The genomes of positive-strand RNA viruses undergo conformational shifts that complicate efforts to equate structures with function. We have initiated a detailed analysis of secondary and tertiary elements within the 3 end of Turnip crinkle virus (TCV) that are required for viral accumulation in vivo. MPGAfold, a massively parallel genetic algorithm, suggested the presence of five hairpins (H4a, H4b, and previously identified hairpins H4, H5, and Pr) and one H-type pseudoknot (⌿ 3 ) within the 3-terminal 194 nucleotides (nt). In vivo compensatory mutagenesis analyses confirmed the existence of H4a, H4b, ⌿ 3 and a second pseudoknot (⌿ 2 ) previously identified in a TCV satellite RNA. In-line structure probing of the 194-nt fragment supported the coexistence of H4, H4a, H4b, ⌿ 3 and a pseudoknot that connects H5 and the 3 end (⌿ 1 ). Stepwise replacements of TCV elements with the comparable elements from Cardamine chlorotic fleck virus indicated that the complete 142-nt 3 end, and subsets containing ⌿ 3 , H4a, and H4b or ⌿ 3 , H4a, H4b, H5, and ⌿ 2 , form functional domains for virus accumulation in vivo. A new 3-D molecular modeling protocol (RNA2D3D) predicted that H4a, H4b, H5, ⌿ 3 , and ⌿ 2 are capable of simultaneous existence and bears some resemblance to a tRNA. The related Japanese iris necrotic ring virus does not have comparable domains. These results provide a framework for determining how interconnected elements participate in processes that require 3 untranslated region sequences such as translation and replication.Replication of plus-strand RNA viruses initially requires translation of the genomic RNA to produce the virus-encoded, RNA-dependent RNA polymerase (RdRp) and any auxiliary viral proteins necessary for transcription. In a process that is poorly defined but likely dictated by viral and/or cellular factors, translation is terminated and the genomic RNA becomes available for reiterative synthesis of complementary strands, a process that requires membrane association (1, 2, 26). For some viruses, subsequent viral plus-strand synthesis occurs in virus-specific membrane invaginations known as spherules, which contain a limited number of minus-sense genomes and whose formation is induced by specific viral proteins (1,15,26). Although the process of producing viral progeny has been extensively studied using many different viral systems, it remains poorly understood. For example, fundamental questions, such as the role that conformational shifts in RNA structure play in switching the template from translation to replication, the proteins required to enact such events, and if cis-acting core promoters, enhancers, and repressors are organized into functional, interacting modules, remain virtually unanswered.Recent reports that portions of RNA viral genomes undergo conformational shifts to execute different functions (10, 13, 21, 28) complicate efforts to assign biological roles to groups of cis-acting elements that may not structurally coexist. The ability of viral RNAs to assume multiple conformation...

Precise temporal control is needed for RNA viral genomes to translate sufficient replication-required products before clearing ribosomes and initiating replication. A 39 translational enhancer in Turnip crinkle virus (TCV) overlaps an internal T-shaped structure (TSS) that binds to 60S ribosomal subunits. The higher-order structure in the region was examined through alteration of critical sequences revealing novel interactions between an H-type pseudoknot and upstream residues, and between the TSS and internal and terminal loops of an upstream hairpin. Our results suggest that the TSS forms a stable scaffold that allows for simultaneous interactions with external sequences through base pairings on both sides of its large internal symmetrical loop. Binding of TCV RNA-dependent RNA polymerase (RdRp) to the region potentiates a widespread conformational shift with substantial rearrangement of the TSS region, including the element required for efficient ribosome binding. Degrading the RdRp caused the RNA to resume its original conformation, suggesting that the initial conformation is thermodynamically favored. These results suggest that the 39 end of TCV folds into a compact, highly interactive structure allowing RdRp access to multiple elements including the 39 end, which causes structural changes that potentiate the shift between translation and replication.

