A phylogenetically conserved hairpin-type 3' untranslated region pseudoknot functions in coronavirus RNA replication

Diverse effects of coronavirus-defective viral genomes on the synthesis of IFNβ and ISG15 mRNAs and coronavirus replication

BACKGROUND

The mechanism by which coronavirus-defective viral genomes (DVGs) affect coronavirus and host cells during infection remains unclear. A variety of DVGs with different RNA structures can be synthesized from coronavirus-infected cells, and these DVGs can also encode proteins. Consequently, in the present study, we first dissected the effects of individual DVGs on the synthesis of IFNβ and ISG15 mRNAs at the RNA, protein and combined levels, and then examined whether different coronavirus-DVGs have different effects on the synthesis of IFNβ and ISG15 mRNAs and coronavirus replication both individually and collectively under different infection conditions.

METHODS

To dissect the effects of individual DVGs on the synthesis of IFNβ and ISG15 mRNAs at the RNA, protein and combined levels, DVG 2.2 and DVG 5.1, which were previously identified in coronavirus-infected cells, and their mutants were constructed followed by transfection. Western blot and RT‒qPCR were used to detect the synthesis of protein and to quantify the synthesis of IFNβ and ISG15 mRNAs, respectively. To examined whether different coronavirus-DVGs have different effects on the synthesis of IFNβ and ISG15 mRNAs and coronavirus replication both individually and collectively under different infection conditions, different naturally occurring DVGs were selected and constructed followed by transfection after or before coronavirus infection and by RT‒qPCR and hemagglutination assay.

RESULTS

These results suggested that (i) coronavirus-DVGs at the RNA, protein and combined levels have different effects on the synthesis of IFNβ and ISG15 mRNAs, (ii) coronavirus-DVGs can inhibit coronavirus replication at least partly through interferon signaling and (iii) different DVGs have different effects on the synthesis of IFNβ and ISG15 mRNAs and coronavirus replication both individually and collectively under different infection conditions.

CONCLUSIONS

Coronavirus replication can be regulated by diverse coronavirus-derived DVGs at least partly through innate immunity. Such regulation may contribute to the pathogenesis of coronavirus. The DVG populations in coronavirus-infected cells with the ability to inhibit coronavirus replication are expected to be potential resources for the identification of antivirals at the level of RNA, protein or in combination, and the methods used in the current study can be used as a platform for this purpose.

Linking the function of cis-acting RNA elements to coronavirus replication using interactomes

RNA structures that are functionally important are defined as cis-acting RNA elements because their functions cannot be compensated for in trans. The cis-acting RNA elements in the 3' UTR of coronaviruses are important for replication; however, the mechanism linking the cis-acting RNA elements to their replication function remains to be established. In the present study, a comparison of the biological processes of the interactome and the replication efficiency between the 3' UTR cis-acting RNA elements in coronaviruses, including severe acute respiratory syndrome coronavirus 2, suggests that (i) the biological processes, including translation, protein folding and protein stabilization, derived from the analysis of the cis-acting RNA element interactome and (ii) the architecture of the cis-acting RNA elements and their interactomes are highly correlated with coronavirus replication. In addition, alteration of the interactome using the compound 5-benzyloxygramine can cause reduced coronavirus replication, reinforcing the connection between cis-acting RNA elements and replication by interactome. Together, these results link cis-acting RNA elements to the coronavirus replication and establish a model to analyse the cis-acting RNA elements in the replication of RNA viruses by interactome.

Untangling the pseudoknots of SARS-CoV-2: Insights into structural heterogeneity and plasticity

Since the onset of the COVID-19 pandemic, one productive area of research has focused on the intricate two- and three-dimensional structures taken on by SARS-CoV-2's RNA genome. These structures control essential viral processes, making them tempting targets for therapeutic intervention. This review focuses on two such structured regions, the frameshift stimulation element (FSE), which controls the translation of viral protein, and the 3' untranslated region (3' UTR), which is thought to regulate genome replication. For the FSE, we discuss its canonical pseudoknot's threaded and unthreaded topologies, as well as the diversity of competing two-dimensional structures formed by local and long-distance base pairing. For the 3' UTR, we review the evidence both for and against the formation of its replication-enabling pseudoknot.

Characterization of the SARS-CoV-2 Genome 3'-Untranslated Region Interactions with Host MicroRNAs

The 2019 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has marked the spread of a novel human coronavirus. SARS-CoV-2 has exhibited increased disease severity and immune evasion across its variants, and the molecular mechanisms behind these phenomena remain largely unknown. Conserved elements of the viral genome, such as secondary structures within the 3'-untranslated region (UTR), could prove crucial in furthering our understanding of the host-virus interface. Analysis of the SARS-CoV-2 viral genome 3'-UTR revealed the potential for host microRNA (miR) binding sites, allowing for sequence-specific interactions. In this study, we demonstrate that the SARS-CoV-2 genome 3'-UTR binds the host cellular miRs miR-34a-5p, miR-34b-5p, and miR-760-3p in vitro. Native gel electrophoresis and steady-state fluorescence spectroscopy were utilized to biophysically characterize the binding of these miRs to their predicted sites within the SARS-CoV-2 genome 3'-UTR. Additionally, we investigated 2'-fluoro-d-arabinonucleic acid (FANA) analogs as competitive binding inhibitors for these interactions. These miRs modulate the translation of granulin (GRN), interleukin-6 (IL-6), and the IL-6 receptor (IL-6R), all of which are key modulators and activators of JAK/STAT3 signaling and are implicated in regulation of the immune response. Thus, we propose that hijacking of these miRs by SARS-CoV-2 could identify a mechanism of host immune modulation by the virus. The mechanisms detailed in this study have the potential to drive the development of antiviral treatments for SARS-CoV-2, through direct targeting of the virus-host interface.

Genomic Evolution Strategy in SARS-CoV-2 Lineage B: Coevolution of Cis Elements

In the SARS-CoV-2 lineage, RNA elements essential for its viral life cycle, including genome replication and gene expression, have been identified. Still, the precise structures and functions of these RNA regions in coronaviruses remain poorly understood. This lack of knowledge points out the need for further research to better understand these crucial aspects of viral biology and, in time, prepare for future outbreaks. In this research, the in silico analysis of the cis RNA structures that act in the alpha-, beta-, gamma-, and deltacoronavirus genera has provided a detailed view of the presence and adaptation of the structures of these elements in coronaviruses. The results emphasize the importance of these cis elements in viral biology and their variability between different viral variants. Some coronavirus variants in some groups, depending on the cis element (stem-loop1 and -2; pseudoknot stem-loop1 and -2, and s2m), exhibited functional adaptation. Additionally, the conformation flexibility of the s2m element in the SARS variants was determined, suggesting a coevolution of this element in this viral group. The variability in secondary structures suggests genomic adaptations that may be related to replication processes, genetic regulation, as well as the specific pathogenicity of each variant. The results suggest that RNA structures in coronaviruses can adapt and evolve toward different viral variants, which has important implications for viral adaptation, pathogenicity, and future therapeutic strategies.

NMR Studies of Genomic RNA in 3' Untranslated Region Unveil Pseudoknot Structure that Initiates Viral RNA Replication in SARS-CoV-2

In the 3' untranslated region of the SARS-CoV-2 virus RNA genome, genomic RNA replication is initiated in the highly conserved region called 3'PK, containing three stem structures (P1pk, P2, and P5). According to one proposed mechanism, P1pk and distal P2 stems switch their structure to a pseudoknot through base-pairing, thereby initiating transcription by recruiting RNA-dependent RNA polymerase complexed with nonstructural proteins (nsp)7 and nsp8. However, experimental evidence of pseudoknot formation or structural switching is unavailable. Using SARS-CoV-2 3'PK fragments, we show that 3'PK adopted stem-loop and pseudoknot forms in a mutually exclusive manner. When P1pk and P2 formed a pseudoknot, the P5 stem, which includes a sequence at the 3' end, exited from the stem-loop structure and opened up. Interaction with the nsp7/nsp8 complex destabilized the stem-loop form but did not alter the pseudoknot form. These results suggest that the interaction between the pseudoknot and nsp7/nsp8 complex transformed the 3' end of viral genomic RNA into single-stranded RNA ready for synthesis, presenting the unique pseudoknot structure as a potential pharmacological target.

