Doing a spin with DNA

A Dutch led international team of researchers has unravelled how nature releases the torque built up in DNA at the molecular level. The researchers from Delft University of Technology, the Ecole Normale Supérieure in Paris and the Sloan-Kettering institute in New York published their findings in the 31 March 2005 issue of Nature. An artistic impression of the enzyme at work is featured on the cover of this issue.

The enzyme topoisomerase IB releases the torsion built up in DNA strands. During their investigations, the researchers could follow a single topoisomerase-enzyme molecule over time as it acted on a single DNA molecule. The topoisomerase clamps onto the DNA, cuts through one of the two DNA strands, and then lets the DNA unwind before sticking the broken ends back together again. With the help of sensitive measuring devices, the researchers could measure various parameters such as the friction of the rotating DNA in a cavity of the enzyme. The research has provided new insights into the interactions between DNA and the enzyme, which are of fundamental importance for understanding cell division.

DNA consists of two long strands joined together by pairs of bases. Both strands wind around each other in the form of a double helix with the base pairs acting as the 'stairs' in a staircase. The sequence of these base pairs stores genetic information. During cell division genetic material is copied and the enzymes responsible for this must be able to transcribe the base sequences. This is only possible if the portion of DNA to be transcribed is unwound. This winding and unwinding of the DNA gives rise to torsional forces in the DNA, the magnitude of which increases as cell division progresses. These forces can delay the process of cell division and under certain conditions even stop it. Topoisomerase IB can reduce these torsional forces.

The enzyme releases the torsion from the DNA as follows: The enzyme surrounds the double-stranded DNA like a clamp and then temporarily cuts through one of the two DNA strands. The accumulated torsional forces in the DNA are then spun out around the intact strand. After a number of turns the topoisomerase ones again firmly grabs the spinning DNA and 'glues' (ligates) the broken stands neatly back together again. The researchers were able to determine the exact number of turns removed by the topisomerase between 'cutting' and 'gluing'.

The precise mechanism of topoisomerase IB is also important for cancer research. Drugs which inhibit the functioning of topoisomerase IB are already in clinical use, but can possibly be improved using the knowledge from this study.

Source : Netherlands Organization for Scientific Research