A -1 ribosomal frameshift in a double-stranded RNA virus of yeast forms a gag-pol fusion protein

Microbial Matryoshka: Addressing the Relationship between Pathogenic Flagellated Protozoans and Their RNA Viral Endosymbionts (Family Totiviridae)

Three genera of viruses of the family Totiviridae establish endosymbiotic associations with flagellated protozoa responsible for parasitic diseases of great impact in the context of One Health. Giardiavirus, Trichomonasvirus, and Leishmaniavirus infect the protozoa Giardia sp., Trichomonas vaginalis, and Leishmania sp., respectively. In the present work, we review the characteristics of the endosymbiotic relationships established, the advantages, and the consequences caused in mammalian hosts. Among the common characteristics of these double-stranded RNA viruses are that they do not integrate into the host genome, do not follow a lytic cycle, and do not cause cytopathic effects. However, in cases of endosymbiosis between Leishmaniavirus and Leishmania species from the Americas, and between Trichomonasvirus and Trichomonas vaginalis, it seems that it can alter their virulence (degree of pathogenicity). In a mammalian host, due to TLR3 activation of immune cells upon the recognition of viral RNA, uncontrolled inflammatory signaling responses are triggered, increasing pathological damage and the risk of failure of conventional standard treatment. Endosymbiosis with Giardiavirus can cause the loss of intestinal adherence of the protozoan, resulting in a benign disease. The current knowledge about viruses infecting flagellated protozoans is still fragmentary, and more research is required to unravel the intricacies of this three-way relationship. We need to develop early and effective diagnostic methods for further development in the field of translational medicine. Taking advantage of promising biotechnological advances, the aim is to develop ad hoc therapeutic strategies that focus not only on the disease-causing protozoan but also on the virus.

Metatranscriptomic Sequencing of Sheath Blight-Associated Isolates of Rhizoctonia solani Revealed Multi-Infection by Diverse Groups of RNA Viruses

Rice sheath blight, caused by the soil-borne fungus Rhizoctonia solani (teleomorph: Thanatephorus cucumeris, Basidiomycota), is one of the most devastating phytopathogenic fungal diseases and causes yield loss. Here, we report on a very high prevalence (100%) of potential virus-associated double-stranded RNA (dsRNA) elements for a collection of 39 fungal strains of R. solani from the rice sheath blight samples from at least four major rice-growing areas in the Philippines and a reference isolate from the International Rice Research Institute, showing different colony phenotypes. Their dsRNA profiles suggested the presence of multiple viral infections among these Philippine R. solani populations. Using next-generation sequencing, the viral sequences of the three representative R. solani strains (Ilo-Rs-6, Tar-Rs-3, and Tar-Rs-5) from different rice-growing areas revealed the presence of at least 36 viruses or virus-like agents, with the Tar-Rs-3 strain harboring the largest number of viruses (at least 20 in total). These mycoviruses or their candidates are believed to have single-stranded RNA or dsRNA genomes and they belong to or are associated with the orders Martellivirales, Hepelivirales, Durnavirales, Cryppavirales, Ourlivirales, and Ghabrivirales based on their coding-complete RNA-dependent RNA polymerase sequences. The complete genome sequences of two novel RNA viruses belonging to the proposed family Phlegiviridae and family Mitoviridae were determined.

Complete genome sequence and phylogenetic analysis of a newly discovered fusagra-like virus infecting Carica papaya in Ecuador

A new fusagra-like virus infecting papaya (Carica papaya L.) was genetically characterized. The genome of the virus, provisionally named "papaya sticky fruit-associated virus" (PSFaV), is a single molecule of double-stranded RNA, 9,199 nucleotides (nt) in length, containing two discontinuous open reading frames. Pairwise sequence comparisons based on complete RNA-dependent-RNA-polymerase (RdRp) sequences revealed identity of 79.4% and 83.3% at the nt and amino acid (aa) level, respectively, to babaco meleira-like virus (BabMelV), an uncharacterized virus sequence discovered in babaco (Vasconcellea x heilbornii) in Ecuador. Additional plant-associated viruses with sequence identity in the 50% range included papaya meleira virus (PMeV) isolates from Brazil. Phylogenetic analysis based on the amino acid sequences of the capsid protein (CP), RdRp, and CP-RdRp fusion protein genes placed PSFaV in a group within a well-supported clade that shares a recent ancestor with Sclerotium rolfsii RNA virus 2 and Phlebiopsis gigantea mycovirus dsRNA 2, two fungus-associated fusagraviruses. Genomic features and phylogenetic relatedness suggest that PSFaV, along with its closest relative BabMelV, represent a species of novel plant-associated virus classified within the recently established family Fusagraviridae.

Complete genome sequence of vaccinium-associated virus C, a new member of the family Totiviridae from Vaccinium floribundum

Blueberries (Vaccinium sp.) are a major crop grown in the Pacific Northwest region. Currently, there are at least 17 known viruses that infect blueberry plants, and some of them cause a wide range of symptoms and economic losses. A new virus, vaccinium-associated virus C (VaVC) (family Totiviridae, genus Totivirus) was identified in an imported blueberry accession from the USDA-ARS National Clonal Germplasm Repository in Corvallis, Oregon. The complete genomic sequence of VaVC was determined, but the biological significance of VaVC is unknown and requires further study. Additional Vaccinium sp. accessions should be screened to investigate the incidence of this new virus.

The Deficiency of Hypusinated eIF5A Decreases the Putrescine/Spermidine Ratio and Inhibits +1 Programmed Ribosomal Frameshifting during the Translation of Ty1 Retrotransposon in Saccharomyces cerevisiae

Programmed ribosomal frameshifting (PRF) exists in all branches of life that regulate gene expression at the translational level. The eukaryotic translation initiation factor 5A (eIF5A) is a highly conserved protein essential in all eukaryotes. It is identified initially as an initiation factor and functions broadly in translation elongation and termination. The hypusination of eIF5A is specifically required for +1 PRF at the shifty site derived from the ornithine decarboxylase antizyme 1 (OAZ1) in Saccharomyces cerevisiae. However, whether the regulation of +1 PRF by yeast eIF5A is universal remains unknown. Here, we found that Sc-eIF5A depletion decreased the putrescine/spermidine ratio. The re-introduction of Sc-eIF5A in yeast eIF5A mutants recovered the putrescine/spermidine ratio. In addition, the Sc-eIF5A depletion decreases +1 PRF during the decoding of Ty1 retrotransposon mRNA, but has no effect on -1 PRF during the decoding of L-A virus mRNA. The re-introduction of Sc-eIF5A in yeast eIF5A mutants restored the +1 PRF rate of Ty1. The inhibition of the hypusine modification of yeast eIF5A by GC7 treatment or by mutating the hypusination site Lys to Arg caused decreases of +1 PRF rates in the Ty1 retrotransposon. Furthermore, mutational studies of the Ty1 frameshifting element support a model where the efficient removal of ribosomal subunits at the first Ty1 frame 0 stop codon is required for the frameshifting of trailing ribosomes. This dependency is likely due to the unique position of the frame 0 stop codon distance from the slippery sequence of Ty1. The results showed that eIF5A is a trans-regulator of +1 PRF for Ty1 retrotransposon and could function universally in yeast.

Coronavirus takeover of host cell translation and intracellular antiviral response: a molecular perspective

Coronaviruses are a group of related RNA viruses that cause respiratory diseases in humans and animals. Understanding the mechanisms of translation regulation during coronaviral infections is critical for developing antiviral therapies and preventing viral spread. Translation of the viral single-stranded RNA genome in the host cell cytoplasm is an essential step in the life cycle of coronaviruses, which affects the cellular mRNA translation landscape in many ways. Here we discuss various viral strategies of translation control, including how members of the Betacoronavirus genus shut down host cell translation and suppress host innate immune functions, as well as the role of the viral non-structural protein 1 (Nsp1) in the process. We also outline the fate of viral RNA, considering stress response mechanisms triggered in infected cells, and describe how unique viral RNA features contribute to programmed ribosomal -1 frameshifting, RNA editing, and translation shutdown evasion.

