Structural basis of viral RNA-dependent RNA polymerase catalysis and translocation

An adenosine analog shows high antiviral potency against coronavirus and arenavirus mainly through an unusual base pairing mode

By targeting the essential viral RNA-dependent RNA polymerase (RdRP), nucleoside analogs (NAs) have exhibited great potential in antiviral therapy for RNA virus-related diseases. However, most ribose-modified NAs do not present broad-spectrum features, likely due to differences in ribose-RdRP interactions across virus families. Here, we show that HNC-1664, an adenosine analog with modifications both in ribose and base, has broad-spectrum antiviral activity against positive-strand coronaviruses and negative-strand arenaviruses. Importantly, treatment with HNC-1664 demonstrate anti-SARS-CoV-2 efficacy in infected K18-human ACE2 mice, with reduced viral titer and mortality, as well as improved lung injury. Enzymology data demonstrate that HNC-1664 inhibits RNA synthesis mainly at the pre-catalysis stage. The cryo-EM structures of HNC-1664-bound RdRP-RNA complexes from both SARS-CoV-2 and LASV reveal an unusual base pairing mode of HNC-1664 in part due to its base modification, thus revealing its great potency in binding but not catalysis. Under certain circumstances, 1664-TP can be slowly incorporated by RdRP through regular Watson-Crick base pairing, as evidenced by enzymology data and an HNC-1664-incorporated crystal structure of the RdRP-RNA complex. Overall, HNC-1664 achieves broad-spectrum characteristics by favoring an alternative base pairing strategy to non-catalytically block RNA synthesis, providing a novel concept for the rational development of NA drugs.

Tertiary Structures of Haseki Tick Virus Nonstructural Proteins Are Similar to Those of Orthoflaviviruses

Currently, a large number of novel tick-borne viruses potentially pathogenic to humans are discovered. Studying many of them by classical methods of virology is difficult due to the absence of live viral particles or a sufficient amount of their genetic material. In this case, the use of modern methods of bioinformatics and synthetic and structural biology can help. Haseki tick virus (HSTV) is a recently discovered tick-borne unclassified ssRNA(+) virus. HSTV-positive patients experienced fever and an elevated temperature. However, at the moment, there is no information on the tertiary structure and functions of its proteins. In this work, we used AlphaFold 3 and other bioinformatic tools for the annotation of HSTV nonstructural proteins, based on the principle that the tertiary structure of a protein is inextricably linked with its molecular function. We were the first to obtain models of tertiary structures and describe the putative functions of HSTV nonstructural proteins (NS3 helicase, NS3 protease, NS5 RNA-dependent RNA-polymerase, and NS5 methyltransferase), which play a key role in viral genome replication. Our results may help in further taxonomic identification of HSTV and the development of direct-acting antiviral drugs, POC tests, and vaccines.

Binding Mechanism of the Active Form of Molnupiravir to RdRp of SARS-CoV-2 and Designing Potential Analogues: Insights from Molecular Dynamics Simulations

Molnupiravir, an FDA-approved nucleoside prodrug for treating COVID-19, converts into N4-hydroxycytidine triphosphate (NHC-TP), which integrates into SARS-CoV-2 RNA by its RNA-dependent RNA polymerase (RdRp) causing lethal mutations in viral proteins. Due to the risk of RdRp-mediated drug resistance and potential off-target effects on host polymerases (e.g., human polymerase II/HPolII), it is crucial to understand NHC-TP interactions at polymerase active sites for developing new, resistance-proof treatments. In this study, we used molecular dynamics (MD) simulations to probe key interactions between NHC-TP and SARS-CoV-2 RdRp and designed novel NHC-TP analogues with greater selectivity for SARS-CoV-2 RdRp over HPolII by a virtual screening workflow. We docked NHC-TP to a modified SARS-CoV-2 RdRp-Remdesivir triphosphate structure (PDB ID: 7BV2) and generated 71 NHC-TP analogues with bulky substituents to increase the interaction with RdRP and to reduce HPolII incorporation. MD simulations assessed the stability, binding affinity, and site interactions of these analogues. The top 7 candidates, with favorable ADMET properties, likely inhibit replication via potential dual mechanisms (the replicative stalling and the induction of lethal mutations) while maintaining selectivity for SARS-CoV-2 RdRp.

Viral replication organelles: the highly complex and programmed replication machinery

Viral infections usually induce the rearrangement of cellular cytoskeletal proteins and organelle membrane structures, thus creating independent compartments [termed replication organelles (ROs)] to facilitate viral genome replication. Within the ROs, viral replicases, including polymerases, helicases, and ligases, play functional roles during viral replication. These viral replicases are pivotal in the virus life cycle, and numerous studies have demonstrated that the viral replicases could be the potential targets for drugs development. Here, we summarize primarily the key replicases within viral ROs and emphasize the advancements of antiviral drugs targeting crucial viral replicases, providing novel insights into the future development of antiviral strategies.

Viral RNA Replication Suppression of SARS-CoV-2: Atomistic Insights into Inhibition Mechanisms of RdRp Machinery by ddhCTP

The nonstructural protein 12, known as RNA-dependent RNA polymerase (RdRp), is essential for both replication and repair of the viral genome. The RdRp of SARS-CoV-2 has been used as a promising candidate for drug development since the inception of the COVID-19 spread. In this work, we performed an in silico investigation on the insertion of the naturally modified pyrimidine nucleobase ddhCTP into the SARS-CoV-2 RdRp active site, in a comparative analysis with the natural one (CTP). The modification in ddhCTP involves the removal of the 3'-hydroxyl group that prevents the addition of subsequent nucleotides into the nascent strand, acting as an RNA chain terminator inhibitor. Quantum mechanical investigations helped to shed light on the mechanistic source of RdRp activity on the selected nucleobases, and comprehensive all-atom simulations provided insights about the structural rearrangements occurring in the active-site region when inorganic pyrophosphate (PPi) is formed. Subsequently, the intricate pathways for the release of PPi, the catalytic product of RdRp, were investigated using Umbrella Sampling simulations. The results are in line with the available experimental data and contribute to a more comprehensive point of view on such an important viral enzyme.

The Influenza A Virus Replication Cycle: A Comprehensive Review

Influenza A virus (IAV) is the primary causative agent of influenza, colloquially called the flu. Each year, it infects up to a billion people, resulting in hundreds of thousands of human deaths, and causes devastating avian outbreaks with worldwide losses worth billions of dollars. Always present is the possibility that a highly pathogenic novel subtype capable of direct human-to-human transmission will spill over into humans, causing a pandemic as devastating if not more so than the 1918 influenza pandemic. While antiviral drugs for influenza do exist, they target very few aspects of IAV replication and risk becoming obsolete due to antiviral resistance. Antivirals targeting other areas of IAV replication are needed to overcome this resistance and combat the yearly epidemics, which exact a serious toll worldwide. This review aims to summarise the key steps in the IAV replication cycle, along with highlighting areas of research that need more focus.

Recent advancements in the discovery of small-molecule non-nucleoside inhibitors targeting SARS-CoV-2 RdRp

The RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2 plays a pivotal role in the life cycle of the novel coronavirus and stands as a significant and promising target for anti-SARS-CoV-2 drugs. Non-nucleoside inhibitors (NNIs), as a category of compounds directed against SARS-CoV-2 RdRp, exhibit a unique and highly effective mechanism, effectively overcoming various factors contributing to drug resistance against nucleoside inhibitors (NIs). This review investigates various NNIs, including both natural and synthetic inhibitors, that closely interacting with the SARS-CoV-2 RdRp with valid evidences from in vitro and in silico studies.

