The internal ribosome entry site (IRES) of the hepatitis C virus (HCV) RNA is known to interact with the 40S ribosomal subunit alone, in the absence of any additional initiation factors or Met-tRNAi. Previous work from this laboratory on the 80S and 48S ribosomal initiation complexes involving the HCV IRES showed that stem-loop III, the pseudoknot domain, and some coding sequence were protected from pancreatic RNase digestion. Stem-loop II is never protected by these complexes. Furthermore, there is no prior evidence reported showing extensive direct binding of stem-loop II to ribosomes or subunits. Using direct analysis of RNase-protected HCV IRES domains bound to 40S ribosomal subunits, we have determined that stem-loops II and III and the pseudoknot of the HCV IRES are involved in this initial binding step. The start AUG codon is only minimally protected. The HCV-40S subunit binary complex thus involves recognition and binding of stem-loop II, revealing its role in the first step of a multistep initiation process that may also involve rearrangement of the bound IRES RNA as it progresses.

Enhanced translation of giardiavirus-luciferase chimeric mRNA in Giardia lamblia requires the initial 264-nt viral capsid coding region as a putative internal ribosomal entry site (IRES). Essential structural elements in this site include (1) a downstream box (DB) complementary to the anti-DB at the 3′ end of 16S-like rRNA, (2) stem-loops I, II, III, and IVA, and (3) a pentanucleotide 5′-UCUCC-3′ immediately downstream from stem loop IVA. A search for the structural role of the pentanucleotide suggested that it may form a pseudoknot with another pentanucleotide 5′-GGAGA-3′ in loop II. Alteration of the two pentanucleotides by site-directed mutagenesis resulted in a drastic reduction in translation of the transcript. But the loss was recovered by compensatory changes in the two sequences, suggesting Watson–Crick base pairings between them. Results from in vitro enzymatic and chemical structural probing supported the presence of such a pseudoknot 143 nt downstream from the initiation codon. Minor repositioning of this codon led invariably to a complete loss of translation, suggesting that the initiation site is confined within a rigid position defined by all the structural elements in the IRES including the pseudoknot. This is the first pseudoknot of its kind shown to play an important role in a downstream IRES of a viral transcript. The finding is particularly interesting because it could reflect a unique feature of translation initiation in Giardia, which is known to have exceedingly short (1–6 nt) 5′ untranslated regions in its mRNAs.

Translation of hepatitis C virus (HCV) RNA is initiated via the internal ribosome entry site (IRES), located within the 5′ untranslated region. Although the secondary structure of this element has been predicted, little information on the tertiary structure is available. Here we report the first structural characterization of the HCV IRES using electron microscopy. In vitro transcribed RNA appeared as particles with characteristic morphology and gold labeling using a specific oligonucleotide confirmed them to be HCV IRES. Dimerization of the IRES by hybridization with tandem repeat oligonucleotides allowed the identification of domain III and an assignment of domains II and IV to distinct regions within the molecule. Using immunogold labeling, the pyrimidine tract binding protein (PTB) was shown to bind to domain III. Structure–function relationships based on the flexible hinge between domains II and III are suggested. Finally, the architecture of the HCV IRES was seen to be markedly different from that of a picornavirus, foot-and-mouth disease virus (FMDV).

Many viruses and certain cellular mRNAs initiate protein synthesis from a highly structured RNA sequence in the 5′ untranslated region, called the internal ribosome entry site (IRES). In hepatitis C virus (HCV), the IRES RNA functionally replaces several large initiation factor proteins by directly recruiting the 43S particle. Using quantitative binding assays, modification interference of binding, and chemical and enzymatic footprinting experiments, we show that three independently folded tertiary structural domains in the IRES RNA make intimate contacts to two purified components of the 43S particle: the 40S ribosomal subunit and eukaryotic initiation factor 3 (eIF3). We measure the affinity and demonstrate the specificity of these interactions for the first time and show that the high affinity interaction of IRES RNA with the 40S subunit drives formation of the IRES RNA[bull ]40S[bull ]eIF3 ternary complex. Thus, the HCV IRES RNA recruits 43S particles in a mode distinct from both eukaryotic cap-dependent and prokaryotic ribosome recruitment strategies, and is architecturally and functionally unique from other large folded RNAs that have been characterized to date.