Double-stranded RNA viruses of Saccharomyces cerevisiae

The Viral K1 Killer Yeast System: Toxicity, Immunity, and Resistance

Killer yeasts, such as the K1 killer strain of S. Cerevisiae, express a secreted anti-competitive toxin whose production and propagation require the presence of two vertically-transmitted dsRNA viruses. In sensitive cells lacking killer virus infection, toxin binding to the cell wall results in ion pore formation, disruption of osmotic homeostasis, and cell death. However, the exact mechanism(s) of K1 toxin killing activity, how killer yeasts are immune to their own toxin, and which factors could influence adaptation and resistance to K1 toxin within formerly sensitive populations are still unknown. Here, we describe the state of knowledge about K1 killer toxin, including current models of toxin processing and killing activity, and a summary of known modifiers of K1 toxin immunity and resistance. In addition, we discuss two key signaling pathways, HOG (high osmolarity glycerol) and CWI (cell wall integrity), whose involvement in an adaptive response to K1 killer toxin in sensitive cells has been previously documented but requires further study. As both host-virus and sensitive-killer competition have been documented in killer systems like K1, further characterization of K1 killer yeasts may provide a useful model system for study of both intracellular genetic conflict and counter-adaptation between competing sensitive and killer populations.

Evaluating Atlantic Salmon (Salmo salar) as a Natural or Alternative Host for Piscine Myocarditis Virus (PMCV) Infection

Cardiomyopathy syndrome (CMS) caused by piscine myocarditis virus (PMCV) has emerged with the rise of the aquaculture of Atlantic salmon (Salmo salar). The lack of cell culture cultivation has hampered the study of this infection. In this study, samples from naturally PMCV-infected Atlantic salmon from different commercial farms were collected and used. In situ hybridization (ISH) revealed intense staining of PMCV RNA in myocardial cells in the spongiform layer of the heart ventricle but almost no staining in the compact layer. In the kidneys, only sporadic staining was seen. Viral RNA was present in all organs, with the highest loads in the heart, kidney, and spleen. The high viral PMCV RNA loads in the heart were due to extensive viral mRNA transcription. The high ratio of viral mRNA to viral genomic dsRNA indicated active transcription but limited production of new viral particles. This suggests that the histopathological changes in the heart are caused by viral mRNA and corresponding viral proteins and not by virus particle formation. The production of full-length transcripts is regulated, with a reduction in the relative number of ORF3-containing transcripts at high transcription rates. Efforts to identify alternative hosts, such as fungi, were inconclusive, as fungal sequences were found inconsistently in the salmon tissue samples. The results of this study reinforce the need for further research to fully understand PMCV's life cycle and potential alternative hosts and its whereabouts when it is not infecting the hearts of the Atlantic salmon.

The RNA Helicase Ded1 from Yeast Is Associated with the Signal Recognition Particle and Is Regulated by SRP21

The DEAD-box RNA helicase Ded1 is an essential yeast protein involved in translation initiation that belongs to the DDX3 subfamily. The purified Ded1 protein is an ATP-dependent RNA-binding protein and an RNA-dependent ATPase, but it was previously found to lack substrate specificity and enzymatic regulation. Here we demonstrate through yeast genetics, yeast extract pull-down experiments, in situ localization, and in vitro biochemical approaches that Ded1 is associated with, and regulated by, the signal recognition particle (SRP), which is a universally conserved ribonucleoprotein complex required for the co-translational translocation of polypeptides into the endoplasmic reticulum lumen and membrane. Ded1 is physically associated with SRP components in vivo and in vitro. Ded1 is genetically linked with SRP proteins. Finally, the enzymatic activity of Ded1 is inhibited by SRP21 in the presence of SCR1 RNA. We propose a model where Ded1 actively participates in the translocation of proteins during translation. Our results provide a new understanding of the role of Ded1 during translation.

Delineating organizational principles of the endogenous L-A virus by cryo-EM and computational analysis of native cell extracts

The high abundance of most viruses in infected host cells benefits their structural characterization. However, endogenous viruses are present in low copy numbers and are therefore challenging to investigate. Here, we retrieve cell extracts enriched with an endogenous virus, the yeast L-A virus. The determined cryo-EM structure discloses capsid-stabilizing cation-π stacking, widespread across viruses and within the Totiviridae, and an interplay of non-covalent interactions from ten distinct capsomere interfaces. The capsid-embedded mRNA decapping active site trench is supported by a constricting movement of two flexible opposite-facing loops. tRNA-loaded polysomes and other biomacromolecules, presumably mRNA, are found in virus proximity within the cell extract. Mature viruses participate in larger viral communities resembling their rare in-cell equivalents in terms of size, composition, and inter-virus distances. Our results collectively describe a 3D-architecture of a viral milieu, opening the door to cell-extract-based high-resolution structural virology.

SKI complex: A multifaceted cytoplasmic RNA exosome cofactor in mRNA metabolism with links to disease, developmental processes, and antiviral responses

RNA stability and quality control are integral parts of gene expression regulation. A key factor shaping eukaryotic transcriptomes, mainly via 3'-5' exoribonucleolytic trimming or degradation of diverse transcripts in nuclear and cytoplasmic compartments, is the RNA exosome. Precise exosome targeting to various RNA molecules requires strict collaboration with specialized auxiliary factors, which facilitate interactions with its substrates. The predominant class of cytoplasmic RNA targeted by the exosome are protein-coding transcripts, which are carefully scrutinized for errors during translation. Normal, functional mRNAs are turned over following protein synthesis by the exosome or by Xrn1 5'-3'-exonuclease, acting in concert with Dcp1/2 decapping complex. In turn, aberrant transcripts are eliminated by dedicated surveillance pathways, triggered whenever ribosome translocation is impaired. Cytoplasmic 3'-5' mRNA decay and surveillance are dependent on the tight cooperation between the exosome and its evolutionary conserved co-factor-the SKI (superkiller) complex (SKIc). Here, we summarize recent findings from structural, biochemical, and functional studies of SKIc roles in controlling cytoplasmic RNA metabolism, including links to various cellular processes. Mechanism of SKIc action is illuminated by presentation of its spatial structure and details of its interactions with exosome and ribosome. Furthermore, contribution of SKIc and exosome to various mRNA decay pathways, usually converging on recycling of ribosomal subunits, is delineated. A crucial physiological role of SKIc is emphasized by describing association between its dysfunction and devastating human disease-a trichohepatoenteric syndrome (THES). Eventually, we discuss SKIc functions in the regulation of antiviral defense systems, cell signaling and developmental transitions, emerging from interdisciplinary investigations. This article is categorized under: RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms RNA Turnover and Surveillance > Regulation of RNA Stability RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes.

RNA viruses, M satellites, chromosomal killer genes, and killer/nonkiller phenotypes in the 100-genomes S. cerevisiae strains

We characterized previously identified RNA viruses (L-A, L-BC, 20S, and 23S), L-A-dependent M satellites (M1, M2, M28, and Mlus), and M satellite-dependent killer phenotypes in the Saccharomyces cerevisiae 100-genomes genetic resource population. L-BC was present in all strains, albeit in 2 distinct levels, L-BChi and L-BClo; the L-BC level is associated with the L-BC genotype. L-BChi, L-A, 20S, 23S, M1, M2, and Mlus (M28 was absent) were in fewer strains than the similarly inherited 2µ plasmid. Novel L-A-dependent phenotypes were identified. Ten M+ strains exhibited M satellite-dependent killing (K+) of at least 1 of the naturally M0 and cured M0 derivatives of the 100-genomes strains; in these M0 strains, sensitivities to K1+, K2+, and K28+ strains varied. Finally, to complement our M satellite-encoded killer toxin analysis, we assembled the chromosomal KHS1 and KHR1 killer genes and used naturally M0 and cured M0 derivatives of the 100-genomes strains to assess and characterize the chromosomal killer phenotypes.

