A metastable rRNA junction essential for bacterial 30S biogenesis

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Tertiary sequence motifs in ribosomal RNAs (rRNAs) dictate the 3D structures of ribosomal subunits.
• A conserved Right Angle motif (J5/6) at the junction of 16S rRNA helices 5 and 6 is essential for small subunit rRNA structure.
• This motif forms a stable right angle in minimal RNAs but not in the mature ribosome, suggesting a role in ribosome biogenesis.

Purpose of the Study:

• To investigate the role of the J5/6 Right Angle motif in 30S ribosome assembly and 16S rRNA maturation.
• To determine how J5/6 motif mutations affect ribosome structure and protein composition.

Main Methods:

• Site-directed mutagenesis of the J5/6 motif in 16S rRNA.
• Ribosome assembly and 16S rRNA maturation assays in Escherichia coli.
• In-cell X-ray footprinting to analyze RNA folding intermediates.
• Quantitative mass spectrometry to assess ribosome protein composition.
• Electron microscopy to visualize pre-30S ribosome structures.

Main Results:

• J5/6 mutations blocked 30S ribosome assembly and 16S rRNA maturation.
• Mutations favored an early assembly intermediate and prevented central pseudoknot formation.
• Mutant pre-30S ribosomes lacked protein uS12 and were depleted of uS5 and uS2.
• Impaired J5/6 folding led to a global collapse of the 30S structure.

Conclusions:

• The J5/6 motif is critical for establishing inter-domain interactions during 30S ribosome biogenesis.
• Improper folding of the J5/6 motif results in structural defects and assembly failure.
• The J5/6 motif is part of a conformational switching RNA spine essential for ensuring 30S ribosome integrity.