Gao Feng, Kasprzak Wojciech K, Szarko Christine, Shapiro Bruce A, Simon Anne E
Department of Cell Biology and Molecular Genetics, University of Maryland College Park, College Park, Maryland, USA.
Basic Science Program, Leidos Biomedical Research, Inc., Frederick National Laboratory for Cancer Research, Frederick, Maryland, USA.
J Virol. 2014 Oct;88(20):11696-712. doi: 10.1128/JVI.01433-14. Epub 2014 Aug 6.
Many plant viruses without 5' caps or 3' poly(A) tails contain 3' proximal, cap-independent translation enhancers (3'CITEs) that bind to ribosomal subunits or translation factors thought to assist in ribosome recruitment. Most 3'CITEs participate in a long-distance kissing-loop interaction with a 5' proximal hairpin to deliver ribosomal subunits to the 5' end for translation initiation. Pea Enation Mosaic Virus (PEMV) contains two adjacent 3'CITEs in the center of its 703-nucleotide 3' untranslated region (3'UTR), the ribosome-binding, kissing-loop T-shaped structure (kl-TSS) and eukaryotic translation initiation factor 4E-binding Panicum mosaic virus-like translation enhance (PTE). We now report that PEMV contains a third, independent 3'CITE located near the 3' terminus. This 3'CITE is composed of three hairpins and two pseudoknots, similar to the TSS 3'CITE of the carmovirus Turnip crinkle virus (TCV). As with the TCV TSS, the PEMV 3'TSS is predicted to fold into a T-shaped structure that binds to 80S ribosomes and 60S ribosomal subunits. A small hairpin (kl-H) upstream of the 3'TSS contains an apical loop capable of forming a kissing-loop interaction with a 5' proximal hairpin and is critical for the accumulation of full-length PEMV in protoplasts. Although the kl-H and 3'TSS are dispensable for the translation of a reporter construct containing the complete PEMV 3'UTR in vitro, deleting the normally required kl-TSS and PTE 3'CITEs and placing the kl-H and 3'TSS proximal to the reporter termination codon restores translation to near wild-type levels. This suggests that PEMV requires three 3'CITEs for proper translation and that additional translation enhancers may have been missed if reporter constructs were used in 3'CITE identification. Importance: The rapid life cycle of viruses requires efficient translation of viral-encoded proteins. Many plant RNA viruses contain 3' cap-independent translation enhancers (3'CITEs) to effectively compete with ongoing host translation. Since only single 3'CITEs have been identified for the vast majority of individual viruses, it is widely accepted that this is sufficient for a virus's translational needs. Pea Enation Mosaic Virus possesses a ribosome-binding 3'CITE that can connect to the 5' end through an RNA-RNA interaction and an adjacent eukaryotic translation initiation factor 4E-binding 3'CITE. We report the identification of a third 3'CITE that binds weakly to ribosomes and requires an upstream hairpin to form a bridge between the 3' and 5' ends. Although both ribosome-binding 3'CITEs are critical for virus accumulation in vivo, only the CITE closest to the termination codon of a reporter open reading frame is active, suggesting that artificial constructs used for 3'CITE identification may underestimate the number of CITEs that participate in translation.
许多没有5'帽或3'多聚腺苷酸尾巴的植物病毒含有3'近端、不依赖帽的翻译增强子(3'CITEs),这些增强子与核糖体亚基或被认为有助于核糖体募集的翻译因子结合。大多数3'CITEs通过与5'近端发夹结构进行长距离的吻式环相互作用,将核糖体亚基递送至5'端以启动翻译。豌豆耳突花叶病毒(PEMV)在其703个核苷酸的3'非翻译区(3'UTR)中心含有两个相邻的3'CITEs,即核糖体结合的吻式环T形结构(kl-TSS)和真核翻译起始因子4E结合的黍花叶病毒样翻译增强子(PTE)。我们现在报告,PEMV在3'末端附近含有第三个独立的3'CITE。这个3'CITE由三个发夹结构和两个假结组成,类似于番茄皱缩病毒(TCV)的TSS 3'CITE。与TCV TSS一样,PEMV 3'TSS预计会折叠成T形结构,与80S核糖体和60S核糖体亚基结合。3'TSS上游的一个小发夹结构(kl-H)含有一个顶端环,能够与5'近端发夹结构形成吻式环相互作用,对原生质体中全长PEMV的积累至关重要。虽然kl-H和3'TSS对于体外含有完整PEMV 3'UTR的报告构建体的翻译不是必需的,但删除通常所需的kl-TSS和PTE 3'CITEs,并将kl-H和3'TSS置于报告终止密码子附近,可使翻译恢复到接近野生型水平。这表明PEMV需要三个3'CITEs才能进行正确翻译,并且如果在3'CITE鉴定中使用报告构建体,可能会遗漏其他翻译增强子。重要性:病毒快速的生命周期需要高效翻译病毒编码的蛋白质。许多植物RNA病毒含有3'不依赖帽的翻译增强子(3'CITEs),以有效竞争宿主正在进行的翻译。由于绝大多数单个病毒仅鉴定出单个3'CITEs,因此人们普遍认为这足以满足病毒的翻译需求。豌豆耳突花叶病毒拥有一个核糖体结合的3'CITE,它可以通过RNA-RNA相互作用连接到5'端,以及一个相邻的真核翻译起始因子4E结合的3'CITE。我们报告鉴定出第三个3'CITE,它与核糖体的结合较弱,并且需要一个上游发夹结构在3'端和5'端之间形成桥梁。虽然两个核糖体结合的3'CITEs对体内病毒积累都至关重要,但只有最接近报告开放阅读框终止密码子的CITE是有活性的,这表明用于3'CITE鉴定的人工构建体可能低估了参与翻译的CITEs数量。