Plus-strand RNA viruses without 5 caps require noncanonical mechanisms for ribosome recruitment. A translational enhancer in the 3 untranslated region (UTR) of Turnip crinkle virus (TCV) contains an internal T-shaped structure (TSS) that binds to 60S ribosomal subunits. We now report that the 63-nucleotide (nt) 5 UTR of TCV contains a 19-nt pyrimidine-rich element near the initiation codon that supports translation of an internal open reading frame (ORF) independent of upstream 5 UTR sequences. Addition of 80S ribosomes to the 5 UTR reduced the flexibility of the polypyrimidine residues and generated a toeprint consistent with binding to this region. Binding of salt-washed 40S ribosomal subunits was reduced 6-fold when the pyrimidinerich sequence was mutated. 40S subunit binding generated the same toeprint as 80S ribosomes but also additional ones near the 5 end. Generation of out-of-frame AUGs upstream of the polypyrimidine region reduced translation, which suggests that 5-terminal entry of 40S subunits is followed by scanning and that the polypyrimidine region is needed for an alternative function that requires ribosome binding. No evidence for RNA-RNA interactions between 5 and 3 sequences was found, suggesting that TCV utilizes an alternative means for circularizing its genome. Combining 5 and 3 UTR fragments in vitro had no discernible effect on the structures of the RNAs. In contrast, when 80S ribosomes were added to both fragments, structural changes were found in the 5 UTR polypyrimidine tract that were not evident when ribosomes interacted with the individual fragments. This suggests that ribosomes can promote an interaction between the 5 and 3 UTRs of TCV.

Circularization of cellular mRNAs is a key event prior to translation initiation. We report that efficient translation of Saguaro cactus virus (SCV) requires a 3' translational enhancer (PTE) located partially in coding sequences. Unlike a similar PTE reported in the 3' UTR of Pea enation mosaic virus that does not engage in an RNA:RNA interaction (Wang Z. et al., J. Biol. Chem. 284, 14189–14202, 2009), the SCV PTE participates in long distance RNA:RNA interactions with hairpins located in the p26 ORF and in the 5' UTR of one subgenomic RNA. At least two additional RNA:RNA interactions are also present, one of which involves the p26 initiation codon. Similar PTE can be found in six additional carmoviruses that can putatively form long-distance interactions with 5' hairpins located in comparable positions.

Nonsense-mediated decay (NMD) is a host RNA control pathway that removes aberrant transcripts with long 3’ untranslated regions (UTRs) due to premature termination codons (PTCs) that arise through mutation or defective splicing. To maximize coding potential, RNA viruses often contain internally located stop codons that should also be prime targets for NMD. Using an agroinfiltration-based NMD assay in Nicotiana benthamiana, we identified two segments conferring NMD-resistance in the carmovirus Turnip crinkle virus (TCV) genome. The ribosome readthrough structure just downstream of the TCV p28 termination codon stabilized an NMD-sensitive reporter as did a frameshifting element from umbravirus Pea enation mosaic virus. In addition, a 51-nt unstructured region (USR) at the beginning of the TCV 3’ UTR increased NMD-resistance 3-fold when inserted into an unrelated NMD-sensitive 3’ UTR. Several additional carmovirus 3’ UTRs also conferred varying levels of NMD resistance depending on the construct despite no sequence similarity in the analogous region. Instead, these regions displayed a marked lack of RNA structure immediately following the NMD-targeted stop codon. NMD-resistance was only slightly reduced by conversion of 19 pyrimidines in the USR to purines, but resistance was abolished when a 2-nt mutation was introduced downstream of the USR that substantially increased the secondary structure in the USR through formation of a stable hairpin. The same 2-nt mutation also enhanced the NMD susceptibility of a subgenomic RNA expressed independently of the genomic RNA. The conserved lack of RNA structure among most carmoviruses at the 5’ end of their 3’ UTR could serve to enhance subgenomic RNA stability, which would increase expression of the encoded capsid protein that also functions as the RNA silencing suppressor. These results demonstrate that the TCV genome has features that are inherently NMD-resistant and these strategies could be widespread among RNA viruses and NMD-resistant host mRNAs with long 3’ UTRs.

