RNA Guide Complementarity Prevents Self-Targeting in Type VI CRISPR Systems.

Area of Science:

• Microbiology
• Molecular Biology
• Immunology

Background:

• CRISPR-Cas systems provide adaptive immunity in prokaryotes against foreign nucleic acids.
• These systems utilize guide RNAs to direct Cas nucleases for target DNA or RNA cleavage.
• Type VI-A CRISPR-Cas systems, including the RNA-targeting Cas13 nuclease, have unique target recognition mechanisms.

Purpose of the Study:

• To investigate the target RNA requirements of the type VI-A CRISPR-Cas system in its natural host, Listeria seeligeri.
• To determine how protospacer flanking sequences and guide RNA complementarity influence Cas13 activity.
• To establish design principles for Cas13-based technologies and understand self-targeting prevention.

Main Methods:

• In vivo and in vitro assays were used to assess Cas13 nuclease activity.
• Target RNAs with varying complementarity to the protospacer flanking sequence and guide RNA repeat were designed and tested.
• Analysis of cleavage activity in the natural host Listeria seeligeri.

Main Results:

• The type VI-A Cas13 nuclease did not cleave target RNAs exhibiting extended complementarity between the protospacer flanking sequence and the guide RNA repeat sequence.
• This inhibition of cleavage was observed in both in vivo and in vitro experimental settings.
• The findings reveal specific target recognition rules for this RNA-targeting CRISPR-Cas system.

Conclusions:

• Extended complementarity between the protospacer flanking sequence and the guide RNA repeat sequence acts as a negative determinant for Cas13-mediated RNA cleavage.
• These results provide fundamental insights into the mechanism of type VI-A CRISPR-Cas immunity.
• The discovered rules are crucial for the rational design of novel Cas13-based gene editing and diagnostic tools and explain self-targeting avoidance.