Quantifying ATP-Independent Nucleosome Chaperone Activity with Single-Molecule Methods.

Area of Science:

• Molecular Biology
• Chromatin Dynamics
• Biophysics

Background:

• Histone-DNA interactions are crucial for chromosome organization and gene regulation.
• Understanding nucleosome dynamics is key to deciphering transcription, replication, and repair.
• The FACT complex is a known regulator of chromatin structure during transcription.

Purpose of the Study:

• To quantitatively measure the energies and kinetics of DNA binding to histones.
• To elucidate the role of the FACT complex in nucleosome disruption and reformation.
• To understand the molecular mechanisms by which FACT influences histone-DNA interactions.

Main Methods:

• Utilized three single-molecule techniques: force disruption (FD) with optical tweezers, confocal imaging (CI), and survival probability (SP) measurements.
• Employed short arrays of positioned nucleosomes for kinetic parameter quantification.
• Studied the effect of the FACT complex on nucleosome stability and histone dynamics.

Main Results:

• FACT binding destabilizes histone-DNA interactions, driving off outer DNA wraps.
• The SPT16 MD domain stabilizes contacts, while the SSRP1 HMGB box destabilizes the nucleosome.
• FACT increases histone release rates and acts as a catalyst for nucleosome reformation.

Conclusions:

• FACT functions as a nucleosome catalyst, lowering energy barriers for both disruption and reformation.
• The distinct domains of FACT play specific roles in modulating histone-DNA interactions.
• These findings provide molecular insights into how FACT facilitates chromatin accessibility.