snoRNA的结构与功能

核仁小分子RNA（snoRNA）是一类广泛分布于真核生物细胞核仁的小分子非编码RNA，具有保守的结构元件，并以此划分为3大类：boxC／DsnoRNA、boxH／ACAsnoRNA和MRPRNA。其中boxC／D和boxH／ACA是已知snoRNA的主要类型，以碱基配对的方式分别指导着核糖体RNA的甲基化和假尿嘧啶化修饰。研究发现，snoRNA除了在核糖体RNA的生物合成中发挥作用之外，还能够指导snRNA、tRNA和mRNA的转录后修饰。此外，还有相当数量的snoRNA功能不明，被称为孤儿sn0RNA（orphansnoRNA）。在哺乳动物的孤儿snoRNA中，印迹snoRNA（imprintedsnoRNA）是最为特殊的一群，由基因组印迹区编码，具有明显的组织表达特异性。原核生物古细菌中类snoRNA的鉴定表明这些非编码RNA家族成员的古老起源；而哺乳动物中大量的snoRNA反转座子的存在更为人们探索snoRNA在基因组中扩增和功能进化提供了新的思路。

Structure and function of snoRNAs

Small nucleolar RNAs （snoRNAs） represent an abundant group of noncoding RNAs with existence in the nucleolus of eukaryotes. Based on the conserved structure elements, the majority of snoRNAs can be divided into two families, box C/D snoRNAs and box H/ACA snoRNAs, which function on 2′-O- methylation and pseudouridylation of rRNA, respectively, through the formation of a canonical guide RNA duplex at the modification site. However, recent studies show that the complexity of the two snoRNA guide families is far beyond expectation. Dozes of novel snoRNAs target other cellular RNAs, including snRNAs, tRNAs and even mRNAs. In addition, an increasing number Of orphan snoRNAs without known targets were identified, and remarkably, imprinted snoRNAs were among them, which are encoded by genomic imprinting region and expressed in a tissue-specific pattern. Furthermore, the ancient origin of these noncoding RNA families was reflected by the identification of snoRNA homologs in Archaea, and latest characterization of snoRNA retrogenes offers new perspectives to their proliferation and evolution.