Aggregates of oligo(dG) bind and inhibit topoisomerase II activity and induce formation of large networks

DNA cleavage by eukaryotic type II DNA topoisomerase (EC 5.99.1.3) was strongly inhibited by an oligonucleotide containing 10 dGua residues. Catalytic activities of topoisomerase II, as measured by relaxation and decatenation reactions, were also inhibited by oligo(dG)₁₀. Inhibition was specific to oligo(dG)₁₀; other oligonucleotides, nucleotides, or single-stranded DNAs tested did not influence the activity of topoisomerase II. Oligo(dG)₁₀ did not inhibit other activities such as restriction enzymes. Although the enzyme neither binds nor cleaves oligo(dG)₁₀, inhibition can be explained by the finding that topoisomerase II binds tightly with aggregated oligo(dG) structures (estimated to contain between 20 and 30 molecules of monomeric oligo(dG)₁₀) that form spontaneously prior to addition of enzyme. These aggregated oligo(dG)-topoisomerase complexes are large networks that can be pelleted by a 20-min centrifugation step in a Microfuge. Western blotting with a monoclonal antibody confirmed that topoisomerase II is trapped in these pellets. The ability of the enzyme to form large DNA-protein networks could be a biochemical mechanism by which topoisomerase II might promote or participate in chromosome condensation in vivo prior to mitosis.