Distinct chikungunya virus polymerase palm subdomains contribute to virus replication and virion assembly

Alphaviruses encode an error-prone RNA-dependent RNA polymerase (RdRp), nsP4, required for genome synthesis, yet how the RdRp functions in the complete alphavirus life cycle is not well-defined. Previous work using chikungunya virus (CHIKV) has established the importance of the nsP4 residue cysteine 483 in maintaining viral genetic fidelity. Given the location of residue C483 in the nsP4 palm domain, we hypothesized that other residues within this domain and surrounding subdomains would also contribute to polymerase function. To test this hypothesis, we designed a panel of nsP4 variants via homology modeling based on the Coxsackievirus B3 3 polymerase. We rescued each variant in both mammalian and mosquito cells and discovered that the palm domain and ring finger subdomain contribute to polymerase host-specific replication and genetic stability. Surprisingly, in mosquito cells, these variants in the ring finger and palm domain were replication competent and produced viral structural proteins, but they were unable to produce infectious progeny, indicating a yet uncharacterized role for the polymerase in viral assembly. Finally, we have identified additional residues in the nsP4 palm domain that influence the genetic diversity of the viral progeny, potentially via an alteration in NTP binding and/or discrimination by the polymerase. Taken together, these studies highlight that distinct nsP4 subdomains regulate multiple processes of the alphavirus life cycle, placing nsP4 in a central role during the switch from RNA synthesis to packaging and assembly.

NYC metropolitan wastewater reveals links between SARS-CoV-2 amino acid mutations and disease outcomes

Since late 2020, diverse SARS-CoV-2 variants with enhanced infectivity and transmissibility have emerged. In contrast to the focus on amino acid mutations in the spike protein, mutations in non-spike proteins and their associated impacts remain relatively understudied. New York City metropolitan wastewater revealed over 60 % of the most frequently occurring amino acid mutations in regions outside the spike protein. Strikingly, ~50 % of the mutations detected herein remain uncharacterized for functional impacts. Our results suggest that there are several understudied mutations within non-spike proteins N, ORF1a, ORF1b, ORF9b, and ORF9c, that could increase transmissibility, and infectivity among human populations. We also demonstrate significant correlations of P314L, D614G, T95I, G50E, G50R, G204R, R203K, G662S, P10S, and P13L with documented mortality rates, hospitalization rates, and percent positivity suggesting that amino acid mutations are likely to be indicators of COVID-19 infection outcomes.

A Literature Review Focusing on the Antiviral Activity of [1,2,4] and [1,2,3]-triazoles

Out of a variety of heterocycles, triazole scaffolds have been shown to play a significant part in a wide array of biological functions. Many drug compounds containing a triazole moiety with important antimicrobial, anticancer and antidepressant properties have been commercialized. In addition, the triazole scaffold exhibits remarkable antiviral activity either incorporated into nucleoside analogs or non-nucleosides. Many synthetic techniques have been produced by scientists around the world as a result of their wide-ranging biological function. In this review, we have tried to summarize new synthetic methods produced by diverse research groups as well as provide a comprehensive description of the function of [1,2,4] and [1,2,3]-triazole derivatives as antiviral agents. Antiviral triazole compounds have been shown to target a wide variety of molecular proteins. In addition, several strains of viruses, including the human immunodeficiency virus, SARS virus, hepatitis B and C viruses, influenza virus, Hantavirus, and herpes virus, were discovered to be susceptible to triazole derivatives. This review article covered the reports for antiviral activity of both 1,2,3- and 1,2,4-triazole moieties up to 2022.

SARS-CoV-2 RNA-Dependent RNA Polymerase Follows Asynchronous Translocation Pathway for Viral Transcription and Replication

Translocation is one essential step for the SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) to exert viral replication and transcription. Although cryo-EM structures of SARS-CoV-2 RdRp are available, the molecular mechanisms of dynamic translocation remain elusive. Herein, we constructed a Markov state model based on extensive molecular dynamics simulations to elucidate the translocation dynamics of the SARS-CoV-2 RdRp. We identified two intermediates that pinpoint the rate-limiting step of translocation and characterize the asynchronous movement of the template-primer duplex. The 3'-terminal nucleotide in the primer strand lags behind due to the uneven distribution of protein-RNA interactions, while the translocation of the template strand is delayed by the hurdle residue K500. Even so, the two strands share the same "ratchet" to stabilize the polymerase in the post-translocation state, suggesting a Brownian-ratchet model. Overall, our study provides intriguing insights into SARS-CoV-2 replication and transcription, which would open a new avenue for drug discoveries.

Current understanding of nucleoside analogs inhibiting the SARS-CoV-2 RNA-dependent RNA polymerase

Since the outbreak of the COVID-19 pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA-dependent RNA polymerase (RdRp) has become a main target for antiviral therapeutics due to its essential role in viral replication and transcription. Thus, nucleoside analogs structurally resemble the natural RdRp substrate and hold great potential as inhibitors. Until now, extensive experimental investigations have been performed to explore nucleoside analogs to inhibit the RdRp, and concerted efforts have been made to elucidate the underlying molecular mechanisms further. This review begins by discussing the nucleoside analogs that have demonstrated inhibition in the experiments. Second, we examine the current understanding of the molecular mechanisms underlying the action of nucleoside analogs on the SARS-CoV-2 RdRp. Recent findings in structural biology and computational research are presented through the classification of inhibitory mechanisms. This review summarizes previous experimental findings and mechanistic investigations of nucleoside analogs inhibiting SARS-CoV-2 RdRp. It would guide the rational design of antiviral medications and research into viral transcriptional mechanisms.

COVID-19: An Overview of SARS-CoV-2 Variants-The Current Vaccines and Drug Development

The world is presently in crisis facing an outbreak of a health-threatening microorganism known as COVID-19, responsible for causing uncommon viral pneumonia in humans. The virus was first reported in Wuhan, China, in early December 2019, and it quickly became a global concern due to the pandemic. Challenges in this regard have been compounded by the emergence of several variants such as B.1.1.7, B.1.351, P1, and B.1.617, which show an increase in transmission power and resistance to therapies and vaccines. Ongoing researches are focused on developing and manufacturing standard treatment strategies and effective vaccines to control the pandemic. Despite developing several vaccines such as Pfizer/BioNTech and Moderna approved by the U.S. Food and Drug Administration (FDA) and other vaccines in phase 4 clinical trials, preventive measures are mandatory to control the COVID-19 pandemic. In this review, based on the latest findings, we will discuss different types of drugs as therapeutic options and confirmed or developing vaccine candidates against SARS-CoV-2. We also discuss in detail the challenges posed by the variants and their effect on therapeutic and preventive interventions.

Unusual substructure conformations observed in crystal structures of a dicistrovirus RNA-dependent RNA polymerase suggest contribution of the N-terminal extension in proper folding

The Dicistroviridae is a virus family that includes many insect pathogens. These viruses contain a positive-sense RNA genome that is replicated by the virally encoded RNA-dependent RNA polymerase (RdRP) also named 3Dpol. Compared with the Picornaviridae RdRPs such as poliovirus (PV) 3Dpol, the Dicistroviridae representative Israeli acute paralysis virus (IAPV) 3Dpol has an additional N-terminal extension (NE) region that is about 40-residue in length. To date, both the structure and catalytic mechanism of the Dicistroviridae RdRP have remain elusive. Here we reported crystal structures of two truncated forms of IAPV 3Dpol, namely Δ85 and Δ40, both missing the NE region, and the 3Dpol protein in these structures exhibited three conformational states. The palm and thumb domains of these IAPV 3Dpol structures are largely consistent with those of the PV 3Dpol structures. However, in all structures, the RdRP fingers domain is partially disordered, while different conformations of RdRP substructures and interactions between them are also present. In particular, a large-scale conformational change occurred in the motif B-middle finger region in one protein chain of the Δ40 structure, while a previously documented alternative conformation of motif A was observed in all IAPV structures. These experimental data on one hand show intrinsic conformational variances of RdRP substructures, and on the other hand suggest possible contribution of the NE region in proper RdRP folding in IAPV.