Shiftless Is a Novel Member of the Ribosome Stress Surveillance Machinery That Has Evolved to Play a Role in Innate Immunity and Cancer Surveillance

A longstanding paradox in molecular biology has centered on the question of how very long proteins are synthesized, despite numerous measurements indicating that ribosomes spontaneously shift reading frame at rates that should preclude their ability completely translate their mRNAs. Shiftless (SFL; C19orf66) was originally identified as an interferon responsive gene encoding an antiviral protein, indicating that it is part of the innate immune response. This activity is due to its ability to bind ribosomes that have been programmed by viral sequence elements to shift reading frame. Curiously, Shiftless is constitutively expressed at low levels in mammalian cells. This study examines the effects of altering Shiftless homeostasis, revealing how it may be used by higher eukaryotes to identify and remove spontaneously frameshifted ribosomes, resolving the apparent limitation on protein length. Data also indicate that Shiftless plays a novel role in the ribosome-associated quality control program. A model is proposed wherein SFL recognizes and arrests frameshifted ribosomes, and depending on SFL protein concentrations, either leads to removal of frameshifted ribosomes while leaving mRNAs intact, or to mRNA degradation. We propose that SFL be added to the growing pantheon of proteins involved in surveilling translational fidelity and controlling gene expression in higher eukaryotes.

Identification and molecular characterization of a novel member of the genus Totivirus from Areca catechu L

In previous work, RNA-seq was applied to identify the causal agent of yellow leaf disease (YLD) in areca palm (Areca catechu L.), resulting in the identification of areca palm velarivirus 1 (APV1) associated with YLD. Additionally, RNA-seq revealed a totivirus-like virus in areca palm. This work revealed that the totivirus-like virus is prevalent in asymptomatic areca palms. Therefore, it was tentatively named "areca palm latent totivirus 1" (APLTV1). The complete genome sequence of APLTV1 was determined and found to be 4754 base pairs (bp) in length, containing two ORFs whose encoded proteins share 55% and 69% amino acid (aa) sequence identity with the capsid protein (CP) and RNA-dependent RNA polymerase (RdRp), respectively, of Bursera graveolens-associated totivirus 1 (BgAT1). Phylogenetic analysis based on alignment of the CP and RdRp sequences revealed that APLTV1 clustered with other members of the genus Totivirus, suggesting that APLTV1 represents a novel species of the genus Totivirus, family Totiviridae.

Programmable eukaryotic protein synthesis with RNA sensors by harnessing ADAR

Programmable approaches to sense and respond to the presence of specific RNAs in biological systems have broad applications in research, diagnostics, and therapeutics. Here we engineer a programmable RNA-sensing technology, reprogrammable ADAR sensors (RADARS), which harnesses RNA editing by adenosine deaminases acting on RNA (ADAR) to gate translation of a cargo protein by the presence of endogenous RNA transcripts. Introduction of a stop codon in a guide upstream of the cargo makes translation contingent on binding of an endogenous transcript to the guide, leading to ADAR editing of the stop codon and allowing translational readthrough. Through systematic sensor engineering, we achieve 277 fold improvement in sensor activation and engineer RADARS with diverse cargo proteins, including luciferases, fluorescent proteins, recombinases, and caspases, enabling detection sensitivity on endogenous transcripts expressed at levels as low as 13 transcripts per million. We show that RADARS are functional as either expressed DNA or synthetic mRNA and with either exogenous or endogenous ADAR. We apply RADARS in multiple contexts, including tracking transcriptional states, RNA-sensing-induced cell death, cell-type identification, and control of synthetic mRNA translation.

Novel insights into virus-host interactions using the model organism C. elegans

Viruses continue to pose a public health threat raising the need for effective management strategies. Currently existing antiviral therapeutics are often specific to only a single viral species, and resistance to the therapeutic can often arise, and therefore new therapeutics are needed. The C. elegans-Orsay virus system offers a powerful platform for studying RNA virus-host interactions that could ultimately lead to novel targets for antiviral therapy. The relative simplicity of C. elegans, the well-established experimental tools, and its extensive evolutionary conservation of genes and pathways with mammals are key features of this model. Orsay virus, a bisegmented positive sense RNA virus, is a natural pathogen of C. elegans. Orsay virus infection can be studied in a multicellular organismal context, overcoming some of the limitations inherent to tissue culture-based systems. Moreover, compared to mice, the rapid generation time of C. elegans enables robust and facile forward genetics. This review aims to summarize studies that have laid the foundation for the C. elegans-Orsay virus experimental system, experimental tools, and key examples of C. elegans host factors that impact Orsay virus infection that have evolutionarily conserved function in mammalian virus infection.

Deep mutational analysis of elongation factor eEF2 residues implicated in human disease to identify functionally important contacts with the ribosome

An emerging body of research is revealing mutations in elongation factor eEF2 that are implicated in both inherited and de novo neurodevelopmental disorders. Previous structural analysis has revealed that most pathogenic amino acid substitutions map to the three main points of contact between eEF2 and critical large subunit rRNA elements of the ribosome, specifically to contacts with Helix 69, Helix 95, also known as the sarcin-ricin loop, and Helix 43 of the GTPase-associated center. In order to further investigate these eEF2-ribosome interactions, we identified a series of yeast eEF2 amino acid residues based on their proximity to these functionally important rRNA elements. Based on this analysis, we constructed mutant strains to sample the full range of amino acid sidechain biochemical properties, including acidic, basic, nonpolar, and deletion (alanine) residues. These were characterized with regard to their effects on cell growth, sensitivity to ribosome-targeting antibiotics, and translational fidelity. We also biophysically characterized one mutant from each of the three main points of contact with the ribosome using CD. Collectively, our findings from these studies identified functionally critical contacts between eEF2 and the ribosome. The library of eEF2 mutants generated in this study may serve as an important resource for biophysical studies of eEF2/ribosome interactions going forward.

Identification of novel totiviruses from the ascomycetous fungus Geotrichum candidum

Mycoviruses are widely distributed across the kingdom Fungi, including ascomycetous yeast strains of the class Saccharomycetes. Geotrichum candidum is an important fungal pathogen belonging to Saccharomycetes and has a diverse host range. Here, we report the characterization of four new classical totiviruses from two distinct Geotrichum candidum strains from Pakistan. The four identified viruses were tentatively named "Geotrichum candidum totivirus 1, 2, 3a, and 3b" (GcTV1-3b). The complete dsRNA genomes of the identified totiviruses are 4621, 4592, 4576, and 4576 bp in length, respectively. All totivirus genomes have two open reading frames, encoding a capsid protein (CP) and an RNA-dependent RNA polymerase (RdRP), respectively. The downstream RdRP domain is assumed to be expressed as a CP-RdRP fusion product via -1 frameshifting mediated by a heptameric slippery site. Sequence comparisons and phylogenetic analysis showed that each of the discovered viruses belongs to a new species of the genus Totivirus in the family Totiviridae, with GcTV1 and GcTV3 (a and b strains) clustering in one subgroup and GcTV2 in another subgroup.

Molecular Characterization of Novel Mycoviruses in Seven Umbelopsis Strains

The presence of viruses is less explored in Mucoromycota as compared to other fungal groups such as Ascomycota and Basidiomycota. Recently, more and more mycoviruses are identified from the early-diverging lineages of fungi. We have determined the genome of 11 novel dsRNA viruses in seven different Umbelopsis strains with next-generation sequencing (NGS). The identified viruses were named Umbelopsis ramanniana virus 5 (UrV5), 6a (UrV6a); 6b (UrV6b); 7 (UrV7); 8a (UrV8a); 8b (UrV8b); Umbelopsis gibberispora virus 1 (UgV1); 2 (UgV2) and Umbelopsis dimorpha virus 1a (UdV1a), 1b (UdV1b) and 2 (UdV2). All the newly identified viruses contain two open reading frames (ORFs), which putatively encode the coat protein (CP) and the RNA-dependent RNA polymerase (RdRp), respectively. Based on the phylogeny inferred from the RdRp sequences, eight viruses (UrV7, UrV8a, UrV8b, UgV1, UgV2, UdV1a, UdV1b and UdV2) belong to the genus Totivirus, while UrV5, UrV6a and UrV6b are placed into a yet unclassified but well-defined Totiviridae-related group. In UrV5, UgV1, UgV2, UrV8b, UdV1a, UdV2 and UdV1b, ORF2 is predicted to be translated as a fusion protein via a rare +1 (or -2) ribosomal frameshift, which is not characteristic to most members of the Totivirus genus. Virus particles 31 to 32 nm in diameter could be detected in the examined fungal strains by transmission electron microscopy. Through the identification and characterization of new viruses of Mucoromycota fungi, we can gain insight into the diversity of mycoviruses, as well as into their phylogeny and genome organization.