Unveiling hidden structural patterns in the SARS-CoV-2 genome: Computational insights and comparative analysis

SARS-CoV-2, the causative agent of COVID-19, is known to exhibit secondary structures in its 5' and 3' untranslated regions, along with the frameshifting stimulatory element situated between ORF1a and 1b. To identify additional regions containing conserved structures, we utilized a multiple sequence alignment with related coronaviruses as a starting point. We applied a computational pipeline developed for identifying non-coding RNA elements. Our pipeline employed three different RNA structural prediction approaches. We identified forty genomic regions likely to harbor structures, with ten of them showing three-way consensus substructure predictions among our predictive utilities. We conducted intracomparisons of the predictive utilities within the pipeline and intercomparisons with four previously published SARS-CoV-2 structural datasets. While there was limited agreement on the precise structure, different approaches seemed to converge on regions likely to contain structures in the viral genome. By comparing and combining various computational approaches, we can predict regions most likely to form structures, as well as a probable structure or ensemble of structures. These predictions can be used to guide surveillance, prophylactic measures, or therapeutic efforts. Data and scripts employed in this study may be found at https://doi.org/10.5281/zenodo.8298680.

Targeting the conserved coronavirus octamer motif GGAAGAGC is a strategy for the development of coronavirus vaccine

BACKGROUND

Coronaviruses are pathogens of humans and animals that cause widespread and costly diseases. The development of effective strategies to combat the threat of coronaviruses is therefore a top priority. The conserved coronavirus octamer motif 5'GGAAGAGC3' exists in the 3' untranslated region of all identified coronaviruses. In the current study, we aimed to examine whether targeting the coronavirus octamer motif GGAAGAGC is a promising approach to develop coronavirus vaccine.

METHODS

Plaque assays were used to determine the titers of mouse hepatitis virus (MHV)-A59 octamer mutant (MHVoctm) and wild-type (wt) MHV-A59 (MHVwt). Western blotting was used for the determination of translation efficiency of MHVoctm and MHVwt. Plaque assays and RT-qPCR were employed to examine whether MHVoctm was more sensitive to interferon treatment than MHVwt. Weight loss, clinical signs, survival rate, viral RNA detection and histopathological examination were used to evaluate whether MHVoctm was a vaccine candidate against MHVwt infection in BALB/c mice.

RESULTS

In this study, we showed that (i) the MHVoctm with mutation of coronavirus octamer was able to grow to high titers but attenuated in mice, (ii) with the reduced multiplicity of infection (MOI), the difference in gene expression between MHVoctm and MHVwt became more evident in cultured cells, (iii) MHVoctm was more sensitive to interferon treatment than MHVwt and (iv) mice inoculated with MHVoctm were protected from MHVwt infection.

CONCLUSIONS

Based on the results obtained from cultured cells, it was suggested that the synergistic effects of octamer mutation, multiplicity of infection and immune response may be a mechanism explaining the distinct phenotypes of octamer-mutated coronavirus in cell culture and mice. In addition, targeting the conserved coronavirus octamer motif is a strategy for development of coronavirus vaccine. Since the conserved octamer exists in all coronaviruses, this strategy of targeting the conserved octamer motif can also be applied to other human and animal coronaviruses for the development of coronavirus vaccines, especially the emergence of novel coronaviruses such as SARS-CoV-2, saving time and cost for vaccine development and disease control.

The coronavirus recombination pathway

Recombination is thought to be a mechanism that facilitates cross-species transmission in coronaviruses, thus acting as a driver of coronavirus spillover and emergence. Despite its significance, the mechanism of recombination is poorly understood, limiting our potential to estimate the risk of novel recombinant coronaviruses emerging in the future. As a tool for understanding recombination, here, we outline a framework of the recombination pathway for coronaviruses. We review existing literature on coronavirus recombination, including comparisons of naturally observed recombinant genomes as well as in vitro experiments, and place the findings into the recombination pathway framework. We highlight gaps in our understanding of coronavirus recombination illustrated by the framework and outline how further experimental research is critical for disentangling the molecular mechanism of recombination from external environmental pressures. Finally, we describe how an increased understanding of the mechanism of recombination can inform pandemic predictive intelligence, with a retrospective emphasis on SARS-CoV-2.

Deletion of the s2m RNA Structure in the Avian Coronavirus Infectious Bronchitis Virus and Human Astrovirus Results in Sequence Insertions

Coronaviruses infect a wide variety of host species, resulting in a range of diseases in both humans and animals. The coronavirus genome consists of a large positive-sense single-stranded molecule of RNA containing many RNA structures. One structure, denoted s2m and consisting of 41 nucleotides, is located within the 3' untranslated region (3' UTR) and is shared between some coronavirus species, including infectious bronchitis virus (IBV), severe acute respiratory syndrome coronavirus (SARS-CoV), and SARS-CoV-2, as well as other pathogens, including human astrovirus. Using a reverse genetic system to generate recombinant viruses, we investigated the requirement of the s2m structure in the replication of IBV, a globally distributed economically important Gammacoronavirus that infects poultry causing respiratory disease. Deletion of three nucleotides predicted to destabilize the canonical structure of the s2m or the deletion of the nucleotides corresponding to s2m impacted viral replication in vitro. In vitro passaging of the recombinant IBV with the s2m sequence deleted resulted in a 36-nucleotide insertion in place of the deletion, which was identified to be composed of a duplication of flanking sequences. A similar result was observed following serial passage of human astrovirus with a deleted s2m sequence. RNA modeling indicated that deletion of the nucleotides corresponding to the s2m impacted other RNA structures present in the IBV 3' UTR. Our results indicated for both IBV and human astrovirus a preference for nucleotide occupation in the genome location corresponding to the s2m, which is independent of the specific s2m sequence. IMPORTANCE Coronaviruses infect many species, including humans and animals, with substantial effects on livestock, particularly with respect to poultry. The coronavirus RNA genome consists of structural elements involved in viral replication whose roles are poorly understood. We investigated the requirement of the RNA structural element s2m in the replication of the Gammacoronavirus infectious bronchitis virus, an economically important viral pathogen of poultry. Using reverse genetics to generate recombinant IBVs with either a disrupted or deleted s2m, we showed that the s2m is not required for viral replication in cell culture; however, replication is decreased in tracheal tissue, suggesting a role for the s2m in the natural host. Passaging of these viruses as well as human astrovirus lacking the s2m sequence demonstrated a preference for nucleotide occupation, independent of the s2m sequence. RNA modeling suggested deletion of the s2m may negatively impact other essential RNA structures.

COVID-19: a wreak havoc across the globe

Coronavirus disease (COVID-19) is an infectious airborne viral pneumonia caused by a novel virus belonging to the family coronaviridae. On February 11, 2019, the Internal Committee on Taxonomy of Virus (ICTV) announced the name of the novel virus as "severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). One of the proteins present on its membrane i.e. the Spike protein is responsible for the attachment of the virus to the host. It spreads through the salivary droplets released when an infected person sneezes or coughs. The best way to slow down the disease is by protecting self by washing hands and using the disinfectant. Most of the infected people experience mild to moderate breathing issues. Serious illness might develop in people with underlying cardiovascular problems, diabetes and other immuno-compromised diseases. To date, there is no effective medicine available in the market which is effective in COVID-19. However, healthcare professionals are using ritonavir, flavipiravir, lopinavir, hydroxychloroquine and remdesivir. Along with the medicines, some countries are using convalescent plasma and mesenchymal stem cells for treatment. Till date, it has claimed millions of death worldwide. In this detailed review, we have discussed the structure of SARS-CoV-2, essential proteins, its lifecycle, transmission, symptoms, pathology, clinical features, diagnosis, prevention, treatment and epidemiology of the disease.

Structural and Functional RNA Motifs of SARS-CoV-2 and Influenza A Virus as a Target of Viral Inhibitors

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for the COVID-19 pandemic, whereas the influenza A virus (IAV) causes seasonal epidemics and occasional pandemics. Both viruses lead to widespread infection and death. SARS-CoV-2 and the influenza virus are RNA viruses. The SARS-CoV-2 genome is an approximately 30 kb, positive sense, 5' capped single-stranded RNA molecule. The influenza A virus genome possesses eight single-stranded negative-sense segments. The RNA secondary structure in the untranslated and coding regions is crucial in the viral replication cycle. The secondary structure within the RNA of SARS-CoV-2 and the influenza virus has been intensively studied. Because the whole of the SARS-CoV-2 and influenza virus replication cycles are dependent on RNA with no DNA intermediate, the RNA is a natural and promising target for the development of inhibitors. There are a lot of RNA-targeting strategies for regulating pathogenic RNA, such as small interfering RNA for RNA interference, antisense oligonucleotides, catalytic nucleic acids, and small molecules. In this review, we summarized the knowledge about the inhibition of SARS-CoV-2 and influenza A virus propagation by targeting their RNA secondary structure.