Structural basis for control of bacterial RNA polymerase pausing by a riboswitch and its ligand

Folding of nascent transcripts can be modulated by the RNA polymerase (RNAP) that carries out their transcription, and vice versa. A pause of RNAP during transcription of a preQ1 riboswitch (termed que-PEC) is stabilized by a previously characterized template consensus sequence and the ligand-free conformation of the nascent RNA. Ligand binding to the riboswitch induces RNAP pause release and downstream transcription termination; however, the mechanism by which riboswitch folding modulates pausing is unclear. Here, we report single-particle cryo-electron microscopy reconstructions of que-PEC in ligand-free and ligand-bound states. In the absence of preQ1, the RNA transcript is in an unexpected hyper-translocated state, preventing downstream nucleotide incorporation. Strikingly, on ligand binding, the riboswitch rotates around its helical axis, expanding the surrounding RNAP exit channel and repositioning the transcript for elongation. Our study reveals the tight coupling by which nascent RNA structures and their ligands can functionally regulate the macromolecular transcription machinery.

Structure-Based Drug Design of RdRp Inhibitors against SARS-CoV-2

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a worldwide pandemic since 2019, spreading rapidly and posing a significant threat to human health and life. With over 6 billion confirmed cases of the virus, the need for effective therapeutic drugs has become more urgent than ever before. RNA-dependent RNA polymerase (RdRp) is crucial in viral replication and transcription, catalysing viral RNA synthesis and serving as a promising therapeutic target for developing antiviral drugs. In this article, we explore the inhibition of RdRp as a potential treatment for viral diseases, analysing the structural information of RdRp in virus proliferation and summarizing the reported inhibitors' pharmacophore features and structure-activity relationship profiles. We hope that the information provided by this review will aid in structure-based drug design and aid in the global fight against SARS-CoV-2 infection.

Favipiravir Suppresses Zika Virus (ZIKV) through Activity as a Mutagen

In a companion paper, we demonstrated that the nucleoside analogue favipiravir (FAV) suppressed Zika virus (ZIKV) replication in three human-derived cell lines-HeLa, SK-N-MC, and HUH-7. Our results revealed that FAV's effect was most pronounced in HeLa cells. In this work, we aimed to explain variation in FAV activity, investigating its mechanism of action and characterizing host cell factors relevant to tissue-specific differences in drug effect. Using viral genome sequencing, we show that FAV therapy was associated with an increase in the number of mutations and promoted the production of defective viral particles in all three cell lines. Our findings demonstrate that defective viral particles made up a larger portion of the viral population released from HeLa cells both at increasing FAV concentrations and at increasing exposure times. Taken together, our companion papers show that FAV acts via lethal mutagenesis against ZIKV and highlight the host cell's influence on the activation and antiviral activity of nucleoside analogues. Furthermore, the information gleaned from these companion papers can be applied to gain a more comprehensive understanding of the activity of nucleoside analogues and the impact of host cell factors against other viral infections for which we currently have no approved antiviral therapies.

An ensemble of interconverting conformations of the elemental paused transcription complex creates regulatory options

Transcriptional pausing underpins the regulation of cellular RNA synthesis, but its mechanism remains incompletely understood. Sequence-specific interactions of DNA and RNA with the dynamic, multidomain RNA polymerase (RNAP) trigger reversible conformational changes at pause sites that temporarily interrupt the nucleotide addition cycle. These interactions initially rearrange the elongation complex (EC) into an elemental paused EC (ePEC). ePECs can form longer-lived PECs by further rearrangements or interactions of diffusible regulators. For both bacterial and mammalian RNAPs, a half-translocated state in which the next DNA template base fails to load into the active site appears central to the ePEC. Some RNAPs also swivel interconnected modules that may stabilize the ePEC. However, it is unclear whether swiveling and half-translocation are requisite features of a single ePEC state or if multiple ePEC states exist. Here, we use cryo-electron microscopy (cryo-EM) analysis of ePECs with different RNA-DNA sequences combined with biochemical probes of ePEC structure to define an interconverting ensemble of ePEC states. ePECs occupy either pre- or half-translocated states but do not always swivel, indicating that difficulty in forming the posttranslocated state at certain RNA-DNA sequences may be the essence of the ePEC. The existence of multiple ePEC conformations has broad implications for transcriptional regulation.

Structural basis of transition from initiation to elongation in de novo viral RNA-dependent RNA polymerases

De novo viral RNA-dependent RNA polymerases (RdRPs) utilize their priming element (PE) to facilitate accurate initiation. Upon transition to elongation, the PE has to retreat from the active site to give room to the template-product RNA duplex. However, PE conformational change upon this transition and the role of PE at elongation both remain elusive. Here, we report crystal structures of RdRP elongation complex (EC) from dengue virus serotype 2 (DENV2), demonstrating a dramatic refolding of PE that allows establishment of interactions with the RNA duplex backbone approved to be essential for EC stability. Enzymology data from both DENV2 and hepatitis C virus (HCV) RdRPs suggest that critical transition of the refolding likely occurs after synthesis of a 4- to 5-nucleotide (nt) product together providing a key basis in understanding viral RdRP transition from initiation to elongation.

Plant-Derived Natural Non-Nucleoside Analog Inhibitors (NNAIs) against RNA-Dependent RNA Polymerase Complex (nsp7/nsp8/nsp12) of SARS-CoV-2

The emergence of fast-spreading SARS-CoV-2 mutants has sparked a new phase of COVID-19 pandemic. There is a dire necessity for antivirals targeting highly conserved genomic domains on SARS-CoV-2 that are less prone to mutation. The nsp12, also known as the RNA-dependent RNA-polymerase (RdRp), the core component of 'SARS-CoV-2 replication-transcription complex', is a potential well-conserved druggable antiviral target. Several FDA-approved RdRp 'nucleotide analog inhibitors (NAIs)' such as remdesivir, have been repurposed to treat COVID-19 infections. The NAIs target RdRp protein translation and competitively block the nucleotide insertion into the RNA chain, resulting in the inhibition of viral replication. However, the replication proofreading function of nsp14-ExoN could provide resistance to SARS-CoV-2 against many NAIs. Conversely, the 'non-nucleoside analog inhibitors (NNAIs)' bind to allosteric sites on viral polymerase surface, change the redox state; thereby, exert antiviral activity by altering interactions between the enzyme substrate and active core catalytic site of the RdRp. NNAIs neither require metabolic activation (unlike NAIs) nor compete with intracellular pool of nucleotide triphosphates (NTPs) for anti-RdRp activity. The NNAIs from phytonutrient origin are potential antiviral candidates compared to their synthetic counterparts. Several in-silico studies reported the antiviral spectrum of natural phytonutrient-NNAIs such as Suramin, Silibinin (flavonolignan), Theaflavin (tea polyphenol), Baicalein (5,6,7-trihydroxyflavone), Corilagin (gallotannin), Hesperidin (citrus bioflavonoid), Lycorine (pyrrolidine alkaloid), with superior redox characteristics (free binding energy, hydrogen-bonds, etc.) than antiviral drugs (i.e. remdesivir, favipiravir). These phytonutrient-NNAIs also exert anti-inflammatory, antioxidant, immunomodulatory and cardioprotective functions, with multifunctional therapeutic benefits in the clinical management of COVID-19.

