The structure of SV 40 chromatin

Area of Science:

• Molecular Biology
• Virology
• Biochemistry

Background:

• Simian virus 40 (SV40) is a well-characterized model system for studying eukaryotic DNA replication and chromatin structure.
• SV40 nucleoprotein complexes, or chromatin, are composed of viral DNA and host cell histones, forming higher-order structures.

Purpose of the Study:

• To investigate the structural conformations of SV40 chromatin under varying ionic strengths.
• To determine the relationship between chromatin structure, histone composition, and DNA topology.

Main Methods:

• Electron microscopy was used to visualize SV40 nucleoprotein complexes.
• Sucrose gradient sedimentation was employed to analyze the sedimentation behavior of the complexes.
• Histone content was assessed in relation to structural changes.

Main Results:

• SV40 chromatin displayed two conformations: compact globular structures at 0.15 M NaCl and 'beads-on-a-string' structures at 0.6 M NaCl.
• The compact form contained both nucleosomal histones and histone H1, sedimenting at 70S.
• The open form, lacking histone H1, sedimented at 40S, with no change in DNA superhelicity.

Conclusions:

• Ionic strength influences SV40 chromatin conformation by affecting histone H1 association.
• The structural transition between condensed and open SV40 chromatin does not involve alterations in DNA topological winding number.
• DNA relaxing enzyme activity is associated with SV40 nucleoprotein complexes.