Genomic Analysis of Non-B Nucleic Acids Structures in SARS-CoV-2: Potential Key Roles for These Structures in Mutability, Translation, and Replication?

Non-B nucleic acids structures have arisen as key contributors to genetic variation in SARS-CoV-2. Herein, we investigated the presence of defining spike protein mutations falling within inverted repeats (IRs) for 18 SARS-CoV-2 variants, discussed the potential roles of G-quadruplexes (G4s) in SARS-CoV-2 biology, and identified potential pseudoknots within the SARS-CoV-2 genome. Surprisingly, there was a large variation in the number of defining spike protein mutations arising within IRs between variants and these were more likely to occur in the stem region of the predicted hairpin stem-loop secondary structure. Notably, mutations implicated in ACE2 binding and propagation (e.g., ΔH69/V70, N501Y, and D614G) were likely to occur within IRs, whilst mutations involved in antibody neutralization and reduced vaccine efficacy (e.g., T19R, ΔE156, ΔF157, R158G, and G446S) were rarely found within IRs. We also predicted that RNA pseudoknots could predominantly be found within, or next to, 29 mutations found in the SARS-CoV-2 spike protein. Finally, the Omicron variants BA.2, BA.4, BA.5, BA.2.12.1, and BA.2.75 appear to have lost two of the predicted G4-forming sequences found in other variants. These were found in nsp2 and the sequence complementary to the conserved stem-loop II-like motif (S2M) in the 3' untranslated region (UTR). Taken together, non-B nucleic acids structures likely play an integral role in SARS-CoV-2 evolution and genetic diversity.

Evolution of naturally arising SARS-CoV-2 defective interfering particles

Defective interfering (DI) particles arise during virus propagation, are conditional on parental virus for replication and packaging, and interfere with viral expansion. There is much interest in developing DIs as anti-viral agents. Here we characterize DI particles that arose following serial passaging of SARS-CoV-2 at high multiplicity of infection. The prominent DIs identified have lost ~84% of the SARS-CoV-2 genome and are capable of attenuating parental viral titers. Synthetic variants of the DI genomes also interfere with infection and can be used as conditional, gene delivery vehicles. In addition, the DI genomes encode an Nsp1-10 fusion protein capable of attenuating viral replication. These results identify naturally selected defective viral genomes that emerged and stably propagated in the presence of parental virus.

Genomes from defective interfering (DI) particles following serial passaging of SARS-CoV-2 reveal a fusion protein that attenuates viral replication. Synthetic, recombinant DI genomes are designed to interfere with SARS-CoV-2 replication.

Computational Pipeline for Reference-Free Comparative Analysis of RNA 3D Structures Applied to SARS-CoV-2 UTR Models

RNA is a unique biomolecule that is involved in a variety of fundamental biological functions, all of which depend solely on its structure and dynamics. Since the experimental determination of crystal RNA structures is laborious, computational 3D structure prediction methods are experiencing an ongoing and thriving development. Such methods can lead to many models; thus, it is necessary to build comparisons and extract common structural motifs for further medical or biological studies. Here, we introduce a computational pipeline dedicated to reference-free high-throughput comparative analysis of 3D RNA structures. We show its application in the RNA-Puzzles challenge, in which five participating groups attempted to predict the three-dimensional structures of 5'- and 3'-untranslated regions (UTRs) of the SARS-CoV-2 genome. We report the results of this puzzle and discuss the structural motifs obtained from the analysis. All simulated models and tools incorporated into the pipeline are open to scientific and academic use.

Lessons Learned and Yet-to-Be Learned on the Importance of RNA Structure in SARS-CoV-2 Replication

SARS-CoV-2, the etiological agent responsible for the COVID-19 pandemic, is a member of the virus family Coronaviridae, known for relatively extensive (~30-kb) RNA genomes that not only encode for numerous proteins but are also capable of forming elaborate structures. As highlighted in this review, these structures perform critical functions in various steps of the viral life cycle, ultimately impacting pathogenesis and transmissibility. We examine these elements in the context of coronavirus evolutionary history and future directions for curbing the spread of SARS-CoV-2 and other potential human coronaviruses. While we focus on structures supported by a variety of biochemical, biophysical, and/or computational methods, we also touch here on recent evidence for novel structures in both protein-coding and noncoding regions of the genome, including an assessment of the potential role for RNA structure in the controversial finding of SARS-CoV-2 integration in “long COVID” patients. This review aims to serve as a consolidation of previous works on coronavirus and more recent investigation of SARS-CoV-2, emphasizing the need for improved understanding of the role of RNA structure in the evolution and adaptation of these human viruses.

Conserved 3′ UTR of Severe Acute Respiratory Syndrome Coronavirus 2: Potential Therapeutic Targets

Our previous paper showed that microRNAs (miRNAs) present within human placental or mesenchymal stem cell-derived extracellular vesicles (EVs) directly interacted with the RNA genome of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), inhibiting viral replication. In this paper, we analyzed whether these miRNAs could exert antiviral activity against other variants of SARS-CoV-2. We downloaded compete SARS-CoV-2 genome data submitted to the National Center for Biotechnology Information for each SARS-CoV-2 variant, aligned the data to the reference SARS-CoV-2 genome sequence, and then confirmed the presence of 3′ untranslated region (UTR) mutations. We identified one type of 3′ UTR mutation in the Alpha variant, four in the Beta variant, four in the Gamma variant, three in the Delta variant, and none in the Omicron variant. Our findings indicate that 3′ UTR mutations rarely occur as persistent mutations. Interestingly, we further confirmed that this phenomenon could suppress virus replication in the same manner as the previously discovered interaction of placental-EV-derived miRNA with 3′ UTRs of SARS-CoV-2. Because the 3′ UTR of the SARS-CoV-2 RNA genome has almost no mutations, it is expected to be an effective therapeutic target regardless of future variants. Thus, a therapeutic strategy targeting the 3′ UTR of SARS-CoV-2 is likely to be extremely valuable, and such an approach is also expected to be applied to all RNA-based virus therapeutics.

Development of RNA G-quadruplex (rG4)-targeting L-RNA aptamers by rG4-SELEX

RNA G-quadruplex (rG4)-SELEX is a method that generates L-RNA aptamers to target an rG4 structure of interest, which can be applied to inhibit G-quadruplex-mediated interactions that have important roles in gene regulation and function. Here we present a Protocol Extension substantially modifying an existing SELEX protocol to describe in detail the procedures involved in performing rG4-SELEX to identify rG4-specific binders that can effectively suppress rG4-peptide and rG4-protein associations. This Protocol Extension improves the speed of aptamer discovery and identification, offering a suite of techniques to characterize the aptamer secondary structure and monitor binding affinity and specificity, and demonstrating the utility of the L-RNA aptamer. The previous protocol mainly describes the identification of RNA aptamers against proteins of interest, whereas in this Protocol Extension we present the development of an unnatural RNA aptamer against an RNA structure of interest, with the potential to be applicable to other nucleic acid motifs or biomolecules. rG4-SELEX starts with a random D-RNA library incubated with the L-rG4 target of interest, followed by binding, washing and elution of the library. Enriched D-aptamer candidates are sequenced and structurally characterized. Then, the L-aptamer is synthesized and used for different applications. rG4-SELEX can be carried out by an experienced molecular biologist with a basic understanding of nucleic acids. The development of rG4-targeting L-RNA aptamers expands the current rG4 toolkit to explore innovative rG4-related applications, and opens new doors to discovering novel rG4 biology in the near future. The duration of each selection cycle as outlined in the protocol is ~2 d.

Genetic Engineering Systems to Study Human Viral Pathogens from the Coronaviridae Family

The COVID-19 pandemic caused by the previously unknown SARS-CoV-2 Betacoronavirus made it extremely important to develop simple and safe cellular systems which allow manipulation of the viral genome and high-throughput screening of its potential inhibitors. In this review, we made an attempt at summarizing the currently existing data on genetic engineering systems used to study not only SARS-CoV-2, but also other viruses from the Coronaviridae family. In addition, the review covers the basic knowledge about the structure and the life cycle of coronaviruses.