Mechanisms and Consequences of Genetic Variation in Hepatitis C Virus (HCV)

Chronic infection with hepatitis C virus (HCV) is an important contributor to the global incidence of liver diseases, including liver cirrhosis and hepatocellular carcinoma. Although common for single-stranded RNA viruses, HCV displays a remarkable high level of genetic diversity, produced primarily by the error-prone viral polymerase and host immune pressure. The high genetic heterogeneity of HCV has led to the evolution of several distinct genotypes and subtypes, with important consequences for pathogenesis, and clinical outcomes. Genetic variability constitutes an evasion mechanism against immune suppression, allowing the virus to evolve epitope escape mutants that avoid immune recognition. Thus, heterogeneity and variability of the HCV genome represent a great hindrance for the development of vaccines against HCV. In addition, the high genetic plasticity of HCV allows the virus to rapidly develop antiviral resistance mutations, leading to treatment failure and potentially representing a major hindrance for the cure of chronic HCV patients. In this chapter, we will present the central role that genetic diversity has in the viral life cycle and epidemiology of HCV. Incorporation errors and recombination, both the result of HCV polymerase activity, represent the main mechanisms of HCV evolution. The molecular details of both mechanisms have been only partially clarified and will be presented in the following sections. Finally, we will discuss the major consequences of HCV genetic diversity, namely its capacity to rapidly evolve antiviral and immunological escape variants that represent an important limitation for clearance of acute HCV, for treatment of chronic hepatitis C and for broadly protective vaccines.

Inspection on the Mechanism of SARS-CoV-2 Inhibition by Penciclovir: A Molecular Dynamic Study

In recent years, humanity has had to face a critical pandemic due to SARS-CoV-2. In the rapid search for effective drugs against this RNA-positive virus, the repurposing of already existing nucleotide/nucleoside analogs able to stop RNA replication by inhibiting the RNA-dependent RNA polymerase enzyme has been evaluated. In this process, a valid contribution has been the use of in silico experiments, which allow for a rapid evaluation of the possible effectiveness of the proposed drugs. Here we propose a molecular dynamic study to provide insight into the inhibition mechanism of Penciclovir, a nucleotide analog on the RNA-dependent RNA polymerase enzyme. Besides the presented results, in this article, for the first time, molecular dynamic simulations have been performed considering not only the RNA-dependent RNA polymerase protein, but also its cofactors (fundamental for RNA replication) and double-strand RNA.

Crystal structure of a pre-chemistry viral RNA-dependent RNA polymerase suggests participation of two basic residues in catalysis

The nucleic acid polymerase-catalyzed nucleotidyl transfer reaction associated with polymerase active site closure is a key step in the nucleotide addition cycle (NAC). Two proton transfer events can occur in such a nucleotidyl transfer: deprotonation of the priming nucleotide 3'-hydroxyl nucleophile and protonation of the pyrophosphate (PPi) leaving group. In viral RNA-dependent RNA polymerases (RdRPs), whether and how active site residues participate in this two-proton transfer reaction remained to be clarified. Here we report a 2.5 Å resolution crystal structure of enterovirus 71 (EV71) RdRP in a catalytically closed pre-chemistry conformation, with a proposed proton donor candidate K360 in close contact with the NTP γ-phosphate. Enzymology data reveal that K360 mutations not only reduce RdRP catalytic efficiency but also alter pH dependency profiles in both elongation and pre-elongation synthesis modes. Interestingly, mutations at R174, an RdRP-invariant residue in motif F, had similar effects with additional impact on the Michaelis constant of NTP (KM,NTP). However, direct participation in protonation was not evident for K360 or R174. Our data suggest that both K360 and R174 participate in nucleotidyl transfer, while their possible roles in acid-base or positional catalysis are discussed in comparison with other classes of nucleic acid polymerases.

Generation of viperin-knockout zebrafish by CRISPR/Cas9-mediated genome engineering and the effect of this mutation under VHSV infection

Viperin is an important virus-induced protein in animals that negatively participates in RNA viral replication and transcription. The reactive machinery of viperin suggests that it produces a regulatory molecule ddhCTP, which may affect immune regulation. In this study, we investigated the expression pattern of viperin in larval and adult stages of zebrafish by whole-mount in situ hybridization and reverse transcription-quantitative PCR (RT-qPCR). To elucidate the function of viperin, we generated a zebrafish knockout model using the CRISPR/Cas9 method and evaluated the mutation's effects under viral hemorrhagic septicemia virus (VHSV) infections. In zebrafish larvae, viperin was expressed in the brain region, eye, and pharynx, which was confirmed by cryosectioning. In adult zebrafish, blood cells showed the highest levels of viperin expression. In 5 dpf fish challenged with VHSV, the expression of the viral NP protein was significantly enhanced in viperin^-/- compared to wild-type fish. In vitro VHSV propagation analysis indicated comparatively higher levels of virus propagation in viperin^-/- fish. Mortality analysis confirmed higher mortality rates, and interferon gene expression analysis showed a strong upregulation of interferon (ifn)φ1 and 3 gene in viperin^-/- fish infected with VHSV. This study describes the successful generation of a viperin-knockout model and the role of viperin during VHSV infections.

Structural Homology-Based Drug Repurposing Approach for Targeting NSP12 SARS-CoV-2

The severe acute respiratory syndrome coronavirus 2, also known as SARS-CoV-2, is the causative agent of the COVID-19 global pandemic. SARS-CoV-2 has a highly conserved non-structural protein 12 (NSP-12) involved in RNA-dependent RNA polymerase (RdRp) activity. For the identification of potential inhibitors for NSP-12, computational approaches such as the identification of homologous proteins that have been previously targeted by FDA-approved antivirals can be employed. Herein, homologous proteins of NSP-12 were retrieved from Protein DataBank (PDB) and the evolutionary conserved sequence and structure similarity of the active site of the RdRp domain of NSP-12 was characterized. The identified homologous structures of NSP-12 belonged to four viral families: Coronaviridae, Flaviviridae, Picornaviridae, and Caliciviridae, and shared evolutionary conserved relationships. The multiple sequences and structural alignment of homologous structures showed highly conserved amino acid residues that were located at the active site of the RdRp domain of NSP-12. The conserved active site of the RdRp domain of NSP-12 was evaluated for binding affinity with the FDA-approved antivirals, i.e., Sofosbuvir and Dasabuvir in a molecular docking study. The molecular docking of Sofosbuvir and Dasabuvir with the active site that contains conserved motifs (motif A-G) of the RdRp domain of NSP-12 revealed significant binding affinity. Furthermore, MD simulation also inferred the potency of Sofosbuvir and Dasabuvir. In conclusion, targeting the active site of the RdRp domain of NSP-12 with Dasabuvir and Sofosbuvir might reduce viral replication and pathogenicity and could be further studied for the treatment of SARS-CoV-2.

Elucidation of the inhibitory activity of ivermectin with host nuclear importin α and several SARS-CoV-2 targets

Ivermectin (IVM) is an FDA-approved drug that has shown antiviral activity against a wide variety of viruses in recent years. IVM inhibits the formation of the importin-α/β1 heterodimeric complex responsible for the translocation and replication of various viral species proteins. Also, IVM hampers SARS-CoV-2 replication in vitro; however, the molecular mechanism through which IVM inhibits SARS-CoV-2 is not well understood. Previous studies have explored the molecular mechanism through which IVM inhibits importin-α and several potential targets associated with COVID-19 by using docking approaches and MD simulations to corroborate the docked complexes. This study explores the energetic and structural properties through which IVM inhibits importin-α and five targets associated with COVID-19 by using docking and MD simulations combined with the molecular mechanics generalized Born surface area (MMGBSA) approach. Energetic and structural analysis showed that the main protease 3CLpro reached the most favorable affinity, followed by importin-α and Nsp9, which shared a similar relationship. Therefore, in vitro activity of IVM can be explained by acting as an inhibitor of importin-α, dimeric 3CLpro, and Nsp9, but mainly over dimeric 3CLpro.Communicated by Ramaswamy H. Sarma.

