Negative supercoiling facilitates human TOP3α-mediated decatenation of DNA braids via catenation junction melting
This study demonstrates that negative supercoiling facilitates human TOP3α-mediated decatenation of DNA braids by inducing local duplex melting at crossover junctions, thereby generating single-stranded DNA substrates accessible to the type IA topoisomerase and its regulatory partners.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Every time a cell divides, it must make an exact copy of its genetic code, a long, twisted ladder of DNA. As the copying machinery moves along the strand, it creates a new pair of ladders that are initially intertwined, like two strands of a rope that have been twisted together. Before the cell can split into two, these tangled strands must be untangled completely. If they remain linked, the chromosomes cannot separate, and the cell cannot divide properly. For decades, scientists believed that only a specific class of enzymes, known as type II topoisomerases, could perform this heavy lifting. These enzymes work like molecular scissors that cut both strands of the DNA ladder, pass the other strand through the gap, and then reseal the cut. This double-strand cutting mechanism seemed essential for untangling two fully formed, double-stranded DNA molecules.
However, another class of enzymes, called type IA topoisomerases, operates differently. These enzymes can only cut a single strand of the DNA ladder. Because they cannot cut both sides of the double helix at once, they were thought to be unable to untangle two fully intact DNA molecules. They seemed restricted to working on single strands or specific gaps in the DNA. This created a puzzle: if cells rely on these single-strand cutters for certain tasks, how do they untangle the complex knots formed between two complete DNA strands? The question was whether there was a way for the physical state of the DNA itself to change the rules, allowing these single-strand cutters to do the job usually reserved for the double-strand cutters.
Researchers at the MRC Laboratory of Medical Sciences in London set out to answer this question by watching the process happen in real time. They used a sophisticated setup involving optical tweezers, which are essentially beams of light that can grab and pull on tiny objects. In this case, the objects were microscopic beads attached to long strands of DNA. By manipulating these beads, the scientists could twist the DNA, stretch it, and create specific knots, known as braids, where two DNA molecules crossed over each other. They then introduced the human enzyme complex known as TOP3α, along with its helper proteins RMI1 and RMI2, to see if it could untangle these knots.
The team discovered that the key to unlocking this process was not a change in the enzyme, but a change in the tension and twisting of the DNA itself. When the DNA strands were relaxed and held in a standard, untwisted state, the enzyme complex could not untangle the knots. The DNA strands remained locked together, and the enzyme simply could not find a way in. However, when the researchers introduced negative supercoiling—a specific type of twisting that stores energy in the DNA—the situation changed dramatically. Negative supercoiling is a state where the DNA is under-twisted, creating a torsional stress that wants to unwind.
Under this stress, something remarkable happened at the exact point where the two DNA strands crossed each other. The twisting force caused the double helix to locally melt or open up right at the crossing point. This melting created a tiny, temporary bubble of single-stranded DNA. Although the rest of the molecule remained a sturdy double helix, this small opening was enough for the TOP3α enzyme to grab onto. Once the enzyme latched onto this single-stranded bubble, it could perform its standard function: cutting the single strand, passing the other DNA molecule through the gap, and resealing it. This action successfully untangled the two DNA strands.
The researchers found that this process was highly efficient. When the DNA was negatively supercoiled, the enzyme complex untangled the knots in a matter of seconds, even when the DNA was being pulled with significant force. In contrast, when the DNA was relaxed, the enzyme failed to untangle the knots and often caused the DNA strands to break instead. The study also showed that a protein called RPA, which naturally binds to single-stranded DNA, helped the process by stabilizing these tiny melted bubbles, making it easier for the enzyme to work. Another helper protein, RMI2, improved the efficiency of the reaction, though the core enzyme complex could still function without it.
To confirm that the enzyme was indeed using the single-stranded opening to untangle the DNA, the team created a special test. They used a DNA strand that already had a pre-made gap, a missing section of one side of the ladder, and twisted it around a normal, fully intact DNA strand. Even without any twisting force to create a new bubble, the enzyme successfully untangled the two strands. This proved that the enzyme does not need to cut both strands of the DNA; it only needs access to a single strand, which can be provided either by a pre-existing gap or by the local melting caused by twisting stress.
These findings suggest that the physical environment of the DNA determines which enzyme gets to do the work. In a relaxed state, the DNA is too tight for the single-strand cutter to enter, so the cell must rely on the double-strand cutter. But when the DNA is under torsional stress, such as during replication or when chromosomes are being pulled apart, the stress can force the DNA to open up just enough for the single-strand cutter to step in. This provides a physical mechanism that links the mechanical state of the DNA to the enzymatic pathway used to resolve it. The study reveals that the cell does not need to rely on a single tool for every job; instead, the tension and twisting of the DNA itself can transform a locked knot into a solvable puzzle, allowing different enzymes to work together to ensure the faithful transmission of genetic information.
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