A Novel Motif in the 3′-UTR of PRRSV-2 Is Critical for Viral Multiplication and Contributes to Enhanced Replication Ability of Highly Pathogenic or L1 PRRSV

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) with enhanced replication capability emerged in China and has become dominant epidemic strain since 2006. Up to now, the replication-regulated genes of PRRSV have not been fully clarified. Here, by swapping the genes or elements between HP-PRRSV and classical PRRSV based on infectious clones, NSP1, NSP2, NSP7, NSP9 and 3′-UTR are found to contribute to the high replication efficiency of HP-PRRSV. Further study revealed that mutations at positions 117th or 119th in the 3′-UTR are significantly related to replication efficiency, and the nucleotide at position 120th is critical for viral rescue. The motif composed by 117–120th nucleotides was quite conservative within each lineage of PRRSV; mutations in the motif of HP-PRRSV and currently epidemic lineage 1 (L1) PRRSV showed higher synthesis ability of viral negative genomic RNA, suggesting that those mutations were beneficial for viral replication. RNA structure analysis revealed that this motif maybe involved into a pseudoknot in the 3′-UTR. The results discovered a novel motif, 117–120th nucleotide in the 3′-UTR, that is critical for replication of PRRSV-2, and mutations in the motif contribute to the enhanced replicative ability of HP-PRRSV or L1 PRRSV. Our findings will help to understand the molecular basis of PRRSV replication and find the potential factors resulting in an epidemic strain of PRRSV.

The Ensembl COVID-19 resource: ongoing integration of public SARS-CoV-2 data

The COVID-19 pandemic has seen unprecedented use of SARS-CoV-2 genome sequencing for epidemiological tracking and identification of emerging variants. Understanding the potential impact of these variants on the infectivity of the virus and the efficacy of emerging therapeutics and vaccines has become a cornerstone of the fight against the disease. To support the maximal use of genomic information for SARS-CoV-2 research, we launched the Ensembl COVID-19 browser; the first virus to be encompassed within the Ensembl platform. This resource incorporates a new Ensembl gene set, multiple variant sets, and annotation from several relevant resources aligned to the reference SARS-CoV-2 assembly. Since the first release in May 2020, the content has been regularly updated using our new rapid release workflow, and tools such as the Ensembl Variant Effect Predictor have been integrated. The Ensembl COVID-19 browser is freely available at https://covid-19.ensembl.org.

Comprehensive mapping of SARS-CoV-2 interactions in vivo reveals functional virus-host interactions

SARS-CoV-2 is a major threat to global health. Here, we investigate the RNA structure and RNA-RNA interactions of wildtype (WT) and a mutant (Δ382) SARS-CoV-2 in cells using Illumina and Nanopore platforms. We identify twelve potentially functional structural elements within the SARS-CoV-2 genome, observe that subgenomic RNAs can form different structures, and that WT and Δ382 virus genomes fold differently. Proximity ligation sequencing identify hundreds of RNA-RNA interactions within the virus genome and between the virus and host RNAs. SARS-CoV-2 genome binds strongly to mitochondrial and small nucleolar RNAs and is extensively 2'-O-methylated. 2'-O-methylation sites are enriched in viral untranslated regions, associated with increased virus pair-wise interactions, and are decreased in host mRNAs upon virus infection, suggesting that the virus sequesters methylation machinery from host RNAs towards its genome. These studies deepen our understanding of the molecular and cellular basis of SARS-CoV-2 pathogenicity and provide a platform for targeted therapy.

In silico analysis of the aggregation propensity of the SARS-CoV-2 proteome: Insight into possible cellular pathologies

The SARS-CoV-2 virus causes the coronavirus disease 19 emerged in 2020. The pandemic triggered a turmoil in public health and is having a tremendous social and economic impact around the globe. Upon entry into host cells, the SARS-CoV-2 virus hijacks cellular machineries to produce and maintain its own proteins, spreading the infection. Although the disease is known for prominent respiratory symptoms, accumulating evidence is also demonstrating the involvement of the central nervous system, with possible mid- and long-term neurological consequences. In this study, we conducted a detailed bioinformatic analysis of the SARS-CoV-2 proteome aggregation propensity by using several complementary computational tools. Our study identified 10 aggregation prone proteins in the reference SARS-CoV-2 strain: the non-structural proteins Nsp4, Nsp6 and Nsp7 as well as ORF3a, ORF6, ORF7a, ORF7b, ORF10, CovE and CovM. By searching for the available mutants of each protein, we have found that most proteins are conserved, while ORF3a and ORF7b are variable and characterized by the occurrence of a large number of mutants with increased aggregation propensity. The geographical distribution of the mutants revealed interesting differences in the localization of aggregation-prone mutants of each protein. Aggregation-prone mutants of ORF7b were found in 7 European countries, whereas those of ORF3a in only 2. Aggregation-prone sequences of ORF7b, but not of ORF3a, were identified in Australia, India, Nepal, China, and Thailand. Our results are important for future analysis of a possible correlation between higher transmissibility and infection, as well as the presence of neurological symptoms with aggregation propensity of SARS-CoV-2 proteins.

A synthetic defective interfering SARS-CoV-2

Viruses thrive by exploiting the cells they infect, but in order to replicate and infect other cells they must produce viral proteins. As a result, viruses are also susceptible to exploitation by defective versions of themselves that do not produce such proteins. A defective viral genome with deletions in protein-coding genes could still replicate in cells coinfected with full-length viruses. Such a defective genome could even replicate faster due to its shorter size, interfering with the replication of the virus. We have created a synthetic defective interfering version of SARS-CoV-2, the virus causing the Covid-19 pandemic, assembling parts of the viral genome that do not code for any functional protein but enable the genome to be replicated and packaged. This synthetic defective genome replicates three times faster than SARS-CoV-2 in coinfected cells, and interferes with it, reducing the viral load of infected cells by half in 24 hours. The synthetic genome is transmitted as efficiently as the full-length genome, suggesting the location of the putative packaging signal of SARS-CoV-2. A version of such a synthetic construct could be used as a self-promoting antiviral therapy: by enabling replication of the synthetic genome, the virus would promote its own demise.

A Novel SARS-CoV-2 Viral Sequence Bioinformatic Pipeline Has Found Genetic Evidence That the Viral 3' Untranslated Region (UTR) Is Evolving and Generating Increased Viral Diversity

An unprecedented amount of SARS-CoV-2 sequencing has been performed, however, novel bioinformatic tools to cope with and process these large datasets is needed. Here, we have devised a bioinformatic pipeline that inputs SARS-CoV-2 genome sequencing in FASTA/FASTQ format and outputs a single Variant Calling Format file that can be processed to obtain variant annotations and perform downstream population genetic testing. As proof of concept, we have analyzed over 229,000 SARS-CoV-2 viral sequences up until November 30, 2020. We have identified over 39,000 variants worldwide with increased polymorphisms, spanning the ORF3a gene as well as the 3' untranslated (UTR) regions, specifically in the conserved stem loop region of SARS-CoV-2 which is accumulating greater observed viral diversity relative to chance variation. Our analysis pipeline has also discovered the existence of SARS-CoV-2 hypermutation with low frequency (less than in 2% of genomes) likely arising through host immune responses and not due to sequencing errors. Among annotated non-sense variants with a population frequency over 1%, recurrent inactivation of the ORF8 gene was found. This was found to be present in the newly identified B.1.1.7 SARS-CoV-2 lineage that originated in the United Kingdom. Almost all VOC-containing genomes possess one stop codon in ORF8 gene (Q27∗), however, 13% of these genomes also contains another stop codon (K68∗), suggesting that ORF8 loss does not interfere with SARS-CoV-2 spread and may play a role in its increased virulence. We have developed this computational pipeline to assist researchers in the rapid analysis and characterization of SARS-CoV-2 variation.

A map of the SARS-CoV-2 RNA structurome

SARS-CoV-2 has exploded throughout the human population. To facilitate efforts to gain insights into SARS-CoV-2 biology and to target the virus therapeutically, it is essential to have a roadmap of likely functional regions embedded in its RNA genome. In this report, we used a bioinformatics approach, ScanFold, to deduce the local RNA structural landscape of the SARS-CoV-2 genome with the highest likelihood of being functional. We recapitulate previously-known elements of RNA structure and provide a model for the folding of an essential frameshift signal. Our results find that SARS-CoV-2 is greatly enriched in unusually stable and likely evolutionarily ordered RNA structure, which provides a large reservoir of potential drug targets for RNA-binding small molecules. Results are enhanced via the re-analyses of publicly-available genome-wide biochemical structure probing datasets that are broadly in agreement with our models. Additionally, ScanFold was updated to incorporate experimental data as constraints in the analysis to facilitate comparisons between ScanFold and other RNA modelling approaches. Ultimately, ScanFold was able to identify eight highly structured/conserved motifs in SARS-CoV-2 that agree with experimental data, without explicitly using these data. All results are made available via a public database (the RNAStructuromeDB: https://structurome.bb.iastate.edu/sars-cov-2) and model comparisons are readily viewable at https://structurome.bb.iastate.edu/sars-cov-2-global-model-comparisons.

