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Superrepression through Altered Corepressor-Activated Protein:Protein Interactions.

Chenlu He1, Gregory Custer1, Jingheng Wang1

  • 1Department of Chemistry & Biochemistry and ‡Fischell Department of Bioengineering, University of Maryland , College Park, Maryland 20742, United States.

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Superrepressor mutants of the E. coli biotin repressor show altered homodimerization, leading to increased sensitivity to small molecule corepressors. This change in protein dimerization explains the enhanced repression of biotin operon transcription.

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Area of Science:

  • Molecular Biology
  • Biochemistry
  • Genetics

Background:

  • Small molecules regulate gene transcription by modulating transcription regulatory complex assembly.
  • Allosteric transcription repressors can have superrepressor mutants with heightened sensitivity to corepressors.
  • Complex assembly often involves multiple steps, making superrepressor phenotypes complex to interpret.

Purpose of the Study:

  • Investigate the molecular basis of superrepressor phenotypes in the Escherichia coli biotin operon repression complex.
  • Determine how mutations affect corepressor binding and homodimerization of the BirA repressor.
  • Understand the relationship between altered protein properties and changes in transcription regulation.

Main Methods:

  • Genetic screening to identify superrepressor mutants.
  • Isothermal titration calorimetry and sedimentation measurements to assess protein binding and dimerization.
  • Molecular dynamics simulations to explore structural and energetic changes.
  • Modeling of multistep complex assembly.

Main Results:

  • Six superrepressor mutants were identified, repressing transcription at lower biotin concentrations.
  • All variants showed similar biotin binding affinities to wild-type BirA.
  • Five of six superrepressors exhibited altered homodimerization energetics.
  • Altered dimerization was linked to perturbations in an electrostatic network crucial for allosteric activation.
  • Modeling confirmed that altered homodimerization alone explains the enhanced sensitivity to biotin.

Conclusions:

  • Altered homodimerization energetics, not altered corepressor binding, is the primary driver of the superrepressor phenotype in the biotin operon.
  • Coupled equilibria in complex assembly allow indirect mechanisms for tuning transcription responses.
  • This study provides insights into allosteric regulation and the multi-step assembly of transcription factors.