Riley D, Weintraub H
Cell. 1978 Feb;13(2):281-93. doi: 10.1016/0092-8674(78)90197-6.
Nucleosomes were treated with increasing concentrations of exonuclease III (Exo III) from E. coli. At low levels of Exo III, the heterogeneous distribution of monomers (with associated DNA fragments ranging in size between 140 and 170 bp) is "trimmed" down to a discrete core of 140 bp. The "trimming" of monomers to 140 bp results from a 3' exonucleolytic digestion accompanied by a 5' clipping activity which is specific for the conformation of internucleosomal DNA. At higher concentrations of Exo III, the enzyme digests the 140 bp "trimmed" nucleosome core from both 3' ends without associated 5' nuclease activity. Most striking is the observation that the fragments produced during such a digestion display discrete single-stranded lengths that are integer multiples of 10 bases. For some dimer nucleosomes, Exo III can digest as many as 200 bases from at least one 3' end and produce a 10 base interval ladder from about 400 bases down to 180 bases. This suggests that the enzyme can traverse the length of an entire nucleosome without destroying whatever structural features are necessary to produce a 10 base DNA ladder.
用来自大肠杆菌的核酸外切酶III（Exo III）浓度递增处理核小体。在低水平的Exo III时，单体的异质分布（伴有大小在140至170碱基对之间的相关DNA片段）被“修剪”成140碱基对的离散核心。单体“修剪”成140碱基对是由3'核酸外切消化伴随5'剪切活性导致的，该5'剪切活性对核小体间DNA的构象具有特异性。在较高浓度的Exo III时，该酶从两个3'末端消化140碱基对的“修剪”核小体核心，而没有相关的5'核酸酶活性。最引人注目的是观察到在这种消化过程中产生的片段显示出离散的单链长度，这些长度是10个碱基的整数倍。对于一些二聚体核小体来说，Exo III可以从至少一个3'末端消化多达200个碱基，并产生从约400个碱基到180个碱基的10碱基间隔梯状条带。这表明该酶可以穿过整个核小体而不破坏产生10碱基DNA梯状条带所需的任何结构特征。