RNA-Dependent RNA Polymerase of the Second Human Pegivirus Exhibits a High-Fidelity Feature

The virus-encoded RNA-dependent RNA polymerase (RdRp) is responsible for viral replication, and its fidelity is closely related to viral diversity, pathogenesis, virulence, and fitness. Hepatitis C virus (HCV) and the second human pegivirus (HPgV-2) belong to the family Flaviviridae and share some features, including similar viral genome structure. Unlike HCV, HPgV-2 preserves a highly conserved genome sequence and low intrahost variation. However, the underlying mechanism remains to be elucidated. In this study, we evaluated the fidelity of HPgV-2 and HCV RdRp in an in vitro RNA polymerase reaction system. The results showed higher fidelity of HPgV-2 RdRp than HCV NS5B with respect to the misincorporation rate due to their difference in recognizing nucleoside triphosphate (NTP) substrates. Furthermore, HPgV-2 RdRp showed lower sensitivity than HCV to sofosbuvir, a nucleotide inhibitor against HCV RdRp, which explained the insusceptibility of HPgV-2 to direct-acting antiviral (DAA) therapy against HCV infection. Our results indicate that HPgV-2 could be an excellent model for studying the mechanisms involved in viral polymerase fidelity as well as RNA virus diversity and evolution. IMPORTANCE RNA viruses represent the most important pathogens for humans and animals and exhibit rapid evolution and high adaptive capacity, which is due to the high mutation rates for using the error-prone RNA-dependent RNA polymerase (RdRp) during replication. The fidelity of RdRp is closely associated with viral diversity, fitness, and pathogenesis. Previous studies have shown that the second human pegivirus (HPgV-2) exhibits a highly conserved genome sequence and low intrahost variation, which might be due to the fidelity of HPgV-2 RdRp. In this work, we used a series of in vitro RNA polymerase assays to evaluate the in vitro fidelity of HPgV-2 RdRp and compared it with that of HCV RdRp. The results indicated that HPgV-2 RdRp preserves significantly higher fidelity than HCV RdRp, which might contribute to the conservation of the HPgV-2 genome. The unique feature of HPgV-2 RdRp fidelity provides a new model for investigation of viral RdRp fidelity.

Translocation pause of remdesivir-containing primer/template RNA duplex within SARS-CoV-2’s RNA polymerase complexes

The mechanism of remdesivir incorporation into the RNA primer by the RNA-dependent RNA polymerase (RdRp) of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) remains to be fully established at the molecular level. Here, we compare molecular dynamics (MD) simulations after incorporation of either remdesivir monophosphate (RMP) or adenosine monophosphate (AMP). We find that the Mg²⁺-pyrophosphate (PPi) binds more tightly to the polymerase when the added RMP is at the third primer position than in the AMP added complex. The increased affinity of Mg²⁺-PPi to the RMP-added primer/template (P/T) RNA duplex complex introduces a new hydrogen bond of a substituted cyano group in RMP with the K593 sidechain. The new interactions disrupt a switching mechanism of a hydrogen bond network that is essential for translocation of the P/T duplex product and for opening of a vacant NTP-binding site necessary for next primer extension. Furthermore, steric interactions between the sidechain of S861 and the 1′-cyano group of RMP at position i+3 hinders translocation of RMP to the i + 4 position, where i labels the insertion site. These findings are particularly valuable to guide the design of more effective inhibitors of SARS-CoV-2 RNA polymerase.

Prevalence and phylogenetic analysis of human enteric emerging viruses in porcine stool samples in the Republic of Korea

Emerging infectious diseases (EID) in humans and animals are proving to be a serious health concern. This study investigated the prevalence of emerging or re-emerging human enteric viruses in porcine stools and swabs. Eleven enteric EID viruses were selected as target viruses for the current study and ranked based on their impact on public health and food safety: enterovirus (EV), hepatitis E virus, norovirus GI and GII, sapovirus (SaV), adenovirus (AdV), astrovirus, rotavirus, hepatitis A virus, aichivirus, and bocavirus. Using real-time RT-PCR or real-time PCR, EID viruses were detected in 129 (86.0%) of 150 samples. The most prevalent virus was EV, which was detected in 68.0% of samples, followed by AdV with a detection rate of 38.0%. In following sequencing and phylogenetic analyses, 33.0% (58/176) of the detected viruses were associated with human enteric EID viruses, including AdV-41, coxsackievirus-A2, echovirus-24, and SaV. Our results show that porcine stools frequently contain human enteric viruses, and that few porcine enteric viruses are genetically related to human enteric viruses. These findings suggest that enteric re-emerging or EID viruses could be zoonoses, and that continuous monitoring and further studies are needed to ensure an integrated "One Health" approach that aims to balance and optimize the health of humans, animals, and ecosystems.

The SARS-CoV nsp12 Polymerase Active Site Is Tuned for Large-Genome Replication

Positive-strand RNA viruses replicate their genomes using virally encoded RNA-dependent RNA polymerases (RdRP) with a common active-site structure and closure mechanism upon which replication speed and fidelity can evolve to optimize virus fitness. Coronaviruses (CoV) form large multicomponent RNA replication-transcription complexes containing a core RNA synthesis machine made of the nsp12 RdRP protein with one nsp7 and two nsp8 proteins as essential subunits required for activity. We show that assembly of this complex can be accelerated 5-fold by preincubation of nsp12 with nsp8 and further optimized with the use of a novel nsp8L7 heterodimer fusion protein construct. Using rapid kinetics methods, we measure elongation rates of up to 260 nucleotides (nt)/s for the core replicase, a rate that is unusually fast for a viral polymerase. To address the origin of this fast rate, we examined the roles of two CoV-specific residues in the RdRP active site: Ala547, which replaces a conserved glutamate above the bound NTP, and Ser759, which mutates the palm domain GDD sequence to SDD. Our data show that Ala547 allows for a doubling of replication rate, but this comes at a fidelity cost that is mitigated by using a SDD sequence in the palm domain. Our biochemical data suggest that fixation of mutations in polymerase motifs F and C played a key role in nidovirus evolution by tuning replication rate and fidelity to accommodate their large genomes. IMPORTANCE Replicating large genomes represents a challenge for RNA viruses because fast RNA synthesis is needed to escape innate immunity defenses, but faster polymerases are inherently low-fidelity enzymes. Nonetheless, the coronaviruses replicate their ≈30-kb genomes using the core polymerase structure and mechanism common to all positive-strand RNA viruses. The classic explanation for their success is that the large-genome nidoviruses have acquired an exonuclease-based repair system that compensates for the high polymerase mutation rate. In this work, we establish that the nidoviral polymerases themselves also play a key role in maintaining genome integrity via mutations at two key active-site residues that enable very fast replication rates while maintaining typical mutation rates. Our findings further demonstrate the evolutionary plasticity of the core polymerase platform by showing how it has adapted during the expansion from short-genome picornaviruses to long-genome nidoviruses.

