Hellen C U, Wimmer E
Department of Microbiology and Immunology, SUNY Health Sciences Center at Brooklyn 11203-2098, USA.
Curr Top Microbiol Immunol. 1995;203:31-63. doi: 10.1007/978-3-642-79663-0_2.
Picornavirus 5' NCRs contain IRES elements that have been divided into two groups, exemplified by PV (type 1) and EMCV (type 2). These elements are functionally related and have an intriguing level of structural and sequence similarity. Some conserved RNA sequences and/or structures may correspond to cis-acting elements involved in IRES function, so that there may also be similarities in the mechanism by which the two types or IRES promote initiation. The function of both types of IRES element appears to depend on a cellular 57 kDa polypeptide, which has been identified as the predominantly nuclear hnRNP protein PTB. However, a specific function for p57/PTB in translation has not yet been established. These two groups can be differentiated on the basis of their requirements for trans-acting factors. The EMCV IRES functions efficiently in a broader range of eukaryotic cell types than type 1 IRES elements, probably because the latter require additional factor(s). A second distinction between these IRES element is that initiation occurs directly at the 3' border of type 2 IRES elements, whereas a nonessential spacer of between 30 nt and 154 nt separates type 1 IRES elements from the downstream initiation codon.
小核糖核酸病毒5'非编码区含有内部核糖体进入位点（IRES）元件，这些元件已被分为两组，以脊髓灰质炎病毒（PV，1型）和脑心肌炎病毒（EMCV，2型）为代表。这些元件在功能上相关，并且在结构和序列相似性方面具有引人关注的程度。一些保守的RNA序列和/或结构可能对应于参与IRES功能的顺式作用元件，因此两种类型的IRES促进起始的机制也可能存在相似性。两种类型的IRES元件的功能似乎都依赖于一种细胞57 kDa多肽，该多肽已被鉴定为主要位于细胞核的不均一核糖核蛋白PTB。然而，p57/PTB在翻译中的具体功能尚未确定。这两组可以根据它们对反式作用因子的需求来区分。EMCV IRES在比1型IRES元件更广泛的真核细胞类型中高效发挥作用，可能是因为后者需要额外的因子。这些IRES元件之间的另一个区别是，起始直接发生在2型IRES元件的3'边界，而1型IRES元件与下游起始密码子之间由30个核苷酸至154个核苷酸的非必需间隔区隔开。
