Evidence for an Internal Ribosome Entry Site Within the 5' Non-translated Region of Turnip Mosaic Potyvirus RNA

Abstract

The genomic RNA ofpotyviruses has a characteristic 5' non-translated region (5'NTR) to which a viral protein, VPg, is covalently attached. This suggests that the viral RNA lacks a conventional cap structure and thus its translation may not proceed in the same way as most cellular mRNAs. To investigate the role of the 5'NTR during translation, various derivatives of the turnip mosaic potyvirus (TuMV) leader were fused to the reporter gene fl-glucuronidase (GUS). These constructs were used to monitor the efficiency of translation in vitro in a rabbit reticulocyte lysate and in planta following microprojectile DNA delivery into tobacco cell suspensions. GUS transcripts fused with the TuMV 5'NTR, whether they were capped or not, were efficiently translated, whereas GUS transcripts without the viral leader needed to be capped for expression. When transcripts of the viral leader were supplied in excess over functional transcripts, translation was inhibited in a dose-dependent manner. Similarly, transcripts synthesized from the reverse complement of the 5'NTR inhibited translation to the same extent as the wild-type sequence, indicating that cap independence was not conferred by a specific sequence within the viral leader. A stable hairpin loop was placed in front or after the viral sequence. This hairpin loop normally prevented translation of control GUS transcripts but when the viral leader was positioned after it a significant level of GUS activity was measured, whether the transcripts were capped or not. On the other hand, when the hairpin loop was positioned after the viral leader, no GUS activity was measured. These results suggested that ribosomes bound to an internal site within the TuMV 5'NTR and then presumably scanned the sequence for the initiator AUG.

Ribosomal Protein P0 Promotes Potato Virus A Infection and Functions in Viral Translation Together with VPg and eIF(iso)4E

“…The sensitivity of DCV infection to knockdown of ribosomal proteins was shown to be connected to a capindependent internal ribosomal entry site (IRES) translation initiation mechanism. Similar to DCV, PVA is an RNA virus that potentially employs IRES-dependent translation mechanisms, as do other potyviruses such as Tobacco etch virus (TEV) and TuMV (53)(54)(55). TuMV and TMV (genus Tobamovirus) infections were highly reduced by knockdown of several ribosomal proteins (56).…”

The 3′ cap-independent translation element of Barley yellow dwarf virus binds eIF4F via the eIF4G subunit to initiate translation

“…These allow them to compete with host mRNAs and to avoid defense mechanisms that act at the level of translation. For example, uncapped RNAs of dicistroviruses (Sasaki and Nakashima 1999;Wilson et al 2000;Jan 2006), picornaviruses (Jang et al 1988;Pelletier and Sonenberg 1988), potyviruses (Levis and Astier-Manifacier 1993;Basso et al 1994;Niepel and Gallie 1999), and capped mRNAs of retroviruses (Herbreteau et al 2005;Nicholson et al 2006), recruit ribosomes at an internal ribosome entry site (IRES). The IRES structure engages the 40S ribosomal subunit, independent of the 59 end of the mRNA and brings it directly to close proximity of the initiation codon.…”

A Unique 5′ Translation Element Discovered in Triticum Mosaic Virus

“…Members of the Potyviridae family, which is comprised of ϳ190 plant viruses distributed in at least 6 genera (6,(11)(12)(13)(14), have a genome structure similar to that of the animal-infecting picornaviruses. Like the picornaviruses, potyviruses possess a viral protein covalently linked to their 5= end (VPg) instead of a 5= cap and are polyadenylated at their 3= end, and their genomic RNA encodes a single large polyprotein that is cleaved into several functional proteins (15).…”