Investigating molecular mechanisms of 2A-stimulated ribosomal pausing and frameshifting in Theilovirus

Programmable modulation of ribosomal frameshifting by mRNA targeting CRISPR-Cas12a system

Minus 1 programmed ribosomal frameshifting (-1 PRF) is a conserved translational regulation event essential for critical biological processes, including the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) replication. Efficient trans-modulation of the structured RNA element crucial to -1 PRF will endow the therapeutic application. Here, we demonstrate that CRISPR RNA can stimulate efficient -1 PRF. Assembled CRISPR-Cas12a, but not CRISPR-Cas9, complex further enhances -1 PRF efficiency through its higher capacity to stall translating ribosomes. We additionally perform CRISPR-Cas12a targeting to impair the SARS-CoV-2 frameshifting pseudoknot structure via a focused screening. We demonstrate that targeting CRISPR-Cas12a results in more than 70% suppression of -1 PRF in vitro and about 50% suppression in mammalian cells. Our results show the expanded function of the CRISPR-Cas12 system in modulating -1 PRF efficiency through stalling ribosomes and deforming frameshifting stimulatory signals, which could serve as a new strategy for future coronavirus pandemics.

-1 Programmed ribosomal frameshifting in Class 2 umbravirus-like RNAs uses multiple long-distance interactions to shift between active and inactive structures and destabilize the frameshift stimulating element

Plus-strand RNA viruses frequently employ -1 programmed ribosomal frameshifting (-1 PRF) to maximize their coding capacity. Ribosomes can frameshift at a slippery sequence if progression is impeded by a frameshift stimulating element (FSE), which is generally a stable, complex, dynamic structure with multiple conformations that contribute to the efficiency of -1 PRF. As FSE are usually analyzed separate from the viral genome, little is known about cis-acting long-distance interactions. Using full-length genomic RNA of umbravirus-like (ula)RNA citrus yellow vein associated virus (CY1) and translation in wheat germ extracts, six tertiary interactions were found associated with the CY1 FSE that span nearly three-quarters of the 2.7 kb genomic RNA. All six tertiary interactions are conserved in other Class 2 ulaRNAs and two are conserved in all ulaRNAs. Two sets of interactions comprise local and distal pseudoknots that involve overlapping FSE nucleotides and thus are structurally incompatible, suggesting that Class 2 FSEs assume multiple conformations. Importantly, two long-distance interactions connect with sequences on opposite sides of the critical FSE central stem, which would unzip the stem and destabilize the FSE. These latter interactions could allow a frameshifting ribosome to translate through a structurally disrupted upstream FSE that no longer blocks ribosome progression.

Structural and Functional Insights into Viral Programmed Ribosomal Frameshifting

Protein synthesis by the ribosome is the final stage of biological information transfer and represents an irreversible commitment to gene expression. Accurate translation of messenger RNA is therefore essential to all life, and spontaneous errors by the translational machinery are highly infrequent (∼1/100,000 codons). Programmed -1 ribosomal frameshifting (-1PRF) is a mechanism in which the elongating ribosome is induced at high frequency to slip backward by one nucleotide at a defined position and to continue translation in the new reading frame. This is exploited as a translational regulation strategy by hundreds of RNA viruses, which rely on -1PRF during genome translation to control the stoichiometry of viral proteins. While early investigations of -1PRF focused on virological and biochemical aspects, the application of X-ray crystallography and cryo-electron microscopy (cryo-EM), and the advent of deep sequencing and single-molecule approaches have revealed unexpected structural diversity and mechanistic complexity. Molecular players from several model systems have now been characterized in detail, both in isolation and, more recently, in the context of the elongating ribosome. Here we provide a summary of recent advances and discuss to what extent a general model for -1PRF remains a useful way of thinking.

Decoding and Recoding of mRNA Sequences by the Ribosome

Faithful translation of messenger RNA (mRNA) into protein is essential to maintain protein homeostasis in the cell. Spontaneous translation errors are very rare due to stringent selection of cognate aminoacyl transfer RNAs (tRNAs) and the tight control of the mRNA reading frame by the ribosome. Recoding events, such as stop codon readthrough, frameshifting, and translational bypassing, reprogram the ribosome to make intentional mistakes and produce alternative proteins from the same mRNA. The hallmark of recoding is the change of ribosome dynamics. The signals for recoding are built into the mRNA, but their reading depends on the genetic makeup of the cell, resulting in cell-specific changes in expression programs. In this review, I discuss the mechanisms of canonical decoding and tRNA-mRNA translocation; describe alternative pathways leading to recoding; and identify the links among mRNA signals, ribosome dynamics, and recoding.

