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Related Experiment Videos

Stability of a Lac repressor mediated "looped complex".

M Brenowitz1, A Pickar, E Jamison

  • 1Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461.

Biochemistry
|June 18, 1991
PubMed
Summary

Lac repressor protein forms looped DNA complexes with moderate cooperativity. Analysis reveals ~65% of molecules are looped complexes, suggesting tetramers dissociate into dimers for stability.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Structural Biology

Background:

  • Lac repressor protein regulates gene expression by binding to DNA.
  • Protein-mediated DNA looping is crucial for gene regulation and DNA organization.
  • Understanding protein-DNA interactions, including cooperativity and complex formation, is fundamental in molecular biology.

Purpose of the Study:

  • To quantify the stability and formation of Lac repressor-DNA looped complexes.
  • To investigate the cooperativity of Lac repressor binding to DNA with two binding sites.
  • To elucidate the relationship between Lac repressor dimer-tetramer equilibrium and DNA looping.

Main Methods:

  • Footprint titration techniques to assess protein binding and DNA accessibility.
  • Gel mobility-shift assays to resolve different protein-DNA complex formations.
  • Analysis of DNase I hypersensitivity to detect DNA structural changes indicative of looping.
  • Mathematical modeling to interpret binding data and determine equilibrium constants.

Main Results:

  • Lac repressor binding to the looped DNA structure exhibited moderate cooperativity (-1.0 kcal/mol).
  • Approximately 65% of Lac repressor-DNA complexes were determined to be looped structures under experimental conditions.
  • Data analysis supported a model where Lac repressor tetramers dissociate into dimers in solution to facilitate looping.
  • The proportion of looped complexes was found to be highly dependent on the dimer-tetramer association constant.

Conclusions:

  • The formation of Lac repressor-DNA looped complexes is favored, with a significant proportion existing in solution.
  • Lac repressor tetramer dissociation into dimers is a key factor in achieving high levels of DNA looping.
  • The binding affinity of tetramers versus dimers requires further investigation to reconcile with observed DNA looping and cooperativity.

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