Preferential counterion binding to A-tract DNA oligomers

Area of Science:

• Molecular Biology
• Biophysics
• Structural Biology

Background:

• A-tracts are DNA sequences known for their unique structural properties, including bending.
• The hypothesis suggests that the narrow minor groove of A-tracts facilitates site-specific monovalent counterion binding.
• This binding is proposed to cause DNA bending through asymmetric charge neutralization and backbone collapse.

Purpose of the Study:

• To investigate the hypothesis of preferential counterion binding in the minor groove of A-tract DNA.
• To determine if counterion binding affects the solution mobility of DNA oligomers.
• To correlate specific A-tract sequences with the degree of counterion binding.

Main Methods:

• Measurement of free solution mobility of four 20 bp DNA oligomers using capillary electrophoresis.
• Comparison of DNA oligomers with and without A-tracts in Tris-acetate buffer.
• Analysis of mobility changes with varying NaCl concentrations (1-20 mM).

Main Results:

• DNA oligomers containing A-tracts exhibited reduced electrophoretic mobility compared to non-A-tract sequences.
• The slowest mobility, indicating greatest counterion binding, was observed in a 20-mer with phased A3T3 runs.
• Phased CACA sequences showed the fastest mobility, while A5 and T3A3 tracts displayed intermediate mobilities.
• Similar mobility differences persisted with the addition of NaCl, supporting preferential binding.

Conclusions:

• The study provides evidence supporting preferential counterion binding within the minor groove of A-tract DNA.
• The AnTn sequence motif appears particularly prone to this preferential counterion binding.
• Counterion binding significantly influences the solution mobility of DNA oligomers, impacting their structural behavior.