Cas6 processes tight and relaxed repeat RNA via multiple mechanisms: A hypothesis

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Proteins interacting with RNA must accommodate its dual nature of flexibility and folding.
• Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-mediated RNA immunity involves complex protein-RNA interactions.
• Existing models suggest CRISPR-Cas6 endoribonucleases recognize RNA via stem-loops, but many CRISPR RNAs present structural variations.

Purpose of the Study:

• To investigate how CRISPR-Cas6 enzymes process structurally diverse RNA molecules.
• To propose novel mechanisms explaining CRISPR-Cas6 interaction with varied RNA repeat structures.
• To elucidate the strategies employed by proteins to bind and process flexible RNA.

Main Methods:

• Literature review of recent studies on CRISPR-Cas6 endoribonucleases and RNA interactions.
• Analysis of structural diversity in CRISPR RNA repeats, categorizing them as 'Relaxed' or 'Tight'.
• Proposal of mechanistic models based on existing structural and biochemical data.

Main Results:

• Identified that many CRISPR RNA repeats exhibit non-canonical stem-loop structures ('Relaxed' or 'Tight') that challenge recognition.
• Proposed a 'bivalent trapping' mechanism for CRISPR-Cas6 to process 'Relaxed' RNA repeats.
• Proposed an 'unwinding' mechanism for CRISPR-Cas6 to process 'Tight' RNA repeats.

Conclusions:

• CRISPR-Cas6 employs distinct mechanisms to overcome structural heterogeneity in RNA substrates.
• Both proposed mechanisms ensure a conserved RNA conformation at the cleavage site for precise processing.
• These mechanisms highlight protein adaptability in interacting with structurally diverse nucleic acids.