Secondary structural elements within the 3' untranslated region of mouse hepatitis virus strain JHM genomic RNA

Evaluation of RNA Secondary Stem-Loop Structures in the UTRs of Mouse Hepatitis Virus as New Therapeutic Targets

MHV-A59 is a beta-coronavirus that causes demyelinating encephalitis and hepatitis in mice. Recently, the mouse infection model of MHV-A59 has been used as an alternative animal infection model for SARS-CoV and SARS-CoV-2, aiding the development of new antiviral drugs. In this study, the MHV-A59 model was employed to investigate the potential of SARS-CoV-2 UTRs as new targets for antiviral drugs. Optimal targets within the MHV-A59 UTRs were identified using a shRNA and siRNA design tool, focusing on RNA secondary stem-loop (SL) structures in the UTRs. We then examined whether the designed RNAi constructs could inhibit MHV-A59 replication. In the 5'UTR, the stem-loop 1 (SL1) was identified as the most effective target, while in the 3'UTR, the minimal element for the initiation of negative-strand RNA synthesis (MIN) proved to be the most effective. Importantly, siRNAs targeting SL1 and MIN structures significantly reduced total RNA synthesis, negative-strand genomic RNA synthesis, subgenomic (sg) RNA synthesis, viral titer, and the plaque size of MHV-A59 compared to the control. Although not statistically significant, the combination of siSL1 and siMIN had a stronger effect on inhibiting MHV-A59 replication than either siRNA monotherapy. Interestingly, while the SL1 structure is present in both MHV and SARS-CoV-2, the MIN structure is unique to MHV. Thus, the SL1 of SARS-CoV-2 may represent a novel and promising target for RNAi-based antiviral drugs.

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.

Accumulation Dynamics of Defective Genomes during Experimental Evolution of Two Betacoronaviruses

Virus-encoded replicases often generate aberrant RNA genomes, known as defective viral genomes (DVGs). When co-infected with a helper virus providing necessary proteins, DVGs can multiply and spread. While DVGs depend on the helper virus for propagation, they can in some cases disrupt infectious virus replication, impact immune responses, and affect viral persistence or evolution. Understanding the dynamics of DVGs alongside standard viral genomes during infection remains unclear. To address this, we conducted a long-term experimental evolution of two betacoronaviruses, the human coronavirus OC43 (HCoV-OC43) and the murine hepatitis virus (MHV), in cell culture at both high and low multiplicities of infection (MOI). We then performed RNA-seq at regular time intervals, reconstructed DVGs, and analyzed their accumulation dynamics. Our findings indicate that DVGs evolved to exhibit greater diversity and abundance, with deletions and insertions being the most common types. Notably, some high MOI deletions showed very limited temporary existence, while others became prevalent over time. We observed differences in DVG abundance between high and low MOI conditions in HCoV-OC43 samples. The size distribution of HCoV-OC43 genomes with deletions differed between high and low MOI passages. In low MOI lineages, short and long DVGs were the most common, with an additional cluster in high MOI lineages which became more prevalent along evolutionary time. MHV also showed variations in DVG size distribution at different MOI conditions, though they were less pronounced compared to HCoV-OC43, suggesting a more random distribution of DVG sizes. We identified hotspot regions for deletions that evolved at a high MOI, primarily within cistrons encoding structural and accessory proteins. In conclusion, our study illustrates the widespread formation of DVGs during betacoronavirus evolution, influenced by MOI and cell- and virus-specific factors.

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.

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.

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.

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.

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.

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.

Genomic characterization and pathogenicity of a porcine hemagglutinating encephalomyelitis virus strain isolated in China

Porcine hemagglutinating encephalomyelitis virus (PHEV) is a member of the genus betacoronavirus within the family coronaviridae, which invades the central nervous system (CNS) via peripheral nervous system and causes encephalomyelitis or vomiting and wasting disease (VWD) in sucking piglets. Up to now, although few complete nucleotide sequences of PHEV have been reported, they are not annotated. This study aimed to illuminate genome characterization, phylogenesis and pathogenicity of the PHEV/2008 strain. The full length of the PHEV/2008 strain genome was 30,684 bp, with a G + C content of 37.27%. The genome included at a minimum of 11 predicted open reading frames (ORFs) flanked by 5' and 3' untranslated regions (UTR) of 211 and 289 nucleotides. The replicase polyproteins pp1a and pp1ab, which had 4382 and 7094 amino acid residues, respectively, were predicted to be cleaved into 16 subunits by two viral proteinases. Phylogenetic analysis based on the complete genome sequence revealed that PHEV/2008 strain was genetically different from other known PHEV types, which represented a novel genotype (GI-1). In addition, we found that PHEV/2008 was neurotropic and highly pathogenic to 4-week-old BALB/c mice. Taken together, this is the first detailed annotated, complete genomic sequence of a new genotype PHEV strain in China.

