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Chromatin conformation and salt-induced compaction: three-dimensional structural information from cryoelectron
J Bednar1, R A Horowitz, J Dubochet
1Département d'Analyse Ultrastructurale, Université de Lausanne, Switzerland.
The Journal of Cell Biology
|December 1, 1995
Summary
Cryoelectron microscopy reveals chicken erythrocyte oligonucleosomes adopt a zig-zag conformation. Ionic strength influences compaction by altering linker DNA angles, not spacing, supporting irregular chromatin structures.
Area of Science:
- Structural Biology
- Molecular Biology
- Biophysics
Background:
- Chromatin, the complex of DNA and proteins that forms chromosomes, plays a crucial role in gene regulation.
- Understanding the three-dimensional (3-D) conformation of oligonucleosomes is essential for elucidating chromatin structure and function.
- Previous models of chromatin conformation have been based on indirect evidence and bulk measurements.
Purpose of the Study:
- To investigate the 3-D conformation of oligonucleosomes from chicken erythrocyte nuclei using cryoelectron microscopy.
- To determine how varying ionic strengths affect the structural organization of oligonucleosomes.
- To compare in-solution conformations with existing data from bulk chromatin studies.
Main Methods:
- Vitrification of oligonucleosomes in solutions of differing ionic strengths.
- Cryoelectron microscopy of vitrified samples.
- Analysis of tilt pairs of micrographs to reconstruct 3-D structures.
- Examination of nucleosome disk arrangement and linker DNA paths.
Main Results:
- At low ionic strength, trinucleosomes form an equilateral triangle with nucleosomes at vertices and approximately 46 bp of linker DNA.
- Removal of histones H1 and H5 leads to more variable trinucleosome structures.
- Compaction at increased ionic strength (20 mM NaCl) occurs via reduction of the linker DNA angle, not nucleosome spacing.
- Polynucleosomes maintain a 3-D zig-zag conformation across ionic strengths, with individual nucleosomes remaining separated.
- No evidence of solenoidal arrangements was observed in polynucleosomes.
Conclusions:
- Oligonucleosomes adopt an irregular zig-zag conformation in solution across a range of ionic strengths.
- The findings support existing in situ and in vitro data on chromatin structure.
- Cryoelectron microscopy provides a direct method to study chromatin conformation, aiding in understanding transcriptional regulation.
- Reinterpretation of bulk chromatin data may be necessary in light of these direct structural observations.