Bryant Zev, Stone Michael D, Gore Jeff, Smith Steven B, Cozzarelli Nicholas R, Bustamante Carlos
Department of Molecular and Cell Biology, Department of Physics, Howard Hughes Medical Institute, University of California, Berkeley, California 94720, USA.
Nature. 2003 Jul 17;424(6946):338-41. doi: 10.1038/nature01810.
Knowledge of the elastic properties of DNA is required to understand the structural dynamics of cellular processes such as replication and transcription. Measurements of force and extension on single molecules of DNA have allowed direct determination of the molecule's mechanical properties, provided rigorous tests of theories of polymer elasticity, revealed unforeseen structural transitions induced by mechanical stresses, and established an experimental and conceptual framework for mechanical assays of enzymes that act on DNA. However, a complete description of DNA mechanics must also consider the effects of torque, a quantity that has hitherto not been directly measured in micromanipulation experiments. We have measured torque as a function of twist for stretched DNA--torsional strain in over- or underwound molecules was used to power the rotation of submicrometre beads serving as calibrated loads. Here we report tests of the linearity of DNA's twist elasticity, direct measurements of the torsional modulus (finding a value approximately 40% higher than generally accepted), characterization of torque-induced structural transitions, and the establishment of a framework for future assays of torque and twist generation by DNA-dependent enzymes. We also show that cooperative structural transitions in DNA can be exploited to construct constant-torque wind-up motors and force-torque converters.
要理解诸如复制和转录等细胞过程的结构动力学，就需要了解DNA的弹性特性。对单个DNA分子进行力和伸长的测量，使得能够直接测定分子的力学性质，对聚合物弹性理论进行了严格测试，揭示了由机械应力诱导的意外结构转变，并为作用于DNA的酶的力学分析建立了实验和概念框架。然而，对DNA力学的完整描述还必须考虑扭矩的影响，而扭矩这一量在微观操作实验中迄今尚未被直接测量。我们测量了拉伸DNA时扭矩随扭转的变化——过旋或欠旋分子中的扭转应变被用来驱动作为校准负载的亚微米珠子的旋转。在此，我们报告了对DNA扭转弹性线性度的测试、扭转模量的直接测量（发现其值比普遍接受的值高约40%）、扭矩诱导结构转变的表征，以及为未来对依赖DNA的酶产生扭矩和扭转的分析建立的框架。我们还表明，DNA中的协同结构转变可被用于构建恒扭矩卷绕电机和力 - 扭矩转换器。