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.

Continuous and Discontinuous RNA Synthesis in Coronaviruses

Replication of the coronavirus genome requires continuous RNA synthesis, whereas transcription is a discontinuous process unique among RNA viruses. Transcription includes a template switch during the synthesis of subgenomic negative-strand RNAs to add a copy of the leader sequence. Coronavirus transcription is regulated by multiple factors, including the extent of base-pairing between transcription-regulating sequences of positive and negative polarity, viral and cell protein-RNA binding, and high-order RNA-RNA interactions. Coronavirus RNA synthesis is performed by a replication-transcription complex that includes viral and cell proteins that recognize cis-acting RNA elements mainly located in the highly structured 5' and 3' untranslated regions. In addition to many viral nonstructural proteins, the presence of cell nuclear proteins and the viral nucleocapsid protein increases virus amplification efficacy. Coronavirus RNA synthesis is connected with the formation of double-membrane vesicles and convoluted membranes. Coronaviruses encode proofreading machinery, unique in the RNA virus world, to ensure the maintenance of their large genome size.

Coronaviruses: an 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. Coronaviruses cause a variety of diseases in mammals and birds ranging from enteritis in cows and pigs and upper respiratory disease in chickens to potentially lethal human respiratory infections. Here we provide a brief introduction to coronaviruses discussing their replication and pathogenicity, and current prevention and treatment strategies. We also discuss the outbreaks of the highly pathogenic Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and the recently identified Middle Eastern Respiratory Syndrome Coronavirus (MERS-CoV).

The structure and functions of coronavirus genomic 3' and 5' ends

Coronaviruses (CoVs) are an important cause of illness in humans and animals. Most human coronaviruses commonly cause relatively mild respiratory illnesses; however two zoonotic coronaviruses, SARS-CoV and MERS-CoV, can cause severe illness and death. Investigations over the past 35 years have illuminated many aspects of coronavirus replication. The focus of this review is the functional analysis of conserved RNA secondary structures in the 5' and 3' of the betacoronavirus genomes. The 5' 350 nucleotides folds into a set of RNA secondary structures which are well conserved, and reverse genetic studies indicate that these structures play an important role in the discontinuous synthesis of subgenomic RNAs in the betacoronaviruses. These cis-acting elements extend 3' of the 5'UTR into ORF1a. The 3'UTR is similarly conserved and contains all of the cis-acting sequences necessary for viral replication. Two competing conformations near the 5' end of the 3'UTR have been shown to make up a potential molecular switch. There is some evidence that an association between the 3' and 5'UTRs is necessary for subgenomic RNA synthesis, but the basis for this association is not yet clear. A number of host RNA proteins have been shown to bind to the 5' and 3' cis-acting regions, but the significance of these in viral replication is not clear. Two viral proteins have been identified as binding to the 5' cis-acting region, nsp1 and N protein. A genetic interaction between nsp8 and nsp9 and the region of the 3'UTR that contains the putative molecular switch suggests that these two proteins bind to this region.

Coronaviruses: an 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. Coronaviruses cause a variety of diseases in mammals and birds ranging from enteritis in cows and pigs and upper respiratory disease in chickens to potentially lethal human respiratory infections. Here we provide a brief introduction to coronaviruses discussing their replication and pathogenicity, and current prevention and treatment strategies. We also discuss the outbreaks of the highly pathogenic Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and the recently identified Middle Eastern Respiratory Syndrome Coronavirus (MERS-CoV).

RNA structure analysis of alphacoronavirus terminal genome regions

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Review of current knowledge of cis-acting RNA elements essential to coronavirus replication.

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Identification of RNA structural elements in alphacoronavirus terminal genome regions.

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Discussion of intra- and intergeneric conservation of genomic cis-acting RNA elements in alpha- and betacoronaviruses.

Coronavirus genome replication is mediated by a multi-subunit protein complex that is comprised of more than a dozen virally encoded and several cellular proteins. Interactions of the viral replicase complex with cis-acting RNA elements located in the 5′ and 3′-terminal genome regions ensure the specific replication of viral RNA. Over the past years, boundaries and structures of cis-acting RNA elements required for coronavirus genome replication have been extensively characterized in betacoronaviruses and, to a lesser extent, other coronavirus genera. Here, we review our current understanding of coronavirus cis-acting elements located in the terminal genome regions and use a combination of bioinformatic and RNA structure probing studies to identify and characterize putative cis-acting RNA elements in alphacoronaviruses. The study suggests significant RNA structure conservation among members of the genus Alphacoronavirus but also across genus boundaries. Overall, the conservation pattern identified for 5′ and 3′-terminal RNA structural elements in the genomes of alpha- and betacoronaviruses is in agreement with the widely used replicase polyprotein-based classification of the Coronavirinae, suggesting co-evolution of the coronavirus replication machinery with cognate cis-acting RNA elements.

