Real-Time Observation of Target Search by the CRISPR Surveillance Complex Cascade

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• CRISPR-Cas systems provide adaptive immunity in prokaryotes against foreign genetic elements.
• Surveillance complexes, like type I-E Cascade, are crucial for recognizing and targeting foreign DNA using guide RNAs.
• The mechanism by which these complexes efficiently search vast genomes for specific targets remains largely unknown.

Purpose of the Study:

• To elucidate the real-time DNA search mechanism of the type I-E CRISPR-Cas surveillance complex, Cascade.
• To understand how Cascade rapidly locates target DNA sequences within a complex genome.

Main Methods:

• Single-molecule Förster Resonance Energy Transfer (smFRET) was employed to visualize Cascade-DNA interactions in real time.
• Analysis of Cascade motifs involved in DNA sampling and target recognition.

Main Results:

• Cascade exhibits rapid, random DNA sampling via nonspecific electrostatic interactions.
• The complex pauses at short Protospacer Adjacent Motif (PAM) sites, which can be near the actual target.
• Specific Cascade motifs were identified for DNA sampling and PAM recognition.

Conclusions:

• A comprehensive structural and kinetic model for Cascade's DNA target search has been established.
• CRISPR surveillance complexes efficiently scan large DNA regions through a combination of random diffusion and specific pausing.
• This mechanism ensures timely and effective bacterial defense against invading nucleic acids.