Nucleomeric organization of chromatin

Area of Science:

• Molecular Biology
• Chromatin Structure
• Biochemistry

Background:

• Chromatin, the complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells, exists as globular nucleomeric fibrils.
• These fibrils are approximately 20-25 nm in diameter in situ and in low-ionic-strength solutions with magnesium ions.

Purpose of the Study:

• To investigate the structural organization of chromatin fibrils.
• To understand the role of magnesium ions and histone H1 in chromatin structure and folding.
• To elucidate the substructure of nucleomers and their unfolding into nucleosomes.

Main Methods:

• Enzymatic digestion of chromatin using staphylococcal or endogenous nucleases.
• Sucrose density gradient sedimentation analysis in the presence of EDTA and MgCl2.
• Determination of DNA fragment lengths within digested chromatin.

Main Results:

• Chromatin fibrils are cleaved into nucleomers and multinucleomers.
• Nucleomers unfold into chains of 4, 6, or 8 nucleosomes upon removal of magnesium ions.
• Mononucleomers, dinucleomers, and trinucleomers sediment as distinct particles (37-S, 47-S, 55-S in EDTA; 45-S for mononucleomers in MgCl2).
• A nucleomer contains approximately 8 nucleosomes, with dimers and trimers containing 14-16 and 21-24 nucleosomes, respectively.
• The compactness of nucleomers (45-S to 37-S) is lost upon Mg2+ removal but is reversible with histone H1.
• Histone H1 and magnesium ions are crucial for maintaining nucleomer compactness; their removal leads to complete unfolding.

Conclusions:

• Chromatin nucleomers are compact globular structures dependent on magnesium ions and histone H1.
• The proposed model suggests periodically discontinued helicity in nucleosome chains within nucleomers.
• This organization may hinder site-specific DNA recognition but allows local chromatin changes.