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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
PubMed
Summary
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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).

Related Experiment Videos

  • 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.