Structural basis for Cas9-directed spacer acquisition in type II-A CRISPR-Cas systems

Area of Science:

• Molecular Biology
• Microbiology
• Structural Biology

Background:

• CRISPR-Cas systems provide prokaryotes with adaptive immunity against foreign genetic elements.
• Type II-A CRISPR-Cas systems, including Cas9, Csn2, and Cas1-Cas2, are crucial for integrating foreign DNA fragments (prespacers) into the host genome.
• The structural mechanisms underlying spacer acquisition in Type II-A systems were previously unknown.

Purpose of the Study:

• To elucidate the structural basis of spacer acquisition in the Type II-A CRISPR-Cas system from Enterococcus faecalis.
• To understand the roles of Cas9, Csn2, and Cas1-Cas2 in recognizing protospacer-adjacent motifs (PAM) and prespacers.
• To reveal the mechanism of directional prespacer integration into the CRISPR array.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine the structures of the Enterococcus faecalis Cas9-Csn2-Cas1-Cas2 supercomplex.
• Structures were solved for both the apo (resting) state and a DNA-bound (prespacer-catching) state.
• Structural analysis focused on protein-DNA interactions and conformational changes during the adaptation process.

Main Results:

• The apo state revealed a resting supercomplex (Cas9₂-Csn2₈-Cas1₈-Cas2₄).
• DNA binding induced a prespacer-catching complex where DNA threads through Csn2, allowing Cas9 to scan for the PAM sequence.
• Cas9 and Csn2 together define a 30-bp DNA segment, and Cas9 dissociation exposes DNA for Cas1-Cas2 binding and integration.

Conclusions:

• The study reveals the coupled mechanism of PAM recognition and prespacer selection by Cas9, Csn2, and Cas1-Cas2.
• Structural insights explain how the Type II-A system ensures fidelity during the integration of foreign DNA.
• This work provides a mechanistic understanding of the adaptation step in CRISPR-mediated prokaryotic immunity.