Bead size effects on protein-mediated DNA looping in tethered-particle motion experiments

Area of Science:

• Biophysics
• Single-molecule biophysics
• Molecular mechanisms

Background:

• Tethered particle motion (TPM) is a key technique for observing biomolecular dynamics at the single-molecule level.
• Concerns exist regarding potential artifacts introduced by the microsphere used in TPM, possibly altering native biophysical processes.
• The lac repressor system provides a model for studying protein-mediated DNA looping.

Purpose of the Study:

• To investigate the influence of microsphere size on the dynamics of protein-mediated DNA loop formation and breakdown.
• To compare the effects of conventional large beads versus smaller nanoparticles in TPM experiments.
• To assess the reliability of TPM for studying the lac repressor system under varying bead conditions.

Main Methods:

• Utilized conventional tethered particle motion (TPM) with 800 nm polystyrene beads.
• Employed dark-field TPM with 50 nm gold (Au) nanoparticles for comparison.
• Analyzed the lifetimes of looped and unlooped states in the lac repressor DNA looping system.

Main Results:

• The presence of the microsphere had a minimal, less than two-fold, modification on the lifetimes of looped and unlooped states.
• Results indicate weak excluded-volume effects and hydrodynamic surface interactions from the cover glass and microsphere.
• Smaller gold nanoparticles showed comparable or less perturbation than larger polystyrene beads.

Conclusions:

• Microsphere size in TPM has a limited impact on the dynamics of lac repressor-mediated DNA looping.
• TPM remains a robust method for single-molecule studies, with minimal alteration of biomolecular dynamics.
• The findings support the validity of TPM for investigating complex protein-DNA interactions.