生物谷報道：在復制和轉(zhuǎn)錄過程中發(fā)生的各種變化導致DNA分子中扭曲張力的積累,，而這種張力的釋放要靠“拓樸異構(gòu)酶”來完成,。IB-型“拓樸異構(gòu)酶”（TopIB）先是將一個DNA鏈切開,，然后再重新連接起來，并將正,、負“超螺旋”都去除掉?，F(xiàn)在，對單獨一個TopIB所做的直接監(jiān)測表明,，與其他“拓樸異構(gòu)酶”不同的是,，它并不去除固定數(shù)量的“超螺旋”，被去除的“超螺旋”的數(shù)量取決于DNA中儲存的扭矩,，它們是通過一個旋轉(zhuǎn)機制被去除的,，該機制涉及旋轉(zhuǎn)的DNA和酶空腔之間的摩擦力。將摩擦力作為一個因素引入DNA與“拓樸異構(gòu)酶”之間的相互作用中,，對于研究DNA與酶之間相互作用的普遍規(guī)律具有重要意義,。封面圖片所示為正在發(fā)揮作用的TopIB,。
本文發(fā)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. ([email protected]).
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.
