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Structural changes in positively and negatively supercoiled DNA
S Brahms1, S Nakasu, A Kikuchi
1Institut Jacques Monod, Centre National de la Recherche Scientifique, Université Paris VII, France.
European Journal of Biochemistry
|September 15, 1989
Summary
Superhelical constraint affects covalently closed circular DNA (cccDNA) structure. Negative supercoiling can induce B-form DNA or left-handed DNA, while positive supercoiling lacks cruciform structures.
Area of Science:
- Molecular Biology
- Biochemistry
- Structural Biology
Background:
- Covalently closed circular DNA (cccDNA) exists in various superhelical states.
- Superhelical constraint, quantified by superhelical density (sigma), influences DNA conformation.
- Understanding these structural changes is crucial for DNA replication and transcription.
Purpose of the Study:
- To investigate the structural impact of superhelical constraint on pBR322 cccDNA.
- To compare the effects of positive and negative supercoiling on DNA structure.
- To identify conformational changes at varying superhelical densities.
Main Methods:
- Studied pBR322 covalently closed circular DNA (cccDNA).
- Manipulated superhelical constraint by altering specific linking difference (superhelical density, sigma).
- Analyzed structural changes under both positive and negative supercoiling conditions.
Main Results:
- At low to moderate negative superhelical densities, cccDNA maintains an unwound B-form.
- Overwound cccDNA (positive writhe) lacks the cruciform structures observed in negatively supercoiled DNA.
- High negative supercoiling induces conformational changes, including twist handedness reversal, forming left-handed DNA segments (Form V DNA).
Conclusions:
- Superhelical constraint significantly dictates cccDNA structure.
- Positive and negative supercoiling lead to distinct structural outcomes.
- DNA can adopt alternative conformations like left-handed DNA under extreme superhelical stress.