Topoisomerase II-Mediated Cleavage of Plasmid DNA

Integral to all catalytic functions of topoisomerase II is the ability of the enzyme to generate transient double-stranded breaks in the backbone of DNA (1 –3 ). When topoisomerase II cleaves DNA, it maintains the topological integrity of the genetic material by forming covalent bonds between its active site tyrosyl residues (one on each subunit of the homodimeric enzyme) and the newly generated 5′-terminal phosphates (4 –6 ). The enzyme acts by making two coordinated nicks on opposite strands of the double helix (7 –9 ). These points of cleavage are staggered by 4 bases, such that scission results in 5′-overhanging cohesive ends (4 ).