Translational readthrough of the stop codon of the capsid protein (CP) open reading frame (ORF) is used by members of the to produce their minor capsid protein as a readthrough protein (RTP). The elements regulating RTP expression are not well understood, but they involve long-distance interactions between RNA domains. Using high-resolution mass spectrometry, glutamine and tyrosine were identified as the primary amino acids inserted at the stop codon of (PLRV) CP ORF. We characterized the contributions of a cytidine-rich domain immediately downstream and a branched stem-loop structure 600 to 700 nucleotides downstream of the CP stop codon. Mutations predicted to disrupt and restore the base of the distal stem-loop structure prevented and restored stop codon readthrough. Motifs in the downstream readthrough element (DRTE) are predicted to base pair to a site within 27 nucleotides (nt) of the CP ORF stop codon. Consistent with a requirement for this base pairing, the DRTE of was not compatible with the stop codon-proximal element of PLRV in facilitating readthrough. Moreover, deletion of the complementary tract of bases from the stop codon-proximal region or the DRTE of PLRV prevented readthrough. In contrast, the distance and sequence composition between the two domains was flexible. Mutants deficient in RTP translation moved long distances in plants, but fewer infection foci developed in systemically infected leaves. Selective 2'-hydroxyl acylation and primer extension (SHAPE) probing to determine the secondary structure of the mutant DRTEs revealed that the functional mutants were more likely to have bases accessible for long-distance base pairing than the nonfunctional mutants. This study reveals a heretofore unknown combination of RNA structure and sequence that reduces stop codon efficiency, allowing translation of a key viral protein. Programmed stop codon readthrough is used by many animal and plant viruses to produce key viral proteins. Moreover, such "leaky" stop codons are used in host mRNAs or can arise from mutations that cause genetic disease. Thus, it is important to understand the mechanism(s) of stop codon readthrough. Here, we shed light on the mechanism of readthrough of the stop codon of the coat protein ORFs of viruses in the by identifying the amino acids inserted at the stop codon and RNA structures that facilitate this "leakiness" of the stop codon. Members of the encode a C-terminal extension to the capsid protein known as the readthrough protein (RTP). We characterized two RNA domains in (PLRV), located 600 to 700 nucleotides apart, that are essential for efficient RTP translation. We further determined that the PLRV readthrough process involves both local structures and long-range RNA-RNA interactions. Genetic manipulation of the RNA structure altered the ability of PLRV to translate RTP and systemically infect the plant. This demonstrates that plant virus RNA contains multiple layers of information beyond the primary sequence and extends our understanding of stop codon readthrough. Strat...

We report the biological and structural characterization of umbravirus-like associated RNAs (ulaRNAs), a new category of coat-protein dependent subviral RNA replicons that infect plants. These RNAs encode an RNA-dependent RNA polymerase (RdRp) following a −1 ribosomal frameshift event, are 2.7–4.6 kb in length, and are related to umbraviruses, unlike similar RNA replicons that are related to tombusviruses. Three classes of ulaRNAs are proposed, with citrus yellow vein associated virus (CYVaV) placed in Class 2. With the exception of CYVaV, Class 2 and Class 3 ulaRNAs encode an additional open reading frame (ORF) with movement protein-like motifs made possible by additional sequences just past the RdRp termination codon. The full-length secondary structure of CYVaV was determined using Selective 2’ Hydroxyl Acylation analyzed by Primer Extension (SHAPE) structure probing and phylogenic comparisons, which was used as a template for determining the putative structures of the other Class 2 ulaRNAs, revealing a number of distinctive structural features. The ribosome recoding sites of nearly all ulaRNAs, which differ significantly from those of umbraviruses, may exist in two conformations and are highly efficient. The 3′ regions of Class 2 and Class 3 ulaRNAs have structural elements similar to those of nearly all umbraviruses, and all Class 2 ulaRNAs have a unique, conserved 3′ cap-independent translation enhancer. CYVaV replicates independently in protoplasts, demonstrating that the reported sequence is full-length. Additionally, CYVaV contains a sequence in its 3′ UTR that confers protection to nonsense mediated decay (NMD), thus likely obviating the need for umbravirus ORF3, a known suppressor of NMD. This initial characterization lays down a road map for future investigations into these novel virus-like RNAs.