Cross-Kingdom Interactions Between Plant and Fungal Viruses

The large genetic and structural divergences between plants and fungi may hinder the transmission of viruses between these two kingdoms to some extent. However, recent accumulating evidence from virus phylogenetic analyses and the discovery of naturally occurring virus cross-infection suggest the occurrence of past and current transmissions of viruses between plants and plant-associated fungi. Moreover, artificial virus inoculation experiments showed that diverse plant viruses can multiply in fungi and vice versa. Thus, virus cross-infection between plants and fungi may play an important role in the spread, emergence, and evolution of both plant and fungal viruses and facilitate the interaction between them. In this review, we summarize current knowledge related to cross-kingdom virus infection in plants and fungi and further discuss the relevance of this new virological topic in the context of understanding virus spread and transmission in nature as well as developing control strategies for crop plant diseases.

The DEAD-Box RNA Helicase Ded1 Is Associated with Translating Ribosomes

DEAD-box RNA helicases are ATP-dependent RNA binding proteins and RNA-dependent ATPases that possess weak, nonprocessive unwinding activity in vitro, but they can form long-lived complexes on RNAs when the ATPase activity is inhibited. Ded1 is a yeast DEAD-box protein, the functional ortholog of mammalian DDX3, that is considered important for the scanning efficiency of the 48S pre-initiation complex ribosomes to the AUG start codon. We used a modified PAR-CLIP technique, which we call quicktime PAR-CLIP (qtPAR-CLIP), to crosslink Ded1 to 4-thiouridine-incorporated RNAs in vivo using UV light centered at 365 nm. The irradiation conditions are largely benign to the yeast cells and to Ded1, and we are able to obtain a high efficiency of crosslinking under physiological conditions. We find that Ded1 forms crosslinks on the open reading frames of many different mRNAs, but it forms the most extensive interactions on relatively few mRNAs, and particularly on mRNAs encoding certain ribosomal proteins and translation factors. Under glucose-depletion conditions, the crosslinking pattern shifts to mRNAs encoding metabolic and stress-related proteins, which reflects the altered translation. These data are consistent with Ded1 functioning in the regulation of translation elongation, perhaps by pausing or stabilizing the ribosomes through its ATP-dependent binding.

A special satellite-like RNA of a novel hypovirus from Pestalotiopsis fici broadens the definition of fungal satellite

Satellites associated with plant or animal viruses have been largely detected and characterized, while those from mycoviruses together with their roles remain far less determined. Three dsRNA segments (dsRNA 1 to 3 termed according to their decreasing sizes) were identified in a strain of phytopathogenic fungus Pestalotiopsis fici AH1-1 isolated from a tea leaf. The complete sequences of dsRNAs 1 to 3, with the sizes of 10316, 5511, and 631 bp, were determined by random cloning together with a RACE protocol. Sequence analyses support that dsRNA1 is a genome of a novel hypovirus belonging to genus Alphahypovirus of the family Hypoviridae, tentatively named Pestalotiopsis fici hypovirus 1 (PfHV1); dsRNA2 is a defective RNA (D-RNA) generating from dsRNA1 with septal deletions; and dsRNA3 is the satellite component of PfHV1 since it could be co-precipitated with other dsRNA components in the same sucrose fraction by ultra-centrifuge, suggesting that it is encapsulated together with PfHV1 genomic dsRNAs. Moreover, dsRNA3 shares an identical stretch (170 bp) with dsRNAs 1 and 2 at their 5' termini and the remaining are heterogenous, which is distinct from a typical satellite that generally has very little or no sequence similarity with helper viruses. More importantly, dsRNA3 lacks a substantial open reading frame (ORF) and a poly (A) tail, which is unlike the known satellite RNAs of hypoviruses, as well as unlike those in association with Totiviridae and Partitiviridae since the latters are encapsidated in coat proteins. As up-regulated expression of RNA3, dsRNA1 was significantly down-regulated, suggesting that dsRNA3 negatively regulates the expression of dsRNA1, whereas dsRNAs 1 to 3 have no obvious impact on the biological traits of the host fungus including morphologies and virulence. This study indicates that PfHV1 dsRNA3 is a special type of satellite-like nucleic acid that has substantial sequence homology with the host viral genome without encapsidation in a coat protein, which broadens the definition of fungal satellite.

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.

Three-Layered Complex Interactions among Capsidless (+)ssRNA Yadokariviruses, dsRNA Viruses, and a Fungus

We have previously discovered a virus neo-lifestyle exhibited by a capsidless positive-sense (+), single-stranded (ss) RNA virus YkV1 (family Yadokariviridae) and an unrelated double-stranded (ds) RNA virus YnV1 (proposed family "Yadonushiviridae") in a phytopathogenic ascomycete, Rosellinia necatrix. YkV1 has been proposed to replicate in the capsid provided by YnV1 as if it were a dsRNA virus and enhance YnV1 replication in return. Recently, viruses related to YkV1 (yadokariviruses) have been isolated from diverse ascomycetous fungi. However, it remains obscure whether such viruses generally show the YkV1-like lifestyle. Here, we identified partner viruses for three distinct yadokariviruses, YkV3, YkV4a, and YkV4b, isolated from R. necatrix that were coinfected with multiple dsRNA viruses phylogenetically distantly related to YnV1. We first established transformants of R. necatrix carrying single yadokarivirus cDNAs and fused them with infectants by single partner candidate dsRNA viruses. Consequently, YkV3 and YkV4s replicated only in the presence of RnMBV3 (family Megabirnaviridae) and RnMTV1 (proposed family "Megatotiviridae"), respectively. The partners were mutually interchangeable between the two YkV4 strains and three RnMTV1 strains but not between other combinations involving YkV1 or YkV3. In contrast to YkV1 enhancing YnV1 accumulation, YkV4s reduced RnMTV1 accumulation to different degrees according to strains. Interestingly, YkV4 rescued the host R. necatrix from impaired growth induced by RnMTV1. YkV3 exerted no apparent effect on its partner (RnMBV3) or host fungus. Overall, we revealed that while yadokariviruses generally require partner dsRNA viruses for replication, each yadokarivirus partners with a different dsRNA virus species in the three diverse families and shows a distinct symbiotic relation in a fungus. IMPORTANCE A capsidless (+)ssRNA virus YkV1 (family Yadokariviridae) highjacks the capsid of an unrelated dsRNA virus YnV1 (proposed family "Yadonushiviridae") in a phytopathogenic ascomycete, while YkV1 trans-enhances YnV1 replication. Herein, we identified the dsRNA virus partners of three yadokariviruses (YkV3, YkV4a, and YkV4b) with genome organization different from YkV1 as being different from YnV1 at the suborder level. Their partners were mutually interchangeable between the two YkV4 strains and three strains of the partner virus RnMTV1 (proposed family "Megatotiviridae") but not between other combinations involving YkV1 or YkV3. Unlike YkV1, YkV4s reduced RnMTV1 accumulation and rescued the host fungus from impaired growth induced by RnMTV1. YkV3 exerted no apparent effect on its partner (RnMBV3, family Megabirnaviridae) or host fungus. These revealed that while each yadokarivirus has a species-specific partnership with a dsRNA virus, yadokariviruses collectively partner extremely diverse dsRNA viruses and show three-layered complex mutualistic/antagonistic interactions in a fungus.