Structures of L-BC virus and its open particle provide insight into Totivirus capsid assembly

L-BC virus persists in the budding yeast Saccharomyces cerevisiae, whereas other viruses from the family Totiviridae infect a diverse group of organisms including protists, fungi, arthropods, and vertebrates. The presence of totiviruses alters the fitness of the host organisms, for example, by maintaining the killer system in yeast or increasing the virulence of Leishmania guyanensis. Despite the importance of totiviruses for their host survival, there is limited information about Totivirus structure and assembly. Here we used cryo-electron microscopy to determine the structure of L-BC virus to a resolution of 2.9 Å. The L-BC capsid is organized with icosahedral symmetry, with each asymmetric unit composed of two copies of the capsid protein. Decamers of capsid proteins are stabilized by domain swapping of the C-termini of subunits located around icosahedral fivefold axes. We show that capsids of 9% of particles in a purified L-BC sample were open and lacked one decamer of capsid proteins. The existence of the open particles together with domain swapping within a decamer provides evidence that Totiviridae capsids assemble from the decamers of capsid proteins. Furthermore, the open particles may be assembly intermediates that are prepared for the incorporation of the virus (+) strand RNA.

A 2.9 Å resolution structure of the L-BC virus provides insight into the contacts between capsid proteins and the mechanism of capsid assembly.

A Transfectable Fusagravirus from a Japanese Strain of Cryphonectria carpinicola with Spherical Particles

A novel dsRNA virus (Cryphonectria carpinicola fusagravirus 1, CcFGV1), isolated from a Japanese strain (JS13) of Cryphonectria carpinicola, was thoroughly characterized. The biological comparison of a set of isogenic CcFGV1-infected and -free (JS13VF) strains indicated asymptomatic infection by CcFGV1. The sequence analysis showed that the virus has a two open reading frame (ORF) genome of 9.6 kbp with the RNA-directed RNA polymerase domain encoded by ORF2. The N-terminal sequencing and peptide mass fingerprinting showed an N-terminally processed or degraded product (150 kDa) of the 5′-proximal ORF1-encoded protein (1462 amino acids) to make up the CcFGV1 spherical particles of ~40 nm in diameter. Interestingly, a portion of CcFGV1 dsRNA co-fractionated with a host protein of 70 kDa. The purified CcFGV1 particles were used to transfect protoplasts of JS13VF as well as the standard strain of an experimental model filamentous fungal host Cryphonectria parasitica. CcFGV1 was confirmed to be associated with asymptomatic infection of both fungi. RNA silencing was shown to target the virus in C. parasitica, resulting in reduced CcFGV1 accumulation by comparing the CcFGV1 content between RNA silencing-competent and -deficient strains. These results indicate the transfectability of spherical particles of a fusagravirus associated with asymptomatic infection.

Infection of Nigrospora nonsegmented RNA Virus 1 Has Important Biological Impacts on a Fungal Host

Nigrospora nonsegmented RNA virus 1 (NoNRV1) has been reported previously in the fungus Nigrospora oryzae, but its biological effects on its host are unknown. In this work, we isolated a strain 9-1 of N. oryzae from a chrysanthemum leaf and identified NoNRV1 infection in the isolated strain. The genome sequence of NoNRV1 identified here is highly homologous to that of the isolate HN-21 of NoNRV1 previously reported; thus, we tentatively designated the newly identified NoNRV1 as NoNRV1-ZJ. Drug treatment with Ribavirin successfully removed NoNRV1-ZJ from the strain 9-1, which provided us with an ideal control to determine the biological impacts of NoNRV1 infection on host fungi. By comparing the virus-carrying (9-1) and virus-cured (9-1C) strains, our results indicated that infection with NoNRV1 promoted the pigmentation of the host cells, while it had no discernable effects on host growth on potato dextrose agar plates when subjected to osmotic or oxidative stress. Interestingly, we observed inhibitory impacts of virus infection on the thermotolerance of N. oryzae and the pathogenicity of the host fungus in cotton leaves. Collectively, our work provides clear evidence of the biological relevance of NoNRV1 infection in N. oryzae, including pigmentation, hypovirulence, and thermotolerance.

Expression of the K74 Killer Toxin from Saccharomyces paradoxus Is Modulated by the Toxin-Encoding M74 Double-Stranded RNA 5' Untranslated Terminal Region

Yeast killer toxins are widely distributed in nature, conferring a competitive advantage to the producer yeasts over nonkiller ones when nutrients are scarce. Most of these toxins are encoded on double-stranded RNAs (dsRNAs) generically called M. L-A members of the viral family Totiviridae act as helper viruses to maintain M, providing the virion proteins that separately encapsidate and replicate L-A and M genomes. M genomes are organized in three regions, a 5' region coding the preprotoxin, followed by an internal poly(A) stretch and a 3' noncoding region. In this work, we report the characterization of K74 toxin encoded on M74 dsRNA from Saccharomyces paradoxus Q74.4. In M74, there is a 5' upstream sequence of 141 nucleotides (nt), which contains regulatory signals for internal translation of the preprotoxin open reading frame (ORF) at the second AUG codon. The first AUG close to the 5' end is not functional. For K74 analysis, M74 viruses were first introduced into laboratory strains of Saccharomyces cerevisiae. We show here that the mature toxin is an α/β heterodimer linked by disulfide bonds. Though the toxin (or preprotoxin) confers immunity to the carrier, cells with increased K74 loads have a sick phenotype that may lead to cell death. Thus, a fine-tuning of K74 production by the upstream regulatory sequence is essential for the host cell to benefit from the toxin it produces and, at the same time, to safely avoid damage by an excess of toxin. IMPORTANCE Killer yeasts produce toxins to which they are immune by mechanisms not well understood. This self-immunity, however, is compromised in certain strains, which secrete an excess of toxin, leading to sick cells or suicidal phenotypes. Thus, a fine-tuning of toxin production has to be achieved to reach a balance between the beneficial effect of toxin production and the stress imposed on the host metabolism. K74 toxin from S. paradoxus is very active against Saccharomyces uvarum, among other yeasts, but an excess of toxin production is deleterious for the host. Here, we report that the presence of a 5' 141-nt upstream sequence downregulates K74 toxin precursor translation, decreasing toxin levels 3- to 5-fold. Thus, this is a special case of translation regulation performed by sequences on the M74 genome itself, which have been presumably incorporated into the viral RNA during evolution for that purpose.

The Characterization of a Novel Virus Discovered in the Yeast Pichia membranifaciens

Mycoviruses are widely distributed across fungi, including the yeasts of the Saccharomycotina subphylum. This manuscript reports the first double-stranded RNA (dsRNA) virus isolated from Pichia membranifaciens. This novel virus has been named Pichia membranifaciens virus L-A (PmV-L-A) and is a member of the Totiviridae. PmV-L-A is 4579 bp in length, with RNA secondary structures similar to the packaging, replication, and frameshift signals of totiviruses that infect Saccharomycotina yeasts. PmV-L-A was found to be part of a monophyletic group within the I-A totiviruses, implying a shared ancestry between mycoviruses isolated from the Pichiaceae and Saccharomycetaceae yeasts. Energy-minimized AlphaFold2 molecular models of the PmV-L-A Gag protein revealed structural conservation with the Gag protein of Saccharomyces cerevisiae virus L-A (ScV-L-A). The predicted tertiary structure of the PmV-L-A Pol and other homologs provided a possible mechanism for totivirus RNA replication due to structural similarities with the RNA-dependent RNA polymerases of mammalian dsRNA viruses. Insights into the structure, function, and evolution of totiviruses gained from yeasts are essential because of their emerging role in animal disease and their parallels with mammalian viruses.