An overview of basic molecular biology of SARS-CoV-2 and current COVID-19 prevention strategies

Coronavirus Disease 2019 (COVID-19) manifests as extreme acute respiratory conditions caused by a novel beta coronavirus named severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) which is reported to be the seventh coronavirus to infect humans. Like other SARS-CoVs it has a large positive-stranded RNA genome. But, specific furin site in the spike protein, mutation prone and phylogenetically mess open reading frame1ab (Orf1ab) separates SARS-CoV-2 from other RNA viruses. Since the outbreak (February-March 2020), researchers, scientists, and medical professionals are inspecting all possible facts and aspects including its replication, detection, and prevention strategies. This led to the prompt identification of its basic biology, genome characterization, structural and expression based functional information of proteins, and utilization of this information in optimizing strategies to prevent its spread. This review summarizes the recent updates on the basic molecular biology of SARS-CoV-2 and prevention strategies undertaken worldwide to tackle COVID-19. This recent information can be implemented for the development and designing of therapeutics against SARS-CoV-2.

Analysis of Emerging Variants in Structured Regions of the SARS-CoV-2 Genome

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has motivated a widespread effort to understand its epidemiology and pathogenic mechanisms. Modern high-throughput sequencing technology has led to the deposition of vast numbers of SARS-CoV-2 genome sequences in curated repositories, which have been useful in mapping the spread of the virus around the globe. They also provide a unique opportunity to observe virus evolution in real time. Here, we evaluate two sets of SARS-CoV-2 genomic sequences to identify emerging variants within structured cis-regulatory elements of the SARS-CoV-2 genome. Overall, 20 variants are present at a minor allele frequency of at least 0.5%. Several enhance the stability of Stem Loop 1 in the 5' untranslated region (UTR), including a group of co-occurring variants that extend its length. One appears to modulate the stability of the frameshifting pseudoknot between ORF1a and ORF1b, and another perturbs a bi-ss molecular switch in the 3'UTR. Finally, 5 variants destabilize structured elements within the 3'UTR hypervariable region, including the S2M (stem loop 2 m) selfish genetic element, raising questions as to the functional relevance of these structures in viral replication. Two of the most abundant variants appear to be caused by RNA editing, suggesting host-viral defense contributes to SARS-CoV-2 genome heterogeneity. Our analysis has implications for the development of therapeutics that target viral cis-regulatory RNA structures or sequences.

Structure and regulation of coronavirus genomes: state-of-the-art and novel insights from SARS-CoV-2 studies

Coronaviruses (CoV) are positive-sense single-stranded RNA viruses, harboring the largest viral RNA genomes known to date. Apart from the primary sequence encoding for all the viral proteins needed for the generation of new viral particles, certain regions of CoV genomes are known to fold into stable structures, controlling several aspects of CoV life cycle, from the regulation of the discontinuous transcription of subgenomic mRNAs, to the packaging of the genome into new virions. Here we review the current knowledge on CoV RNA structures, discussing it in light of the most recent discoveries made possible by analyses of the SARS-CoV-2 genome.

Different mutations in SARS-CoV-2 associate with severe and mild outcome

INTRODUCTION

Genomic alterations in a viral genome can lead to either better or worse outcome and identifying these mutations is of utmost importance. Here, we correlated protein-level mutations in the SARS-CoV-2 virus to clinical outcome.

METHODS

Mutations in viral sequences from the GISAID virus repository were evaluated by using "hCoV-19/Wuhan/WIV04/2019" as the reference. Patient outcomes were classified as mild disease, hospitalization and severe disease (death or documented treatment in an intensive-care unit). Chi-square test was applied to examine the association between each mutation and patient outcome. False discovery rate was computed to correct for multiple hypothesis testing and results passing FDR cutoff of 5% were accepted as significant.

RESULTS

Mutations were mapped to amino acid changes for 3,733 non-silent mutations. Mutations correlated to mild outcome were located in the ORF8, NSP6, ORF3a, NSP4, and in the nucleocapsid phosphoprotein N. Mutations associated with inferior outcome were located in the surface (S) glycoprotein, in the RNA dependent RNA polymerase, in ORF3a, NSP3, ORF6 and N. Mutations leading to severe outcome with low prevalence were found in the ORF3A and in NSP7 proteins. Four out of 22 of the most significant mutations mapped onto a 10 amino acid long phosphorylated stretch of N indicating that in spite of obvious sampling restrictions the approach can find functionally relevant sites in the viral genome.

CONCLUSIONS

We demonstrate that mutations in the viral genes may have a direct correlation to clinical outcome. Our results help to quickly identify SARS-CoV-2 infections harboring mutations related to severe outcome.

A Comprehensive Analysis of cis-Acting RNA Elements in the SARS-CoV-2 Genome by a Bioinformatics Approach

The emergence of a new coronavirus (CoV), severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), responsible for severe respiratory disease in humans termed coronavirus disease of 2019 (COVID-19), became a new global threat for health and the economy. The SARS-CoV-2 genome is about a 29,800-nucleotide-long plus-strand RNA that can form functionally important secondary and higher-order structures called cis-acting RNA elements. These elements can interact with viral proteins, host proteins, or other RNAs and be involved in regulating translation and replication processes of the viral genome and encapsidation of the virus. However, the cis-acting RNA elements and their biological roles in SARS-CoV-2 as well as their comparative analysis in the closely related viral genome have not been well explored, which is very important to understand the molecular mechanism of viral infection and pathogenies. In this study, we used a bioinformatics approach to identify the cis-acting RNA elements in the SARS-CoV-2 genome. Initially, we aligned the full genomic sequence of six different CoVs, and a phylogenetic analysis was performed to understand their evolutionary relationship. Next, we predicted the cis-acting RNA elements in the SARS-CoV-2 genome using the structRNAfinder tool. Then, we annotated the location of these cis-acting RNA elements in different genomic regions of SARS-CoV-2. After that, we analyzed the sequence conservation patterns of each cis-acting RNA element among the six CoVs. Finally, the presence of cis-acting RNA elements across different CoV genomes and their comparative analysis was performed. Our study identified 12 important cis-acting RNA elements in the SARS-CoV-2 genome; among them, Corona_FSE, Corona_pk3, and s2m are highly conserved across most of the studied CoVs, and Thr_leader, MAT2A_D, and MS2 are uniquely present in SARS-CoV-2. These RNA structure elements can be involved in viral translation, replication, and encapsidation and, therefore, can be potential targets for better treatment of COVID-19. It is imperative to further characterize these cis-acting RNA elements experimentally for a better mechanistic understanding of SARS-CoV-2 infection and therapeutic intervention.

The RNA Architecture of the SARS-CoV-2 3'-Untranslated Region

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for the current COVID-19 pandemic. The 3' untranslated region (UTR) of this β-CoV contains essential cis-acting RNA elements for the viral genome transcription and replication. These elements include an equilibrium between an extended bulged stem-loop (BSL) and a pseudoknot. The existence of such an equilibrium is supported by reverse genetic studies and phylogenetic covariation analysis and is further proposed as a molecular switch essential for the control of the viral RNA polymerase binding. Here, we report the SARS-CoV-2 3' UTR structures in cells that transcribe the viral UTRs harbored in a minigene plasmid and isolated infectious virions using a chemical probing technique, namely dimethyl sulfate (DMS)-mutational profiling with sequencing (MaPseq). Interestingly, the putative pseudoknotted conformation was not observed, indicating that its abundance in our systems is low in the absence of the viral nonstructural proteins (nsps). Similarly, our results also suggest that another functional cis-acting element, the three-helix junction, cannot stably form. The overall architectures of the viral 3' UTRs in the infectious virions and the minigene-transfected cells are almost identical.

