Nucleosome dissociation at physiological ionic strengths

Area of Science:

• Molecular Biology
• Biochemistry
• Structural Biology

Background:

• Nucleosomes are the fundamental units of DNA packaging in eukaryotes.
• Understanding nucleosome stability is crucial for comprehending DNA accessibility and gene regulation.

Purpose of the Study:

• To investigate the dissociation dynamics of monomer nucleosomes under varying ionic strength and DNA concentration.
• To determine the reversibility of nucleosome dissociation and identify equilibrium conditions.

Main Methods:

• Purification of monomer nucleosomes using isokinetic sucrose gradients.
• Analysis of nucleosome dissociation via sedimentation velocity in the presence of varying NaCl concentrations.
• Radiolabeling (3H-thymidine) to track DNA components during dissociation.

Main Results:

• Monomer nucleosomes (11S) dissociate into DNA and histones below 20 µg/ml DNA concentration at physiological ionic strength.
• Dissociation is concentration-dependent, increasing with NaCl concentration (0.15-0.35 M) and decreasing with DNA concentration (>1 µg/ml).
• Dissociated components rapidly reassociate to form 11S monomers under high DNA concentration and low ionic strength, indicating reversibility.

Conclusions:

• Nucleosome dissociation is a reversible process establishing an equilibrium between intact monomers and free DNA/histones.
• The stability of monomer nucleosomes is sensitive to ionic strength and DNA concentration.
• These findings provide insights into the dynamic nature of chromatin structure and its regulation.