Characterization of Two Novel Toti-Like Viruses Co-infecting the Atlantic Blue Crab, Callinectes sapidus, in Its Northern Range of the United States

The advancement of high throughput sequencing has greatly facilitated the exploration of viruses that infect marine hosts. For example, a number of putative virus genomes belonging to the Totiviridae family have been described in crustacean hosts. However, there has been no characterization of the most newly discovered putative viruses beyond description of their genomes. In this study, two novel double-stranded RNA (dsRNA) virus genomes were discovered in the Atlantic blue crab (Callinectes sapidus) and further investigated. Sequencing of both virus genomes revealed that they each encode RNA dependent RNA polymerase proteins (RdRps) with similarities to toti-like viruses. The viruses were tentatively named Callinectes sapidus toti-like virus 1 (CsTLV1) and Callinectes sapidus toti-like virus 2 (CsTLV2). Both genomes have typical elements required for −1 ribosomal frameshifting, which may induce the expression of an encoded ORF1–ORF2 (gag-pol) fusion protein. Phylogenetic analyses of CsTLV1 and CsTLV2 RdRp amino acid sequences suggested that they are members of two new genera in the family Totiviridae. The CsTLV1 and CsTLV2 genomes were detected in muscle, gill, and hepatopancreas of blue crabs by real-time reverse transcription quantitative PCR (RT-qPCR). The presence of ~40 nm totivirus-like viral particles in all three tissues was verified by transmission electron microscopy, and pathology associated with CsTLV1 and CsTLV2 infections were observed by histology. PCR assays showed the prevalence and geographic range of these viruses, to be restricted to the northeast United States sites sampled. The two virus genomes co-occurred in almost all cases, with the CsTLV2 genome being found on its own in 8.5% cases, and the CsTLV1 genome not yet found on its own. To our knowledge, this is the first report of toti-like viruses in C. sapidus. The information reported here provides the knowledge and tools to investigate transmission and potential pathogenicity of these viruses.

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

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

Genome Features of a New Double-Stranded RNA Helper Virus (LBCbarr) from Wine Torulaspora delbrueckii Killer Strains

The killer phenotype of Torulaspora delbrueckii (Td) and Saccharomyces cerevisiae (Sc) is encoded in the genome of medium-size dsRNA viruses (V-M). Killer strains also contain a helper large size (4.6 kb) dsRNA virus (V-LA) which is required for maintenance and replication of V-M. Another large-size (4.6 kb) dsRNA virus (V-LBC), without known helper activity to date, may join V-LA and V-M in the same yeast. T. delbrueckii Kbarr1 killer strain contains the killer virus Mbarr1 in addition to two L viruses, TdV-LAbarr1 and TdV-LBCbarr1. In contrast, the T. delbrueckii Kbarr2 killer strain contains two M killer viruses (Mbarr1 and M1) and a LBC virus (TdV-LBCbarr2), which has helper capability to maintain both M viruses. The genomes of TdV-LBCbarr1 and TdV-LBCbarr2 were characterized by high-throughput sequencing (HTS). Both RNA genomes share sequence identity and similar organization with their ScV-LBC counterparts. They contain all conserved motifs required for translation, packaging, and replication of viral RNA. Their Gag-Pol amino-acid sequences also contain the features required for cap-snatching and RNA polymerase activity. However, some of these motifs and features are similar to those of LA viruses, which may explain that at least TdV-LBCbarr2 has a helper ability to maintain M killer viruses. Newly sequenced ScV-LBC genomes contained the same motifs and features previously found in LBC viruses, with the same genome location and secondary structure. Sequence comparison showed that LBC viruses belong to two clusters related to each species of yeast. No evidence for associated co-evolution of specific LBC with specific M virus was found. The presence of the same M1 virus in S. cerevisiae and T. delbrueckii raises the possibility of cross-species transmission of M viruses.

Potential 3-chymotrypsin-like cysteine protease cleavage sites in the coronavirus polyproteins pp1a and pp1ab and their possible relevance to COVID-19 vaccine and drug development

Coronavirus (CoV) 3-chymotrypsin (C)-like cysteine protease (3CLpro ) is a target for anti-CoV drug development and drug repurposing because along with papain-like protease, it cleaves CoV-encoded polyproteins (pp1a and pp1ab) into nonstructural proteins (nsps) for viral replication. However, the cleavage sites of 3CLpro and their relevant nsps remain unclear, which is the subject of this perspective. Here, we address the subject from three standpoints. First, we explore the inconsistency in the cleavage sites and relevant nsps across CoVs, and investigate the function of nsp11. Second, we consider the nsp16 mRNA overlapping of the spike protein mRNA, and analyze the effect of this overlapping on mRNA vaccines. Finally, we study nsp12, whose existence depends on ribosomal frameshifting, and investigate whether 3CLpro requires a large number of inhibitors to achieve full inhibition. This perspective helps us to clarify viral replication and is useful for developing anti-CoV drugs with 3CLpro as a target in the current coronavirus disease 2019 (COVID-19) pandemic.

Specificity Determination in Saccharomyces cerevisiae Killer Virus Systems

Saccharomyces yeasts are widely distributed in the environment and microbiota of higher organisms. The killer phenotype of yeast, encoded by double-stranded RNA (dsRNA) virus systems, is a valuable trait for host survival. The mutual relationship between the different yet clearly defined LA and M virus pairs suggests complex fitting context. To define the basis of this compatibility, we established a system devoted to challenging inherent yeast viruses using viral proteins expressed in trans. Virus exclusion by abridged capsid proteins was found to be complete and nonspecific, indicating the presence of generic mechanisms of Totiviridae maintenance in yeast cells. Indications of specificity in both the exclusion of LA viruses and the maintenance of M viruses by viral capsid proteins expressed in trans were observed. This precise specificity was further established by demonstrating the importance of the satellite virus in the maintenance of LA virus, suggesting the selfish behavior of M dsRNA.

Conformational Shannon Entropy of mRNA Structures from Force Spectroscopy Measurements Predicts the Efficiency of -1 Programmed Ribosomal Frameshift Stimulation

-1 programmed ribosomal frameshifting (-1  PRF) is stimulated by structures in messenger RNA (mRNA), but the factors determining -1  PRF efficiency are unclear. We show that -1  PRF efficiency varies directly with the conformational heterogeneity of the stimulatory structure, quantified as the Shannon entropy of the state occupancy, for a panel of stimulatory structures with efficiencies from 2% to 80%. The correlation is force dependent and vanishes at forces above those applied by the ribosome. These results support the hypothesis that heterogeneous conformational dynamics are a key factor in stimulating -1  PRF.

Structure and assembly of double-stranded RNA mycoviruses

Mycoviruses are a diverse group that includes ssRNA, dsRNA, and ssDNA viruses, with or without a protein capsid, as well as with a complex envelope. Most mycoviruses are transmitted by cytoplasmic interchange and are thought to lack an extracellular phase in their infection cycle. Structural analysis has focused on dsRNA mycoviruses, which usually package their genome in a 120-subunit T=1 icosahedral capsid, with a capsid protein (CP) dimer as the asymmetric unit. The atomic structure is available for four dsRNA mycovirus from different families: Saccharomyces cerevisiae virus L-A (ScV-L-A), Penicillium chrysogenum virus (PcV), Penicillium stoloniferum virus F (PsV-F), and Rosellinia necatrix quadrivirus 1 (RnQV1). Their capsids show structural variations of the same framework, with asymmetric or symmetric CP dimers respectively for ScV-L-A and PsV-F, dimers of similar domains of a single CP for PcV, or of two different proteins for RnQV1. The CP dimer is the building block, and assembly proceeds through dimers of dimers or pentamers of dimers, in which the genome is packed as ssRNA by interaction with CP and/or viral polymerase. These capsids remain structurally undisturbed throughout the viral cycle. The T=1 capsid participates in RNA synthesis, organizing the viral polymerase (1-2 copies) and a single loosely packaged genome segment. It also acts as a molecular sieve, to allow the passage of viral transcripts and nucleotides, but to prevent triggering of host defense mechanisms. Due to the close mycovirus-host relationship, CP evolved to allocate peptide insertions with enzyme activity, as reflected in a rough outer capsid surface.