New Insights into the Genome Organization of Yeast Double-Stranded RNA LBC Viruses

The yeasts Torulaspora delbrueckii (Td) and Saccharomyces cerevisiae (Sc) may show a killer phenotype that is encoded in dsRNA M viruses (V-M), which require the helper activity of another dsRNA virus (V-LA or V-LBC) for replication. Recently, two TdV-LBCbarr genomes, which share sequence identity with ScV-LBC counterparts, were characterized by high-throughput sequencing (HTS). They also share some similar characteristics with Sc-LA viruses. This may explain why TdV-LBCbarr has helper capability to maintain M viruses, whereas ScV-LBC does not. We here analyze two stretches with low sequence identity (LIS I and LIS II) that were found in TdV-LBCbarr Gag-Pol proteins when comparing with the homologous regions of ScV-LBC. These stretches may result from successive nucleotide insertions or deletions (indels) that allow compensatory frameshift events required to maintain specific functions of the RNA-polymerase, while modifying other functions such as the ability to bind V-M (+)RNA for packaging. The presence of an additional frameshifting site in LIS I may ensure the synthesis of a certain amount of RNA-polymerase until the new compensatory indel appears. Additional 5'- and 3'-extra sequences were found beyond V-LBC canonical genomes. Most extra sequences showed high identity to some stretches of the canonical genomes and can form stem-loop structures. Further, the 3'-extra sequence of two ScV-LBC genomes contains rRNA stretches. The origin and possible functions of these extra sequences are here discussed.

Eukaryotic Microbial RNA Viruses-Acute or Persistent? Insights into Their Function in the Aquatic Ecosystem

Isolated RNA viruses mainly parasitize eukaryotes. RNA viruses either expand horizontally by infecting hosts (acute type) or coexist with the host and are vertically inherited (persistent type). The significance of persistent-type RNA viruses in environmental viromes (the main hosts are expected to be microbes) was only recently reported because they had previously been overlooked in virology. In this review, we summarize the host-virus relationships of eukaryotic microbial RNA viruses. Picornavirales and Reoviridae are recognized as representative acute-type virus families, and most of the microbial viruses in Narnaviridae, Totiviridae, and Partitiviridae are categorized as representative persistent-type viruses. Acute-type viruses have only been found in aquatic environments, while persistent-type viruses are present in various environments, including aquatic environments. Moreover, persistent-type viruses are potentially widely spread in the RNA viral sequence space. This emerging evidence provides novel insights into RNA viral diversity, host-virus relationships, and their history of co-evolution.

Infection by dsRNA viruses is associated with enhanced sporulation efficiency in Saccharomyces cerevisiae

Upon starvation diploid cells of the facultative sexual yeast Saccharomyces cerevisiae undergo sporulation, forming four metabolically quiescent and robust haploid spores encased in a degradable ascus. All endosymbionts, whether they provide net benefits or costs, utilize host resources; in yeast, this should induce an earlier onset of sporulation. Here, we tested whether the presence of endosymbiotic dsRNA viruses (M satellite and L-A helper) correspond with higher sporulation rate of their host, S. cerevisiae. We find that S. cerevisiae hosting both the M and L-A viruses (so-called "killer yeasts") have significantly higher sporulation efficiency than those without. We also found that the removal of the M virus did not reduce sporulation frequency, possibly because the L-A virus still utilizes host resources with and without the M virus. Our findings indicate that either virulent resource use by endosymbionts induces sporulation, or that viruses are spread more frequently to sporulating strains. Further exploration is required to distinguish cause from effect.

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.

Scent of a killer: How could killer yeast boost its dispersal?

Vector-borne parasites often manipulate hosts to attract uninfected vectors. For example, parasites causing malaria alter host odor to attract mosquitoes. Here, we discuss the ecology and evolution of fruit-colonizing yeast in a tripartite symbiosis-the so-called "killer yeast" system. "Killer yeast" consists of Saccharomyces cerevisiae yeast hosting two double-stranded RNA viruses (M satellite dsRNAs, L-A dsRNA helper virus). When both dsRNA viruses occur in a yeast cell, the yeast converts to lethal toxin‑producing "killer yeast" phenotype that kills uninfected yeasts. Yeasts on ephemeral fruits attract insect vectors to colonize new habitats. As the viruses have no extracellular stage, they depend on the same insect vectors as yeast for their dispersal. Viruses also benefit from yeast dispersal as this promotes yeast to reproduce sexually, which is how viruses can transmit to uninfected yeast strains. We tested whether insect vectors are more attracted to killer yeasts than to non‑killer yeasts. In our field experiment, we found that killer yeasts were more attractive to Drosophila than non-killer yeasts. This suggests that vectors foraging on yeast are more likely to transmit yeast with a killer phenotype, allowing the viruses to colonize those uninfected yeast strains that engage in sexual reproduction with the killer yeast. Beyond insights into the basic ecology of the killer yeast system, our results suggest that viruses could increase transmission success by manipulating the insect vectors of their host.

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.

Saccharomyces cerevisiae and its industrial applications

Saccharomyces cerevisiae is the best studied eukaryote and a valuable tool for most aspects of basic research on eukaryotic organisms. This is due to its unicellular nature, which often simplifies matters, offering the combination of the facts that nearly all biological functions found in eukaryotes are also present and well conserved in S. cerevisiae. In addition, it is also easily amenable to genetic manipulation. Moreover, unlike other model organisms, S. cerevisiae is concomitantly of great importance for various biotechnological applications, some of which date back to several thousands of years. S. cerevisiae's biotechnological usefulness resides in its unique biological characteristics, i.e., its fermentation capacity, accompanied by the production of alcohol and CO2 and its resilience to adverse conditions of osmolarity and low pH. Among the most prominent applications involving the use of S. cerevisiae are the ones in food, beverage -especially wine- and biofuel production industries. This review focuses exactly on the function of S. cerevisiae in these applications, alone or in conjunction with other useful microorganisms involved in these processes. Furthermore, various aspects of the potential of the reservoir of wild, environmental, S. cerevisiae isolates are examined under the perspective of their use for such applications.

Development of Drugs Based on High-Polymeric Double-Stranded RNA for Antiviral and Antitumor Therapy

Abstract-The review summarizes literature data on the development of drugs based on natural and synthetic high-polymeric double-stranded RNA (dsRNA), their antiviral, immunoadjuvant, and antitumor properties. Special attention is paid to cell receptors responding to exogenous dsRNA, pathways of dsRNA-dependent antiviral reaction, ability of dsRNA to inhibit growth and induce apoptosis of malignant cells. It has been shown that enhancing the innate immune response with dsRNA can be an effective component in improving methods for treating and preventing infectious and cancer diseases. The further use of dsRNA for the correction of pathological processes of different origin is discussed.

Characterization of Cronartium ribicola dsRNAs reveals novel members of the family Totiviridae and viral association with fungal virulence

Background

Mycoviruses were recently discovered in the white pine blister rust (WPBR) fungus Cronartium ribicola (J.C. Fisch.). Detection and characterization of their double stranded RNA (dsRNA) would facilitate understanding of pathogen virulence and disease pathogenesis in WPBR systems.

Methods

Full-length cDNAs were cloned from the dsRNAs purified from viral-infected C. ribicola, and their cDNA sequences were determined by DNA sequencing. Evolutionary relationships of the dsRNAs with related mycoviruses were determined by phylogenetic analysis. Dynamic distributions of the viral RNAs within samples of their fungal host C. ribicola were investigated by measurement of viral genome prevalence and viral gene expression.

Results

In this study we identified and characterized five novel dsRNAs from C. ribicola, designated as Cronartium ribicola totivirus 1–5 (CrTV1 to CrTV5). These dsRNA sequences encode capsid protein and RNA-dependent RNA polymerase with significant homologies to dsRNA viruses of the family Totiviridae. Phylogenetic analysis showed that the CrTVs were grouped into two distinct clades. CrTV2 through CrTV5 clustered within the genus Totivirus. CrTV1 along with a few un-assigned dsRNAs constituted a distinct phyletic clade that is genetically distant from presently known genera in the Totiviridae family, indicating that CrTV1 represents a novel genus in the Totiviridae family. The CrTVs were prevalent in fungal samples obtained from infected western white pine, whitebark pine, and limber pines. Viral RNAs were generally expressed at higher levels during in planta mycelium growth than in aeciospores and urediniospores. CrTV4 was significantly associated with C. ribicola virulent pathotype and specific C. ribicola host tree species, suggesting dsRNAs as potential tools for dissection of pathogenic mechanisms of C. ribicola and diagnosis of C. ribicola pathotypes.

