Salt-induced conformational transitions in chromatin. A flow linear dichroism study

Area of Science:

• Molecular Biology
• Biophysics
• Structural Biology

Background:

• Chromatin, the complex of DNA and proteins that forms chromosomes, plays a crucial role in genome organization and regulation.
• Understanding chromatin structure in solution is essential for deciphering its functional mechanisms.
• Ionic strength is a key environmental factor influencing macromolecular conformation and interactions.

Purpose of the Study:

• To investigate the impact of varying ionic strengths on chromatin structure and nucleosome orientation in solution.
• To elucidate the conformational transitions of chromatin as a function of salt concentration.
• To assess the role of histone H1 in salt-induced chromatin structural changes.

Main Methods:

• Flow linear dichroism (LD) spectroscopy was employed to study chromatin structure.
• Experiments were conducted across a wide range of ionic strengths, from 0.25 mM Na2EDTA to 100 mM NaCl.
• Comparative studies were performed on both native and H1-depleted chromatin.

Main Results:

• Increasing ionic strength from 0.25 mM to 100 mM NaCl significantly reduced the LD amplitude of chromatin.
• A notable inversion of the LD sign from negative to positive occurred around 2 mM NaCl.
• Chromatin displayed a maximal positive LD value between 10-20 mM NaCl, indicating a reorientation of nucleosome discs.

Conclusions:

• At very low ionic strength, nucleosome discs are oriented parallel to the chromatin filament axis.
• Increasing ionic strength up to 20 mM NaCl induces a conformational transition, reorienting nucleosome discs to be inclined relative to the fibril axis.
• This salt-induced reorientation is dependent on the presence of histone H1 and is less pronounced in H1-depleted chromatin.