A Novel Mycovirus Evokes Transcriptional Rewiring in the Fungus Malassezia and Stimulates Beta Interferon Production in Macrophages

Malassezia species represent the most common fungal inhabitant of the mammalian skin microbiome and are natural skin commensal flora. However, these fungi are also associated with a variety of clinical skin disorders. Recent studies have reported associations of Malassezia with Crohn’s disease and pancreatic cancer, further implicating this fungal genus in inflammatory and neoplastic disease states. Because M. sympodialis has lost genes involved in RNA interference (RNAi), we hypothesized Malassezia could harbor dsRNA mycoviruses. Indeed, we identified a novel mycovirus of the totivirus family in several Malassezia species and characterized the MsMV1 mycovirus of M. sympodialis. We found conditions that lead to curing of the virus and analyzed isogenic virus-infected/virus-cured strains to determine MsMV1 genetic and pathogenic impacts. MsMV1 induces a strong overexpression of transcription factors and ribosomal genes, while downregulating cellular metabolism. Moreover, MsMV1 induced a significantly higher level of beta interferon expression in cultured macrophages. This study sheds light on the mechanisms of pathogenicity of Malassezia, focusing on a previously unidentified novel mycovirus.

Mycoviruses infect fungi, and while most persist asymptomatically, there are examples of mycoviruses having both beneficial and detrimental effects on their host. Virus-infected Saccharomyces and Ustilago strains exhibit a killer phenotype conferring a growth advantage over uninfected strains and other competing yeast species, whereas hypovirus-infected Cryphonectria parasitica displays defects in growth, sporulation, and virulence. In this study, we identify a double-stranded RNA (dsRNA) mycovirus in five Malassezia species. Sequence analysis reveals it to be a totivirus with two dsRNA segments: a larger 4.5-kb segment with genes encoding components for viral replication and maintenance, and a smaller 1.4-kb segment encoding a novel protein. Furthermore, transcriptome sequencing (RNA-seq) of virus-infected versus virus-cured Malassezia sympodialis revealed an upregulation of dozens of ribosomal components in the cell, suggesting the virus modifies the transcriptional and translational landscapes of the cell. Given that Malassezia is the most abundant fungus on human skin, we assessed the impact of the mycovirus in a murine epicutaneous infection model. Although infection with virus-infected strains was not associated with an increased inflammatory response, we did observe enhanced skin colonization in one of two virus-infected M. sympodialis strains. Noteworthy, beta interferon expression was significantly upregulated in bone marrow-derived macrophages when challenged with virus-infected, compared to virus-cured, M. sympodialis, suggesting that the presence of the virus can induce an immunological response. Although many recent studies have illuminated how widespread mycoviruses are, there are relatively few in-depth studies about their impact on disease caused by the host fungus. We describe here a novel mycovirus in Malassezia and its possible implications in pathogenicity.

A Novel Virus Alters Gene Expression and Vacuolar Morphology in Malassezia Cells and Induces a TLR3-Mediated Inflammatory Immune Response

Malassezia is the most dominant fungal genus on the human skin surface and is associated with various skin diseases including dandruff and seborrheic dermatitis. Among Malassezia species, Malassezia restricta is the most widely observed species on the human skin. In the current study, we identified a novel dsRNA virus, named MrV40, in M. restricta and characterized the sequence and structure of the viral genome along with an independent satellite dsRNA viral segment. Moreover, expression of genes involved in ribosomal synthesis and programmed cell death was altered, indicating that virus infection affected the physiology of the fungal host cells. Our data also showed that the viral nucleic acid from MrV40 induces a TLR3-mediated inflammatory immune response in bone marrow-derived dendritic cells, indicating that a viral element likely contributes to the pathogenicity of Malassezia. This is the first study to identify and characterize a novel mycovirus in Malassezia.

Most fungal viruses have been identified in plant pathogens, whereas the presence of viral particles in human-pathogenic fungi is less well studied. In the present study, we observed extrachromosomal double-stranded RNA (dsRNA) segments in various clinical isolates of Malassezia species. Malassezia is the most dominant fungal genus on the human skin surface, and species in this group are considered etiological factors of various skin diseases including dandruff, seborrheic dermatitis, and atopic dermatitis. We identified novel dsRNA segments, and our sequencing results revealed that the virus, named MrV40, belongs to the Totiviridae family and contains an additional satellite dsRNA segment encoding a novel protein. The transcriptome of virus-infected Malassezia restricta cells was compared to that of virus-cured cells, and the results showed that transcripts involved in ribosomal biosynthesis were downregulated and those involved in energy production and programmed cell death were upregulated. Moreover, transmission electron microscopy revealed significantly larger vacuoles in virus-infected M. restricta cells, indicating that MrV40 infection dramatically altered M. restricta physiology. Our analysis also revealed that viral nucleic acid from MrV40 induced a TLR3 (Toll-like receptor 3)-mediated inflammatory immune response in bone marrow-derived dendritic cells, suggesting that a viral element contributes to the pathogenicity of Malassezia.

Disome and Trisome Profiling Reveal Genome-wide Targets of Ribosome Quality Control

The ribosome-associated protein quality control (RQC) system that resolves stalled translation events is activated when ribosomes collide and form disome, trisome, or higher-order complexes. However, it is unclear whether this system distinguishes collision complexes formed on defective mRNAs from those with functional roles on endogenous transcripts. Here, we performed disome and trisome footprint profiling in yeast and found collisions were enriched on diverse sequence motifs known to slow translation. When 60S recycling was inhibited, disomes accumulated at stop codons and could move into the 3' UTR to reinitiate translation. The ubiquitin ligase and RQC factor Hel2/ZNF598 generally recognized collisions but did not induce degradation of endogenous transcripts. However, loss of Hel2 triggered the integrated stress response, via phosphorylation of eIF2α, thus linking these pathways. Our results suggest that Hel2 has a role in sensing ribosome collisions on endogenous mRNAs, and such events may be important for cellular homeostasis.

Yeast Viral Killer Toxin K1 Induces Specific Host Cell Adaptions via Intrinsic Selection Pressure

The killer phenomenon in yeast (Saccharomyces cerevisiae) not only provides the opportunity to study host-virus interactions in a eukaryotic model but also represents a powerful tool to analyze potential coadaptional events and the role of killer yeast in biological diversity. Although undoubtedly having a crucial impact on the abundance and expression of the killer phenotype in killer-yeast harboring communities, the influence of a particular toxin on its producing host cell has not been addressed sufficiently. In this study, we describe a model system of two K1 killer yeast strains with distinct phenotypical differences pointing to substantial selection pressure in response to the toxin secretion level. Transcriptome and lipidome analyses revealed specific and intrinsic host cell adaptions dependent on the amount of K1 toxin produced. High basal expression of genes coding for osmoprotectants and stress-responsive proteins in a killer yeast strain secreting larger amounts of active K1 toxin implies a generally increased stress tolerance. Moreover, the data suggest that immunity of the host cell against its own toxin is essential for the balanced virus-host interplay providing valuable hints to elucidate the molecular mechanisms underlying K1 immunity and implicating an evolutionarily conserved role for toxin immunity in natural yeast populations.IMPORTANCE The killer phenotype in Saccharomyces cerevisiae relies on the cytoplasmic persistence of two RNA viruses. In contrast to bacterial toxin producers, killer yeasts necessitate a specific immunity mechanism against their own toxin because they bear the same receptor populations as sensitive cells. Although the killer phenomenon is highly abundant and has a crucial impact on the structure of yeast communities, the influence of a particular toxin on its host cell has been barely addressed. In our study, we used two derivatives secreting different amount of the killer toxin K1 to analyze potential coadaptional events in this particular host/virus system. Our data underline the dependency of the host cell's ability to cope with extracellular toxin molecules and intracellular K1 molecules provided by the virus. Therefore, this research significantly advances the current understanding of the evolutionarily conserved role of this molecular machinery as an intrinsic selection pressure in yeast populations.

A Novel Totivirus Naturally Occurring in Two Different Fungal Genera

Mycoviruses are widely distributed across different phyla of the fungal kingdom. Viruses that share significant sequence similarities have been reported in different fungi, suggesting descent from a common ancestor. In this study, two fungal genera isolated from the same sample, Trichoderma koningiopsis isolate Mg10 and Clonostachys rosea isolate Mg06, were reported to have identical double-stranded RNA (dsRNA) profiles that consist of two virus-like, dsRNA elements (dsRNA-L and dsRNA-S). The complete sequence and genome organization of dsRNA-L from isolate Mg10 was determined. It is 4712 nucleotides (nt) long and contains two non-overlapping open reading frames (ORFs) that code for proteins with similarities to totiviruses. Consequently the virus was given the proposed name Trichoderma koningiopsis totivirus 1 (TkTV1/Mg10). The TkTV1/Mg10 genome structure resembles that of yeast totiviruses in which the region preceding the stop codon of ORF1 contains the elements required for -1 ribosomal frameshifting which may induce the expression of an ORF1–ORF2 (CP-RdRp) fusion protein. Sequence analyses of viral dsRNA-L from C. rosea isolate Mg06 revealed that it is nearly identical with that of TkTV1/Mg10. This relatedness was confirmed by northern blot hybridization and indicates very recent natural horizontal transmission of this virus between unrelated fungi. TkTV1 purified isometric virions were ∼38–40 nm in diameter and were able to transfect T. koningiopsis and C. rosea protoplasts. This is another report of a mycovirus present naturally in two taxonomically distinct fungi.