Conclusion

Phylogenetic and expression analyses of viruses in the WPBR pathogen, C. ribicola, have enchanced our understanding of virus diversity in the family Totiviridae, and provided a potential strategy to utilize pathotype-associated mycoviruses to control fungal forest diseases.

Substitution of cysteines in the yeast viral killer toxin K1 precursor reveals novel insights in heterodimer formation and immunity

The killer toxin K1 is a virally encoded fungal A/B toxin acting by disrupting plasma membrane integrity. The connection of α and β constitutes a critical feature for toxin biology and for decades the formation of three disulphide bonds linking the major toxin subunits was accepted as status quo. Due to the absence of experimental evidence, the involvement of each cysteine in heterodimer formation, K1 lethality and immunity was systematically analysed. Substitution of any cysteine in α led to a complete loss of toxin dimer secretion and toxicity, whereas K1 toxin derivatives carrying mutations of C248, C312 or the double mutation C248-312 were active against spheroplasted cells. Importantly, substitution of the C95 and C107 in the toxin precursor completely abolished the mediation of functional immunity. In contrast, K1 toxicity, i.e. its ionophoric effect, does not depend on the cysteine residues at all. In contrast to the literature, our data imply the formation of a single disulphide bond involving C92 in α and C239 in β. This finding not only refines the current model stated for decades but also provides new opportunities to elucidate the mechanisms underlying K1 toxicity and immunity at the molecular level.

The ecology of killer yeasts: Interference competition in natural habitats

Killer yeasts are ubiquitous in the environment: They have been found in diverse habitats ranging from ocean sediment to decaying cacti to insect bodies and on all continents including Antarctica. However, environmental killer yeasts are poorly studied compared with laboratory and domesticated killer yeasts. Killer yeasts secrete so-called killer toxins that inhibit nearby sensitive yeasts, and the toxins are frequently assumed to be tools for interference competition in diverse yeast communities. The diversity and ubiquity of killer yeasts imply that interference competition is crucial for shaping yeast communities. Additionally, these toxins may have ecological functions beyond use in interference competition. This review introduces readers to killer yeasts in environmental systems, with a focus on what is and is not known about their ecology and evolution. It also explores how results from experimental killer systems in laboratories can be extended to understand how competitive strategies shape yeast communities in nature. Overall, killer yeasts are likely to occur everywhere yeasts are found, and the killer phenotype has the potential to radically shape yeast diversity in nature.

Structure of the Ty3/Gypsy retrotransposon capsid and the evolution of retroviruses

Retroviruses evolved from long terminal repeat (LTR) retrotransposons by acquisition of envelope functions, and subsequently reinvaded host genomes. Together, endogenous retroviruses and LTR retrotransposons represent major components of animal, plant, and fungal genomes. Sequences from these elements have been exapted to perform essential host functions, including placental development, synaptic communication, and transcriptional regulation. They encode a Gag polypeptide, the capsid domains of which can oligomerize to form a virus-like particle. The structures of retroviral capsids have been extensively described. They assemble an immature viral particle through oligomerization of full-length Gag. Proteolytic cleavage of Gag results in a mature, infectious particle. In contrast, the absence of structural data on LTR retrotransposon capsids hinders our understanding of their function and evolutionary relationships. Here, we report the capsid morphology and structure of the archetypal Gypsy retrotransposon Ty3. We performed electron tomography (ET) of immature and mature Ty3 particles within cells. We found that, in contrast to retroviruses, these do not change size or shape upon maturation. Cryo-ET and cryo-electron microscopy of purified, immature Ty3 particles revealed an irregular fullerene geometry previously described for mature retrovirus core particles and a tertiary and quaternary arrangement of the capsid (CA) C-terminal domain within the assembled capsid that is conserved with mature HIV-1. These findings provide a structural basis for studying retrotransposon capsids, including those domesticated in higher organisms. They suggest that assembly via a structurally distinct immature capsid is a later retroviral adaptation, while the structure of mature assembled capsids is conserved between LTR retrotransposons and retroviruses.

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.

Host Pah1p phosphatidate phosphatase limits viral replication by regulating phospholipid synthesis

Replication of positive-strand RNA viruses [(+)RNA viruses] takes place in membrane-bound viral replication complexes (VRCs). Formation of VRCs requires virus-mediated manipulation of cellular lipid synthesis. Here, we report significantly enhanced brome mosaic virus (BMV) replication and much improved cell growth in yeast cells lacking PAH1 (pah1Δ), the sole yeast ortholog of human LIPIN genes. PAH1 encodes Pah1p (phosphatidic acid phosphohydrolase), which converts phosphatidate (PA) to diacylglycerol that is subsequently used for the synthesis of the storage lipid triacylglycerol. Inactivation of Pah1p leads to altered lipid composition, including high levels of PA, total phospholipids, ergosterol ester, and free fatty acids, as well as expansion of the nuclear membrane. In pah1Δ cells, BMV replication protein 1a and double-stranded RNA localized to the extended nuclear membrane, there was a significant increase in the number of VRCs formed, and BMV genomic replication increased by 2-fold compared to wild-type cells. In another yeast mutant that lacks both PAH1 and DGK1 (encodes diacylglycerol kinase converting diacylglycerol to PA), which has a normal nuclear membrane but maintains similar lipid compositional changes as in pah1Δ cells, BMV replicated as efficiently as in pah1Δ cells, suggesting that the altered lipid composition was responsible for the enhanced BMV replication. We further showed that increased levels of total phospholipids play an important role because the enhanced BMV replication required active synthesis of phosphatidylcholine, the major membrane phospholipid. Moreover, overexpression of a phosphatidylcholine synthesis gene (CHO2) promoted BMV replication. Conversely, overexpression of PAH1 or plant PAH1 orthologs inhibited BMV replication in yeast or Nicotiana benthamiana plants. Competing with its host for limited resources, BMV inhibited host growth, which was markedly alleviated in pah1Δ cells. Our work suggests that Pah1p promotes storage lipid synthesis and thus represses phospholipid synthesis, which in turn restricts both viral replication and cell growth during viral infection.

Enhancing Efficacy and Stability of an Antiheroin Vaccine: Examination of Antinociception, Opioid Binding Profile, and Lethality

In recent years, drug conjugate vaccines have shown promise as therapeutics for substance use disorder. As a means to improve the efficacy of a heroin conjugate vaccine, we systematically explored 20 vaccine formulations with varying combinations of carrier proteins and adjuvants. In regard to adjuvants, we explored a Toll-like receptor 9 (TLR9) agonist and a TLR3 agonist in the presence of alum. The TLR9 agonist was cytosine-guanine oligodeoxynucleotide 1826 (CpG ODN 1826), while the TLR3 agonist was virus-derived genomic doubled-stranded RNA (dsRNA). The vaccine formulations containing TLR3 or TLR9 agonist alone elicited strong antiheroin antibody titers and blockade of heroin-induced antinociception when formulated with alum; however, a combination of TLR3 and TLR9 adjuvants did not result in improved efficacy. Investigation of month-long stability of the two lead formulations revealed that the TLR9 but not the TLR3 formulation was stable when stored as a lyophilized solid or as a liquid over 30 days. Furthermore, mice immunized with the TLR9 + alum heroin vaccine gained significant protection from lethal heroin doses, suggesting that this vaccine formulation is suitable for mitigating the harmful effects of heroin, even following month-long storage at room temperature.

