Structure of the 300A chromatin filament: X-ray diffraction from oriented samples.

Area of Science:

• Molecular Biology
• Biophysics
• Structural Biology

Background:

• Chromatin, the complex of DNA and proteins, is organized into filaments.
• Understanding the higher-order structure of chromatin is crucial for gene regulation and cellular processes.
• Previous models proposed different arrangements for chromatin fibers.

Purpose of the Study:

• To investigate the structural organization of 300 Angstrom (Å) chromatin filaments.
• To determine the packing arrangement of nucleosomes within these filaments.
• To evaluate existing models of chromatin higher-order structure.

Main Methods:

• Obtaining X-ray diffraction patterns from partially oriented 300Å chromatin filaments.
• Preparing chromatin using methods that preserve its native structure.
• Inducing spontaneous formation of ordered chromatin aggregates for analysis.

Main Results:

• Observed a meridional band at 110Å and equatorial bands at 340Å, 57Å, 37Å, and 27Å in diffraction patterns.
• Integrated diffraction data with the known 7Å electron density map of the nucleosome core particle.
• Inferred side-to-side nucleosome packing along the filament axis and radial packing around it.

Conclusions:

• The observed diffraction patterns strongly support the "solenoid" model of chromatin structure proposed by Finch and Klug.
• The findings are inconsistent with several alternative models for chromatin higher-order organization.
• This study provides key evidence for a specific, ordered arrangement of nucleosomes in chromatin fibers.