The Short- and Long-Range RNA-RNA Interactome of SARS-CoV-2

The Coronaviridae is a family of positive-strand RNA viruses that includes SARS-CoV-2, the etiologic agent of the COVID-19 pandemic. Bearing the largest single-stranded RNA genomes in nature, coronaviruses are critically dependent on long-distance RNA-RNA interactions to regulate the viral transcription and replication pathways. Here we experimentally mapped the in vivo RNA-RNA interactome of the full-length SARS-CoV-2 genome and subgenomic mRNAs. We uncovered a network of RNA-RNA interactions spanning tens of thousands of nucleotides. These interactions reveal that the viral genome and subgenomes adopt alternative topologies inside cells and engage in different interactions with host RNAs. Notably, we discovered a long-range RNA-RNA interaction, the FSE-arch, that encircles the programmed ribosomal frameshifting element. The FSE-arch is conserved in the related MERS-CoV and is under purifying selection. Our findings illuminate RNA structure-based mechanisms governing replication, discontinuous transcription, and translation of coronaviruses and will aid future efforts to develop antiviral strategies.

Non-Coding RNAs and SARS-Related Coronaviruses

The emergence of SARS-CoV-2 in 2019 has caused a major health and economic crisis around the globe. Gaining knowledge about its attributes and interactions with human host cells is crucial. Non-coding RNAs (ncRNAs) are involved in the host cells' innate antiviral immune response. In RNA interference, microRNAs (miRNAs) may bind to complementary sequences of the viral RNA strand, forming an miRNA-induced silencing complex, which destroys the viral RNA, thereby inhibiting viral protein expression. There are several targets for human miRNAs on SARS-CoV-2's RNA, most of which are in the 5' and 3' untranslated regions. Mutations of the viral genome causing the creation or loss of miRNA binding sites may have crucial effects on SARS-CoV-2 pathogenicity. In addition to mediating immunity, the ncRNA landscape of host cells further influences their susceptibility to virus infection, as certain miRNAs are essential in the regulation of cellular receptors that are necessary for virus invasion. Conversely, virus infection also changes the host ncRNA expression patterns, possibly augmenting conditions for viral replication and dissemination. Hence, ncRNAs typically upregulated in SARS-CoV-2 infection could be useful biomarkers for disease progression and severity. Understanding these mechanisms could provide further insight into the pathogenesis and possible treatment options against COVID-19.

SARS-CoV-2: A critical review of its history, pathogenesis, transmission, diagnosis and treatment

The outbreak of the deadly virus (novel coronavirus or Severe Acute Respiratory Syndrome Coronavirus-2) that emerged in December 2019, remained a controversial subject of intense speculations regarding its origin, became a worldwide health problem resulting in serious coronavirus disease 2019 (acronym COVID-19). The concern regarding this new viral strain "Severe Acute Respiratory Syndrome Coronavirus-2" (acronym SARS-CoV-2) and diseases it causes (COVID-19) is well deserved at all levels. The incidence of COVID-19 infection and infectious patients are increasing at a high rate. Coronaviruses (CoVs), enclosed positive-sense RNA viruses, are distinguished by club-like spikes extending from their surface, an exceptionally large genome of RNA, and a special mechanism for replication. Coronaviruses are associated with a broad variety of human and other animal diseases spanning from enteritis in cattle and pigs and upper chicken respiratory disease to extremely lethal human respiratory infections. With World Health Organization (WHO) declaring COVID-19 as pandemic, we deemed it necessary to provide a detailed review of coronaviruses discussing their history, current situation, coronavirus classification, pathogenesis, structure, mode of action, diagnosis and treatment, the effect of environmental factors, risk reduction and guidelines to understand the virus and develop ways to control it.

Structural analysis of SARS-CoV-2 genome and predictions of the human interactome

Specific elements of viral genomes regulate interactions within host cells. Here, we calculated the secondary structure content of >2000 coronaviruses and computed >100 000 human protein interactions with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The genomic regions display different degrees of conservation. SARS-CoV-2 domain encompassing nucleotides 22 500-23 000 is conserved both at the sequence and structural level. The regions upstream and downstream, however, vary significantly. This part of the viral sequence codes for the Spike S protein that interacts with the human receptor angiotensin-converting enzyme 2 (ACE2). Thus, variability of Spike S is connected to different levels of viral entry in human cells within the population. Our predictions indicate that the 5' end of SARS-CoV-2 is highly structured and interacts with several human proteins. The binding proteins are involved in viral RNA processing, include double-stranded RNA specific editases and ATP-dependent RNA-helicases and have strong propensity to form stress granules and phase-separated assemblies. We propose that these proteins, also implicated in viral infections such as HIV, are selectively recruited by SARS-CoV-2 genome to alter transcriptional and post-transcriptional regulation of host cells and to promote viral replication.

Structural analysis of SARS-CoV-2 genome and predictions of the human interactome

Specific elements of viral genomes regulate interactions within host cells. Here, we calculated the secondary structure content of >2000 coronaviruses and computed >100 000 human protein interactions with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The genomic regions display different degrees of conservation. SARS-CoV-2 domain encompassing nucleotides 22 500-23 000 is conserved both at the sequence and structural level. The regions upstream and downstream, however, vary significantly. This part of the viral sequence codes for the Spike S protein that interacts with the human receptor angiotensin-converting enzyme 2 (ACE2). Thus, variability of Spike S is connected to different levels of viral entry in human cells within the population. Our predictions indicate that the 5' end of SARS-CoV-2 is highly structured and interacts with several human proteins. The binding proteins are involved in viral RNA processing, include double-stranded RNA specific editases and ATP-dependent RNA-helicases and have strong propensity to form stress granules and phase-separated assemblies. We propose that these proteins, also implicated in viral infections such as HIV, are selectively recruited by SARS-CoV-2 genome to alter transcriptional and post-transcriptional regulation of host cells and to promote viral replication.

Evolutionary dynamics of the SARS-CoV-2 ORF8 accessory gene

The new SARS-CoV-2 poses a significant threat to human health but many aspects of its basic biology remain unknown. Its genome encodes accessory genes that differ significantly within coronaviruses and contribute to the virus pathogenicity. Among accessory genes, open reading frame 8 (ORF8) stands out by being highly variable and showing structural changes suspected to be related with the virus ability to spread. However, the function of ORF8 remains to be elucidated, making it less studied than other SARS-CoV-2 genes. Here I show that ORF8 is poorly conserved among related coronaviruses. The ORF8 phylogeny built using 11,113 SARS-CoV-2 sequences revealed traces of a typical expanding population with a small number of highly frequent lineages. Interestingly, I detected several nonsense mutations and three main deletions in the ORF8 gene that either remove or significantly change the ORF8 protein. These findings suggest that SARS-CoV-2 can persist without a functional ORF8 protein. Deletion breakpoints were found located in predicted hairpins suggesting a possible involvement of these elements in the rearrangement process. Although the function of ORF8 remains to be elucidated, its structural plasticity and high diversity suggest an important role in SARS-CoV-2 pathogenicity.

RNA structure prediction using positive and negative evolutionary information

Knowing the structure of conserved structural RNAs is important to elucidate their function and mechanism of action. However, predicting a conserved RNA structure remains unreliable, even when using a combination of thermodynamic stability and evolutionary covariation information. Here we present a method to predict a conserved RNA structure that combines the following three features. First, it uses significant covariation due to RNA structure and removes spurious covariation due to phylogeny. Second, it uses negative evolutionary information: basepairs that have variation but no significant covariation are prevented from occurring. Lastly, it uses a battery of probabilistic folding algorithms that incorporate all positive covariation into one structure. The method, named CaCoFold (Cascade variation/covariation Constrained Folding algorithm), predicts a nested structure guided by a maximal subset of positive basepairs, and recursively incorporates all remaining positive basepairs into alternative helices. The alternative helices can be compatible with the nested structure such as pseudoknots, or overlapping such as competing structures, base triplets, or other 3D non-antiparallel interactions. We present evidence that CaCoFold predictions are consistent with structures modeled from crystallography.

The availability of deeper comparative sequence alignments and recent advances in statistical analysis of RNA sequence covariation have made it possible to identify a reliable set of conserved base pairs, as well as a reliable set of non-basepairs (positions that vary without covarying). Predicting an overall consensus secondary structure consistent with a set of individual inferred pairs and non-pairs remains a problem. Current RNA structure prediction algorithms that predict nested secondary structures cannot use the full set of inferred covarying pairs, because covariation analysis also identifies important non-nested pairing interactions such as pseudoknots, base triples, and alternative structures. Moreover, although algorithms for incorporating negative constraints exist, negative information from covariation analysis (inferred non-pairs) has not been systematically exploited. Here I introduce an efficient approximate RNA structure prediction algorithm that incorporates all inferred pairs and excludes all non-pairs. Using this, and an improved visualization tool, I show that the method correctly identifies many non-nested structures in agreement with known crystal structures, and improves many curated consensus secondary structure annotations in RNA sequence alignment databases.