Leishmania guyanensis parasites block the activation of the inflammasome by inhibiting maturation of IL-1β

The various symptomatic outcomes of cutaneous leishmaniasis relates to the type and potency of its underlying inflammatory responses. Presence of the cytoplasmic Leishmania RNA virus-1 (LRV1) within Leishmania guyanensis, worsens lesional inflammation and parasite burden, as the viral dsRNA genome acts as a potent innate immunogen stimulating Toll-Like-Receptor-3 (TLR3). Here we investigated other innate pattern recognition receptors capable of reacting to dsRNA and potentially contributing to LRV1-mediated inflammatory pathology. We included the cytoplasmic dsRNA sensors, namely, the RIG-like receptors (RLRs) and the inflammasome-dependent and -independent Nod-like-receptors (NLRs). Our study found no role for RLRs or inflammasome-dependent NLRs in the pathology of L. guyanensis infection irrespective of its LRV1-status. Further, neither LRV1-bearing L. guyanensis (LgyLRV1+) nor LRV1-negative L. guyanensis (LgyLRV1-) activated the inflammasome in vitro. Interestingly, similarly to L. donovani, L. guyanensis infection induced the up-regulation of the A20 protein, known to be involved in the evasion of inflammasome activation. Moreover, we observed that LgyLRV1+ promoted the transcription of inflammasome-independent NLRC2 (also called NOD2) and NLRC5. However, only NLRC2 showed some contribution to LRV1-dependent pathology. These data confirmed that the endosomal TLR3 pathway is the dominant route of LRV1-dependent signalling, thus excluding the cytosolic and inflammasome pathways. We postulate that avoidance of the inflammasome pathways is likely an important mechanism of virulence in Leishmania infection irrespective of the LRV1-status.

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.

Yeast for virus research

Budding yeast (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) are two popular model organisms for virus research. They are natural hosts for viruses as they carry their own indigenous viruses. Both yeasts have been used for studies of plant, animal and human viruses. Many positive sense (+) RNA viruses and some DNA viruses replicate with various levels in yeasts, thus allowing study of those viral activities during viral life cycle. Yeasts are single cell eukaryotic organisms. Hence, many of the fundamental cellular functions such as cell cycle regulation or programed cell death are highly conserved from yeasts to higher eukaryotes. Therefore, they are particularly suited to study the impact of those viral activities on related cellular activities during virus-host interactions. Yeasts present many unique advantages in virus research over high eukaryotes. Yeast cells are easy to maintain in the laboratory with relative short doubling time. They are non-biohazardous, genetically amendable with small genomes that permit genome-wide analysis of virologic and cellular functions. In this review, similarities and differences of these two yeasts are described. Studies of virologic activities such as viral translation, viral replication and genome-wide study of virus-cell interactions in yeasts are highlighted. Impacts of viral proteins on basic cellular functions such as cell cycle regulation and programed cell death are discussed. Potential applications of using yeasts as hosts to carry out functional analysis of small viral genome and to develop high throughput drug screening platform for the discovery of antiviral drugs are presented.

Cryo-electron tomography reveals novel features of a viral RNA replication compartment

Positive-strand RNA viruses, the largest genetic class of viruses, include numerous important pathogens such as Zika virus. These viruses replicate their RNA genomes in novel, membrane-bounded mini-organelles, but the organization of viral proteins and RNAs in these compartments has been largely unknown. We used cryo-electron tomography to reveal many previously unrecognized features of Flock house nodavirus (FHV) RNA replication compartments. These spherular invaginations of outer mitochondrial membranes are packed with electron-dense RNA fibrils and their volumes are closely correlated with RNA replication template length. Each spherule’s necked aperture is crowned by a striking cupped ring structure containing multifunctional FHV RNA replication protein A. Subtomogram averaging of these crowns revealed twelve-fold symmetry, concentric flanking protrusions, and a central electron density. Many crowns were associated with long cytoplasmic fibrils, likely to be exported progeny RNA. These results provide new mechanistic insights into positive-strand RNA virus replication compartment structure, assembly, function and control.

DOI:http://dx.doi.org/10.7554/eLife.25940.001

Exacerbated Leishmaniasis Caused by a Viral Endosymbiont can be Prevented by Immunization with Its Viral Capsid

Recent studies have shown that a cytoplasmic virus called Leishmaniavirus (LRV) is present in some Leishmania species and acts as a potent innate immunogen, aggravating lesional inflammation and development in mice. In humans, the presence of LRV in Leishmania guyanensis and in L. braziliensis was significantly correlated with poor treatment response and symptomatic relapse. So far, no clinical effort has used LRV for prophylactic purposes. In this context, we designed an original vaccine strategy that targeted LRV nested in Leishmania parasites to prevent virus-related complications. To this end, C57BL/6 mice were immunized with a recombinant LRV1 Leishmania guyanensis viral capsid polypeptide formulated with a T helper 1-polarizing adjuvant. LRV1-vaccinated mice had significant reduction in lesion size and parasite load when subsequently challenged with LRV1+ Leishmania guyanensis parasites. The protection conferred by this immunization could be reproduced in naïve mice via T-cell transfer from vaccinated mice but not by serum transfer. The induction of LRV1 specific T cells secreting IFN-γ was confirmed in vaccinated mice and provided strong evidence that LRV1-specific protection arose via a cell mediated immune response against the LRV1 capsid. Our studies suggest that immunization with LRV1 capsid could be of a preventive benefit in mitigating the elevated pathology associated with LRV1 bearing Leishmania infections and possibly avoiding symptomatic relapses after an initial treatment. This novel anti-endosymbiotic vaccine strategy could be exploited to control other infectious diseases, as similar viral infections are largely prevalent across pathogenic pathogens and could consequently open new vaccine opportunities.

About 80% of leishmaniasis infections result in cutaneous manifestations with a broad symptomatic spectrum, ranging from self-healing localized to disseminated lesions. The mechanism behind these latter aggravated forms of leishmaniasis is still poorly understood. One possible factor is an endosymbiotic RNA virus identified in the cytoplasm of several Leishmania species (Leishmania RNA virus 1). LRV1 acts as a virulence factor, causing a destructive hyper-inflammatory response. In this study, we tested the prophylactic potential of a vaccine formulated with a recombinant LRV1 capsid and a T helper 1-polarizing adjuvant. Our approach conferred significant protection against LRV1+ Leishmania guyanensis infection, decreasing lesional inflammation and parasite burden. Further analysis demonstrated that this vaccine induced a potent T helper 1 response. Consequently, we propose that the LRV1-capsid is a promising vaccine component in order to reduce clinical complications (e.g. symptomatic relapses) in areas endemic to LRV1 co-infected Leishmania species. Taken together, we present an original strategy, whereby targeting the pervasive intracellular viruses within pathogens may reduce pathologic inflammation and offer an extra-genetic candidate that may circumvent escape mutations or poor response to drug treatment.

Heterodimers as the Structural Unit of the T=1 Capsid of the Fungal Double-Stranded RNA Rosellinia necatrix Quadrivirus 1

Most double-stranded RNA (dsRNA) viruses are transcribed and replicated in a specialized icosahedral capsid with a T=1 lattice consisting of 60 asymmetric capsid protein (CP) dimers. These capsids help to organize the viral genome and replicative complex(es). They also act as molecular sieves that isolate the virus genome from host defense mechanisms and allow the passage of nucleotides and viral transcripts. Rosellinia necatrix quadrivirus 1 (RnQV1), the type species of the family Quadriviridae, is a dsRNA fungal virus with a multipartite genome consisting of four monocistronic segments (segments 1 to 4). dsRNA-2 and dsRNA-4 encode two CPs (P2 and P4, respectively), which coassemble into ∼450-Å-diameter capsids. We used three-dimensional cryo-electron microscopy combined with complementary biophysical techniques to determine the structures of RnQV1 virion strains W1075 and W1118. RnQV1 has a quadripartite genome, and the capsid is based on a single-shelled T=1 lattice built of P2-P4 dimers. Whereas the RnQV1-W1118 capsid is built of full-length CP, P2 and P4 of RnQV1-W1075 are cleaved into several polypeptides, maintaining the capsid structural organization. RnQV1 heterodimers have a quaternary organization similar to that of homodimers of reoviruses and other dsRNA mycoviruses. The RnQV1 capsid is the first T=1 capsid with a heterodimer as an asymmetric unit reported to date and follows the architectural principle for dsRNA viruses that a 120-subunit capsid is a conserved assembly that supports dsRNA replication and organization.