Maize chlorotic mottle virus (MCMV) combines with a potyvirus in maize lethal necrosis disease (MLND), a serious emerging disease worldwide. To inform resistance strategies, we characterized the translation initiation mechanism of MCMV. We report that MCMV RNA contains a cap-independent translation element (CITE) in its 3′ untranslated region (UTR). The MCMV 3′ CITE (MTE) was mapped to nucleotides 4164-4333 in the genomic RNA. SHAPE probing revealed that the MTE is a distinct variant of the panicum mosaic virus-like 3′ CITE (PTE). Like the PTE, electrophoretic mobility shift assays (EMSAs) indicated that eukaryotic translation initiation factor 4E (eIF4E) binds the MTE despite the absence of a m7GpppN cap structure, which is normally required for eIF4E to bind RNA. Using a luciferase reporter system, mutagenesis to disrupt and restore base pairing revealed that the MTE interacts with the 5′ UTRs of both genomic RNA and subgenomic RNA1 via long-distance kissing stem-loop interaction to facilitate translation. The MTE stimulates a relatively low level of translation and has a weak, if any, pseudoknot, which is present in the most active PTEs, mainly because the MTE lacks the pyrimidine-rich tract that base pairs to a G-rich bulge to form the pseudoknot. However, most mutations designed to form a pseudoknot decreased translation activity. Mutations in the viral genome that reduced or restored translation prevented and restored virus replication, respectively, in maize protoplasts and in plants. In summary, the MTE differs from the canonical PTE but falls into a structurally related class of 3′ CITE. IMPORTANCE In the past decade, maize lethal necrosis disease has caused massive crop losses in East Africa. It has also emerged in China and parts of South America. Maize chlorotic mottle virus (MCMV) infection is required for this disease. While some tolerant maize lines have been identified, there are no known resistance genes that confer immunity to MCMV. In order to improve resistance strategies against MCMV, we focused on how the MCMV genome is translated, the first step of gene expression by all positive-strand RNA viruses. We identified a structure (cap-independent translation element) in the 3′ untranslated region of the viral RNA genome that allows the virus to usurp a host translation initiation factor, eIF4E, in a way that differs from host mRNA interactions with the translational machinery. This difference indicates eIF4E may be a soft target for engineering of – or breeding for – resistance to MCMV.

The student author, whose presentation of the scholarship herein was approved by the program of study committee, is solely responsible for the content of this dissertation. The Graduate College will ensure this dissertation is globally accessible and will not permit alterations after a degree is conferred.

Maize chlorotic mottle virus (MCMV) combines with a potyvirus in maize lethal necrosis disease (MLND), an emerging disease worldwide that often causes catastrophic yield loss. To inform resistance strategies, we characterized the translation initiation mechanism of MCMV. We report that, like other tombusvirids, MCMV RNA contains a cap-independent translation element (CITE) in its 3’ untranslated region (UTR). The MCMV 3’ CITE (MTE) was mapped to nucleotides 4164-4333 in the genomic RNA. SHAPE probing revealed that the MTE is a variant of the panicum mosaic virus-like 3’ CITE (PTE). Like the PTE, electrophoretic mobility shift assays (EMSAs) indicated that eukaryotic translation initiation factor 4E (eIF4E) binds the MTE despite the absence of a m7GpppN cap structure, which is normally required for eIF4E to bind RNA. The MTE interaction with eIF4E suggests eIF4E may be a soft target for engineered resistance to MCMV. Using a luciferase reporter system, mutagenesis to disrupt and restore base pairing revealed that the MTE interacts with the 5’ UTRs of both genomic RNA and the 3’-coterminal subgenomic RNA1 via long-distance kissing stem-loop base pairing to facilitate translation in wheat germ extract and in protoplasts. However, the MTE is a relatively weak stimulator of translation and has a weak, if any, pseudoknot, which is present in the most active PTEs. Most mutations designed to form a pseudoknot decreased translation activity. Mutations in the viral genome that reduced or restored translation prevented and restored virus replication, respectively, in maize protoplasts and in plants. We propose that MCMV, and some other positive strand RNA viruses, favors a weak translation element to allow highly efficient viral RNA synthesis.Author SummaryIn recent years, maize lethal necrosis disease has caused massive crop losses in East Africa and Ecuador. It has also emerged in East Asia. Maize chlorotic mottle virus (MCMV) infection is required for this disease. While some tolerant maize lines have been identified, there are no known resistance genes that confer full immunity to MCMV. In order to design better resistance strategies against MCMV, we focused on how the MCMV genome is translated, the first step of gene expression required for infection by all positive strand RNA viruses. We identified a structure (cap-independent translation element) in the 3’ untranslated region of the viral RNA genome that allows the virus to usurp a host translation initiation factor in a way that differs from host mRNA interactions with the translational machinery. This difference may guide engineering of – or breeding for – resistance to MCMV. Moreover, this work adds to the diversity of known eukaryotic translation initiation mechanisms, as it provides more information on mRNA structural features that permit noncanonical interaction with a translation factor. Finally, owing to the conflict between ribosomes translating and viral replicase copying viral RNA, we propose that MCMV has evolved a relatively weak translation element in order to permit highly efficient RNA synthesis, and that this replication-translation trade-off may apply to other positive strand RNA viruses.