Triple-helix formation at different positions on nucleosomal DNA

Area of Science:

• Molecular Biology
• Epigenetics
• Structural Biology

Background:

• Nucleosomes are the basic units of DNA packaging in eukaryotes.
• Triple-helix formation is a DNA structural motif with potential therapeutic applications.
• Understanding DNA accessibility within nucleosomes is crucial for gene regulation and drug targeting.

Purpose of the Study:

• To investigate the feasibility of forming DNA triple helices on nucleosome core particles.
• To determine how the position of oligopurine tracts within nucleosomal DNA affects triple-helix formation.
• To compare the formation of parallel and antiparallel triplexes at different nucleosomal sites.

Main Methods:

• Preparation of DNA fragments with specific oligopurine tracts based on the tyrT sequence.
• Reconstitution of DNA fragments onto nucleosome core particles.
• DNase I footprinting to assess DNA accessibility and triple-helix formation.

Main Results:

• Triple-helix formation is generally limited to DNA sites located at the ends of nucleosomal fragments.
• Stable parallel triplexes (pH 5.5) and antiparallel triplexes (pH 7.5) can form at accessible sites (positions <33 and >94).
• Oligopurine tracts near the nucleosome dyad can alter nucleosomal positioning, allowing parallel but not antiparallel triplex formation.

Conclusions:

• Nucleosome structure significantly restricts triple-helix formation to specific accessible regions.
• The type of triplex formed (parallel vs. antiparallel) is dependent on both DNA sequence and pH.
• Targeting DNA triple helices within nucleosomes requires careful consideration of DNA accessibility and nucleosome positioning.