The initiation, propagation and dynamics of CRISPR-SpyCas9 R-loop complex

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• The CRISPR-Cas9 system is a powerful tool for genome editing.
• The precise molecular mechanisms of Cas9 endonuclease R-loop formation remain unclear.
• Understanding these mechanisms is crucial for improving CRISPR/Cas9 specificity.

Purpose of the Study:

• To investigate the DNA cleavage activities of Streptococcus pyogenes Cas9 (SpyCas9) and its mutants.
• To study the conformational dynamics of the R-loop structure during target binding using single-molecule fluorescence energy transfer (smFRET).
• To elucidate the molecular details of R-loop formation and its relation to CRISPR/Cas9 off-targeting.

Main Methods:

• Utilized various target sequences to examine DNA cleavage activities of SpyCas9 and its mutants.
• Employed single-molecule fluorescence energy transfer (smFRET) to study R-loop structure dynamics during target binding.
• Performed kinetic analysis of the intermediate state in R-loop formation.

Main Results:

• Identified distinct DNA domains (protospacer adjacent motif, linker, Seed, Middle, Tail) recognized by the Cas9-sgRNA complex.
• Observed that Cas9 transiently adopts a semi-active conformation after seed pairing, leading to strand cleavage.
• Discovered an intermediate state preceding stable R-loop complex formation, with its lifetime dependent on guide-DNA hybrid duplex length (max at 18 bp).

Conclusions:

• The findings provide novel insights into the dynamic process of R-loop formation in CRISPR-Cas9.
• The identified intermediate state and its dynamics offer a molecular explanation for off-targeting events in CRISPR/Cas9 systems.
• This research contributes to a deeper understanding of CRISPR-Cas9 mechanism, potentially aiding in the design of more precise genome editing tools.