Identification of cis-acting elements on positive-strand subgenomic mRNA required for the synthesis of negative-strand counterpart in bovine coronavirus

It has been demonstrated that, in addition to genomic RNA, sgmRNA is able to serve as a template for the synthesis of the negative-strand [(−)-strand] complement. However, the cis-acting elements on the positive-strand [(+)-strand] sgmRNA required for (−)-strand sgmRNA synthesis have not yet been systematically identified. In this study, we employed real-time quantitative reverse transcription polymerase chain reaction to analyze the cis-acting elements on bovine coronavirus (BCoV) sgmRNA 7 required for the synthesis of its (−)-strand counterpart by deletion mutagenesis. The major findings are as follows. (1) Deletion of the 5'-terminal leader sequence on sgmRNA 7 decreased the synthesis of the (−)-strand sgmRNA complement. (2) Deletions of the 3' untranslated region (UTR) bulged stem-loop showed no effect on (−)-strand sgmRNA synthesis; however, deletion of the 3' UTR pseudoknot decreased the yield of (−)-strand sgmRNA. (3) Nucleotides positioned from −15 to −34 of the sgmRNA 7 3'-terminal region are required for efficient (−)-strand sgmRNA synthesis. (4) Nucleotide species at the 3'-most position (−1) of sgmRNA 7 is correlated to the efficiency of (−)-strand sgmRNA synthesis. These results together suggest, in principle, that the 5'- and 3'-terminal sequences on sgmRNA 7 harbor cis-acting elements are critical for efficient (−)-strand sgmRNA synthesis in BCoV.

The 3'-terminal 55 nucleotides of bovine coronavirus defective interfering RNA harbor cis-acting elements required for both negative- and positive-strand RNA synthesis

The synthesis of the negative-strand [(−)-strand] complement of the ∼30 kilobase, positive-strand [(+)-strand] coronaviral genome is a necessary early step for genome replication. The identification of cis-acting elements required for (−)-strand RNA synthesis in coronaviruses, however, has been hampered due to insufficiencies in the techniques used to detect the (−)-strand RNA species. Here, we employed a method of head-to-tail ligation and real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) to detect and quantitate the synthesis of bovine coronavirus (BCoV) defective interfering (DI) RNA (−) strands. Furthermore, using the aforementioned techniques along with Northern blot assay, we specifically defined the cis-acting RNA elements within the 3′-terminal 55 nucleotides (nts) which function in the synthesis of (−)- or (+)-strand BCoV DI RNA. The major findings are as follows: (i) nts from -5 to -39 within the 3′-terminal 55 nts are the cis-acting elements responsible for (−)-strand BCoV DI RNA synthesis, (ii) nts from −3 to −34 within the 3′-terminal 55 nts are cis-acting elements required for (+)-strand BCoV DI RNA synthesis, and (iii) the nucleotide species at the 3′-most position (−1) is important, but not critical, for both (−)- and (+)-strand BCoV DI RNA synthesis. These results demonstrate that the 3′-terminal 55 nts in BCoV DI RNA harbor cis-acting RNA elements required for both (−)- and (+)-strand DI RNA synthesis and extend our knowledge on the mechanisms of coronavirus replication. The method of head-to-tail ligation and qRT-PCR employed in the study may also be applied to identify other cis-acting elements required for (−)-strand RNA synthesis in coronaviruses.

Functional analysis of the stem loop S3 and S4 structures in the coronavirus 3'UTR

We designed a series of mutations to separately destabilize two helical stems (designated S3 and S4) predicted by a covariation-based model of the coronavirus 3′UTR (Zust et al., 2008). Mouse hepatitis virus genomes containing three or four nucleotide mutations that destabilize either S3 or S4 were viable, whereas genomes carrying these mutations in both S3 and S4 were not viable. A genome carrying these mutations in S3 and S4 plus compensatory mutations restoring base-pairing yielded a virus with wild type phenotype. Larger mutations which completely disrupt S3 or S4 generated various phenotypes. Mutations opening up S3 were lethal. Disruptions of S4 generated both viable and lethal mutants. Genomes carrying the original mutations in S3 or S4 plus compensatory mutations restoring base pairing were viable and had robust growth phenotypes. These results support the Zust model for the coronavirus 3′UTR and suggest that the S3 stem is required for virus viability.

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Genetic studies of MHV 3′UTR RNA secondary structure support the Zust et al. model.

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The 3′UTR S3 helical stem is required for virus viability.

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The 3′UTR S4 helical stem is not required for virus viability.

