Polach K J, Widom J
Department of Biochemistry Molecular Biology, and Cell Biology, Northwestern University, Evanston IL 60208-3500, USA.
J Mol Biol. 1995 Nov 24;254(2):130-49. doi: 10.1006/jmbi.1995.0606.
We present evidence for a mechanism by which regulatory proteins may gain access to their target DNA sequences in chromatin. In this model, nucleosomes are dynamic structures, transiently exposing stretches of their DNA. Regulatory proteins gain access to DNA target sites in the exposed state, and bind with an apparent dissociation constant equal to their dissociation constant for naked DNA divided by a position-dependent equilibrium constant for site exposure within the nucleosome. A sensitive assay, based on the kinetics of restriction digestion of sites within nucleosomes, reveals this dynamic behaviour and quantifies the equilibrium constants for site exposure. Our results have implications for many aspects of chromatin function. They offer new mechanisms for cooperativity (synergy) in regulatory protein binding and for active invasion of nucleosomes.
我们提供了一种机制的证据，通过该机制调节蛋白可能进入染色质中的靶DNA序列。在这个模型中，核小体是动态结构，会短暂暴露其DNA片段。调节蛋白在暴露状态下进入DNA靶位点，并以一个表观解离常数结合，该常数等于其对裸露DNA的解离常数除以核小体内位点暴露的位置依赖性平衡常数。一种基于核小体内位点限制性消化动力学的灵敏检测方法揭示了这种动态行为，并量化了位点暴露的平衡常数。我们的结果对染色质功能的许多方面都有影响。它们为调节蛋白结合中的协同作用（协同效应）和核小体的主动侵入提供了新机制。
