Shimizu M, Mori T, Sakurai T, Shindo H
Department of Chemistry, Meisei University, Hino, Tokyo 191-8506 and School of Pharmacy, Tokyo University of Pharmacy and Life Science, Hachioji, Tokyo 192-0392, Japan.
EMBO J. 2000 Jul 3;19(13):3358-65. doi: 10.1093/emboj/19.13.3358.
Poly(dA) small middle dotpoly(dT) tracts are common and often found upstream of genes in eukaryotes. It has been suggested that poly(dA) small middle dotpoly(dT) promotes transcription in vivo by affecting nucleosome formation. On the other hand, in vitro studies show that poly(dA) small middle dotpoly(dT) can be easily incorporated into nucleosomes. Therefore, the roles of these tracts in nucleosome organization in vivo remain to be established. We have developed an assay system that can evaluate nucleosome formation in yeast cells, and demonstrated that relatively longer tracts such as A(15)TATA(16) and A(34) disrupt an array of positioned nucleosomes, whereas a shorter A(5)TATA(4) tract is incorporated in positioned nucleosomes of yeast minichromosomes. Thus, nucleosomes are destabilized by poly(dA) small middle dotpoly(dT) in vivo in a length-dependent manner. Furthermore, in vivo UV footprinting revealed that the longer tracts adopt an unusual DNA structure in yeast cells that corresponds to the B' conformation described in vitro. Our results support a mechanism in which a unique poly(dA) small middle dot poly(dT) conformation presets chromatin structure to which transcription factors are accessible.
聚（dA）·聚（dT）序列很常见，在真核生物中经常在基因上游发现。有人提出聚（dA）·聚（dT）通过影响核小体形成在体内促进转录。另一方面，体外研究表明聚（dA）·聚（dT）可以很容易地整合到核小体中。因此，这些序列在体内核小体组织中的作用仍有待确定。我们开发了一种可以评估酵母细胞中核小体形成的检测系统，并证明相对较长的序列如A(15)TATA(16)和A(34)会破坏一系列定位核小体，而较短的A(5)TATA(4)序列则整合到酵母微型染色体的定位核小体中。因此，在体内聚（dA）·聚（dT）以长度依赖的方式使核小体不稳定。此外，体内紫外线足迹分析表明，较长的序列在酵母细胞中呈现出一种不寻常的DNA结构，与体外描述的B'构象相对应。我们的结果支持一种机制，即独特的聚（dA）·聚（dT）构象预先设定了染色质结构，转录因子可以与之结合。