High-Density Amplicon Sequencing Identifies Community Spread and Ongoing Evolution of SARS-CoV-2 in the Southern United States

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is constantly evolving. Prior studies focused on high-case-density locations, such as the northern and western metropolitan areas of the United States. This study demonstrates continued SARS-CoV-2 evolution in a suburban southern region of the United States by high-density amplicon sequencing of symptomatic cases. 57% of strains carry the spike D614G variant, which is associated with higher genome copy numbers, and its prevalence expands with time. Four strains carry a deletion in a predicted stem loop of the 3' UTR. The data are consistent with community spread within local populations and the larger continental United States. The data instill confidence in current testing sensitivity and validate "testing by sequencing" as an option to uncover cases, particularly nonstandard coronavirus disease 2019 (COVID-19) clinical presentations. This study contributes to the understanding of COVID-19 through an extensive set of genomes from a non-urban setting and informs vaccine design by defining D614G as a dominant and emergent SARS-CoV-2 isolate in the United States.

Multiple novel non-canonically transcribed sub-genomic mRNAs produced by avian coronavirus infectious bronchitis virus

Coronavirus sub-genomic mRNA (sgmRNA) synthesis occurs via a process of discontinuous transcription involving complementary transcription regulatory sequences (TRSs), one (TRS-L) encompassing the leader sequence of the 5' untranslated region (UTR), and the other upstream of each structural and accessory gene (TRS-B). Several coronaviruses have an ORF located between the N gene and the 3'-UTR, an area previously thought to be non-coding in the Gammacoronavirus infectious bronchitis virus (IBV) due to a lack of a canonical TRS-B. Here, we identify a non-canonical TRS-B allowing for a novel sgmRNA relating to this ORF to be produced in several strains of IBV: Beaudette, CR88, H120, D1466, Italy-02 and QX. Interestingly, the potential protein produced by this ORF is prematurely truncated in the Beaudette strain. A single nucleotide deletion was made in the Beaudette strain allowing for the generation of a recombinant IBV (rIBV) that had the potential to express a full-length protein. Assessment of this rIBV in vitro demonstrated that restoration of the full-length potential protein had no effect on viral replication. Further assessment of the Beaudette-derived RNA identified a second non-canonically transcribed sgmRNA located within gene 2. Deep sequencing analysis of allantoic fluid from Beaudette-infected embryonated eggs confirmed the presence of both the newly identified non-canonically transcribed sgmRNAs and highlighted the potential for further yet unidentified sgmRNAs. This HiSeq data, alongside the confirmation of non-canonically transcribed sgmRNAs, indicates the potential of the coronavirus genome to encode a larger repertoire of genes than has currently been identified.

Coronaviruses: An Updated Overview of Their Replication and Pathogenesis

Coronaviruses (CoVs), enveloped positive-sense RNA viruses, are characterized by club-like spikes that project from their surface, an unusually large RNA genome, and a unique replication strategy. CoVs cause a variety of diseases in mammals and birds ranging from enteritis in cows and pigs, and upper respiratory tract and kidney disease in chickens to lethal human respiratory infections. Most recently, the novel coronavirus, SARS-CoV-2, which was first identified in Wuhan, China in December 2019, is the cause of a catastrophic pandemic, COVID-19, with more than 8 million infections diagnosed worldwide by mid-June 2020. Here we provide a brief introduction to CoVs discussing their replication, pathogenicity, and current prevention and treatment strategies. We will also discuss the outbreaks of the highly pathogenic Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and Middle Eastern Respiratory Syndrome Coronavirus (MERS-CoV), which are relevant for understanding COVID-19.

Global cataloguing of variations in untranslated regions of viral genome and prediction of key host RNA binding protein-microRNA interactions modulating genome stability in SARS-CoV-2

BACKGROUND

The world is going through the critical phase of COVID-19 pandemic, caused by human coronavirus, SARS-CoV-2. Worldwide concerted effort to identify viral genomic changes across different sub-types has identified several strong changes in the coding region. However, there have not been many studies focusing on the variations in the 5' and 3' untranslated regions and their consequences. Considering the possible importance of these regions in host mediated regulation of viral RNA genome, we wanted to explore the phenomenon.

METHODS

To have an idea of the global changes in 5' and 3'-UTR sequences, we downloaded 8595 complete and high-coverage SARS-CoV-2 genome sequence information from human host in FASTA format from Global Initiative on Sharing All Influenza Data (GISAID) from 15 different geographical regions. Next, we aligned them using Clustal Omega software and investigated the UTR variants. We also looked at the putative host RNA binding protein (RBP) and microRNA binding sites in these regions by 'RBPmap' and 'RNA22 v2' respectively. Expression status of selected RBPs and microRNAs were checked in lungs tissue.

RESULTS

We identified 28 unique variants in SARS-CoV-2 UTR region based on a minimum variant percentage cut-off of 0.5. Along with 241C>T change the important 5'-UTR change identified was 187A>G, while 29734G>C, 29742G>A/T and 29774C>T were the most familiar variants of 3'UTR among most of the continents. Furthermore, we found that despite the variations in the UTR regions, binding of host RBP to them remains mostly unaltered, which further influenced the functioning of specific miRNAs.

CONCLUSION

Our results, shows for the first time in SARS-Cov-2 infection, a possible cross-talk between host RBPs-miRNAs and viral UTR variants, which ultimately could explain the mechanism of escaping host RNA decay machinery by the virus. The knowledge might be helpful in developing anti-viral compounds in future.

Analysis of Rapidly Emerging Variants in Structured Regions of the SARS-CoV-2 Genome

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has motivated a widespread effort to understand its epidemiology and pathogenic mechanisms. Modern high-throughput sequencing technology has led to the deposition of vast numbers of SARS-CoV-2 genome sequences in curated repositories, which have been useful in mapping the spread of the virus around the globe. They also provide a unique opportunity to observe virus evolution in real time. Here, we evaluate two cohorts of SARS-CoV-2 genomic sequences to identify rapidly emerging variants within structured cis-regulatory elements of the SARS-CoV-2 genome. Overall, twenty variants are present at a minor allele frequency of at least 0.5%. Several enhance the stability of Stem Loop 1 in the 5'UTR, including a set of co-occurring variants that extend its length. One appears to modulate the stability of the frameshifting pseudoknot between ORF1a and ORF1b, and another perturbs a bi-stable molecular switch in the 3'UTR. Finally, five variants destabilize structured elements within the 3'UTR hypervariable region, including the S2M stem loop, raising questions as to the functional relevance of these structures in viral replication. Two of the most abundant variants appear to be caused by RNA editing, suggesting host-viral defense contributes to SARS-CoV-2 genome heterogeneity. This analysis has implications for the development of therapeutics that target viral cis-regulatory RNA structures or sequences, as rapidly emerging variations in these regions could lead to drug resistance.

An in silico map of the SARS-CoV-2 RNA Structurome

SARS-CoV-2 is a positive-sense single-stranded RNA virus that has exploded throughout the global human population. This pandemic coronavirus strain has taken scientists and public health researchers by surprise and knowledge of its basic biology (e.g. structure/function relationships in its genomic, messenger and template RNAs) and modes for therapeutic intervention lag behind that of other human pathogens. In this report we used a recently-developed bioinformatics approach, ScanFold, to deduce the RNA structural landscape of the SARS-CoV-2 transcriptome. We recapitulate known elements of RNA structure and provide a model for the folding of an essential frameshift signal. Our results find that the SARS-CoV-2 is greatly enriched in unusually stable and likely evolutionarily ordered RNA structure, which provides a huge reservoir of potential drug targets for RNA-binding small molecules. Our results also predict regions that are accessible for intermolecular interactions, which can aid in the design of antisense therapeutics. All results are made available via a public database (the RNAStructuromeDB) where they may hopefully drive drug discovery efforts to inhibit SARS-CoV-2 pathogenesis.

