Naturally ornate RNA-only complexes revealed by cryo-EM

Area of Science:

• Structural Biology
• RNA Biology
• Bacterial Genetics

Background:

• The three-dimensional structures of natural RNAs are largely unknown, potentially concealing significant biological functions.
• Large, non-coding RNAs are increasingly recognized for their complex roles in cellular processes.

Purpose of the Study:

• To elucidate the three-dimensional structures of three large, ornate bacterial RNAs using cryo-electron microscopy (cryo-EM).
• To investigate the quaternary structures, assembly, and stability of these RNA complexes.
• To explore the evolutionary conservation and biological significance of these RNA structures.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine high-resolution 3D structures.
• Bioinformatic analyses, including sequence covariation, were used to identify conserved interactions.
• Biochemical assays were performed to assess complex stoichiometry and stability.

Main Results:

• The structures of three large bacterial RNAs (GOLLD, ROOL, and OLE) were determined.
• GOLLD and ROOL RNAs form RNA-only multimeric nanocages larger than the ribosome.
• OLE RNA forms a dimeric complex with extensive coaxial stacking.
• Multiple non-canonical interactions, including A-minor interactions and an A-A helix, stabilize these complexes.
• RNA complex stoichiometries are maintained at lower concentrations than found in cells.

Conclusions:

• Large, ornate RNA structures can form complex, protein-independent quaternary assemblies.
• These RNA nanocages and complexes possess unique architectures stabilized by diverse intramolecular and intermolecular interactions.
• Evolutionary conservation of intermolecular interactions underscores the biological relevance of these complex RNA structures.