Parallel helical domains in DNA branched junctions containing 5',5' and 3',3' linkages

Area of Science:

• Molecular Biology
• Genetics
• Structural Biology

Background:

• The Holliday junction is a critical intermediate structure in genetic recombination.
• Holliday junctions feature four DNA strands forming a branch point with four helical arms.
• In the presence of Mg2+, these arms typically stack into two antiparallel helical domains.

Purpose of the Study:

• To investigate the structural basis for the antiparallel orientation of helical domains in Holliday junctions.
• To differentiate between junction structure and electrostatic repulsion as the cause of antiparallel domain orientation.
• To construct and analyze a novel 'bowtie' junction analogue to test structural hypotheses.

Main Methods:

• Construction and characterization of a bowtie junction with altered strand linkages at the branch point.
• Comparison with conventional immobile branched junctions using Ferguson analysis and thermal transition profiles.
• Hydroxyl radical autofootprinting to confirm linkage positions and domain formation.
• Cooper-Hagerman gel mobility analyses to determine helical domain orientations.

Main Results:

• Bowtie and conventional junctions exhibited nearly identical behavior in Ferguson analysis and thermal profiles.
• Hydroxyl radical autofootprinting confirmed unusual linkages at the branch point and conserved domain stacking.
• Gel mobility analyses revealed that helical domains in the bowtie junction are closer to parallel than antiparallel.

Conclusions:

• The intrinsic structure of the Holliday junction's branch point appears to dominate over electrostatic repulsion in determining domain orientation.
• Control of branch point structure can lead to parallel helical domains, aligning with genetic models of recombination.
• This finding provides insights into the structural mechanisms governing genetic recombination pathways.