The first complete genome sequences of clinical isolates of human coronavirus 229E

Human coronavirus 229E has been identified in the mid-1960s, yet still only one full-genome sequence is available. This full-length sequence has been determined from the cDNA-clone Inf-1 that is based on the lab-adapted strain VR-740. Lab-adaptation might have resulted in genomic changes, due to insufficient pressure to maintain gene integrity of non-essential genes. We present here the first full-length genome sequence of two clinical isolates. Each encoded gene was compared to Inf-1. In general, little sequence changes were noted, most could be attributed to genetic drift, since the clinical isolates originate from 2009 to 2010 and VR740 from 1962. Hot spots of substitutions were situated in the S1 region of the Spike, the nucleocapsid gene, and the non-structural protein 3 gene, whereas several deletions were detected in the 3′UTR. Most notable was the difference in genome organization: instead of an ORF4A and ORF4B, an intact ORF4 was present in clinical isolates.

RNA-RNA and RNA-protein interactions in coronavirus replication and transcription

Coronavirus (CoV) RNA synthesis includes the replication of the viral genome, and the transcription of sgRNAs by a discontinuous mechanism. Both processes are regulated by RNA sequences such as the 5' and 3' untranslated regions (UTRs), and the transcription regulating sequences (TRSs) of the leader (TRS-L) and those preceding each gene (TRS-Bs). These distant RNA regulatory sequences interact with each other directly and probably through protein-RNA and protein-protein interactions involving viral and cellular proteins. By analogy to other plus-stranded RNA viruses, such as polioviruses, in which translation and replication switch involves a cellular factor (PCBP) and a viral protein (3CD) it is conceivable that in CoVs the switch between replication and transcription is also associated with the binding of proteins that are specifically recruited by the replication or transcription complexes. Complexes between RNA motifs such as TRS-L and the TRS-Bs located along the CoV genome are probably formed previously to the transcription start, and most likely promote template-switch of the nascent minus RNA to the TRS-L region. Many cellular proteins interacting with regulatory CoV RNA sequences are members of the heterogeneous nuclear ribonucleoprotein (hnRNP) family of RNA-binding proteins, involved in mRNA processing and transport, which shuttle between the nucleus and the cytoplasm. In the context of CoV RNA synthesis, these cellular ribonucleoproteins might also participate in RNA-protein complexes to bring into physical proximity TRS-L and distant TRS-B, as proposed for CoV discontinuous transcription. In this review, we summarize RNA-RNA and RNA-protein interactions that represent modest examples of complex quaternary RNA-protein structures required for the fine-tuning of virus replication. Design of chemically defined replication and transcription systems will help to clarify the nature and activity of these structures.

RNA Higher-Order Structures Within the Coronavirus 5′ and 3′ Untranslated Regions and Their Roles in Viral Replication

The 5′ and 3′ untranslated regions (UTRs) of all coronaviruses contain RNA higher-order structures which play essential roles in viral transcription and replication. In this chapter we present our current knowledge of how those cis-acting elements were defined and their functional roles in viral transcription and replication. Cellular proteins which have been shown binding to those cis-acting elements and potentially support the RNA discontinuous synthesis model are also discussed. A conserved RNA structure model for the 5′ and 3′ UTRs of group 2 coronaviruses is presented with the known cellular protein binding sites.

Bovine coronavirus nonstructural protein 1 (p28) is an RNA binding protein that binds terminal genomic cis-replication elements

Nonstructural protein 1 (nsp1), a 28-kDa protein in the bovine coronavirus (BCoV) and closely related mouse hepatitis coronavirus, is the first protein cleaved from the open reading frame 1 (ORF 1) polyprotein product of genome translation. Recently, a 30-nucleotide (nt) cis-replication stem-loop VI (SLVI) has been mapped at nt 101 to 130 within a 288-nt 5'-terminal segment of the 738-nt nsp1 cistron in a BCoV defective interfering (DI) RNA. Since a similar nsp1 coding region appears in all characterized groups 1 and 2 coronavirus DI RNAs and must be translated in cis for BCoV DI RNA replication, we hypothesized that nsp1 might regulate ORF 1 expression by binding this intra-nsp1 cistronic element. Here, we (i) establish by mutation analysis that the 72-nt intracistronic SLV immediately upstream of SLVI is also a DI RNA cis-replication signal, (ii) show by gel shift and UV-cross-linking analyses that cellular proteins of approximately 60 and 100 kDa, but not viral proteins, bind SLV and SLVI, (SLV-VI) and (iii) demonstrate by gel shift analysis that nsp1 purified from Escherichia coli does not bind SLV-VI but does bind three 5' untranslated region (UTR)- and one 3' UTR-located cis-replication SLs. Notably, nsp1 specifically binds SLIII and its flanking sequences in the 5' UTR with approximately 2.5 muM affinity. Additionally, under conditions enabling expression of nsp1 from DI RNA-encoded subgenomic mRNA, DI RNA levels were greatly reduced, but there was only a slight transient reduction in viral RNA levels. These results together indicate that nsp1 is an RNA-binding protein that may function to regulate viral genome translation or replication but not by binding SLV-VI within its own coding region.