Abracadabra, One Becomes Two: The Importance of Context in Viral -1 Programmed Ribosomal Frameshifting

The constrained nature of viral genomes has allowed a translational sleight of hand known as −1 Programmed Ribosomal Frameshifting (−1 PRF) to flourish. Numerous studies have sought to tease apart the mechanisms and implications of −1PRF utilizing a few techniques. The dual-luciferase assay and ribosomal profiling have driven the PRF field to make great advances; however, the use of these assays means that the full impact of the genomic and cellular context on −1 PRF is often lost. Here, we discuss how the Minimal Frameshifting Element (MFE) and its constraints can hide contextual effects on −1 PRF. We review how sequence elements proximal to the traditionally defined MFE, such as the coronavirus attenuator sequence, can affect the observed rates of −1 PRF. Further, the MFE-based approach fully obscured −1 PRF in Barley yellow dwarf virus and would render the exploration of −1 PRF difficult in Porcine reproductive and respiratory syndrome virus, Encephalomyocarditis virus, Theiler’s murine encephalomyelitis virus, and Sindbis virus. Finally, we examine how the cellular context of tRNA abundance, miRNAs, and immune response elements can affect −1 PRF. The use of MFE was instrumental in establishing the basic foundations of PRF; however, it has become clear that the contextual impact on −1 PRF is no longer the exception so much as it is the rule and argues for new approaches to study −1PRF that embrace context. We therefore urge our field to expand the strategies and methods used to explore −1 PRF.

Ribosome profiling of porcine reproductive and respiratory syndrome virus reveals novel features of viral gene expression

The arterivirus porcine reproductive and respiratory syndrome virus (PRRSV) causes significant economic losses to the swine industry worldwide. Here we apply ribosome profiling (RiboSeq) and parallel RNA sequencing (RNASeq) to characterise the transcriptome and translatome of both species of PRRSV and to analyse the host response to infection. We calculated programmed ribosomal frameshift (PRF) efficiency at both sites on the viral genome. This revealed the nsp2 PRF site as the second known example where temporally regulated frameshifting occurs, with increasing -2 PRF efficiency likely facilitated by accumulation of the PRF-stimulatory viral protein, nsp1β. Surprisingly, we find that PRF efficiency at the canonical ORF1ab frameshift site also increases over time, in contradiction of the common assumption that RNA structure-directed frameshift sites operate at a fixed efficiency. This has potential implications for the numerous other viruses with canonical PRF sites. Furthermore, we discovered several highly translated additional viral ORFs, the translation of which may be facilitated by multiple novel viral transcripts. For example, we found a highly expressed 125-codon ORF overlapping nsp12, which is likely translated from novel subgenomic RNA transcripts that overlap the 3' end of ORF1b. Similar transcripts were discovered for both PRRSV-1 and PRRSV-2, suggesting a potential conserved mechanism for temporally regulating expression of the 3'-proximal region of ORF1b. We also identified a highly translated, short upstream ORF in the 5' UTR, the presence of which is highly conserved amongst PRRSV-2 isolates. These findings reveal hidden complexity in the gene expression programmes of these important nidoviruses.

Thinking Outside the Frame: Impacting Genomes Capacity by Programmed Ribosomal Frameshifting

Translation facilitates the transfer of the genetic information stored in the genome via messenger RNAs to a functional protein and is therefore one of the most fundamental cellular processes. Programmed ribosomal frameshifting is a ubiquitous alternative translation event that is extensively used by viruses to regulate gene expression from overlapping open reading frames in a controlled manner. Recent technical advances in the translation field enabled the identification of precise mechanisms as to how and when ribosomes change the reading frame on mRNAs containing cis-acting signals. Several studies began also to illustrate that trans-acting RNA modulators can adjust the timing and efficiency of frameshifting illuminating that frameshifting can be a dynamically regulated process in cells. Here, we intend to summarize these new findings and emphasize how it fits in our current understanding of PRF mechanisms as previously described.

Insights from structural studies of the Cardiovirus 2A protein

Cardioviruses are single-stranded RNA viruses of the family Picornaviridae. In addition to being the first example of internal ribosome entry site (IRES) utilization, cardioviruses also employ a series of alternative translation strategies, such as Stop-Go translation and programmed ribosome frameshifting. Here, we focus on cardiovirus 2A protein, which is not only a primary virulence factor, but also exerts crucial regulatory functions during translation, including activation of viral ribosome frameshifting and inhibition of host cap-dependent translation. Only recently, biochemical and structural studies have allowed us to close the gaps in our knowledge of how cardiovirus 2A is able to act in diverse translation-related processes as a novel RNA-binding protein. This review will summarize these findings, which ultimately may lead to the discovery of other RNA-mediated gene expression strategies across a broad range of RNA viruses.