Unraveling the binding mechanism of the active form of Remdesivir to RdRp of SARS-CoV-2 and designing new potential analogues: Insights from molecular dynamics simulations

The binding of the active form of Remdesivir (RTP) to RNA-dependent RNA Polymerase (RdRp) of SARS-CoV-2 was studied using molecular dynamics simulation. The RTP maintained the interactions observed in the experimental cryo-EM structure. Next, we designed new analogues of RTP, which not only binds to the RNA primer strand in a similar pose as that of RTP, but also binds more strongly than RTP does as predicted by MM-PBSA binding energy. This suggest that these analogues might be able to covalently link to the primer strand as RTP, but their 3' modification would terminate the primer strand growth.

Application of Baculovirus Expression Vector system (BEV) for COVID-19 diagnostics and therapeutics: a review

BACKGROUND

The baculovirus expression vector system has been developed for expressing a wide range of proteins, including enzymes, glycoproteins, recombinant viruses, and vaccines. The availability of the SARS-CoV-2 genome sequence has enabled the synthesis of SARS-CoV2 proteins in a baculovirus-insect cell platform for various applications. The most cloned SARS-CoV-2 protein is the spike protein, which plays a critical role in SARS-CoV-2 infection. It is available in its whole length or as subunits like S1 or the receptor-binding domain (RBD). Non-structural proteins (Nsps), another recombinant SARS-CoV-2 protein generated by the baculovirus expression vector system (BEV), are used in the identification of new medications or the repurposing of existing therapies for the treatment of COVID-19. Non-SARS-CoV-2 proteins generated by BEV for SARS-CoV-2 diagnosis or treatment include moloney murine leukemia virus reverse transcriptase (MMLVRT), angiotensin converting enzyme 2 (ACE2), therapeutic proteins, and recombinant antibodies. The recombinant proteins were modified to boost the yield or to stabilize the protein.

CONCLUSION

This review covers the wide application of the recombinant protein produced using the baculovirus expression technology for COVID-19 research. A lot of improvements have been made to produce functional proteins with high yields. However, there is still room for improvement and there are parts of this field of research that have not been investigated yet.

Identification of potential pan-coronavirus therapies using a computational drug repurposing platform

In the past 20 years, there have been several infectious disease outbreaks in humans for which the causative agent has been a zoonotic coronavirus. Novel infectious disease outbreaks, as illustrated by the current coronavirus disease 2019 (COVID-19) pandemic, demand a rapid response in terms of identifying effective treatments for seriously ill patients. The repurposing of approved drugs from other therapeutic areas is one of the most practical routes through which to approach this. Here, we present a systematic network-based drug repurposing methodology, which interrogates virus-human, human protein-protein and drug-protein interactome data. We identified 196 approved drugs that are appropriate for repurposing against COVID-19 and 102 approved drugs against a related coronavirus, severe acute respiratory syndrome (SARS-CoV). We constructed a protein-protein interaction (PPI) network based on disease signatures from COVID-19 and SARS multi-omics datasets. Analysis of this PPI network uncovered key pathways. Of the 196 drugs predicted to target COVID-19 related pathways, 44 (hypergeometric p-value: 1.98e-04) are already in COVID-19 clinical trials, demonstrating the validity of our approach. Using an artificial neural network, we provide information on the mechanism of action and therapeutic value for each of the identified drugs, to facilitate their rapid repurposing into clinical trials.

Evaluating the Impact of gRNA SNPs in CasRx Activity for Reducing Viral RNA in HCoV-OC43

Viruses within a given family often share common essential genes that are highly conserved due to their critical role for the virus's replication and survival. In this work, we developed a proof-of-concept for a pan-coronavirus CRISPR effector system by designing CRISPR targets that are cross-reactive among essential genes of different human coronaviruses (HCoV). We designed CRISPR targets for both the RNA-dependent RNA polymerase (RdRp) gene as well as the nucleocapsid (N) gene in coronaviruses. Using sequencing alignment, we determined the most highly conserved regions of these genes to design guide RNA (gRNA) sequences. In regions that were not completely homologous among HCoV species, we introduced mismatches into the gRNA sequence and tested the efficacy of CasRx, a Cas13d type CRISPR effector, using reverse transcription quantitative polymerase chain reaction (RT-qPCR) in HCoV-OC43. We evaluated the effect that mismatches in gRNA sequences has on the cleavage activity of CasRx and found that this CRISPR effector can tolerate up to three mismatches while still maintaining its nuclease activity in HCoV-OC43 viral RNA. This work highlights the need to evaluate off-target effects of CasRx with gRNAs containing up to three mismatches in order to design safe and effective CRISPR experiments.

Fusarivirus accessory helicases present an evolutionary link for viruses infecting plants and fungi

A significant number of mycoviruses have been identified that are related to plant viruses, but their evolutionary relationships are largely unexplored. A fusarivirus, Rhizoctonia solani fusarivirus 4 (RsFV4), was identified in phytopathogenic fungus Rhizoctonia solani (R. solani) strain XY74 co-infected by an alphaendornavirus. RsFV4 had a genome of 10,833 ​nt (excluding the poly-A tail), and consisted of four non-overlapping open reading frames (ORFs). ORF1 encodes an 825 aa protein containing a conserved helicase domain (Hel1). ORF3 encodes 1550 aa protein with two conserved domains, namely an RNA-dependent RNA polymerase (RdRp) and another helicase (Hel2). The ORF2 and ORF4 likely encode two hypothetical proteins (520 and 542 aa) with unknown functions. The phylogenetic analysis based on Hel2 and RdRp suggest that RsFV4 was positioned within the fusarivirus group, but formed an independent branch with three previously reported fusariviruses of R. solani. Notably, the Hel1 and its relatives were phylogenetically closer to helicases of potyviruses and hypoviruses than fusariviruses, suggesting fusarivirus Hel1 formed an evolutionary link between these three virus groups. This finding provides evidence of the occurrence of a horizontal gene transfer or recombination event between mycoviruses and plant viruses or between mycoviruses. Our findings are likely to enhance the understanding of virus evolution and diversity.

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Rhizoctonia solani strain XY74 hosts two mycoviruses, fusarivirus (RsFV4) and endornavirus (RsAEV1).

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RsFV4 consists of four ORFs and is evolutionarily associated to fusariviruses.

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Two ORFs of RsFV4 encode two helicases belonging to superfamly II.

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The accessory helicase of RsFV4 and its relatives are phylogenetically related to mycoviruses and plant viruses.

Non-nucleoside structured compounds with antiviral activity—past 10 years (2010–2020)

Nucleosides and their derivatives are a well-known and well-described class of compounds with antiviral activity. Currently, in the era of the COVID-19 pandemic, scientists are also looking for compounds not related to nucleosides with antiviral properties. This review aims to provide an overview of selected synthetic antiviral agents not associated to nucleosides developed against human viruses and introduced to preclinical and clinical trials as well as drugs approved for antiviral therapy over the last 10 years. The article describes for the first time the wide classification of such antiviral drugs and drug candidates and briefly summarizes the biological target and clinical applications of the compounds. The described compounds are arranged according to the antiviral mechanism of action. Knowledge of the drug's activity toward specific molecular targets may be the key to researching new antiviral compounds and repositioning drugs already approved for clinical use. The paper also briefly discusses the future directions of antiviral therapy. The described examples of antiviral compounds can be helpful for further drug development.

