Chien Fan-Tso, van Noort John
Physics of Life Processes, LION, Leiden University, Leiden, The Netherlands.
Curr Pharm Biotechnol. 2009 Aug;10(5):474-85. doi: 10.2174/138920109788922128.
The compact, yet dynamic organization of chromatin plays an essential role in regulating gene expression. Although the static structure of chromatin fibers has been studied extensively, the controversy about the higher order folding remains. In the past ten years a number of studies have addressed chromatin folding with single molecule force spectroscopy. By manipulating chromatin fibers individually, the mechanical properties of the fibers were quantified with piconewton and nanometer accuracy. Here, we review the results of force induced chromatin unfolding and compare the differences between experimental conditions and single molecule manipulation techniques like force and position clamps. From these studies, five major features appeared upon forced extension of chromatin fibers: the elastic stretching of chromatin's higher order structure, the breaking of internucleosomal contacts, unwrapping of the first turn of DNA, unwrapping of the second turn of DNA, and the dissociation of histone octamers. These events occur sequentially at the increasing force. Resolving force induced structural changes of chromatin fibers at the single molecule level will help to provide a physical understanding of processes involving chromatin that occur in vivo and will reveal the mechanical constraints that are relevant for processing and maintenance of DNA in eukaryotes.
染色质紧凑而动态的结构在基因表达调控中起着至关重要的作用。尽管对染色质纤维的静态结构已进行了广泛研究，但关于其高级折叠的争议仍然存在。在过去十年中，一些研究利用单分子力谱法研究了染色质折叠。通过单独操纵染色质纤维，以皮牛顿和纳米级精度对纤维的力学性质进行了量化。在此，我们综述了力诱导染色质解折叠的结果，并比较了实验条件以及诸如力钳和位置钳等单分子操纵技术之间的差异。从这些研究中可以看出，染色质纤维在强制拉伸时出现了五个主要特征：染色质高级结构的弹性拉伸、核小体间接触的断裂、DNA第一圈的解旋、DNA第二圈的解旋以及组蛋白八聚体的解离。这些事件在力增加时依次发生。在单分子水平解析力诱导的染色质纤维结构变化，将有助于从物理角度理解体内发生的涉及染色质的过程，并揭示与真核生物中DNA加工和维持相关的力学限制因素。
