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lac repressor forms loops with linear DNA carrying two suitably spaced lac operators
The EMBO Journal
|May 1, 1987
Summary
The lac repressor protein can form DNA loops by binding to two operator sites on a single DNA molecule. This DNA looping is influenced by operator spacing and component concentrations, leading to different structural arrangements.
Area of Science:
- Molecular Biology
- Biochemistry
- Structural Biology
Background:
- The lac repressor protein (LacI) is a key regulator of the lac operon in bacteria.
- Tetrameric LacI can potentially interact with multiple DNA binding sites.
Purpose of the Study:
- To investigate the ability of tetrameric lac repressor to form DNA loops by binding to two lac operators.
- To characterize the structures formed by lac repressor-DNA interactions under varying conditions.
Main Methods:
- Electron microscopy
- Non-denaturing polyacrylamide gel electrophoresis
- DNase I protection assays
Main Results:
- Demonstrated DNA loop formation between two lac operators, with distances ranging from 63 to 535 bp.
- DNA loop formation is favored by correct operator phasing and low component concentrations.
- High repressor excess leads to 'tandem' structures; high repressor and operator concentrations form 'sandwich' structures.
Conclusions:
- Tetrameric lac repressor can mediate DNA looping between two operator sites.
- The specific DNA-protein complex structure depends on the relative concentrations of repressor and DNA, and operator phasing.