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Related Experiment Videos

How Lac repressor finds lac operator in vitro.

R Fickert1, B Müller-Hill

  • 1Institut für Genetik, Universität zu Köln, FRG.

Journal of Molecular Biology
|July 5, 1992
PubMed
Summary

Filter-binding assays for Lac repressor (a protein regulating gene expression) can be misleading. New findings show certain DNA-protein complexes are not retained on filters, impacting kinetic analysis of Lac repressor-DNA interactions.

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Area of Science:

  • Molecular Biology
  • Biochemistry
  • Genetics

Background:

  • The Lac repressor protein controls the lac operon in bacteria.
  • Understanding Lac repressor-DNA interactions is crucial for gene regulation studies.
  • Filter-binding assays are commonly used to study protein-DNA complex kinetics.

Purpose of the Study:

  • To analyze the kinetics of Lac repressor-lac operator interactions using filter-binding and gel mobility shift assays.
  • To compare the reliability of filter-binding assays versus gel mobility shift assays for studying these interactions.
  • To re-evaluate previous interpretations of filter-binding assay data for tetrameric Lac repressor.

Main Methods:

  • Filter-binding assays were performed using nitrocellulose filters.
  • Gel mobility shift assays were utilized to analyze protein-DNA complex formation.
  • Kinetic parameters (association and dissociation rate constants) were measured for Lac repressor-DNA interactions.

Main Results:

  • Filter-binding assays with tetrameric Lac repressor have been misinterpreted due to unequal filter retention of different complex types.
  • Sandwich or loop complexes (two operators bound to one repressor) are poorly retained on filters.
  • Both dimeric and tetrameric Lac repressor exhibit no DNA-length dependence in binding kinetics for fragments <2455 bp with a single operator.
  • Tetrameric Lac repressor shows a ninefold increased association rate to ideal lac operator on lambda DNA, likely via intersegment transfer.

Conclusions:

  • The assumption of equal filter affinity for all Lac repressor-DNA complexes is incorrect.
  • Filter-binding assays require careful interpretation, especially for multimeric proteins like tetrameric Lac repressor.
  • The observed enhanced association rate in tetrameric Lac repressor suggests a role for DNA looping or intersegment transfer mechanisms.

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