Novel Victorivirus from a Pakistani Isolate of Alternaria alternata Lacking a Typical Translational Stop/Restart Sequence Signature

The family Totiviridae currently contains five genera Totivirus, Victorivirus, Leishmavirus, Trichomonasvirus, and Giardiavirus. Members in this family generally have a set of two-open reading frame (ORF) elements in their genome with the 5'-proximal ORF (ORF1) encoding a capsid protein (CP) and the 3'-proximal one (ORF2) for RNA-dependent RNA polymerase (RdRp). How the downstream open reading frames (ORFs) are expressed is genus-specific. All victoriviruses characterized thus far appear to use the stop/restart translation mechanism, allowing for the expression of two separate protein products from bicitronic genome-sized viral mRNA, while the totiviruses use a -1 ribosomal frame-shifting that leads to a fusion product of CP and RdRp. We report the biological and molecular characterization of a novel victorivirus termed Alternaria alternata victorivirus 1 (AalVV1) isolated from Alternaria alternata in Pakistan. The phylogenetic and molecular analyses showed AalVV1 to be distinct from previously reported victoriviruses. AalVV1 appears to have a sequence signature required for the -1 frame-shifting at the ORF1/2 junction region, rather than a stop/restart key mediator. By contrast, SDS-polyacrylamide gel electrophoresis and peptide mass fingerprinting analyses of purified virion preparations suggested the expression of two protein products, not a CP-RdRp fusion product. How these proteins are expressed is discussed in this study. Possible effects of infection by this virus were tested in two fungal species: A. alternata and RNA silencing proficient and deficient strains of Cryphonectria parasitica, a model filamentous fungus. AalVV1 showed symptomless infection in all of these fungal strains, even in the RNA silencing deficient C. parasitica strain.

Translational recoding signals: Expanding the synthetic biology toolbox

Innovation follows discovery. If the 20th century was a golden age of discovery in the biomolecular biosciences, the current century may be remembered by the explosion of beneficial devices and therapies conceived by the bioengineers of the era. Much as the development of solid-state electronic components made possible the information revolution, the rational combining of millions of basic molecular control modules will enable the development of highly sophisticated biomachines that will make today's smartphones appear rudimentary. The molecular toolbox is already well-stocked, particularly in our ability to manipulate DNA, control transcription, generate functionally novel hybrid proteins, and expand the genetic code to include unnatural amino acids. This review focuses on how RNA-based regulatory modules that direct alternative readings of the genetic code can be employed as basic circuit components to expand our ability to control gene expression.

The cap-snatching reaction of yeast L-A double-stranded RNA virus is reversible and the catalytic sites on both Gag and the Gag domain of Gag-Pol are active

The yeast L-A double-stranded RNA virus synthesizes capped transcripts by a unique cap-snatching mechanism in which the m⁷ Gp moiety of host mRNA (donor) is transferred to the diphosphorylated 5' end of the viral transcript (acceptor). This reaction is activated by viral transcription. Here, we show that cap snatching can be reversible. Because only m⁷ Gp is transferred during the reaction, the resulting decapped donor, as expected, retained diphosphates at the 5' end. We also found that the 5' terminal nucleotide of the acceptor needs to be G but not A. Interestingly, the A-initiated molecule when equipped with a cap structure (m⁷ GpppA…) could work as cap donor. Because the majority of host mRNAs in yeast have A after the cap structures at the 5' ends, this finding implies that cap-snatching in vivo is virtually a one-way reaction, in favor of furnishing the viral transcript with a cap. The cap-snatching sites are located on the coat protein Gag and also the Gag domain of Gag-Pol. Here, we demonstrate that both sites are functional, indicating that activation of cap snatching by transcription is not transmitted through the peptide bonding between the Gag and Pol domains of Gag-Pol.

Saccharomyces paradoxus K66 Killer System Evidences Expanded Assortment of Helper and Satellite Viruses

The Saccharomycetaceae yeast family recently became recognized for expanding of the repertoire of different dsRNA-based viruses, highlighting the need for understanding of their cross-dependence. We isolated the Saccharomyces paradoxus AML-15-66 killer strain from spontaneous fermentation of serviceberries and identified helper and satellite viruses of the family Totiviridae, which are responsible for the killing phenotype. The corresponding full dsRNA genomes of viruses have been cloned and sequenced. Sequence analysis of SpV-LA-66 identified it to be most similar to S. paradoxus LA-28 type viruses, while SpV-M66 was mostly similar to the SpV-M21 virus. Sequence and functional analysis revealed significant differences between the K66 and the K28 toxins. The structural organization of the K66 protein resembled those of the K1/K2 type toxins. The AML-15-66 strain possesses the most expressed killing property towards the K28 toxin-producing strain. A genetic screen performed on S. cerevisiae YKO library strains revealed 125 gene products important for the functioning of the S. paradoxus K66 toxin, with 85% of the discovered modulators shared with S. cerevisiae K2 or K1 toxins. Investigation of the K66 protein binding to cells and different polysaccharides implies the β-1,6 glucans to be the primary receptors of S. paradoxus K66 toxin. For the first time, we demonstrated the coherent habitation of different types of helper and satellite viruses in a wild-type S. paradoxus strain.

A Coding Sequence-Embedded Principle Governs Translational Reading Frame Fidelity

Upon initiation at a start codon, the ribosome must maintain the correct reading frame for hundreds of codons in order to produce functional proteins. While some sequence elements are able to trigger programmed ribosomal frameshifting (PRF), very little is known about how the ribosome normally prevents spontaneous frameshift errors that can have dire consequences if uncorrected. Using high resolution ribosome profiling data sets, we discovered that the translating ribosome uses the 3' end of 18S rRNA to scan the AUG-like codons after the decoding process. The postdecoding mRNA:rRNA interaction not only contributes to predominant translational pausing, but also provides a retrospective mechanism to safeguard the ribosome in the correct reading frame. Partially eliminating the AUG-like "sticky" codons in the reporter message leads to increased +1 frameshift errors. Remarkably, mutating the highly conserved CAU triplet of 18S rRNA globally changes the codon "stickiness". Further supporting the role of "sticky" sequences in reading frame maintenance, the codon composition of open reading frames is highly optimized across eukaryotic genomes. These results suggest an important layer of information embedded within the protein-coding sequences that instructs the ribosome to ensure reading frame fidelity during translation.

Capsid Structure of dsRNA Fungal Viruses

Most fungal, double-stranded (ds) RNA viruses lack an extracellular life cycle stage and are transmitted by cytoplasmic interchange. dsRNA mycovirus capsids are based on a 120-subunit T = 1 capsid, with a dimer as the asymmetric unit. These capsids, which remain structurally undisturbed throughout the viral cycle, nevertheless, are dynamic particles involved in the organization of the viral genome and the viral polymerase necessary for RNA synthesis. The atomic structure of the T = 1 capsids of four mycoviruses was resolved: the L-A virus of Saccharomyces cerevisiae (ScV-L-A), Penicillium chrysogenum virus (PcV), Penicillium stoloniferum virus F (PsV-F), and Rosellinia necatrix quadrivirus 1 (RnQV1). These capsids show structural variations of the same framework, with 60 asymmetric or symmetric homodimers for ScV-L-A and PsV-F, respectively, monomers with a duplicated similar domain for PcV, and heterodimers of two different proteins for RnQV1. Mycovirus capsid proteins (CP) share a conserved α-helical domain, although the latter may carry different peptides inserted at preferential hotspots. Insertions in the CP outer surface are likely associated with enzymatic activities. Within the capsid, fungal dsRNA viruses show a low degree of genome compaction compared to reoviruses, and contain one to two copies of the RNA-polymerase complex per virion.