Protein subunit vaccines present a compelling new area of research for control of tuberculosis (TB). Based on the interaction between Mycobacterium tuberculosis and its host, five stage-specific antigens of M. tuberculosis that participate in TB pathogenesis—Rv1813, Rv2660c, Ag85B, Rv2623, and HspX—were selected. These antigens were verified to be recognized by T cells from a total of 42 whole blood samples obtained from active TB patients, patients with latent TB infections (LTBIs), and healthy control donors. The multistage polyprotein A1D4 was developed using the selected five antigens as a potentially more effective novel subunit vaccine. The immunogenicity and protective efficacy of A1D4 emulsified in the adjuvant MTO [monophosphoryl lipid A (MPL), trehalose-6,6′-dibehenate (TDB), components of MF59] was compared with Bacillus Calmette-Guerin (BCG) in C57BL/6 mice. Our results demonstrated that A1D4/MTO could provide more significant protection against M. tuberculosis infection than the PBS control or MTO adjuvant alone judging from the A1D4-specific Th1-type immune response; however, its efficacy was inferior to BCG as demonstrated by the bacterial load in the lung and spleen, and by the pathological changes in the lung. Antigen-specific single IL-2-secreting cells and different combinations with IL-2-secreting CD4+ T cells were beneficial and correlated with BCG vaccine-induced protection against TB. Antigen-specific IFN-γ+IL-2+ CD4+ T cells were the only effective biomarker significantly induced by A1D4/MTO. Among all groups, A1D4/MTO immunization also conferred the highest number of antigen-specific single IFN-γ+ and IFN-γ+TNF-α+ CD4+ T cells, which might be related to the antigen load in vivo, and single IFN-γ+ CD8+ T cells by mimicking the immune patterns of LTBIs or curable TB patients. Our strategy seems promising for the development of a TB vaccine based on multistage antigens, and subunit antigen A1D4 suspended in MTO adjuvant warrants preclinical evaluation in animal models of latent infection and may boost BCG vaccination.

Plus-strand RNA viruses serve as templates for translation and then transcription by newly synthesized RdRp. A ribosome-binding tRNA-shaped structure (TSS) and upstream hairpin H4 in the 3′ UTR of Turnip crinkle virus (TCV) play key roles in translation and transcription. Second-site mutations generated to compensate for altering the critical asymmetric internal loop of H4 included a three-to-two base alteration in the terminal loop of a 3′ proximal hairpin (Pr) located downstream of the TSS. Unlike the non-deleterious three base alteration, single mutations in Pr loop were detrimental for RdRp transcription while enhancing translation and RdRp binding. One deleterious mutation in the Pr loop altered the structures of both the TSS and H4. These complex interactions in the 3′ UTR support a compact structural arrangement likely permitting RdRp access to a number of residues within a 195-base region including the 3′ end that are necessary for efficient transcription initiation.

The majority of the 3= untranslated region (UTR) of Turnip crinkle virus (TCV) was previously identified as forming a highly interactive structure with a ribosome-binding tRNA-shaped structure (TSS) acting as a scaffold and undergoing a widespread conformational shift upon binding to RNA-dependent RNA polymerase (RdRp). Tertiary interactions in the region were explored by identifying two highly detrimental mutations within and adjacent to a hairpin H4 upstream of the TSS that reduce translation in vivo and cause identical structural changes in the loop of the 3= terminal hairpin Pr. Second-site changes that compensate for defects in translation/accumulation and reverse the structural differences in the Pr loop were found in the Pr stem, as well as in a specific stem within the TSS and within the capsid protein (CP) coding region, suggesting that the second-site changes were correcting a conformational defect and not restoring specific base pairing. The RdRp-mediated conformational shift extended upstream through this CP open reading frame (ORF) region after bypassing much of an intervening, largely unstructured region, supporting a connection between 3= elements and coding region elements. These data suggest that the Pr loop, TSS, and H4 are central elements in the regulation of translation and replication in TCV and allow for development of an RNA interactome that maps the higher-order structure of a postulated RNA domain within the 3= region of a plus-strand RNA virus.T he genomes of positive-strand RNA viruses control fundamental processes such as translation and replication through multiple RNA elements that interact dynamically with each other and with viral and host proteins. Upon entry into host cells, the viral genomic RNA (gRNA) is recognized as a template by the host translational apparatus for production of replication-associated proteins, which combine with an increasingly diverse variety of host factors to synthesize negative-strand RNAs that then serve as the templates for synthesis of progeny positive-strand RNAs (1,7,8,19). The widespread positioning of cis-elements that function in translation and/or replication requires short-range or long-range bridges within the RNA to deliver bound elements to locations where the specific processes initiate.Understanding how functional, dynamic RNA structures regulate viral processes requires a detailed knowledge of the topology of important regions of the RNA genome and the canonical and noncanonical tertiary interactions that connect various elements. Attempts to decipher networks of short-and long-distance RNA-RNA interactions have been limited to a few viruses. For dengue virus (DENV), several sets of overlapping 5=-and 3=-interacting sequences have been identified that control the balance between linear and circular forms of the genome, which is critical for replication but not translation (13,38). Tomato bushy stunt virus (TBSV) requires a complex network of long-distance RNA-RNA interactions to promote replication, subgenomic RNA (sgRNA) synthesis, tr...