IMPORTANCE

Given their importance to health, members of the family Reoviridae are the basis of most structural and functional studies and provide much of our knowledge of dsRNA viruses. Analysis of bacterial, protozoal, and fungal dsRNA viruses has improved our understanding of their structure, function, and evolution, as well. Here, we studied a dsRNA virus that infects the fungus Rosellinia necatrix, an ascomycete that is pathogenic to a wide range of plants. Using three-dimensional cryo-electron microscopy and analytical ultracentrifugation analysis, we determined the structure and stoichiometry of Rosellinia necatrix quadrivirus 1 (RnQV1). The RnQV1 capsid is a T=1 capsid with 60 heterodimers as the asymmetric units. The large amount of genetic information used by RnQV1 to construct a simple T=1 capsid is probably related to the numerous virus-host and virus-virus interactions that it must face in its life cycle, which lacks an extracellular phase.

The victorivirus Helminthosporium victoriae virus 190S is the primary cause of disease/hypovirulence in its natural host and a heterologous host

A transmissible disease of the plant pathogenic fungus Helminthosporium victoriae, the causal agent of Victoria blight of oats, was reported more than 50 years ago. Diseased, but not normal, isolates, of H. victoriae contain two distinct viruses designated according to their sedimentation values as victorivirus Helminthosporium victoriae virus 190S (HvV190S) and chrysovirus Helminthosporium victoriae 145S (HvV145S). Although a viral etiology of the disease was previously proposed, conclusive evidence was lacking. Here we present unequivocal evidence based on transfecting virus-free H. victoriae protoplasts with purified virus particles showing that HvV190S is essential for disease development. Furthermore, we show an expansion of the host range of HvV190S to include Cryphonectria parasitica and we also show similarity in a subset of phenotypic traits between HvV190S-infected RNA silencing deficient mutant (Δdcl-2) of C. parasitica and a strain of H. victoriae. In virulence assays on detached American chestnut branches and Red Delicious apple fruits, HvV190S-infected C. parasitica strain Δdcl-2 was markedly less virulent than wild type and virus-free Δdcl-2 C. parasitica strains. Furthermore, the hypovirulent HvV190S-infected C. parasitica Δdcl-2 strain exhibited strong antifungal activity in dual culture with the plant pathogenic fungus Sclerotinia sclerotiorum. No such inhibitory activity was observed in comparable dual cultures with wild type and virus-free Δdcl-2 C. parasitica strains. The discovery that infection with HvV190S induced a hypovirulent phenotype in a heterologous plant pathogenic host is very significant since it might be possible to convert other economically important plant pathogenic fungi to hypovirulence using HvV190S.

Ribosomal protein and biogenesis factors affect multiple steps during movement of the Saccharomyces cerevisiae Ty1 retrotransposon

BACKGROUND

A large number of Saccharomyces cerevisiae cellular factors modulate the movement of the retrovirus-like transposon Ty1. Surprisingly, a significant number of chromosomal genes required for Ty1 transposition encode components of the translational machinery, including ribosomal proteins, ribosomal biogenesis factors, protein trafficking proteins and protein or RNA modification enzymes.

RESULTS

To assess the mechanistic connection between Ty1 mobility and the translation machinery, we have determined the effect of these mutations on ribosome biogenesis and Ty1 transcriptional and post-transcriptional regulation. Lack of genes encoding ribosomal proteins or ribosome assembly factors causes reduced accumulation of the ribosomal subunit with which they are associated. In addition, these mutations cause decreased Ty1 + 1 programmed translational frameshifting, and reduced Gag protein accumulation despite at least normal levels of Ty1 mRNA. Several ribosome subunit mutations increase the level of both an internally initiated Ty1 transcript and its encoded truncated Gag-p22 protein, which inhibits transposition.

CONCLUSIONS

Together, our results suggest that this large class of cellular genes modulate Ty1 transposition through multiple pathways. The effects are largely post-transcriptional acting at a variety of levels that may include translation initiation, protein stability and subcellular protein localization.

Mouse superkiller-2-like helicase DDX60 is dispensable for type I IFN induction and immunity to multiple viruses

IFN-α/β allow cells to fight virus infection by inducing the expression of many genes that encode effectors of antiviral defense. One of these, the Ski2-like DExH-box helicase DDX60, was recently implicated in resistance of human cells to hepatitis C virus, as well as in induction of IFN-α/β by retinoic acid inducible gene 1-like receptors (RLRs) that detect the presence of RNA viruses in a cell-intrinsic manner. Here, we sought to investigate the role of DDX60 in IFN-α/β induction and in resistance to virus infection. Analysis of fibroblasts and myeloid cells from Ddx60-deficient mice revealed no impairment in IFN-α/β production in response to RLR agonists, RNA viruses, or other stimuli. Moreover, overexpression of DDX60 did not potentiate IFN induction and DDX60 did not interact with RLRs or capture RLR agonists from virally infected cells. We also failed to identify any impairment in Ddx60-deficient murine cells or mice in resistance to infection with influenza A virus, encephalomyocarditis virus, Sindbis virus, vaccinia virus, or herpes simplex virus-1. These results put in question the reported role of DDX60 as a broad-acting positive regulator of RLR responses and hint at the possibility that it may function as a restriction factor highly specific for a particular virus or class of viruses.

Bioprotective Role of Yeasts

The yeasts constitute a large group of microorganisms characterized by the ability to grow and survive in different and stressful conditions and then to colonize a wide range of environmental and human ecosystems. The competitive traits against other microorganisms have attracted increasing attention from scientists, who proposed their successful application as bioprotective agents in the agricultural, food and medical sectors. These antagonistic activities rely on the competition for nutrients, production and tolerance of high concentrations of ethanol, as well as the synthesis of a large class of antimicrobial compounds, known as killer toxins, which showed clearly a large spectrum of activity against food spoilage microorganisms, but also against plant, animal and human pathogens. This review describes the antimicrobial mechanisms involved in the antagonistic activity, their applications in the processed and unprocessed food sectors, as well as the future perspectives in the development of new bio-drugs, which may overcome the limitations connected to conventional antimicrobial and drug resistance.

A new wine Torulaspora delbrueckii killer strain with broad antifungal activity and its toxin-encoding double-stranded RNA virus

Wine Torulaspora delbrueckii strains producing a new killer toxin (Kbarr-1) were isolated and selected for wine making. They killed all the previously known Saccharomyces cerevisiae killer strains, in addition to other non-Saccharomyces yeasts. The Kbarr-1 phenotype is encoded by a medium-size 1.7 kb dsRNA, TdV-Mbarr-1, which seems to depend on a large-size 4.6 kb dsRNA virus (TdV-LAbarr) for stable maintenance and replication. The TdV-Mbarr-1 dsRNA was sequenced by new generation sequencing techniques. Its genome structure is similar to those of S. cerevisiae killer M dsRNAs, with a 5'-end coding region followed by an internal A-rich sequence and a 3'-end non-coding region. Mbarr-1 RNA positive strand carries cis acting signals at its 5' and 3' termini for transcription and replication respectively, similar to those RNAs of yeast killer viruses. The ORF at the 5' region codes for a putative preprotoxin with an N-terminal secretion signal, potential Kex2p/Kexlp processing sites, and N-glycosylation sites. No relevant sequence identity was found either between the full sequence of Mbarr-1 dsRNA and other yeast M dsRNAs, or between their respective toxin-encoded proteins. However, a relevant identity of TdV-Mbarr-1 RNA regions to the putative replication and packaging signals of most of the M-virus RNAs suggests that they are all evolutionarily related.