Single-Molecule Mechanical Folding and Unfolding of RNA Hairpins: Effects of Single A-U to A·C Pair Substitutions and Single Proton Binding and Implications for mRNA Structure-Induced -1 Ribosomal Frameshifting

A wobble A·C pair can be protonated at near physiological pH to form a more stable wobble A⁺·C pair. Here, we constructed an RNA hairpin (rHP) and three mutants with one A-U base pair substituted with an A·C mismatch on the top (near the loop, U22C), middle (U25C), and bottom (U29C) positions of the stem, respectively. Our results on single-molecule mechanical (un)folding using optical tweezers reveal the destabilization effect of A-U to A·C pair substitution and protonation-dependent enhancement of mechanical stability facilitated through an increased folding rate, or decreased unfolding rate, or both. Our data show that protonation may occur rapidly upon the formation of an apparent mechanical folding transition state. Furthermore, we measured the bulk -1 ribosomal frameshifting efficiencies of the hairpins by a cell-free translation assay. For the mRNA hairpins studied, -1 frameshifting efficiency correlates with mechanical unfolding force at equilibrium and folding rate at around 15 pN. U29C has a frameshifting efficiency similar to that of rHP (∼2%). Accordingly, the bottom 2-4 base pairs of U29C may not form under a stretching force at pH 7.3, which is consistent with the fact that the bottom base pairs of the hairpins may be disrupted by ribosome at the slippery site. U22C and U25C have a similar frameshifting efficiency (∼1%), indicating that both unfolding and folding rates of an mRNA hairpin in a crowded environment may affect frameshifting. Our data indicate that mechanical (un)folding of RNA hairpins may mimic how mRNAs unfold and fold in the presence of translating ribosomes.

Two novel totiviruses in the white-backed planthopper, Sogatella furcifera

There is little information about commensal viruses in the white-backed planthopper, Sogatella furcifera, although it is an important agricultural insect. Here, two novel double-stranded RNA viruses related to the viruses in the family Totiviridae were identified using next-generation sequencing and tentatively named Sogatella furcifera totivirus 1 and 2 (SfTV1 and SfTV2). Their complete genomes consist of 6310 and 6303 nt, respectively, showing typical genomic features with viruses in the family Totiviridae. Identity, phylogenetic and conserved sequence analyses showed that SfTV1, SfTV2 and three other insect viruses may form a proposed novel genus of the family Totiviridae. Vertical transmission of the two viruses was highly efficient, and they were detected in all insect tissues and developmental stages, with the highest titres in the adult and in the haemolymph and reproductive organs. To our knowledge, this is the first report of viruses in the family Totiviridae found in a hemipteran insect.

Yeast Killer Toxin K28: Biology and Unique Strategy of Host Cell Intoxication and Killing

The initial discovery of killer toxin-secreting brewery strains of Saccharomyces cerevisiae (S. cerevisiae) in the mid-sixties of the last century marked the beginning of intensive research in the yeast virology field. So far, four different S. cerevisiae killer toxins (K28, K1, K2, and Klus), encoded by cytoplasmic inherited double-stranded RNA viruses (dsRNA) of the Totiviridae family, have been identified. Among these, K28 represents the unique example of a yeast viral killer toxin that enters a sensitive cell by receptor-mediated endocytosis to reach its intracellular target(s). This review summarizes and discusses the most recent advances and current knowledge on yeast killer toxin K28, with special emphasis on its endocytosis and intracellular trafficking, pointing towards future directions and open questions in this still timely and fascinating field of killer yeast research.

Variation and Distribution of L-A Helper Totiviruses in Saccharomyces sensu stricto Yeasts Producing Different Killer Toxins

Yeasts within the Saccharomyces sensu stricto cluster can produce different killer toxins. Each toxin is encoded by a medium size (1.5-2.4 Kb) M dsRNA virus, maintained by a larger helper virus generally called L-A (4.6 Kb). Different types of L-A are found associated to specific Ms: L-A in K1 strains and L-A-2 in K2 strains. Here, we extend the analysis of L-A helper viruses to yeasts other than S. cerevisiae, namely S. paradoxus, S. uvarum and S. kudriavzevii. Our sequencing data from nine new L-A variants confirm the specific association of each toxin-producing M and its helper virus, suggesting co-evolution. Their nucleotide sequences vary from 10% to 30% and the variation seems to depend on the geographical location of the hosts, suggesting cross-species transmission between species in the same habitat. Finally, we transferred by genetic methods different killer viruses from S. paradoxus into S. cerevisiae or viruses from S. cerevisiae into S. uvarum or S. kudriavzevii. In the foster hosts, we observed no impairment for their stable transmission and maintenance, indicating that the requirements for virus amplification in these species are essentially the same. We also characterized new killer toxins from S. paradoxus and constructed "superkiller" strains expressing them.

Molecular Characterization of Novel Totivirus-Like Double-Stranded RNAs from Puccinia striiformis f. sp. tritici, the Causal Agent of Wheat Stripe Rust

Characterization of newly isolated mycoviruses may contribute to understanding of the evolution and diversity of viruses. Here, a deep sequencing approach was used to analyze the double-stranded RNA (dsRNA) mycoviruses isolated from field-collected P. striiformis samples in China. Database searches showed the presence of at least four totivirus-like sequences, termed Puccinia striiformis virus 1 to 4 (PsV1 to 4). All of these identified sequences contained two overlapping open reading frames (ORFs) which encode a putative coat protein (CP) and an RNA-dependent RNA polymerase (RdRp) showing similar structures to members of the genus Totivirus. Each PsV contained a -1 ribosomal frameshifting region with a slippery site and a pseudoknot structure in the overlapped regions of these ORFs, indicating that the RdRp is translated as a CP-RdRp fusion. Phylogenetic analyses based on RdRp and CP suggested that these novel viruses belong to the genus Totivirus in the family Totiviridae. The presences of these PsVs were further validated by transmission electron microscope (TEM) and RT-PCR. Taken together, our results demonstrate the presence of diverse, novel totiviruses in the P. striiformis field populations.

The frenemies within: viruses, retrotransposons and plasmids that naturally infect Saccharomyces yeasts

Viruses are a major focus of current research efforts because of their detrimental impact on humanity and their ubiquity within the environment. Bacteriophages have long been used to study host-virus interactions within microbes, but it is often forgotten that the single-celled eukaryote Saccharomyces cerevisiae and related species are infected with double-stranded RNA viruses, single-stranded RNA viruses, LTR-retrotransposons and double-stranded DNA plasmids. These intracellular nucleic acid elements have some similarities to higher eukaryotic viruses, i.e. yeast retrotransposons have an analogous lifecycle to retroviruses, the particle structure of yeast totiviruses resembles the capsid of reoviruses and segregation of yeast plasmids is analogous to segregation strategies used by viral episomes. The powerful experimental tools available to study the genetics, cell biology and evolution of S. cerevisiae are well suited to further our understanding of how cellular processes are hijacked by eukaryotic viruses, retrotransposons and plasmids. This article has been written to briefly introduce viruses, retrotransposons and plasmids that infect Saccharomyces yeasts, emphasize some important cellular proteins and machineries with which they interact, and suggest the evolutionary consequences of these interactions. Copyright © 2017 John Wiley & Sons, Ltd.

Coordination among tertiary base pairs results in an efficient frameshift-stimulating RNA pseudoknot

Frameshifting is an essential process that regulates protein synthesis in many viruses. The ribosome may slip backward when encountering a frameshift motif on the messenger RNA, which usually contains a pseudoknot structure involving tertiary base pair interactions. Due to the lack of detailed molecular explanations, previous studies investigating which features of the pseudoknot are important to stimulate frameshifting have presented diverse conclusions. Here we constructed a bimolecular pseudoknot to dissect the interior tertiary base pairs and used single-molecule approaches to assess the structure targeted by ribosomes. We found that the first ribosome target stem was resistant to unwinding when the neighboring loop was confined along the stem; such constrained conformation was dependent on the presence of consecutive adenosines in this loop. Mutations that disrupted the distal base triples abolished all remaining tertiary base pairs. Changes in frameshifting efficiency correlated with the stem unwinding resistance. Our results demonstrate that various tertiary base pairs are coordinated inside a highly efficient frameshift-stimulating RNA pseudoknot and suggest a mechanism by which mechanical resistance of the pseudoknot may persistently act on translocating ribosomes.

