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Extrachromosomal Nucleolus-Like Compartmentalization by a Plasmid-Borne Ribosomal RNA Operon and Its Role in Nucleoid

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Summary

A plasmid-borne ribosomal RNA operon can form a bacterial nucleolus-like structure, concentrating RNA polymerase and compacting the nucleoid in Escherichia coli mutants lacking chromosomal rRNA operons.

Keywords:
Escherichia coliRNA polymerasebacterial nucleolus-likenucleoid structurerRNA synthesis and ribosome biogenesisthree-dimensional superresolution Structured Illumination Microscopytranscription factories

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Area of Science:

  • Cell Biology
  • Microbiology
  • Molecular Biology

Background:

  • Fast-growing Escherichia coli cells exhibit nucleolus-like structures formed by concentrated RNA polymerase (RNAP) at ribosomal RNA (rRNA) operon clusters.
  • This bacterial nucleolus-like organization is crucial for rRNA synthesis, ribosome biogenesis, and nucleoid compaction.
  • Mutants lacking multiple rRNA operons show uncompacted nucleoids and lack RNAP transcription foci.

Purpose of the Study:

  • To investigate if a plasmid-borne rRNA operon (prrnB) can restore the bacterial nucleolus-like structure in E. coli mutants lacking chromosomal rRNA operons (Δ6rrn and Δ7rrn).
  • To determine if extrachromosomal nucleolus-like structures are organized and functional in trans.
  • To analyze the spatial organization and functional impact of these structures on nucleoid organization.

Main Methods:

  • Utilized multicolor three-dimensional superresolution Structured Illumination Microscopy (3D-SIM).
  • Visualized distributions of RNAP, NusB (a transcription factor), prrnB clusters, and nucleoid structure.
  • Examined mutant strains (Δ6rrn and Δ7rrn) in response to environmental cues.

Main Results:

  • Extrachromosomal nucleolus-like structures formed by prrnB were observed, often localized at cellular poles in the mutants.
  • Formation of RNAP foci at these extrachromosomal structures was confirmed.
  • The presence of these structures led to nucleoid condensation, similar to wild-type cells.

Conclusions:

  • A multicopy plasmid-borne rRNA operon can successfully establish a functional bacterial nucleolus-like structure in E. coli mutants lacking chromosomal copies.
  • Active transcription within these extrachromosomal nucleolus-like structures is a key driver of nucleoid compaction.
  • This finding supports the role of transcription factories in organizing the bacterial chromosome.