生物谷报道：在复制和转录过程中发生的各种变化导致DNA分子中扭曲张力的积累，而这种张力的释放要靠“拓朴异构酶”来完成。IB-型“拓朴异构酶”（TopIB）先是将一个DNA链切开，然后再重新连接起来，并将正、负“超螺旋”都去除掉。现在，对单独一个TopIB所做的直接监测表明，与其他“拓朴异构酶”不同的是，它并不去除固定数量的“超螺旋”，被去除的“超螺旋”的数量取决于DNA中储存的扭矩，它们是通过一个旋转机制被去除的，该机制涉及旋转的DNA和酶空腔之间的摩擦力。将摩擦力作为一个因素引入DNA与“拓朴异构酶”之间的相互作用中，对于研究DNA与酶之间相互作用的普遍规律具有重要意义。封面图片所示为正在发挥作用的TopIB。
本文发表在：Nature 434, 671 - 674 (31 March 2005); doi:10.1038/nature03395
Friction and torque govern the relaxation of DNA supercoils by eukaryotic topoisomerase IB
DANIEL A. KOSTER1, VINCENT CROQUETTE2, CEES DEKKER1, STEWART SHUMAN3 & NYNKE H. DEKKER1
1 Kavli Institute of Nanoscience, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands
2 Laboratoire de Physique Statistique, Ecole Normale Supérieure, 75005 Paris, France
3 Molecular Biology Program, Sloan-Kettering Institute, New York, New York 10021, USA
Correspondence and requests for materials should be addressed to N.H.D. (nynke.dekker@mb.tn.tudelft.nl).
Topoisomerases relieve the torsional strain in DNA that is built up during replication and transcription. They are vital for cell proliferation and are a target for poisoning by anti-cancer drugs. Type IB topoisomerase (TopIB) forms a protein clamp around the DNA duplex and creates a transient nick that permits removal of supercoils. Using real-time single-molecule observation, we show that TopIB releases supercoils by a swivel mechanism that involves friction between the rotating DNA and the enzyme cavity: that is, the DNA does not freely rotate. Unlike a nicking enzyme, TopIB does not release all the supercoils at once, but it typically does so in multiple steps. The number of supercoils removed per step follows an exponential distribution. The enzyme is found to be torque-sensitive, as the mean number of supercoils per step increases with the torque stored in the DNA. We propose a model for topoisomerization in which the torque drives the DNA rotation over a rugged periodic energy landscape in which the topoisomerase has a small but quantifiable probability to religate the DNA once per turn.