Genetic interactions between an essential 3' cis-acting RNA pseudoknot, replicase gene products, and the extreme 3' end of the mouse coronavirus genome

The upstream end of the 3' untranslated region (UTR) of the mouse hepatitis virus genome contains two essential and overlapping RNA secondary structures, a bulged stem-loop and a pseudoknot, which have been proposed to be elements of a molecular switch that is critical for viral RNA synthesis. It has previously been shown that a particular six-base insertion in loop 1 of the pseudoknot is extremely deleterious to the virus. We have now isolated multiple independent second-site revertants of the loop 1 insertion mutant, and we used reverse-genetics methods to confirm the identities of suppressor mutations that could compensate for the original insertion. The suppressors were localized to two separate regions of the genome. Members of one class of suppressor were mapped to the portions of gene 1 that encode nsp8 and nsp9, thereby providing the first evidence for specific interactions between coronavirus replicase gene products and a cis-acting genomic RNA element. The second class of suppressor was mapped to the extreme 3' end of the genome, a result which pointed to the existence of a direct base-pairing interaction between loop 1 of the pseudoknot and the genomic terminus. The latter finding was strongly supported by phylogenetic evidence and by the construction of a deletion mutant that reduced the 3' UTR to its minimal essential elements. Taken together, the interactions revealed by the two classes of suppressors suggest a model for the initiation of coronavirus negative-strand RNA synthesis.

A U-turn motif-containing stem-loop in the coronavirus 5' untranslated region plays a functional role in replication

The 5' untranslated region (UTR) of the mouse hepatitis virus (MHV) genome contains cis-acting sequences necessary for transcription and replication. A consensus secondary structural model of the 5' 140 nucleotides of the 5' UTRs of nine coronaviruses (CoVs) derived from all three major CoV groups is presented and characterized by three major stem-loops, SL1, SL2, and SL4. NMR spectroscopy provides structural support for SL1 and SL2 in three group 2 CoVs, including MHV, BCoV, and HCoV-OC43. SL2 is conserved in all CoVs, typically containing a pentaloop (C47-U48-U49-G50-U51 in MHV) stacked on a 5 base-pair stem, with some sequences containing an additional U 3' to U51; SL2 therefore possesses sequence features consistent with a U-turn-like conformation. The imino protons of U48 in the wild-type RNA, and G48 in the U48G SL2 mutant RNA, are significantly protected from exchange with solvent, consistent with a hydrogen bonding interaction critical to the hairpin loop architecture. SL2 is required for MHV replication; MHV genomes containing point substitutions predicted to perturb the SL2 structure (U48C, U48A) were not viable, while those that maintain the structure (U48G and U49A) were viable. The U48C MHV mutant supports both positive- and negative-sense genome-sized RNA synthesis, but fails to direct the synthesis of positive- or negative-sense subgenomic RNAs. These data support the existence of the SL2 in our models, and further suggest a critical role in coronavirus replication.

A hypervariable region within the 3' cis-acting element of the murine coronavirus genome is nonessential for RNA synthesis but affects pathogenesis

The 3' cis-acting element for mouse hepatitis virus (MHV) RNA synthesis resides entirely within the 301-nucleotide 3' untranslated region (3' UTR) of the viral genome and consists of three regions. Encompassing the upstream end of the 3' UTR are a bulged stem-loop and an overlapping RNA pseudoknot, both of which are essential to MHV and common to all group 2 coronaviruses. At the downstream end of the genome is the minimal signal for initiation of negative-strand RNA synthesis. Between these two ends is a hypervariable region (HVR) that is only poorly conserved between MHV and other group 2 coronaviruses. Paradoxically, buried within the HVR is an octanucleotide motif (oct), 5'-GGAAGAGC-3', which is almost universally conserved in coronaviruses and is therefore assumed to have a critical biological function. We conducted an extensive mutational analysis of the HVR. Surprisingly, this region tolerated numerous deletions, rearrangements, and point mutations. Most striking, a mutant deleted of the entire HVR was only minimally impaired in tissue culture relative to the wild type. By contrast, the HVR deletion mutant was highly attenuated in mice, causing no signs of clinical disease and minimal weight loss compared to wild-type virus. Correspondingly, replication of the HVR deletion mutant in the brains of mice was greatly reduced compared to that of the wild type. Our results show that neither the HVR nor oct is essential for the basic mechanism of MHV RNA synthesis in tissue culture. However, the HVR appears to play a significant role in viral pathogenesis.