Mechanism and Regulation of Protein Synthesis in Saccharomyces cerevisiae

In this review, we provide an overview of protein synthesis in the yeast Saccharomyces cerevisiae The mechanism of protein synthesis is well conserved between yeast and other eukaryotes, and molecular genetic studies in budding yeast have provided critical insights into the fundamental process of translation as well as its regulation. The review focuses on the initiation and elongation phases of protein synthesis with descriptions of the roles of translation initiation and elongation factors that assist the ribosome in binding the messenger RNA (mRNA), selecting the start codon, and synthesizing the polypeptide. We also examine mechanisms of translational control highlighting the mRNA cap-binding proteins and the regulation of GCN4 and CPA1 mRNAs.

Relationships and Evolution of Double-Stranded RNA Totiviruses of Yeasts Inferred from Analysis of L-A-2 and L-BC Variants in Wine Yeast Strain Populations

Saccharomyces cerevisiae killer strains secrete a protein toxin active on nonkiller strains of the same (or other) yeast species. Different killer toxins, K1, K2, K28, and Klus, have been described. Each toxin is encoded by a medium-size (1.5- to 2.3-kb) M double-stranded RNA (dsRNA) located in the cytoplasm. M dsRNAs require L-A helper virus for maintenance. L-A belongs to the Totiviridae family, and its dsRNA genome of 4.6 kb codes for the major capsid protein Gag and a minor Gag-Pol protein, which form the virions that separately encapsidate L-A or the M satellites. Different L-A variants exist in nature; on average, 24% of their nucleotides are different. Previously, we reported that L-A-lus was specifically associated with Mlus, suggesting coevolution, and proposed a role of the toxin-encoding M dsRNAs in the appearance of new L-A variants. Here we confirm this by analyzing the helper virus in K2 killer wine strains, which we named L-A-2. L-A-2 is required for M2 maintenance, and neither L-A nor L-A-lus shows helper activity for M2 in the same genetic background. This requirement is overcome when coat proteins are provided in large amounts by a vector or in ski mutants. The genome of another totivirus, L-BC, frequently accompanying L-A in the same cells shows a lower degree of variation than does L-A (about 10% of nucleotides are different). Although L-BC has no helper activity for M dsRNAs, distinct L-BC variants are associated with a particular killer strain. The so-called L-BC-lus (in Klus strains) and L-BC-2 (in K2 strains) are analyzed.

IMPORTANCE

Killer strains of S. cerevisiae secrete protein toxins that kill nonkiller yeasts. The "killer phenomenon" depends on two dsRNA viruses: L-A and M. M encodes the toxin, and L-A, the helper virus, provides the capsids for both viruses. Different killer toxins exist: K1, K2, K28, and Klus, encoded on different M viruses. Our data indicate that each M dsRNA depends on a specific helper virus; these helper viruses have nucleotide sequences that may be as much as 26% different, suggesting coevolution. In wine environments, K2 and Klus strains frequently coexist. We have previously characterized the association of Mlus and L-A-lus. Here we sequence and characterize L-A-2, the helper virus of M2, establishing the helper virus requirements of M2, which had not been completely elucidated. We also report the existence of two specific L-BC totiviruses in Klus and K2 strains with about 10% of their nucleotides different, suggesting different evolutionary histories from those of L-A viruses.

Structure of a pentameric virion-associated fiber with a potential role in Orsay virus entry to host cells

Despite the wide use of Caenorhabditis elegans as a model organism, the first virus naturally infecting this organism was not discovered until six years ago. The Orsay virus and its related nematode viruses have a positive-sense RNA genome, encoding three proteins: CP, RdRP, and a novel δ protein that shares no homology with any other proteins. δ can be expressed either as a free δ or a CP-δ fusion protein by ribosomal frameshift, but the structure and function of both δ and CP-δ remain unknown. Using a combination of electron microscopy, X-ray crystallography, computational and biophysical analyses, here we show that the Orsay δ protein forms a ~420-Å long, pentameric fiber with an N-terminal α-helical bundle, a β-stranded filament in the middle, and a C-terminal head domain. The pentameric nature of the δ fiber has been independently confirmed by both mass spectrometry and analytical ultracentrifugation. Recombinant Orsay capsid containing CP-δ shows protruding long fibers with globular heads at the distal end. Mutant viruses with disrupted CP-δ fibers were generated by organism-based reverse genetics. These viruses were found to be either non-viable or with poor infectivity according to phenotypic and qRT-PCR analyses. Furthermore, addition of purified δ proteins to worm culture greatly reduced Orsay infectivity in a sequence-specific manner. Based on the structure resemblance between the Orsay CP-δ fiber and the fibers from reovirus and adenovirus, we propose that CP-δ functions as a cell attachment protein to mediate Orsay entry into worm intestine cells.

Functional analysis of the uL11 protein impact on translational machinery

The ribosomal GTPase associated center constitutes the ribosomal area, which is the landing platform for translational GTPases and stimulates their hydrolytic activity. The ribosomal stalk represents a landmark structure in this center, and in eukaryotes is composed of uL11, uL10 and P1/P2 proteins. The modus operandi of the uL11 protein has not been exhaustively studied in vivo neither in prokaryotic nor in eukaryotic cells. Using a yeast model, we have brought functional insight into the translational apparatus deprived of uL11, filling the gap between structural and biochemical studies. We show that the uL11 is an important element in various aspects of 'ribosomal life'. uL11 is involved in 'birth' (biogenesis and initiation), by taking part in Tif6 release and contributing to ribosomal subunit-joining at the initiation step of translation. uL11 is particularly engaged in the 'active life' of the ribosome, in elongation, being responsible for the interplay with eEF1A and fidelity of translation and contributing to a lesser extent to eEF2-dependent translocation. Our results define the uL11 protein as a critical GAC element universally involved in trGTPase 'productive state' stabilization, being primarily a part of the ribosomal element allosterically contributing to the fidelity of the decoding event.

Ribosomal frameshifting and transcriptional slippage: From genetic steganography and cryptography to adventitious use

Genetic decoding is not ‘frozen’ as was earlier thought, but dynamic. One facet of this is frameshifting that often results in synthesis of a C-terminal region encoded by a new frame. Ribosomal frameshifting is utilized for the synthesis of additional products, for regulatory purposes and for translational ‘correction’ of problem or ‘savior’ indels. Utilization for synthesis of additional products occurs prominently in the decoding of mobile chromosomal element and viral genomes. One class of regulatory frameshifting of stable chromosomal genes governs cellular polyamine levels from yeasts to humans. In many cases of productively utilized frameshifting, the proportion of ribosomes that frameshift at a shift-prone site is enhanced by specific nascent peptide or mRNA context features. Such mRNA signals, which can be 5′ or 3′ of the shift site or both, can act by pairing with ribosomal RNA or as stem loops or pseudoknots even with one component being 4 kb 3′ from the shift site. Transcriptional realignment at slippage-prone sequences also generates productively utilized products encoded trans-frame with respect to the genomic sequence. This too can be enhanced by nucleic acid structure. Together with dynamic codon redefinition, frameshifting is one of the forms of recoding that enriches gene expression.

Diphosphates at the 5' end of the positive strand of yeast L-A double-stranded RNA virus as a molecular self-identity tag

The 5'end of RNA conveys important information on self-identity. In mammalian cells, double-stranded RNA (dsRNA) with 5'di- or triphosphates generated during virus infection is recognized as foreign and elicits the host innate immune response. Here, we analyze the 5' ends of the dsRNA genome of the yeast L-A virus. The positive strand has largely diphosphates with a minor amount of triphosphates, while the negative strand has only diphosphates. Although the virus can produce capped transcripts by cap snatching, neither strand carried a cap structure, suggesting that only non-capped transcripts serve as genomic RNA for encapsidation. We also found that the 5' diphosphates of the positive but not the negative strand within the dsRNA genome are crucial for transcription in vitro. Furthermore, the presence of a cap structure in the dsRNA abrogated its template activity. Given that the 5' diphosphates of the transcripts are also essential for cap acquisition and that host cytosolic RNAs (mRNA, rRNA, and tRNA) are uniformly devoid of 5' pp-structures, the L-A virus takes advantage of its 5' terminal diphosphates, using them as a self-identity tag to propagate in the host cytoplasm.