Single-stranded nucleic acid binding and coacervation by linker histone H1

Area of Science:

• Molecular Biology
• Biophysics
• Genetics

Background:

• Histone H1 proteins are abundant in eukaryotic chromatin but their precise functions are not fully understood.
• Understanding H1's role is crucial for comprehending chromosome structure and genome maintenance.

Purpose of the Study:

• To directly visualize the behavior of Histone H1 on nucleic acids and nucleosomes.
• To investigate the phase separation properties of H1 in response to different DNA structures.
• To explore H1's involvement in cellular processes like DNA replication and damage response.

Main Methods:

• Single-molecule fluorescence and force microscopy.
• Optical tweezers-controlled droplet fusion assay.
• Molecular dynamics simulations.
• Live-cell imaging of eGFP-tagged Histone H1.

Main Results:

• Histone H1 coalesces around single-stranded DNA formed during DNA duplex melting.
• Single-stranded nucleic acids induce gel-like H1 droplets, while dsDNA and nucleosomes form liquid-like droplets.
• H1 phase separation is driven by multivalent, transient interactions with unpaired DNA.
• Increased single-stranded DNA in cells leads to fusion-competent H1 puncta.
• H1 colocalizes with Proliferating Cell Nuclear Antigen after DNA damage, suggesting a role at stalled replication forks.

Conclusions:

• Histone H1 exhibits distinct phase separation behaviors dependent on nucleic acid structure.
• H1's interaction with single-stranded DNA is key to its phase separation.
• The findings reveal new insights into H1's roles in genome organization, DNA repair, and replication fork stability.