The molecular biology of coronaviruses

Coronaviruses are large, enveloped RNA viruses of both medical and veterinary importance. Interest in this viral family has intensified in the past few years as a result of the identification of a newly emerged coronavirus as the causative agent of severe acute respiratory syndrome (SARS). At the molecular level, coronaviruses employ a variety of unusual strategies to accomplish a complex program of gene expression. Coronavirus replication entails ribosome frameshifting during genome translation, the synthesis of both genomic and multiple subgenomic RNA species, and the assembly of progeny virions by a pathway that is unique among enveloped RNA viruses. Progress in the investigation of these processes has been enhanced by the development of reverse genetic systems, an advance that was heretofore obstructed by the enormous size of the coronavirus genome. This review summarizes both classical and contemporary discoveries in the study of the molecular biology of these infectious agents, with particular emphasis on the nature and recognition of viral receptors, viral RNA synthesis, and the molecular interactions governing virion assembly.

RNA signals in the 3' terminus of the genome of Equine arteritis virus are required for viral RNA synthesis

RNA virus genomes contain cis-acting sequences and structural elements involved in virus replication. Both full-length and subgenomic negative-strand RNA synthesis are initiated at the 3' terminus of the positive-strand genomic RNA of Equine arteritis virus (EAV). To investigate the molecular mechanism of EAV RNA synthesis, the RNA secondary structure of the 3'-proximal region of the genome was analysed by chemical and enzymic probing. Based on the RNA secondary structure model derived from this analysis, several deletions were engineered in a full-length cDNA copy of the viral genome. Two RNA domains were identified that are essential for virus replication and most likely play a key role in viral RNA synthesis. The first domain, located directly upstream of the 3' untranslated region (UTR) (nt 12610-12654 of the genome), is mainly single-stranded but contains one small stem-loop structure. The second domain is located within the 3' UTR (nt 12661-12690) and folds into a prominent stem-loop structure with a large loop region. The location of this stem-loop structure near the 3' terminus of the genome suggests that it may act as a recognition signal during the initiation of minus-strand RNA synthesis.

Effect of mutations in the mouse hepatitis virus 3'(+)42 protein binding element on RNA replication

The mouse hepatitis virus (MHV) genome's 3' untranslated region contains cis-acting sequences necessary for replication. Studies of MHV and other coronaviruses have indicated a role for RNA secondary and tertiary elements in replication. Previous work in our laboratory has identified four proteins which form ribonucleoprotein complexes with the 3'-terminal 42 nucleotides [3'(+)42] of the MHV genome. Defective interfering (DI) RNA replication assays have demonstrated a role for the 3'(+)42 host protein binding element in the MHV life cycle. Using gel mobility shift RNase T1 protection assays and secondary structure modeling, we have characterized a possible role for RNA secondary structure in host protein binding to the 3'-terminal 42-nucleotide element. Additionally we have identified a role for the 3'-terminal 42-nucleotide host protein binding element in RNA replication and transcription using DI RNA replication assays and targeted recombination and by directly constructing mutants in this protein binding element using a recently described MHV reverse genetic system. DI RNA replication assays demonstrated that mutations in the 3'(+)42 host protein binding element had a deleterious effect on the accumulation of DI RNA. When the identical mutations were directly inserted into the MHV genome, most mutant genomes were viable but formed smaller plaques than the wild-type parent virus. One mutant was not viable. This mutant directed the synthesis of genome-sized negative-sense RNA approximately as efficiently as the wild type did but had a defect in subgenomic mRNA synthesis. These results point to a potential role for sequences at the extreme 3' end of the MHV genome in subgenomic RNA synthesis.

Inhibition, escape, and attenuated growth of severe acute respiratory syndrome coronavirus treated with antisense morpholino oligomers

The recently emerged severe acute respiratory syndrome coronavirus (SARS-CoV) is a potent pathogen of humans and is capable of rapid global spread. Peptide-conjugated antisense morpholino oligomers (P-PMO) were designed to bind by base pairing to specific sequences in the SARS-CoV (Tor2 strain) genome. The P-PMO were tested for their capacity to inhibit production of infectious virus as well as to probe the function of conserved viral RNA motifs and secondary structures. Several virus-targeted P-PMO and a random-sequence control P-PMO showed low inhibitory activity against SARS coronavirus. Certain other virus-targeted P-PMO reduced virus-induced cytopathology and cell-to-cell spread as a consequence of decreasing viral amplification. Active P-PMO were effective when administered at any time prior to peak viral synthesis and exerted sustained antiviral effects while present in culture medium. P-PMO showed low nonspecific inhibitory activity against translation of nontargeted RNA or growth of the arenavirus lymphocytic choriomeningitis virus. Two P-PMO targeting the viral transcription-regulatory sequence (TRS) region in the 5' untranslated region were the most effective inhibitors tested. After several viral passages in the presence of a TRS-targeted P-PMO, partially drug-resistant SARS-CoV mutants arose which contained three contiguous base point mutations at the binding site of a TRS-targeted P-PMO. Those partially resistant viruses grew more slowly and formed smaller plaques than wild-type SARS-CoV. These results suggest PMO compounds have powerful therapeutic and investigative potential toward coronavirus infection.

