Cai L, Chen L, Raghavan S, Ratliff R, Moyzis R, Rich A
Department of Biology, Room 68-233, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
Nucleic Acids Res. 1998 Oct 15;26(20):4696-705. doi: 10.1093/nar/26.20.4696.
The cytosine-rich strand of the Tetrahymena telomere consists of multiple repeats of sequence d(AACCCC). We have solved the crystal structure of the crystalline repeat sequence at 2.5 A resolution. The adenines form two different and previously unknown clusters (A clusters) in orthogonal directions with their counterparts from other strands, each containing a total of eight adenines. The clusters appear to be stable aggregates held together by base stacking and three different base-pairing modes. Two different types of cytosine tetraplexes are found in the crystal. Each four-stranded complex is composed of two intercalated parallel-stranded duplexes pointing in opposite directions, with hemiprotonated cytosine-cytosine (C.C+) base pairs. The outermost C.C+base pairs are from the 5'-end of each strand in one cytosine tetraplex and from the 3'-end of each strand in the other. The A clusters and the cytosine tetraplexes form two alternating stacking patterns, creating continuous base stacking in two perpendicular directions along the x - and z -axes. The adenine clusters could be organizational motifs for macromolecular RNA.
嗜热四膜虫端粒富含胞嘧啶的链由序列d(AACCCC)的多个重复序列组成。我们已经以2.5埃的分辨率解析了该晶体重复序列的晶体结构。腺嘌呤与其他链上的对应物在正交方向上形成了两个不同的、前所未知的簇（A簇），每个簇总共包含八个腺嘌呤。这些簇似乎是通过碱基堆积和三种不同的碱基配对模式结合在一起的稳定聚集体。在晶体中发现了两种不同类型的胞嘧啶四链体。每个四链复合物由两个相互嵌入、方向相反的平行链双链体组成，具有半质子化的胞嘧啶-胞嘧啶（C.C+）碱基对。最外层的C.C+碱基对在一个胞嘧啶四链体中来自每条链的5'端，在另一个胞嘧啶四链体中来自每条链 的3'端。A簇和胞嘧啶四链体形成两种交替的堆积模式，在沿x轴和z轴的两个垂直方向上产生连续的碱基堆积。腺嘌呤簇可能是大分子RNA的组织基序。
