Super-resolution imaging reveals nucleolar encapsulation by single-stranded DNA.

Area of Science:

• Cell Biology
• Genomics
• Molecular Biology

Background:

• Nuclear bodies, such as the nucleolus, are crucial for genomic processes.
• The nucleolus is essential for ribosome biogenesis, involving proteins like RNA polymerase I and nucleophosmin 1 (NPM1).
• While liquid-liquid phase separation (LLPS) is implicated in nucleolar formation, its structural maintenance within the nucleus is unclear.

Purpose of the Study:

• To investigate the structural maintenance of the nucleolus within the intranuclear architecture.
• To identify the molecular components responsible for the structural integrity of nuclear bodies.

Main Methods:

• Super-resolution lattice-structured illumination microscopy (lattice-SIM) to visualize ssDNA distribution.
• In situ digestion of ssDNA to assess its role in nucleolar structure.
• Identification of ssDNA-binding proteins associated with nucleolar encapsulation.

Main Results:

• The nucleolus is encapsulated by a molecular complex based on single-stranded DNA (ssDNA).
• High abundance of ssDNA was observed surrounding the nucleolus.
• Digestion of ssDNA led to nucleolar disruption and release of NPM1, confirming ssDNA's structural role.
• ssDNA was found to form a complex with histone H1 for nucleolar encapsulation.

Conclusions:

• The nucleolus's structural integrity is maintained by an ssDNA-based molecular complex.
• This ssDNA-histone H1 complex provides encapsulation for nuclear bodies.
• This mechanism is vital for coordinating nuclear genomic processes like transcription and replication.