Coronavirus genome structure and replication

In addition to the SARS coronavirus (treated separately elsewhere in this volume), the complete genome sequences of six species in the coronavirus genus of the coronavirus family [avian infectious bronchitis virus-Beaudette strain (IBV-Beaudette), bovine coronavirus-ENT strain (BCoV-ENT), human coronavirus-229E strain (HCoV-229E), murine hepatitis virus-A59 strain (MHV-A59), porcine transmissible gastroenteritis-Purdue 115 strain (TGEV-Purdue 115), and porcine epidemic diarrhea virus-CV777 strain (PEDV-CV777)] have now been reported. Their lengths range from 27,317 nt for HCoV-229E to 31,357 nt for the murine hepatitis virus-A59, establishing the coronavirus genome as the largest known among RNA viruses. The basic organization of the coronavirus genome is shared with other members of the Nidovirus order (the torovirus genus, also in the family Coronaviridae, and members of the family Arteriviridae) in that the nonstructural proteins involved in proteolytic processing, genome replication, and subgenomic mRNA synthesis (transcription) (an estimated 14–16 end products for coronaviruses) are encoded within the 5′-proximal two-thirds of the genome on gene 1 and the (mostly) structural proteins are encoded within the 3′-proximal one-third of the genome (8–9 genes for coronaviruses). Genes for the major structural proteins in all coronaviruses occur in the 5′ to 3′ order as S, E, M, and N. The precise strategy used by coronaviruses for genome replication is not yet known, but many features have been established. This chapter focuses on some of the known features and presents some current questions regarding genome replication strategy, the cis-acting elements necessary for genome replication [as inferred from defective interfering (DI) RNA molecules], the minimum sequence requirements for autonomous replication of an RNA replicon, and the importance of gene order in genome replication.

Viral and cellular proteins involved in coronavirus replication

As the largest RNA virus, coronavirus replication employs complex mechanisms and involves various viral and cellular proteins. The first open reading frame of the coronavirus genome encodes a large polyprotein, which is processed into a number of viral proteins required for viral replication directly or indirectly. These proteins include the RNA-dependent RNA polymerase (RdRp), RNA helicase, proteases, metal-binding proteins, and a number of other proteins of unknown function. Genetic studies suggest that most of these proteins are involved in viral RNA replication. In addition to viral proteins, several cellular proteins, such as heterogeneous nuclear ribonucleoprotein (hnRNP) A1, polypyrimidine-tract-binding (PTB) protein, poly(A)-binding protein (PABP), and mitochondrial aconitase (m-aconitase), have been identified to interact with the critical cis-acting elements of coronavirus replication. Like many other RNA viruses, coronavirus may subvert these cellular proteins from cellular RNA processing or translation machineries to play a role in viral replication.

Characterization of heteroclite subgenomic RNAs associated with PRRSV infection

In this study, porcine reproductive and respiratory syndrome virus (PRRSV) heteroclite (uncommon forms) RNAs were characterized. Nucleotide sequencing of 11 additional defective RNA species verified that heteroclites are formed between the 5' and 3' termini of PRRSV at short stretches of identity, with variability seen between the junction sites utilized. Northern blot and RT-PCR analyses indicated that heteroclite RNA species were likely to be packaged into purified virions. To study whether heteroclite RNAs and viral genomic RNAs could be packaged into the same virions, PRRSV strain VR-2332 was purified by sucrose density gradient centrifugation. RT-PCR amplification of the viral RNAs isolated from three distinct gradient bands, using genomic- and heteroclite-specific primer pairs, demonstrated that heteroclite RNAs could not be readily dissociated from genomic RNA. Partial segregation of full-length and larger heteroclite genomes to the upper two gradient bands was seen, but smaller species could be found in all three fractions. These results strongly suggest that heteroclite RNAs retain the PRRSV RNA packaging signal. In vitro transcription and translation of one heteroclite cDNA clone verified that the RNA could express a predicted 32.6 kDa protein, indicating that these RNA species have the potential to produce abnormal proteins in infected cells.

The 3' cis-acting genomic replication element of the severe acute respiratory syndrome coronavirus can function in the murine coronavirus genome

The 3' untranslated region (3' UTR) of the genome of the severe acute respiratory syndrome coronavirus can functionally replace its counterpart in the prototype group 2 coronavirus mouse hepatitis virus (MHV). By contrast, the 3' UTRs of representative group 1 or group 3 coronaviruses cannot operate as substitutes for the MHV 3' UTR.