Breeding Strategy To Generate Robust Yeast Starter Cultures for Cocoa Pulp Fermentations

Cocoa pulp fermentation is a spontaneous process during which the natural microbiota present at cocoa farms is allowed to ferment the pulp surrounding cocoa beans. Because such spontaneous fermentations are inconsistent and contribute to product variability, there is growing interest in a microbial starter culture that could be used to inoculate cocoa pulp fermentations. Previous studies have revealed that many different fungi are recovered from different batches of spontaneous cocoa pulp fermentations, whereas the variation in the prokaryotic microbiome is much more limited. In this study, therefore, we aimed to develop a suitable yeast starter culture that is able to outcompete wild contaminants and consistently produce high-quality chocolate. Starting from specifically selected Saccharomyces cerevisiae strains, we developed robust hybrids with characteristics that allow them to efficiently ferment cocoa pulp, including improved temperature tolerance and fermentation capacity. We conducted several laboratory and field trials to show that these new hybrids often outperform their parental strains and are able to dominate spontaneous pilot scale fermentations, which results in much more consistent microbial profiles. Moreover, analysis of the resulting chocolate showed that some of the cocoa batches that were fermented with specific starter cultures yielded superior chocolate. Taken together, these results describe the development of robust yeast starter cultures for cocoa pulp fermentations that can contribute to improving the consistency and quality of commercial chocolate production.

Characterization of virus-like particles and identification of capsid proteins in Xanthophyllomyces dendrorhous

Two dsRNAs of estimated lengths of 5 (L1) and 3.7 (L2) kpb are commonly found in strains of the basidiomycetous yeast Xanthophyllomyces dendrorhous, and the presence of virus-like particles (VLPs) have been described in some strains. Recently, two putative totiviruses (XdV-L1A and XdV-L1B) were identified from L1 dsRNA and one (XdV-L2) from L2 dsRNA in the strain UCD 67-385. In some strains, there are smaller dsRNAs (0.9-1.4 kb) that probable are satellite elements. In this work, the VLPs from several strains of X. dendrorhous, which differ in their dsRNAs content, were separated by sucrose gradient and characterized in relation to the dsRNAs and proteins that compose them. It was found that all types of dsRNAs were encapsidated into VLPs, supporting the hypothesis that the smaller dsRNAs are satellite molecules. A main protein of approx. 76 or 37 kDa composed the virions that only have the L1-dsRNA or L2-dsRNA, respectively. In the strain UCD 67-385, these both proteins were identified as viral capsid protein (CP), allow to confirm the gag predicted ORFs in XdV-L1A, XdV-L1B, and XdV-L2, with CPs of 76.6, 76.2, and 38.8 kDa, respectively. Analysis of predicted structures of CPs of XdV-L1A and XdV-L1B, showed high similitudes with the CPs of ScV-L-A and other totiviruses.

Tombusvirus-yeast interactions identify conserved cell-intrinsic viral restriction factors

To combat viral infections, plants possess innate and adaptive immune pathways, such as RNA silencing, R gene and recessive gene-mediated resistance mechanisms. However, it is likely that additional cell-intrinsic restriction factors (CIRF) are also involved in limiting plant virus replication. This review discusses novel CIRFs with antiviral functions, many of them RNA-binding proteins or affecting the RNA binding activities of viral replication proteins. The CIRFs against tombusviruses have been identified in yeast (Saccharomyces cerevisiae), which is developed as an advanced model organism. Grouping of the identified CIRFs based on their known cellular functions and subcellular localization in yeast reveals that TBSV replication is limited by a wide variety of host gene functions. Yeast proteins with the highest connectivity in the network map include the well-characterized Xrn1p 5'-3' exoribonuclease, Act1p actin protein and Cse4p centromere protein. The protein network map also reveals an important interplay between the pro-viral Hsp70 cellular chaperone and the antiviral co-chaperones, and possibly key roles for the ribosomal or ribosome-associated factors. We discuss the antiviral functions of selected CIRFs, such as the RNA binding nucleolin, ribonucleases, WW-domain proteins, single- and multi-domain cyclophilins, TPR-domain co-chaperones and cellular ion pumps. These restriction factors frequently target the RNA-binding region in the viral replication proteins, thus interfering with the recruitment of the viral RNA for replication and the assembly of the membrane-bound viral replicase. Although many of the characterized CIRFs act directly against TBSV, we propose that the TPR-domain co-chaperones function as "guardians" of the cellular Hsp70 chaperone system, which is subverted efficiently by TBSV for viral replicase assembly in the absence of the TPR-domain co-chaperones.

Inferring host gene subnetworks involved in viral replication

Systematic, genome-wide loss-of-function experiments can be used to identify host factors that directly or indirectly facilitate or inhibit the replication of a virus in a host cell. We present an approach that combines an integer linear program and a diffusion kernel method to infer the pathways through which those host factors modulate viral replication. The inputs to the method are a set of viral phenotypes observed in single-host-gene mutants and a background network consisting of a variety of host intracellular interactions. The output is an ensemble of subnetworks that provides a consistent explanation for the measured phenotypes, predicts which unassayed host factors modulate the virus, and predicts which host factors are the most direct interfaces with the virus. We infer host-virus interaction subnetworks using data from experiments screening the yeast genome for genes modulating the replication of two RNA viruses. Because a gold-standard network is unavailable, we assess the predicted subnetworks using both computational and qualitative analyses. We conduct a cross-validation experiment in which we predict whether held-aside test genes have an effect on viral replication. Our approach is able to make high-confidence predictions more accurately than several baselines, and about as well as the best baseline, which does not infer mechanistic pathways. We also examine two kinds of predictions made by our method: which host factors are nearest to a direct interaction with a viral component, and which unassayed host genes are likely to be involved in viral replication. Multiple predictions are supported by recent independent experimental data, or are components or functional partners of confirmed relevant complexes or pathways. Integer program code, background network data, and inferred host-virus subnetworks are available at http://www.biostat.wisc.edu/~craven/chasman_host_virus/.

Yeast virus-derived stimulator of the innate immune system augments the efficacy of virus vector-based immunotherapy

To identify novel stimulators of the innate immune system, we constructed a panel of eight HEK293 cell lines double positive for human Toll-like receptors (TLRs) and an NF-κB-inducible reporter gene. Screening of a large variety of compounds and cellular extracts detected a TLR3-activating compound in a microsomal yeast extract. Fractionation of this extract identified an RNA molecule of 4.6 kb, named nucleic acid band 2 (NAB2), that was sufficient to confer the activation of TLR3. Digests with single- and double-strand-specific RNases showed the double-strand nature of this RNA, and its sequence was found to be identical to that of the genome of the double-stranded RNA (dsRNA) L-BC virus of Saccharomyces cerevisiae. A large-scale process of production and purification of this RNA was established on the basis of chemical cell lysis and dsRNA-specific chromatography. NAB2 complexed with the cationic lipid Lipofectin but neither NAB2 nor Lipofectin alone induced the secretion of interleukin-12(p70) [IL-12(p70)], alpha interferon, gamma interferon-induced protein 10, macrophage inflammatory protein 1β, or IL-6 in human monocyte-derived dendritic cells. While NAB2 activated TLR3, Lipofectin-stabilized NAB2 also signaled via the cytoplasmic sensor for RNA recognition MDA-5. A significant increase of RMA-MUC1 tumor rejection and survival was observed in C57BL/6 mice after prophylactic vaccination with MUC1-encoding modified vaccinia virus Ankara (MVA) and NAB2-Lipofectin. This combination of immunotherapies strongly increased at the injection sites the percentage of infiltrating natural killer (NK) cells and plasmacytoid dendritic cells (pDCs), cell types which can modulate innate and adaptive immune responses.

