Positioning the 3'-DNA terminus for topoisomerase II-mediated religation

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Topoisomerase II (Topo II) enzymes are essential for maintaining genomic integrity through DNA cleavage and religation.
• The mechanism of DNA religation by Topo II, particularly the positioning of 3 -DNA termini, remains incompletely understood.
• Topo II forms covalent bonds with 5 -DNA termini but not 3 -DNA termini, necessitating alternative mechanisms for 3 -end alignment.

Purpose of the Study:

• To elucidate the mechanism by which Topoisomerase II positions 3 -DNA termini during the DNA religation step.
• To differentiate between base pairing, base stacking, and protein-DNA interactions in orienting the 3 -DNA termini.

Main Methods:

• Synthesis of modified oligonucleotides with alterations in base pairing and base stacking at the 3 -terminus.
• Assay of human topoisomerase IIalpha religation activity using these modified substrates.
• Comparative analysis of religation rates based on substrate modifications.

Main Results:

• Oligonucleotides with disrupted terminal base pairing or base stacking, but intact terminal bases, were religated at wild-type rates.
• Substrates lacking the terminal base or with an altered terminal base showed significantly reduced religation rates.
• These findings indicate that the integrity of the terminal base is critical for efficient religation.

Conclusions:

• Topoisomerase II relies on noncovalent protein-DNA interactions to accurately position the 3 -DNA termini for religation.
• Base pairing and base stacking of the terminal 3 -base are not the primary mechanisms for positioning.
• This discovery provides key insights into the fundamental mechanisms of DNA repair and replication fidelity.