Characterization of the RNA components of a putative molecular switch in the 3' untranslated region of the murine coronavirus genome

RNA virus genomes contain cis-acting sequence and structural elements that participate in viral replication. We previously identified a bulged stem-loop secondary structure at the upstream end of the 3' untranslated region (3' UTR) of the genome of the coronavirus mouse hepatitis virus (MHV). This element, beginning immediately downstream of the nucleocapsid gene stop codon, was shown to be essential for virus replication. Other investigators discovered an adjacent downstream pseudoknot in the 3' UTR of the closely related bovine coronavirus (BCoV). This pseudoknot was also shown to be essential for replication, and it has a conserved counterpart in every group 1 and group 2 coronavirus. In MHV and BCoV, the bulged stem-loop and pseudoknot are, in part, mutually exclusive, because of the overlap of the last segment of the stem-loop and stem 1 of the pseudoknot. This led us to hypothesize that they form a molecular switch, possibly regulating a transition occurring during viral RNA synthesis. We have now performed an extensive genetic analysis of the two components of this proposed switch. Our results define essential and nonessential components of these structures and establish the limits to which essential parts of each element can be destabilized prior to loss of function. Most notably, we have confirmed the interrelationship of the two putative switch elements. Additionally, we have identified a pseudoknot loop insertion mutation that appears to point to a genetic interaction between the pseudoknot and a distant region of the genome.

Common RNA replication signals exist among group 2 coronaviruses: evidence for in vivo recombination between animal and human coronavirus molecules

5′ and 3′ UTR sequences on the coronavirus genome are known to carry cis-acting elements for DI RNA replication and presumably also virus genome replication. 5′ UTR-adjacent coding sequences are also thought to harbor cis-acting elements. Here we have determined the 5′ UTR and adjacent 289-nt sequences, and 3′ UTR sequences, for six group 2 coronaviruses and have compared them to each other and to three previously reported group 2 members. Extensive regions of highly similar UTR sequences were found but small regions of divergence were also found indicating group 2 coronaviruses could be subdivided into those that are bovine coronavirus (BCoV)-like (BCoV, human respiratory coronavirus-OC43, human enteric coronavirus, porcine hemagglutinating encephalomyelitis virus, and equine coronavirus) and those that are murine hepatitis virus (MHV)-like (A59, 2, and JHM strains of MHV, puffinosis virus, and rat sialodacryoadenitis virus). The 3′ UTRs of BCoV and MHV have been previously shown to be interchangeable. Here, a reporter-containing BCoV DI RNA was shown to be replicated by all five BCoV-like helper viruses and by MHV-H2 (a human cell-adapted MHV strain), a representative of the MHV-like subgroup, demonstrating group 2 common 5′ and 3′ replication signaling elements. BCoV DI RNA, furthermore, acquired the leader of HCoV-OC43 by leader switching, demonstrating for the first time in vivo recombination between animal and human coronavirus molecules. These results indicate that common replication signaling elements exist among group 2 coronaviruses despite a two-cluster pattern within the group and imply there could exist a high potential for recombination among group members.

Stem-loop III in the 5' untranslated region is a cis-acting element in bovine coronavirus defective interfering RNA replication

Higher-order structures in the 5' untranslated region (UTR) of plus-strand RNA viruses are known in many cases to function as cis-acting elements in RNA translation, replication, or transcription. Here we describe evidence supporting the structure and a cis-acting function in defective interfering (DI) RNA replication of stem-loop III, the third of four predicted higher-order structures mapping within the 210-nucleotide (nt) 5' UTR of the 32-kb bovine coronavirus (BCoV) genome. Stem-loop III maps at nt 97 through 116, has a calculated free energy of -9.1 kcal/mol in the positive strand and -3.0 kcal/mol in the negative strand, and has associated with it beginning at nt 100 an open reading frame (ORF) potentially encoding an 8-amino-acid peptide. Stem-loop III is presumed to function in the positive strand, but its strand of action has not been established. Stem-loop III (i) shows phylogenetic conservation among group 2 coronaviruses and appears to have a homolog in coronavirus groups 1 and 3, (ii) has in all coronaviruses for which sequence is known a closely associated short, AUG-initiated intra-5' UTR ORF, (iii) is supported by enzyme structure-probing evidence in BCoV RNA, (iv) must maintain stem integrity for DI RNA replication in BCoV DI RNA, and (v) shows a positive correlation between maintenance of the short ORF and maximal DI RNA accumulation in BCoV DI RNA. These results indicate that stem-loop III in the BCoV 5' UTR is a cis-acting element for DI RNA replication and that its associated intra-5' UTR ORF may function to enhance replication. It is postulated that these two elements function similarly in the virus genome.