Within and Beyond the Nucleotide Addition Cycle of Viral RNA-dependent RNA Polymerases

Nucleotide addition cycle (NAC) is a fundamental process utilized by nucleic acid polymerases when carrying out nucleic acid biosynthesis. An induced-fit mechanism is usually taken by these polymerases upon NTP/dNTP substrate binding, leading to active site closure and formation of a phosphodiester bond. In viral RNA-dependent RNA polymerases, the post-chemistry translocation is stringently controlled by a structurally conserved motif, resulting in asymmetric movement of the template-product duplex. This perspective focuses on viral RdRP NAC and related mechanisms that have not been structurally clarified to date. Firstly, RdRP movement along the template strand in the absence of catalytic events may be relevant to catalytic complex dissociation or proofreading. Secondly, pyrophosphate or non-cognate NTP-mediated cleavage of the product strand 3′-nucleotide can also play a role in reactivating paused or arrested catalytic complexes. Furthermore, non-cognate NTP substrates, including NTP analog inhibitors, can not only alter NAC when being misincorporated, but also impact on subsequent NACs. Complications and challenges related to these topics are also discussed.

Structures and functions of coronavirus replication-transcription complexes and their relevance for SARS-CoV-2 drug design

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has killed millions of people and continues to cause massive global upheaval. Coronaviruses are positive-strand RNA viruses with an unusually large genome of ~30 kb. They express an RNA-dependent RNA polymerase and a cohort of other replication enzymes and supporting factors to transcribe and replicate their genomes. The proteins performing these essential processes are prime antiviral drug targets, but drug discovery is hindered by our incomplete understanding of coronavirus RNA synthesis and processing. In infected cells, the RNA-dependent RNA polymerase must coordinate with other viral and host factors to produce both viral mRNAs and new genomes. Recent research aiming to decipher and contextualize the structures, functions and interplay of the subunits of the SARS-CoV-2 replication and transcription complex proteins has burgeoned. In this Review, we discuss recent advancements in our understanding of the molecular basis and complexity of the coronavirus RNA-synthesizing machinery. Specifically, we outline the mechanisms and regulation of RNA translation, replication and transcription. We also discuss the composition of the replication and transcription complexes and their suitability as targets for antiviral therapy.

Unique SARS-CoV-2 Variant Exhibiting Plenteous Missense Mutations in Structural and Nonstructural Genes

Determining the variations in SARS-CoV-2 variant is considered main factor for understanding the pathogenic mechanisms, aid in diagnosis, prevention and treatment. The present study aimed to determine the genetic variations of SARS-CoV-2. The sequences of SARS-CoV-2 were obtained from National Center for Biotechnology Information (NCBI) and studied according to the time of isolation and their origin. The genome sequence of SARS-CoV-2 accession number NC_045512 which represented the first isolated sequence of SARS-CoV-2 (Wuhan strain) was used as the reference sequence. The obtained genome sequences of SARS-CoV-2 were aligned against this Wuhan strain and variations among nucleotides and proteins were examined. The sequence of SARS-CoV-2 accession number MT577016 showed very low homology 98.75% compared to Wuhan strain NC_045512. The analysis identified 301 nucleotide changes, which correspond to 258 different mutations; most of them 80% (207/258) were missense point mutations followed by 17.1% (44/258) silent point mutations. The critical mutations occurred in viral structural genes; 16.7% (43/258) mutations reported in S gene and 1 missense mutation was observed in E gene. Our finding showed the lowest homology and relatively distant phylogenetic relation of this SARS-CoV-2 variant with Wuhan strain along with high frequency of mutations including those in spike S and envelope E genes.

Design, synthesis and evaluation of 2'-acetylene-7-deaza-adenosine phosphoamidate derivatives as anti-EV71 and anti-EV-D68 agents

A series of phosphoamidate derivatives of nucleoside 2'-acetylene-7-deaza-adenosine (NITD008) were synthesized and evaluated for their in vitro antiviral activities against the enteroviruses EV71 and EV-D68. The phosphoamidate (15f) containing a hexyl ester of l-alanine exhibited the most promising activity against EV71 (IC₅₀ = 0.13 ± 0.08 μM) and was 4-times more potent than NITD008. Meanwhile, the derivative containing a cyclohexyl ester of l-alanine (15l) exhibited the most potent activity with high selectivity index against both EV71 (IC₅₀ = 0.19 ± 0.27 μM, SI = 117.00) and EV-D68 (IC₅₀ = 0.17 ± 0.16 μM, SI = 130.76), which were both higher than that of NITD008. The results indicated that the phosphoamidate 15l was the most promising candidate for further development as antiviral agents for the treatment of both EV71 and EV-D68 infection.

Top Three Strategies of ss(+)RNA Plant Viruses: Great Opportunists and Ecosystem Tuners with a Small Genome

ss(+)RNA viruses represent the dominant group of plant viruses. They owe their evolutionary superiority to the large number of mutations that occur during replication, courtesy of RNA-dependent RNA polymerase. Natural selection rewards successful viral subtypes, whose effective tuning of the ecosystem regulates the interactions between its participants. Thus, ss(+)RNA viruses act as shuttles for the functionally important genes of the participants in symbiotic relationships within the ecosystem, of which the most common ecological triad is “plant–virus–insect”. Due to their short life cycle and large number of offspring, RNA viruses act as skillful tuners of the ecosystem, which benefits both viruses and the system as a whole. A fundamental understanding of this aspect of the role played by viruses in the ecosystem makes it possible to apply this knowledge to the creation of DNA insecticides. In fact, since the genes that viruses are involved in transferring are functionally important for both insects and plants, silencing these genes (for example, in insects) can be used to regulate the pest population. RNA viruses are increasingly treated not as micropathogens but as necessary regulators of ecosystem balance.

Induced intra- and intermolecular template switching as a therapeutic mechanism against RNA viruses

Viral RNA-dependent RNA polymerases (RdRps) are a target for broad-spectrum antiviral therapeutic agents. Recently, we demonstrated that incorporation of the T-1106 triphosphate, a pyrazine-carboxamide ribonucleotide, into nascent RNA increases pausing and backtracking by the poliovirus RdRp. Here, by monitoring enterovirus A-71 RdRp dynamics during RNA synthesis using magnetic tweezers, we identify the "backtracked" state as an intermediate used by the RdRp for copy-back RNA synthesis and homologous recombination. Cell-based assays and RNA sequencing (RNA-seq) experiments further demonstrate that the pyrazine-carboxamide ribonucleotide stimulates these processes during infection. These results suggest that pyrazine-carboxamide ribonucleotides do not induce lethal mutagenesis or chain termination but function by promoting template switching and formation of defective viral genomes. We conclude that RdRp-catalyzed intra- and intermolecular template switching can be induced by pyrazine-carboxamide ribonucleotides, defining an additional mechanistic class of antiviral ribonucleotides with potential for broad-spectrum activity.

Remdesivir overcomes the S861 roadblock in SARS-CoV-2 polymerase elongation complex

Remdesivir (RDV), a nucleotide analog with broad-spectrum features, has exhibited effectiveness in COVID-19 treatment. However, the precise working mechanism of RDV when targeting the viral RNA-dependent RNA polymerase (RdRP) has not been fully elucidated. Here, we solve a 3.0-Å structure of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RdRP elongation complex (EC) and assess RDV intervention in polymerase elongation phase. Although RDV could induce an “i+3” delayed termination in meta-stable complexes, only pausing and subsequent elongation are observed in the EC. A comparative investigation using an enterovirus RdRP further confirms similar delayed intervention and demonstrates that steric hindrance of the RDV-characteristic 1′-cyano at the −4 position is responsible for the “i+3” intervention, although two representative Flaviviridae RdRPs do not exhibit similar behavior. A comparison of representative viral RdRP catalytic complex structures indicates that the product RNA backbone encounters highly conserved structural elements, highlighting the broad-spectrum intervention potential of 1′-modified nucleotide analogs in anti-RNA virus drug development.

