Yang Janet G, Madrid Tina Shahian, Sevastopoulos Elena, Narlikar Geeta J
Department of Biochemistry and Biophysics, University of California, San Francisco, 600 16th Street, San Francisco, California 94158, USA.
Nat Struct Mol Biol. 2006 Dec;13(12):1078-83. doi: 10.1038/nsmb1170. Epub 2006 Nov 12.
Arrays of regularly spaced nucleosomes directly correlate with closed chromatin structures at silenced loci. The ATP-dependent chromatin-assembly factor (ACF) generates such arrays in vitro and is required for transcriptional silencing in vivo. A key unresolved question is how ACF 'measures' equal spacing between nucleosomes. We show that ACF senses flanking DNA length and transduces length information in an ATP-dependent manner to regulate the rate of nucleosome movement. Using fluorescence resonance energy transfer to follow nucleosome movement, we find that ACF can rapidly sample DNA on either side of a nucleosome and moves the longer flanking DNA across the nucleosome faster than the shorter flanking DNA. This generates a dynamic equilibrium in which nucleosomes having equal DNA on either side accumulate. Our results indicate that ACF generates the characteristic 50- to 60-base-pair internucleosomal spacing in silent chromatin by kinetically discriminating against shorter linker DNAs.
规则间隔的核小体阵列与沉默基因座处的封闭染色质结构直接相关。ATP 依赖的染色质组装因子（ACF）在体外产生这样的阵列，并且是体内转录沉默所必需的。一个关键的未解决问题是 ACF 如何“测量”核小体之间的相等间距。我们表明，ACF 感知侧翼 DNA 长度，并以 ATP 依赖的方式传递长度信息以调节核小体移动速率。利用荧光共振能量转移来跟踪核小体移动，我们发现 ACF 可以快速对核小体两侧的 DNA 进行采样，并且使较长的侧翼 DNA 比短侧翼 DNA 更快地穿过核小体。这产生了一种动态平衡，其中两侧具有相等 DNA 的核小体得以积累。我们的结果表明，ACF 通过动力学上区分较短的连接 DNA，在沉默染色质中产生特征性的 50 至 60 个碱基对的核小体间间距。
