Topoisomerases bind directly to the DNA phosphate backbone, where a catalytic tyrosine residue in the enzyme forms a temporary covalent bond with the DNA strand. But “where” has several layers: the chemical bond itself, the sequences the enzyme prefers, the structural domains that grip the DNA, and the locations within the genome where topoisomerases concentrate during normal cell activity.
The Covalent Bond With DNA
At the most fundamental level, topoisomerases work by cutting DNA and resealing it. To do this, a tyrosine amino acid in the enzyme’s active site attacks a phosphodiester bond in the DNA backbone, temporarily breaking one strand (Type I) or both strands (Type II). This creates a short-lived covalent link between the protein and the cut DNA end.
The two major classes attach to opposite ends of the break. Type IB topoisomerases (like human topoisomerase I, or TOP1) form a bond with the 3′ end of the cut strand. Specifically, Tyr-723 in human TOP1 links to the phosphate group, creating what biochemists call a 3′-phosphotyrosine intermediate. Type IA topoisomerases, by contrast, attach to the 5′ end of the cleaved strand, leaving a free 3′-OH group on the other side. Type II topoisomerases also use a tyrosine to form a 5′-phosphotyrosine bond, but they cut both strands of the double helix simultaneously.
How the Enzyme Grips the Double Helix
Human TOP1 is built from four domains: an unstructured N-terminal region, a conserved core domain, a flexible linker, and a C-terminal domain that contains the catalytic tyrosine. The core domain splits into two functional halves, a capping module and a catalytic module, that together form a donut-shaped (toroidal) fold. This ring-like structure physically encircles the DNA.
A five-residue loop called the hinge connects the two modules and acts like a gate. When the hinge flexes, the enzyme opens wide enough for a DNA strand to enter the central cavity. Once the DNA is inside, two loops on the opposite side of the protein (called the “Lips”) form a salt bridge that locks the enzyme into a closed conformation around the helix. This open-to-closed transition is essential for catalysis, and a conserved tyrosine near the hinge helps mediate the shift.
Beyond the catalytic tyrosine, several surrounding residues make direct contact with the DNA bases flanking the cut site. In human TOP1, Arg-364 hydrogen-bonds with the base across from the cleavage site on the intact strand, Lys-532 contacts the base at position -1 on the cleaved strand, and Thr-718 bonds with the phosphate of the base immediately downstream at position +1. These contacts stabilize the enzyme-DNA complex and help position the cut precisely.
Sequence Preferences at the Cut Site
Topoisomerases are not purely random in where they cut. While they don’t require a strict recognition sequence the way restriction enzymes do, they show measurable preferences. Studies cataloguing thousands of cleavage events by topoisomerase II identified a consensus sequence surrounding the cut site, with specific base preferences at positions flanking the break between the -1 and +1 nucleotides. This consensus can predict cleavage sites in lab experiments, and sequences matching it appear naturally within viral and tissue-specific enhancer elements.
For TOP1, crystallography of the enzyme bound to DNA has used substrates with defined sequences around the cleavage point, with a thymine typically occupying the -1 position on the cut strand. However, the enzyme’s site selection isn’t just about reading a sequence. For topoisomerase II, single-molecule fluorescence experiments have shown that the enzyme actively bends the DNA before cutting it, and the rate of this bending step varies with sequence. Crucially, the bending is not an intrinsic flexibility of the DNA itself. It is driven by how the protein interacts with a given sequence, meaning the enzyme tests potential sites by trying to deform them.
Supercoiled DNA Gets Priority
DNA topology, meaning whether the helix is overwound, underwound, or relaxed, strongly influences where topoisomerases bind in practice. Nearly all topoisomerases tested show higher affinity for supercoiled DNA than for relaxed DNA. Topo IV, a bacterial Type II enzyme, binds negatively supercoiled DNA with roughly five-fold greater affinity than fully relaxed DNA, and its binding affinity increases steadily as the degree of supercoiling increases in either direction.
This preference makes biological sense. Supercoiling is the problem topoisomerases exist to solve: overwound (positive) supercoils pile up ahead of replication forks and transcription machinery, while underwound (negative) supercoils accumulate behind them. By preferentially binding the most stressed DNA, topoisomerases concentrate their activity exactly where it is needed most.
Where Topoisomerases Work in the Genome
Inside living cells, topoisomerases don’t just float around binding random stretches of DNA. They cluster at specific, high-stress locations. Genome-wide mapping in yeast shows that both TOP1 and topoisomerase II (Top2) bind within a narrow window of about 600 base pairs surrounding active replication forks. As the fork moves, the topoisomerases travel with it, staying close to the replication machinery to resolve the torsional stress generated as the helix unwinds.
TOP1 binding is enriched at early-firing replication origins, the spots where DNA copying begins first during cell division. Its presence correlates tightly with active DNA synthesis: regions where TOP1 and Top2 were found also showed new DNA incorporation, confirming the enzymes were working at sites of ongoing replication.
Top2 has an additional role beyond replication. During S phase, it forms clusters at specific intergenic regions, particularly upstream of genes, near promoters. Out of 18 replication-unrelated Top2 clusters mapped on one yeast chromosome, 17 sat upstream of transcription units. The likely explanation is that Top2 relieves the unwinding stress generated when gene transcription initiates, a job that requires cutting and resealing both DNA strands.
Bacterial Gyrase Wraps DNA Differently
Not all topoisomerases grip DNA the same way. Bacterial DNA gyrase, a Type IIA enzyme, is unique in its ability to introduce negative supercoils into DNA rather than simply relaxing them. It accomplishes this through a structural feature that eukaryotic topoisomerase II lacks: a C-terminal domain (CTD) on its GyrA subunit that wraps approximately 120 to 150 base pairs of DNA around itself.
This wrapping positions two segments of the same DNA molecule in a specific orientation. The enzyme then cuts one segment (the G-segment, or gate segment), passes the other segment (the T-segment, or transport segment) through the break, and reseals the cut. In gyrase, DNA binding triggers an upward movement of these C-terminal domains and a narrowing of the enzyme’s N-gate, movements thought to be specific to the intramolecular strand passage required for supercoiling. Eukaryotic topoisomerase II and bacterial Topo IV lack this extensive DNA-wrapping domain, which is why they relax supercoils or decatenate linked DNA circles but cannot actively introduce negative supercoils.
Where Cancer Drugs Target the Complex
The transient covalent bond between topoisomerase and DNA creates a vulnerability that cancer drugs exploit. Topoisomerase inhibitors don’t block the enzyme from binding DNA. Instead, they trap the enzyme after it has already cut the strand, preventing the resealing step and converting a normal intermediate into a permanent DNA break.
These drugs bind at the interface between the enzyme and the DNA, right at the cleavage site. Camptothecin and its derivatives (used clinically as topotecan and irinotecan) target TOP1 by sliding between the base pairs flanking the cut, stacking their flat aromatic rings between the -1 and +1 positions through pi-pi interactions. Additional hydrogen bonds link the drug to TOP1 residues that are also involved in the enzyme’s normal contacts with DNA. This wedge-like intercalation physically blocks the cut ends from realigning, so the strand break persists. Etoposide and the anthracyclines work by a similar interfacial mechanism against topoisomerase II, trapping the double-strand break that the enzyme normally reseals within milliseconds.
Because rapidly dividing cancer cells generate enormous amounts of replication and transcription stress, they depend heavily on topoisomerase activity. Trapping these enzymes on DNA converts an essential cellular tool into a source of lethal DNA damage, which is why topoisomerase inhibitors remain a cornerstone of chemotherapy for cancers ranging from ovarian and lung cancer to leukemia.