Structural basis for a dual-function type II-B CRISPR-Cas9.

Area of Science:

• Molecular Biology
• Genetics
• Biotechnology

Background:

• The Streptococcus pyogenes Cas9 (SpCas9) enzyme is widely used for genome editing but can cause off-target edits due to tolerance of DNA-RNA mismatches.
• High-fidelity genome editing tools are crucial for precise genetic modifications in research and therapeutic applications.

Purpose of the Study:

• To investigate the structural basis for the high-fidelity DNA targeting of Cas9 from Francisella novicida (FnCas9).
• To explore the dual functions of FnCas9 in DNA cleavage and transcriptional repression.
• To assess the potential of FnCas9 for developing advanced genome editing technologies and antibacterial strategies.

Main Methods:

• Kinetic and structural analyses of FnCas9.
• Biochemical assays to determine DNA targeting fidelity.
• X-ray crystallography to elucidate enzyme-RNA-DNA interactions.
• Investigation of FnCas9-mediated transcriptional repression using small CRISPR-associated RNA (scaRNA).

Main Results:

• FnCas9 possesses a unique REC3 clamp, a structural feature responsible for its intrinsic high-fidelity DNA targeting.
• The REC3 clamp interacts with the R-loop, establishing a checkpoint that enhances specificity during enzyme activation.
• FnCas9, guided by scaRNA, can repress gene transcription, a mechanism observed in its native bacterial context for immune evasion.
• Structural data revealed how partial R-loop complementarity prevents cleavage and favors transcriptional repression.

Conclusions:

• The REC3 clamp is a key determinant of FnCas9's high specificity and offers a novel mechanism for precise genome editing.
• FnCas9 exhibits a dual mode of action, enabling both DNA cleavage and transcriptional repression, with implications for engineering precise editors.
• The conserved nature of the REC3 clamp suggests potential for developing novel antibacterial strategies and enhancing the precision of CRISPR-based therapeutics.