Lac repressor hinge flexibility and DNA looping: single molecule kinetics by tethered particle motion

Area of Science:

• Molecular Biophysics
• Biochemistry
• Single-molecule biophysics

Background:

• Tethered particle motion (TPM) is a single-molecule technique for detecting biomolecular activity.
• TPM has been successfully applied to study enzymes like RNA polymerase and other biomolecules.
• Understanding DNA looping kinetics is crucial for gene regulation.

Purpose of the Study:

• To systematically investigate the kinetics of DNA looping by wild-type Lac repressor (wt-LacI) and its hinge mutants.
• To quantify the effects of protein hinge flexibility and DNA loop strain on DNA looping kinetics.
• To develop a novel method for TPM data analysis to reliably measure loop formation and disruption kinetics.

Main Methods:

• Utilized tethered particle motion (TPM) to monitor DNA looping dynamics at the single-molecule level.
• Employed a novel data analysis method for precise measurement of loop formation and disruption rates.
• Investigated wild-type Lac repressor (wt-LacI) and hinge mutants (Q60G, Q60+1) to assess protein flexibility effects.

Main Results:

• Protein hinge flexibility significantly influences the lifetime of the DNA-looped state.
• DNA bending energy has a minor impact on loop disruption kinetics.
• DNA bending energy strongly affects the kinetics of DNA loop formation.

Conclusions:

• The flexibility of the protein hinge is a critical determinant of DNA loop stability.
• Mechanical factors like DNA bending energy play distinct roles in loop formation versus disruption.
• These findings support theoretical models and highlight the in vivo importance of mechanical forces in gene regulation.