SARS-CoV-2 spike protein and RNA dependent RNA polymerase as targets for drug and vaccine development: A review

The present pandemic has posed a crisis to the economy of the world and the health sector. Therefore, the race to expand research to understand some good molecular targets for vaccine and therapeutic development for SARS-CoV-2 is inevitable. The newly discovered coronavirus 2019 (COVID-19) is a positive sense, single-stranded RNA, and enveloped virus, assigned to the beta CoV genus. The virus (SARS-CoV-2) is more infectious than the previously detected coronaviruses (MERS and SARS). Findings from many studies have revealed that S protein and RdRp are good targets for drug repositioning, novel therapeutic development (antibodies and small molecule drugs), and vaccine discovery. Therapeutics such as chloroquine, convalescent plasma, monoclonal antibodies, spike binding peptides, and small molecules could alter the ability of S protein to bind to the ACE-2 receptor, and drugs such as remdesivir (targeting SARS-CoV-2 RdRp), favipir, and emetine could prevent SASR-CoV-2 RNA synthesis. The novel vaccines such as mRNA1273 (Moderna), 3LNP-mRNAs (Pfizer/BioNTech), and ChAdOx1-S (University of Oxford/Astra Zeneca) targeting S protein have proven to be effective in combating the present pandemic. Further exploration of the potential of S protein and RdRp is crucial in fighting the present pandemic.

Developing a direct acting, orally available antiviral agent in a pandemic: the evolution of molnupiravir as a potential treatment for COVID-19

Despite the availability of vaccines, there remains an urgent need for antiviral drugs with potent activity against SARS-CoV-2, the cause of COVID-19. Millions of people are immune-suppressed and may not be able to mount a fully protective immune response after vaccination. There is also an increasingly critical need for a drug to cover emerging SARS-CoV-2 variants, against which existing vaccines may be less effective. Here, we describe the evolution of molnupiravir (EIDD-2801, MK-4482), a broad-spectrum antiviral agent originally designed for the treatment of Alphavirus infections, into a potential drug for the prevention and treatment of COVID-19. When the pandemic began, molnupiravir was in pre-clinical development for the treatment of seasonal influenza. As COVID-19 spread, the timeline for the development program was moved forward significantly, and focus shifted to treatment of coronavirus infections. Real time consultation with regulatory authorities aided in making the acceleration of the program possible.

Discovery, Development, and Patent Trends on Molnupiravir: A Prospective Oral Treatment for COVID-19

The COVID-19 pandemic needs no introduction at present. Only a few treatments are available for this disease, including remdesivir and favipiravir. Accordingly, the pharmaceutical industry is striving to develop new treatments for COVID-19. Molnupiravir, an orally active RdRp inhibitor, is in a phase 3 clinical trial against COVID-19. The objective of this review article is to enlighten the researchers working on COVID-19 about the discovery, recent developments, and patents related to molnupiravir. Molnupiravir was originally developed for the treatment of influenza at Emory University, USA. However, this drug has also demonstrated activity against a variety of viruses, including SARS-CoV-2. Now it is being jointly developed by Emory University, Ridgeback Biotherapeutics, and Merck to treat COVID-19. The published clinical data indicate a good safety profile, tolerability, and oral bioavailability of molnupiravir in humans. The patient-compliant oral dosage form of molnupiravir may hit the market in the first or second quarter of 2022. The patent data of molnupiravir revealed its granted compound patent and process-related patent applications. We also anticipate patent filing related to oral dosage forms, inhalers, and a combination of molnupiravir with marketed drugs like remdesivir, favipiravir, and baricitinib. The current pandemic demands a patient compliant, safe, tolerable, and orally effective COVID-19 treatment. The authors believe that molnupiravir meets these requirements and is a breakthrough COVID-19 treatment.

A novel mechanism of enhanced transcription activity and fidelity for influenza A viral RNA-dependent RNA polymerase

During RNA elongation, the influenza A viral (IAV) RNA-dependent RNA polymerase (RdRp) residues in the active site interact with the triphosphate moiety of nucleoside triphosphate (NTP) for catalysis. The molecular mechanisms by which they control the rate and fidelity of NTP incorporation remain elusive. Here, we demonstrated through enzymology, virology and computational approaches that the R239 and K235 in the PB1 subunit of RdRp are critical to controlling the activity and fidelity of transcription. Contrary to common beliefs that high-fidelity RdRp variants exert a slower incorporation rate, we discovered a first-of-its-kind, single lysine-to-arginine mutation on K235 exhibited enhanced fidelity and activity compared with wild-type. In particular, we employed a single-turnover NTP incorporation assay for the first time on IAV RdRp to show that K235R mutant RdRp possessed a 1.9-fold increase in the transcription activity of the cognate NTP and a 4.6-fold increase in fidelity compared to wild-type. Our all-atom molecular dynamics simulations further elucidated that the higher activity is attributed to the shorter distance between K235R and the triphosphate moiety of NTP compared with wild-type. These results provide novel insights into NTP incorporation and fidelity control mechanisms, which lay the foundation for the rational design of IAV vaccine and antiviral targets.

Watching the bacterial RNA polymerase transcription reaction by time-dependent soak-trigger-freeze X-ray crystallography

RNA polymerase (RNAP) is the central enzyme of gene expression, which transcribes DNA to RNA. All cellular organisms synthesize RNA with highly conserved multi-subunit DNA-dependent RNAPs, except mitochondrial RNA transcription, which is carried out by a single-subunit RNAP. Over 60 years of extensive research has elucidated the structures and functions of cellular RNAPs. In this review, we introduce a brief structural feature of bacterial RNAP, the most well characterized model enzyme, and a novel experimental approach known as "Time-dependent soak-trigger-freeze X-ray crystallography" which can be used to observe the RNA synthesis reaction at atomic resolution in real time. This principle methodology can be used for elucidating fundamental mechanisms of cellular RNAP transcription.

Putative Repurposing of Lamivudine, a Nucleoside/Nucleotide Analogue and Antiretroviral to Improve the Outcome of Cancer and COVID-19 Patients

Lamivudine, also widely known as 3TC belongs to a family of nucleotide/nucleoside analogues of cytidine or cytosine that inhibits the Reverse Transcriptase (RT) of retroviruses such as HIV. Lamivudine is currently indicated in combination with other antiretroviral agents for the treatment of HIV-1 infection or for chronic Hepatitis B (HBV) virus infection associated with evidence of hepatitis B viral replication and active liver inflammation. HBV reactivation in patients with HBV infections who receive anticancer chemotherapy can be a life-threatening complication during and after the completion of chemotherapy. Lamivudine is used, as well as other antiretrovirals, to prevent the reactivation of the Hepatitis B virus during and after chemotherapy. In addition, Lamivudine has been shown to sensitize cancer cells to chemotherapy. Lamivudine and other similar analogues also have direct positive effects in the prevention of cancer in hepatitis B or HIV positive patients, independently of chemotherapy or radiotherapy. Recently, it has been proposed that Lamivudine might be also repurposed against SARS-CoV-2 in the context of the COVID-19 pandemic. In this review we first examine recent reports on the re-usage of Lamivudine or 3TC against the SARS-CoV-2, and we present docking evidence carried out in silico suggesting that Lamivudine may bind and possibly work as an inhibitor of the SARS-CoV-2 RdRp RNA polymerase. We also evaluate and propose assessment of repurposing Lamivudine as anti-SARS-CoV-2 and anti-COVID-19 antiviral. Secondly, we summarize the published literature on the use of Lamivudine or (3TC) before or during chemotherapy to prevent reactivation of HBV, and examine reports of enhanced effectiveness of radiotherapy in combination with Lamivudine treatment against the cancerous cells or tissues. We show that the anti-cancer properties of Lamivudine are well established, whereas its putative anti-COVID effect is under investigation. The side effects of lamivudine and the appearance of resistance to 3TC are also discussed.