The nucleolar shell provides anchoring sites for DNA untwisting.

Area of Science:

• Molecular Biology
• Cell Biology
• Genetics

Background:

• DNA underwinding (untwisting) is critical for transcriptional activation.
• The mechanism constraining DNA axial rotation within the nucleus remained unclear.
• RNA polymerase activity generates torque, necessitating DNA rotation constraints.

Purpose of the Study:

• To identify the nuclear structures that constrain DNA axial rotation.
• To elucidate the role of nuclear protein condensates in DNA underwinding.
• To understand the contribution of the nucleolus to DNA underwinding and transcription.

Main Methods:

• In situ super-resolution imaging of underwound DNA.
• Investigation of DNA underwinding in the nucleolus, a nuclear condensate.
• Perturbation of nucleolar shell component binding to G-quadruplex (G4) structures.

Main Results:

• Underwound DNA accumulates within the nucleolus, particularly in its core.
• RNA polymerase I (RNAPI) activity drives the distribution of underwound DNA in the nucleolar core.
• Disruption of nucleolar shell binding to G4 structures reduces underwound DNA in the core.

Conclusions:

• The nucleolar shell acts as an anchoring site, constraining DNA axial rotation for RNAPI-driven DNA underwinding.
• Nuclear protein condensates provide essential constraints for site-specific DNA underwinding regulation.
• Findings advance the understanding of transcription regulation by nuclear condensates.