Ohlmann T, Jackson R J
Department of Biochemistry, University of Cambridge, United Kingdom.
RNA. 1999 Jun;5(6):764-78. doi: 10.1017/s1355838299982158.
The internal ribosome entry segment (IRES) of picornaviruses consists of approximately 450 nt of 5'-untranslated region, terminating at the 3' end with an approximately 25 nt element consisting of an absolutely conserved UUUC motif followed by a more variable pyrimidine-rich tract and G-poor spacer, and finally an AUG triplet, which is considered to be the actual ribosome entry site. Events following entry at this site differ among picornaviruses: in encephalomyocarditis virus (EMCV) virtually all ribosomes initiate translation at this site (AUG-11); in foot-and-mouth-disease virus (FMDV), one-third of the ribosomes initiate at this AUG (the Lab site), and the rest at the next AUG 84 nt downstream (Lb site); and in poliovirus (PV), the AUG at the 3' end of the IRES (at nt 586 in PV type 1) is considered to be a silent entry site, with all ribosomes initiating translation at the next AUG downstream (nt 743). To investigate what determines this different behavior, chimeras were constructed with a crossover at the conserved UUUC motif: the body of the IRES, the sequences upstream of this UUUC motif, was derived from one species, and the downstream sequences from another. When the body of the FMDV or PV IRESes was replaced by that of EMCV, there was a marked increase in the absolute and relative frequency of initiation at the upstream AUG, the Lab site of FMDV and 586AUG of PV, respectively. In contrast, when the body of the EMCV IRES was replaced by that of PV, initiation occurred with no preference at three AUGs: the normal site (AUG-11), AUG-10 situated 8 nt upstream, and AUG-12, which is 12 nt downstream. Thus although the context of the AUG at the 3' end of the IRES may influence initiation frequency at this site, as was shown by improving the context of 586AUG of PV, the behavior of the ribosome is also highly dependent on the nature of the upstream IRES. Delivery of the ribosome to this AUG in an initiation-competent manner is particularly efficient and accurate with the EMCV IRES.
小核糖核酸病毒的内部核糖体进入片段（IRES）由约450个核苷酸的5'非翻译区组成，在3'端终止于一个约25个核苷酸的元件，该元件由一个绝对保守的UUUC基序、其后一个更具变异性的富含嘧啶的区域和富含鸟嘌呤的间隔区，以及最后一个AUG三联体组成，该AUG三联体被认为是实际的核糖体进入位点。在该位点进入后发生的事件在不同小核糖核酸病毒中有所不同：在脑心肌炎病毒（EMCV）中，几乎所有核糖体都在此位点（AUG-11）起始翻译；在口蹄疫病毒（FMDV）中，三分之一的核糖体在此AUG（Lab位点）起始翻译，其余的在下游84个核苷酸处的下一个AUG（Lb位点）起始翻译；在脊髓灰质炎病毒（PV）中，IRES 3'端的AUG（在1型PV中为核苷酸586处）被认为是一个沉默进入位点，所有核糖体都在下游的下一个AUG（核苷酸743）起始翻译。为了研究是什么决定了这种不同的行为，构建了在保守的UUUC基序处有交叉的嵌合体：IRES的主体，即该UUUC基序上游的序列，来自一个物种，而下游序列来自另一个物种。当FMDV或PV IRES的主体被EMCV的主体取代时，分别在FMDV的上游AUG即Lab位点和PV的586AUG处起始的绝对频率和相对频率均显著增加。相反，当EMCV IRES的主体被PV的主体取代时，在三个AUG处起始无偏好：正常位点（AUG-11）、位于上游8个核苷酸处的AUG-10和位于下游12个核苷酸处的AUG-12。因此，尽管如通过改善PV的586AUG的上下文所显示的那样，IRES 3'端AUG的上下文可能会影响该位点的起始频率，但核糖体的行为也高度依赖于上游IRES的性质。对于EMCV IRES，以起始能力的方式将核糖体递送至该AUG特别有效且准确。
