Cryo-EM captures early ribosome assembly in action

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Ribosome biogenesis is essential for all life, involving rRNA and protein production, processing, and assembly.
• The early assembly phase of the bacterial 50S ribosomal subunit remains largely unexplored.
• Understanding ribosome assembly is crucial for deciphering cellular function and developing targeted therapeutics.

Purpose of the Study:

• To investigate the initial steps of bacterial 50S ribosomal subunit assembly.
• To elucidate the structural pathway of early 50S subunit formation in vitro.
• To understand the role of ribosomal proteins and rRNA structure in assembly cooperativity.

Main Methods:

• Utilized a minimal in vitro reconstitution system with purified ribosomal components.
• Employed time-limited assembly assays to capture dynamic assembly intermediates.
• Performed cryo-electron microscopy (cryo-EM) for high-resolution structural analysis.

Main Results:

• Visualized a complex early assembly pathway starting with a particle containing ~500 nucleotides of 23S rRNA domain I and three proteins.
• Demonstrated that early 50S assembly proceeds in a domain-wise manner, while later stages occur incrementally.
• Identified cooperativity where ribosomal proteins and folded rRNA helices stabilize adjacent rRNA structures.

Conclusions:

• The study provides unprecedented structural insights into the early bacterial 50S ribosomal subunit assembly pathway.
• Early assembly is characterized by domain-wise organization, transitioning to incremental steps later in the process.
• Protein-RNA and RNA-RNA interactions play a critical role in driving and stabilizing the assembly process.