Interaction of coronavirus nucleocapsid protein with the 5'- and 3'-ends of the coronavirus genome is involved in genome circularization and negative-strand RNA synthesis

Synthesis of the negative‐strand ((−)‐strand) counterpart is the first step of coronavirus (CoV) replication; however, the detailed mechanism of the early event and the factors involved remain to be determined. Here, using bovine coronavirus (BCoV)‐defective interfering (DI) RNA, we showed that (a) a poly(A) tail with a length of 15 nucleotides (nt) was sufficient to initiate efficient (−)‐strand RNA synthesis and (b) substitution of the poly(A) tail with poly(U), (C) or (G) only slightly decreased the efficiency of (−)‐strand synthesis. The findings indicate that in addition to the poly(A) tail, other factors acting in trans may also participate in (−)‐strand synthesis. The BCoV nucleocapsid (N) protein, an RNA‐binding protein, was therefore tested as a candidate. Based on dissociation constant (K_d) values, it was found that the binding affinity between N protein, but not poly(A)‐binding protein, and the 3′‐terminal 55 nt plus a poly(A), poly(U), poly(C) or poly(G) tail correlates with the efficiency of (−)‐strand synthesis. Such an association was also evidenced by the binding affinity between the N protein and 5′‐ and 3′‐terminal cis‐acting elements important for (−)‐strand synthesis. Further analysis demonstrated that N protein can act as a bridge to facilitate interaction between the 5′‐ and 3′‐ends of the CoV genome, leading to circularization of the genome. Together, the current study extends our understanding of the mechanism of CoV (−)‐strand RNA synthesis through involvement of N protein and genome circularization and thus may explain why the addition of N protein in trans is required for efficient CoV replication.

Gene Variations in Cis-Acting Elements between the Taiwan and Prototype Strains of Porcine Epidemic Diarrhea Virus Alter Viral Gene Expression

In 2013, the outbreak of porcine epidemic diarrhea (PED) in Taiwan caused serious economic losses. In this study, we examined whether the variations of the cis-acting elements between the porcine epidemic diarrhea virus (PEDV) Taiwan (TW) strain and the prototype strain CV777 alter gene expression. For this aim, we analyzed the variations of the cis-acting elements in the 5' and 3' untranslated regions (UTRs) between the PEDV TW, CV777, and other reference strains. We also determined the previously unidentified transcription regulatory sequence (TRS), a sequence motif required for coronavirus transcription, and found that a nucleotide deletion in the TW strain, in comparison with CV777 strain, immediately downstream of the leader core sequence alters the identity between the leader TRS and the body TRS. Functional analyses using coronavirus defective interfering (DI) RNA revealed that such variations in cis-acting elements for the TW strain compared with the CV777 strain have an influence on the efficiency of gene expression. The current data show for the first time the evolution of PEDV in terms of cis-acting elements and their effects on gene expression, and thus may contribute to our understanding of recent PED outbreaks worldwide.

Genome-wide identification and characterization of long non-coding RNAs involved in the early somatic embryogenesis in Dimocarpus longan Lour

BACKGROUND

Long non-coding RNAs (lncRNAs) are involved in variable cleavage, transcriptional interference, regulation of DNA methylation and protein modification. However, the regulation of lncRNAs in plant somatic embryos remains unclear. The longan (Dimocarpus longan) somatic embryogenesis (SE) system is a good system for research on longan embryo development.

RESULTS

In this study, 7643 lncRNAs obtained during early SE in D. longan were identified by high-throughput sequencing, among which 6005 lncRNAs were expressed. Of the expressed lncRNAs, 4790 were found in all samples and 160 were specifically expressed in embryogenic callus (EC), 154 in incomplete embryogenic compact structures (ICpECs), and 376 in globular embryos (GEs). We annotated the 6005 expressed lncRNAs, and 1404 lncRNAs belonged to 506 noncoding RNA (ncRNA) families and 4682 lncRNAs were predicted to target protein-coding genes. The target genes included 5051 cis-regulated target genes (5712 pairs) and 1605 trans-regulated target genes (3618 pairs). KEGG analysis revealed that most of the differentially expressed target genes (mRNAs) of the lncRNAs were enriched in the "plant-pathogen interaction" and "plant hormone signaling" pathways during early longan SE. Real-time quantitative PCR confirmed that 20 selected lncRNAs showed significant differences in expression and that five lncRNAs were related to auxin response factors. Compared with the FPKM expression trends, 16 lncRNA expression trends were the same in qPCR. In lncRNA-miRNA-mRNA relationship prediction, 40 lncRNAs were predicted to function as eTMs for 15 miRNAs and 7 lncRNAs were identified as potential miRNA precursors. In addition, we verified the lncRNA-miRNA-mRNA regulatory relationships by transient expression of miRNAs (miR172a, miR159a.1 and miR398a).

CONCLUSION

Analyses of lncRNAs during early longan SE showed that differentially expressed lncRNAs were involved in expression regulation at each SE stage, and may form a regulatory network with miRNAs and mRNAs. These findings provide new insights into lncRNAs and lay a foundation for future functional analysis of lncRNAs during early longan SE.

Identification of the RNA Pseudoknot within the 3' End of the Porcine Reproductive and Respiratory Syndrome Virus Genome as a Pathogen-Associated Molecular Pattern To Activate Antiviral Signaling via RIG-I and Toll-Like Receptor 3

Once infected by viruses, cells can detect pathogen-associated molecular patterns (PAMPs) on viral nucleic acid by host pattern recognition receptors (PRRs) to initiate the antiviral response. Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of porcine reproductive and respiratory syndrome (PRRS), characterized by reproductive failure in sows and respiratory diseases in pigs of different ages. To date, the sensing mechanism of PRRSV has not been elucidated. Here, we reported that the pseudoknot region residing in the 3' untranslated regions (UTR) of the PRRSV genome, which has been proposed to regulate RNA synthesis and virus replication, was sensed as nonself by retinoic acid-inducible gene I (RIG-I) and Toll-like receptor 3 (TLR3) and strongly induced type I interferons (IFNs) and interferon-stimulated genes (ISGs) in porcine alveolar macrophages (PAMs). The interaction between the two stem-loops inside the pseudoknot structure was sufficient for IFN induction, since disruption of the pseudoknot interaction powerfully dampened the IFN induction. Furthermore, transfection of the 3' UTR pseudoknot transcripts in PAMs inhibited PRRSV replication in vitro Importantly, the predicted similar structures of other arterivirus members, including equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV), and simian hemorrhagic fever virus (SHFV), also displayed strong IFN induction activities. Together, in this work we identified an innate recognition mechanism by which the PRRSV 3' UTR pseudoknot region served as PAMPs of arteriviruses and activated innate immune signaling to produce IFNs that inhibit virus replication. All of these results provide novel insights into innate immune recognition during virus infection.IMPORTANCE PRRS is the most common viral disease in the pork industry. It is caused by PRRSV, a positive single-stranded RNA virus, whose infection often leads to persistent infection. To date, it is not yet clear how PRRSV is recognized by the host and what is the exact mechanism of IFN induction. Here, we investigated the nature of PAMPs on PRRSV and the associated PRRs. We found that the 3' UTR pseudoknot region of PRRSV, which has been proposed to regulate viral RNA synthesis, could act as PAMPs recognized by RIG-I and TLR3 to induce type I IFN production to suppress PRRSV infection. This report is the first detailed description of pattern recognition for PRRSV, which is important in understanding the antiviral response of arteriviruses, especially PRRSV, and extends our knowledge on virus recognition.

Coronavirus cis-Acting RNA Elements

Coronaviruses have exceptionally large RNA genomes of approximately 30 kilobases. Genome replication and transcription is mediated by a multisubunit protein complex comprised of more than a dozen virus-encoded proteins. The protein complex is thought to bind specific cis-acting RNA elements primarily located in the 5'- and 3'-terminal genome regions and upstream of the open reading frames located in the 3'-proximal one-third of the genome. Here, we review our current understanding of coronavirus cis-acting RNA elements, focusing on elements required for genome replication and packaging. Recent bioinformatic, biochemical, and genetic studies suggest a previously unknown level of conservation of cis-acting RNA structures among different coronavirus genera and, in some cases, even beyond genus boundaries. Also, there is increasing evidence to suggest that individual cis-acting elements may be part of higher-order RNA structures involving long-range and dynamic RNA-RNA interactions between RNA structural elements separated by thousands of nucleotides in the viral genome. We discuss the structural and functional features of these cis-acting RNA elements and their specific functions in coronavirus RNA synthesis.