IMPORTANCE

Virus-based cancer vaccines offer a good alternative to the treatment of cancer but could be improved. Starting from a screening approach, we have identified and characterized an unexplored biological molecule with immunomodulatory characteristics which augments the efficacy of an MVA-based immunotherapeutic agent. The immune modulator consists of the purified dsRNA genome isolated from a commercially used yeast strain, NAB2, mixed with a cationic lipid, Lipofectin. NAB2-Lipofectin stimulates the immune system via TLR3 and MDA-5. When it was injected at the MVA vaccination site, the immune modulator increased survival in a preclinical tumor model. We could demonstrate that NAB2-Lipofectin augments the MVA-induced infiltration of natural killer and plasmacytoid dendritic cells. We suggest indirect mechanisms of activation of these cell types by the influence of NAB2-Lipofectin on innate and adaptive immunity. Detailed analysis of cell migration at the vaccine injection site and the appropriate choice of an immune modulator should be considered to achieve the rational improvement of virus vector-based vaccination by immune modulators.

A novel victorivirus from a phytopathogenic fungus, Rosellinia necatrix, is infectious as particles and targeted by RNA silencing

A novel victorivirus, termed Rosellinia necatrix victorivirus 1 (RnVV1), was isolated from a plant pathogenic ascomycete, white root rot fungus Rosellinia necatrix, coinfected with a partitivirus. The virus was molecularly and biologically characterized using the natural and experimental hosts (chestnut blight fungus, Cryphonectria parasitica). RnVV1 was shown to have typical molecular victorivirus attributes, including a monopartite double-stranded RNA genome with two open reading frames (ORFs) encoding capsid protein (CP) and RNA-dependent RNA polymerase (RdRp), a UAAUG pentamer presumed to facilitate the coupled termination/reinitiation for translation of the two ORFs, a spherical particle structure ~40 nm in diameter, and moderate levels of CP and RdRp sequence identity (34 to 58%) to those of members of the genus Victorivirus within the family Totiviridae. A reproducible transfection system with purified RnVV1 virions was developed for the two distinct fungal hosts. Transfection assay with purified RnVV1 virions combined with virus elimination by hyphal tipping showed that the effects of RnVV1 on the phenotype of the natural host were negligible. Interestingly, comparison of the RNA silencing-competent (standard strain EP155) and -defective (Δdcl-2) strains of C. parasitica infected with RnVV1 showed that RNA silencing acted against the virus to repress its replication, which was restored by coinfection with hypovirus or transgenic expression of an RNA silencing suppressor, hypovirus p29. Phenotypic changes were observed in the Δdcl-2 strain but not in EP155. This is the first reported study on the host range expansion of a Totiviridae member that is targeted by RNA silencing.

Antimicrobial and probiotic properties of yeasts: from fundamental to novel applications

The yeasts constitute a large and heterogeneous group of microorganisms that are currently attracting increased attention from scientists and industry. Numerous and diverse biological activities make them promising candidates for a wide range of applications not limited to the food sector. In addition to their major contribution to flavor development in fermented foods, their antagonistic activities toward undesirable bacteria, and fungi are now widely known. These activities are associated with their competitiveness for nutrients, acidification of their growth medium, their tolerance of high concentrations of ethanol, and release of antimicrobial compounds such as antifungal killer toxins or "mycocins" and antibacterial compounds. While the design of foods containing probiotics (microorganisms that confer health benefits) has focused primarily on Lactobacillus and Bifidobacterium, the yeast Saccharomyces cerevisiae var. boulardii has long been known effective for treating gastroenteritis. In this review, the antimicrobial activities of yeasts are examined. Mechanisms underlying this antagonistic activity as well as recent applications of these biologically active yeasts in both the medical and veterinary sectors are described.

Leishmania RNA virus: when the host pays the toll

The presence of an RNA virus in a South American subgenus of the Leishmania parasite, L. (Viannia), was detected several decades ago but its role in leishmanial virulence and metastasis was only recently described. In Leishmania guyanensis, the nucleic acid of Leishmania RNA virus (LRV1) acts as a potent innate immunogen, eliciting a hyper-inflammatory immune response through toll-like receptor 3 (TLR3). The resultant inflammatory cascade has been shown to increase disease severity, parasite persistence, and perhaps even resistance to anti-leishmanial drugs. Curiously, LRVs were found mostly in clinical isolates prone to infectious metastasis in both their human source and experimental animal model, suggesting an association between the viral hyperpathogen and metastatic complications such as mucocutaneous leishmaniasis (MCL). MCL presents as chronic secondary lesions in the mucosa of the mouth and nose, debilitatingly inflamed and notoriously refractory to treatment. Immunologically, this outcome has many of the same hallmarks associated with the reaction to LRV: production of type 1 interferons, bias toward a chronic Th1 inflammatory state and an impaired ability of host cells to eliminate parasites through oxidative stress. More intriguing, is that the risk of developing MCL is found almost exclusively in infections of the L. (Viannia) subtype, further indication that leishmanial metastasis is caused, at least in part, by a parasitic component. LRV present in this subgenus may contribute to the destructive inflammation of metastatic disease either by acting in concert with other intrinsic "metastatic factors" or by independently preying on host TLR3 hypersensitivity. Because LRV amplifies parasite virulence, its presence may provide a unique target for diagnostic and clinical intervention of metastatic leishmaniasis. Taking examples from other members of the Totiviridae virus family, this paper reviews the benefits and costs of endosymbiosis, specifically for the maintenance of LRV infection in Leishmania parasites, which is often at the expense of its human host.

Characterization of Magnaporthe oryzae chrysovirus 1 structural proteins and their expression in Saccharomyces cerevisiae

Magnaporthe oryzae chrysovirus 1 (MoCV1), which is associated with an impaired growth phenotype of its host fungus, harbors four major proteins: P130 (130 kDa), P70 (70 kDa), P65 (65 kDa), and P58 (58 kDa). N-terminal sequence analysis of each protein revealed that P130 was encoded by double-stranded RNA1 (dsRNA1) (open reading frame 1 [ORF1] 1,127 amino acids [aa]), P70 by dsRNA4 (ORF4; 812 aa), and P58 by dsRNA3 (ORF3; 799 aa), although the molecular masses of P58 and P70 were significantly smaller than those deduced for ORF3 and ORF4, respectively. P65 was a degraded form of P70. Full-size proteins of ORF3 (84 kDa) and ORF4 (85 kDa) were produced in Escherichia coli. Antisera against these recombinant proteins detected full-size proteins encoded by ORF3 and ORF4 in mycelia cultured for 9, 15, and 28 days, and the antisera also detected smaller degraded proteins, namely, P58, P70, and P65, in mycelia cultured for 28 days. These full-size proteins and P58 and P70 were also components of viral particles, indicating that MoCV1 particles might have at least two forms during vegetative growth of the host fungus. Expression of the ORF4 protein in Saccharomyces cerevisiae resulted in cytological changes, with a large central vacuole associated with these growth defects. MoCV1 has five dsRNA segments, as do two Fusarium graminearum viruses (FgV-ch9 and FgV2), and forms a separate clade with FgV-ch9, FgV2, Aspergillus mycovirus 1816 (AsV1816), and Agaricus bisporus virus 1 (AbV1) in the Chrysoviridae family on the basis of their RdRp protein sequences.