Nelson R G, Fangman W L
Proc Natl Acad Sci U S A. 1979 Dec;76(12):6515-9. doi: 10.1073/pnas.76.12.6515.
The eukaryotic microorganism Saccharomyces cerevisiae contains 50-100 copies per cell of a circular plasmid called 2-micrometer DNA. The intracellular structure of these molecules, which represent about 4% of the total DNA, was examined by digestion of total cellular chromatin with micrococcal nuclease (nucleate 3'-oligonucleotidohydrolase, EC 3.1.31.1). Nuclease-resistant DNA fragments were fractionated by gel electrophoresis and 2-micrometer DNA sequences were detected by hybridization. The 2-micrometer and chromosomal DNA digestion patterns were very similar indicating that both types of DNA are condensed into nucleosomes. An analysis of these digestion patterns showed that the kinetics of digestion of 2-micrometer chromatin and total chromatin are similar and that both have the same nucleosome repeat length of about 165 base pairs. Native 2-micrometer plasmids were examined by zone sedimentation in sucrose gradients containing 0.15 M NaCl and were found to have a sedimentation constant of 75 S, about 3 times the sedimentation constant of protein-free 2-micrometer DNA. This sedimentation property is what would be expected for a 2-micrometer DNA minichromosome. We conclude that within the cell 2-micrometer DNA molecules are organized in a chromatin structure very similar to that of the yeast chromosomes.
真核微生物酿酒酵母每个细胞含有50 - 100个拷贝的一种名为2微米DNA的环状质粒。这些分子约占总DNA的4%，其细胞内结构通过用微球菌核酸酶（核酸3'-寡核苷酸水解酶，EC 3.1.31.1）消化总细胞染色质来进行检测。对核酸酶抗性DNA片段进行凝胶电泳分离，并通过杂交检测2微米DNA序列。2微米DNA和染色体DNA的消化模式非常相似，表明这两种类型的DNA都浓缩成核小体。对这些消化模式的分析表明，2微米染色质和总染色质的消化动力学相似，且两者具有相同的约165个碱基对的核小体重复长度。通过在含有0.15 M NaCl的蔗糖梯度中进行区带沉降来检测天然2微米质粒，发现其沉降常数为75 S，约为无蛋白2微米DNA沉降常数的3倍。这种沉降特性是2微米DNA微型染色体所预期的。我们得出结论，在细胞内，2微米DNA分子以与酵母染色体非常相似的染色质结构存在。
