAM region had measurable (albeit modest) effects on cleavage. In addition, the first-base Guanine constraint commonly imposed by gRNA expression systems has little effect on overall cleavage efficiency. Taken together, these studies provide an in vitro understanding of the complexities of Cas9-gRNA interaction and cleavage beyond the general paradigm of site determination based on the 'seed' sequence and PAM.
Insights
This study reveals Streptococcus pyogenes Cas9 (SpCas9) enzyme activity is complex. Beyond the PAM and seed regions, other DNA sequences influence SpCas9 cleavage, impacting gene editing specificity.
Area of Science:
- Molecular Biology
- Biochemistry
- Genetics
Background:
- The Cas9 enzyme from Streptococcus pyogenes is a key tool in genome editing.
- Understanding Cas9's target sequence specificity and kinetics is crucial for precise gene editing applications.
Purpose of the Study:
- To investigate the target sequence specificity, selectivity, and reaction kinetics of Streptococcus pyogenes Cas9 (SpCas9) in vitro.
- To elucidate the factors influencing SpCas9 cleavage beyond the canonical PAM and seed sequences.
Main Methods:
- In vitro cleavage assays using purified SpCas9::guide RNA complexes.
- Challenging SpCas9 with libraries of random variant target DNA sequences.
- Analysis of cleavage kinetics and sequence-dependent activity.
Main Results:
- SpCas9 cleavage kinetics exhibited nonlinear behavior with an initial burst followed by sustained activity.
- Cleavage was impaired by mutations in the PAM and seed sequences (proximal 8 bp of guide), but not completely abolished.
- A secondary region (positions 13-18 relative to PAM) also required homology for efficient cleavage.
- Flanking sequences had modest effects on cleavage, and the first-base Guanine constraint had minimal impact.
Conclusions:
- SpCas9 target recognition and cleavage are more complex than solely relying on PAM and seed sequences.
- These findings provide a deeper in vitro understanding of SpCas9-guide RNA interactions and cleavage mechanisms.
- The study highlights the importance of considering broader sequence contexts for optimizing CRISPR-Cas9 gene editing specificity.
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