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Single-molecule spectroscopic determination of lac repressor-DNA loop conformation
Michael A Morgan1, Kenji Okamoto, Jason D Kahn
1Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA.
Biophysical Journal
|August 9, 2005
Summary
Single-molecule fluorescence resonance energy transfer (FRET) reveals that the Escherichia coli lactose repressor protein (LacI)-DNA loop exists exclusively in a single closed conformation. This finding clarifies the protein-DNA looping dynamics previously thought to involve multiple geometries.
Area of Science:
- Molecular biology
- Biophysics
- Structural biology
Background:
- The Escherichia coli lactose repressor protein (LacI) is a model system for studying protein-induced DNA looping.
- LacI utilizes a tetramerization domain that may function as a flexible hinge, influencing DNA loop geometry.
- Previous studies suggested LacI-DNA loops could adopt multiple conformations, including compact and open forms, with the "9C14" construct being of particular interest.
Purpose of the Study:
- To investigate the conformational states and dynamics of LacI-DNA loops using advanced single-molecule techniques.
- To resolve ambiguities from ensemble measurements regarding the existence of open or intermediate LacI-DNA loop geometries.
- To determine the precise conformational state of the LacI-9C14 DNA loop under specific experimental conditions.
Main Methods:
- Application of single-molecule fluorescence resonance energy transfer (smFRET) to Cy3 and Cy5 double-labeled LacI-DNA loops.
- Utilizing multiple excitation wavelengths to analyze FRET efficiencies and spectral characteristics.
- Monitoring the behavior of the zero-energy transfer (ET) peak during titration with LacI to infer conformational populations.
Main Results:
- Single-molecule FRET experiments demonstrated that the LacI-9C14 DNA loop exists exclusively in a single, compact "closed" form.
- The closed conformation exhibited nearly 100% energy transfer (ET), consistent with a tightly V-shaped repressor structure.
- The study ruled out the significant population of open or intermediate geometries for this specific LacI-DNA construct under the tested conditions.
Conclusions:
- The LacI-9C14 DNA loop adopts a single, stable closed conformation, challenging previous hypotheses of multiple stable states.
- Single-molecule FRET provides unprecedented resolution for characterizing protein-DNA complex dynamics and conformational heterogeneity.
- This work refines our understanding of the structural plasticity and regulatory mechanisms of LacI-mediated gene expression.