Yaniv M, Cereghini S
CRC Crit Rev Biochem. 1986;21(1):1-26. doi: 10.3109/10409238609113607.
Transcriptionally active or potentially active genes can be distinguished by several criteria from inactive sequences. Active genes show both an increased general sensitivity to endonucleases like DNase I or micrococcal nuclease and the presence of nuclease hypersensitive sites. Frequently, the nuclease hypersensitive sites are present just upstream of the transcription initiation site covering sequences that are crucial for the promoter function. Viral or cellular transcription enhancer elements are also associated with DNase I hypersensitive sites. At least for the SV40 enhancer, it was shown by electronmicroscopic studies that the DNase I hypersensitive DNA segment is excluded from nucleosomes. It is highly plausible that the binding of regulatory proteins to enhancer or promoter sequences is responsible for the exclusion of these DNA segments from nucleosomes and for the formation of nuclease hypersensitive sites. We speculate that the binding of such proteins may switch on a change in the conformation and/or the protein composition of a chromatin segment or domain containing one to several genes. Biochemical analysis of fractionated nucleosome particles or of active and inactive chromatin fractions have revealed differences in the composition as well as in the degree of modification of histones in these two subfractions of the chromosome. However, until present it is impossible to define unambiguously what are the crucial structural elements that distinguish between particles present on active and inactive chromatin.
转录活性基因或潜在活性基因可通过多种标准与非活性序列区分开来。活性基因对核酸内切酶（如DNase I或微球菌核酸酶）的总体敏感性增加，且存在核酸酶超敏位点。通常，核酸酶超敏位点位于转录起始位点上游，覆盖对启动子功能至关重要的序列。病毒或细胞转录增强子元件也与DNase I超敏位点相关。至少对于SV40增强子，电子显微镜研究表明，DNase I超敏DNA片段被排除在核小体之外。很有可能是调节蛋白与增强子或启动子序列的结合导致这些DNA片段被排除在核小体之外，并形成核酸酶超敏位点。我们推测，此类蛋白的结合可能会引发包含一个或几个基因的染色质片段或结构域的构象和/或蛋白质组成的变化。对分级分离的核小体颗粒或活性和非活性染色质组分的生化分析揭示了染色体这两个亚组分中组蛋白在组成以及修饰程度上的差异。然而，到目前为止，还无法明确界定区分活性和非活性染色质上的颗粒的关键结构元件是什么。
