Learning Objective: Describe the synthesis, processing, and assembly of eukaryotic and prokaryotic ribosomal RNA (rRNA), including ribosomal subunit structure and key clinical correlations.

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Overview of Ribosomal RNA

Ribosomal RNA (rRNA) is the structural and catalytic component of ribosomes in all cells. It forms the backbone of ribosomal subunits and plays a central role in translation, including ribozyme activity in peptide-bond formation.

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Eukaryotic rRNA: Synthesis and Processing

Site of Synthesis

• rRNA synthesis occurs primarily in the nucleolus.
• The major rRNA transcript is synthesized by RNA polymerase I.

45S Pre-rRNA

• Transcribed as a single 45S precursor.
• This pre-rRNA is later cleaved into:
  • 28S rRNA
  • 18S rRNA
  • 5.8S rRNA

5S rRNA Production

• Produced separately by RNA polymerase III.
• Transcribed outside the nucleolus but transported in for ribosome assembly.

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Assembly of Eukaryotic Ribosomal Subunits

Formation

The rRNA pieces combine with ribosomal proteins (imported from the cytoplasm) in the nucleolus to form subunits:

40S Subunit

• Contains:
  • 18S rRNA

60S Subunit

• Contains:
  • 28S rRNA
  • 5.8S rRNA
  • 5S rRNA

Complete Ribosome

• 80S ribosome (non-additive Svedberg units)
• Joins the cytoplasm during translation.

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Prokaryotic Ribosomal RNA

rRNA Components

• 50S large subunit:
  • 23S rRNA
  • 5S rRNA
• 30S small subunit:
  • 16S rRNA

Complete Ribosome

• 70S ribosome

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Clinical Correlation (High Yield)

Shiga Toxin & Shiga-Like Toxin

• Produced by Shigella dysenteriae and EHEC (enterohemorrhagic E. coli).
• Mechanism:
  • Inactivate 28S rRNA of the 60S subunit in eukaryotes.
  • Function as RNA glycosylases, removing an adenine residue.
  • Blocks aminoacyl-tRNA binding → halts protein synthesis.
• Clinical outcome: severe bloody diarrhea, risk of HUS.

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Comparison Table

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Activity