Energetics of DNA twisting. I. Relation between twist and cyclization probability

Area of Science:

• Biophysics
• Molecular Biology
• Structural Biology

Background:

• DNA's helical structure is fundamental to its function.
• Understanding DNA's mechanical properties, like twisting, is crucial for molecular processes.
• Previous methods for determining DNA twisting potential were indirect.

Purpose of the Study:

• To directly measure the twisting potential of DNA.
• To determine the torsional constant of DNA.
• To investigate the symmetry of the DNA twisting potential.

Main Methods:

• Utilized EcoRI restriction fragments with varying lengths (237-254 bp) to alter DNA twist.
• Measured cyclization probability (j-factor) using phage T4 DNA ligase-mediated covalent closure.
• Analyzed the relationship between DNA length, fractional twist, and cyclization equilibrium.

Main Results:

• The j-factor exhibited an oscillatory dependence on DNA length, with a period of approximately 10 bp, strongly correlating with fractional twist.
• Fitted data to a harmonic twisting potential, yielding a torsional constant (C) of 2.4 X 10(-19) erg cm.
• Demonstrated a symmetric DNA twisting potential and continuity of the helix across single-strand nicks.

Conclusions:

• Provided a direct measurement of DNA's torsional free energy.
• The determined torsional constant aligns with previous estimates, validating the experimental approach.
• The findings support a continuous DNA helix model and a symmetric twisting potential.