The energy landscape for R-loop formation by the CRISPR-Cas Cascade complex

Area of Science:

• Molecular Biology
• Biophysics
• Genetics

Background:

• CRISPR-Cas systems are powerful gene editing tools relying on CRISPR RNA (crRNA) for DNA target recognition.
• Accurate targeting is essential for therapeutic applications, but poorly understood mechanisms of off-target recognition persist.
• R-loop formation, involving DNA unwinding and crRNA-DNA hybridization, is critical for CRISPR-Cas function before DNA cleavage.

Purpose of the Study:

• To investigate the real-time dynamics of R-loop formation by the CRISPR-Cas Cascade effector complex.
• To elucidate the mechanistic basis of DNA target recognition, including the role of mismatches.
• To provide base-pair resolution insights into the energy landscape of R-loop extension.

Main Methods:

• Ultrafast DNA unwinding experiments utilizing plasmonic DNA origami nanorotors.
• Real-time monitoring of R-loop formation at near base-pair resolution.
• Analysis of the energy landscape influenced by base flips and mismatches.

Main Results:

• Observed a weak global downhill bias followed by a steep uphill bias during R-loop formation.
• Demonstrated modulation of the energy landscape by base flips and mismatches.
• Identified distinct timescales for R-loop formation: submillisecond single base-pair steps and longer six base-pair intermediate steps.

Conclusions:

• Cascade-mediated R-loop formation occurs in rapid, stepwise increments.
• The energy landscape dynamics explain the mechanism of target recognition and specificity.
• Findings offer a deeper mechanistic understanding of CRISPR-Cas systems, potentially improving therapeutic applications.