20S RNA narnavirus defies the antiviral activity of SKI1/XRN1 in Saccharomyces cerevisiae

Genetic conflicts in budding yeast: The 2μ plasmid as a model selfish element

Antagonistic coevolution, arising from genetic conflict, can drive rapid evolution and biological innovation. Conflict can arise both between organisms and within genomes. This review focuses on budding yeasts as a model system for exploring intra- and inter-genomic genetic conflict, highlighting in particular the 2-micron (2μ) plasmid as a model selfish element. The 2μ is found widely in laboratory strains and industrial isolates of Saccharomyces cerevisiae and has long been known to cause host fitness defects. Nevertheless, the plasmid is frequently ignored in the context of genetic, fitness, and evolution studies. Here, I make a case for further exploring the evolutionary impact of the 2μ plasmid as well as other selfish elements of budding yeasts, discuss recent advances, and, finally, future directions for the field.

Argonaute-independent, Dicer-dependent antiviral defense against RNA viruses

Antiviral RNA interference (RNAi) is conserved from yeasts to mammals. Dicer recognizes and cleaves virus-derived double-stranded RNA (dsRNA) and/or structured single-stranded RNA (ssRNA) into small-interfering RNAs, which guide effector Argonaute to homologous viral RNAs for digestion and inhibit virus replication. Thus, Argonaute is believed to be essential for antiviral RNAi. Here, we show Argonaute-independent, Dicer-dependent antiviral defense against dsRNA viruses using Cryphonectria parasitica (chestnut blight fungus), which is a model filamentous ascomycetous fungus and hosts a variety of viruses. The fungus has two dicer-like genes (dcl1 and dcl2) and four argonaute-like genes (agl1 to agl4). We prepared a suite of single to quadruple agl knockout mutants with or without dcl disruption. We tested these mutants for antiviral activities against diverse dsRNA viruses and ssRNA viruses. Although both DCL2 and AGL2 worked as antiviral players against some RNA viruses, DCL2 without argonaute was sufficient to block the replication of other RNA viruses. Overall, these results indicate the existence of a Dicer-alone defense and different degrees of susceptibility to it among RNA viruses. We discuss what determines the great difference in susceptibility to the Dicer-only defense.

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.

Novel viruses of the family Partitiviridae discovered in Saccharomyces cerevisiae

It has been 49 years since the last discovery of a new virus family in the model yeast Saccharomyces cerevisiae. A large-scale screen to determine the diversity of double-stranded RNA (dsRNA) viruses in S. cerevisiae has identified multiple novel viruses from the family Partitiviridae that have been previously shown to infect plants, fungi, protozoans, and insects. Most S. cerevisiae partitiviruses (ScPVs) are associated with strains of yeasts isolated from coffee and cacao beans. The presence of partitiviruses was confirmed by sequencing the viral dsRNAs and purifying and visualizing isometric, non-enveloped viral particles. ScPVs have a typical bipartite genome encoding an RNA-dependent RNA polymerase (RdRP) and a coat protein (CP). Phylogenetic analysis of ScPVs identified three species of ScPV, which are most closely related to viruses of the genus Cryspovirus from the mammalian pathogenic protozoan Cryptosporidium parvum. Molecular modeling of the ScPV RdRP revealed a conserved tertiary structure and catalytic site organization when compared to the RdRPs of the Picornaviridae. The ScPV CP is the smallest so far identified in the Partitiviridae and has structural homology with the CP of other partitiviruses but likely lacks a protrusion domain that is a conspicuous feature of other partitivirus particles. ScPVs were stably maintained during laboratory growth and were successfully transferred to haploid progeny after sporulation, which provides future opportunities to study partitivirus-host interactions using the powerful genetic tools available for the model organism S. cerevisiae.

A novel narnavirus is widespread in Saccharomyces cerevisiae and impacts multiple host phenotypes

RNA viruses are a widespread, biologically diverse group that includes the narnaviridiae, a family of unencapsidated RNA viruses containing a single ORF that encodes an RNA-dependent RNA polymerase. In the yeast Saccharomyces cerevisiae, the 20S and 23S RNA viruses are well-studied members of the narnaviridiae, which are present at low intracellular copy numbers, unless induced by stress or unfavorable growth conditions, and are not known to affect host fitness. In this study, we describe a new S. cerevisiae narnavirus that we designate as N1199. We show that N1199 is uniquely present as a double-stranded RNA at a high level relative to other known members of this family in 1 strain background, YJM1199, and is present as a single-stranded RNA at lower levels in 98 of the remaining 100-genomes strains. Furthermore, we see a strong association between the presence of high level N1199 and host phenotype defects, including greatly reduced sporulation efficiency and growth on multiple carbon sources. Finally, we describe associations between N1199 abundance and host phenotype defects, including autophagy.

Mycovirus Diversity and Evolution Revealed/Inferred from Recent Studies

High-throughput virome analyses with various fungi, from cultured or uncultured sources, have led to the discovery of diverse viruses with unique genome structures and even neo-lifestyles. Examples in the former category include splipalmiviruses and ambiviruses. Splipalmiviruses, related to yeast narnaviruses, have multiple positive-sense (+) single-stranded (ss) RNA genomic segments that separately encode the RNA-dependent RNA polymerase motifs, the hallmark of RNA viruses (members of the kingdom Orthornavirae). Ambiviruses appear to have an undivided ssRNA genome of 3∼5 kb with two large open reading frames (ORFs) separated by intergenic regions. Another narna-like virus group has two fully overlapping ORFs on both strands of a genomic segment that span more than 90% of the genome size. New virus lifestyles exhibited by mycoviruses include the yado-kari/yado-nushi nature characterized by the partnership between the (+)ssRNA yadokarivirus and an unrelated dsRNA virus (donor of the capsid for the former) and the hadaka nature of capsidless 10-11 segmented (+)ssRNA accessible by RNase in infected mycelial homogenates. Furthermore, dsRNA polymycoviruses with phylogenetic affinity to (+)ssRNA animal caliciviruses have been shown to be infectious as dsRNA-protein complexes or deproteinized naked dsRNA. Many previous phylogenetic gaps have been filled by recently discovered fungal and other viruses, which haveprovided interesting evolutionary insights. Phylogenetic analyses and the discovery of natural and experimental cross-kingdom infections suggest that horizontal virus transfer may have occurred and continue to occur between fungi and other kingdoms.

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.

RNA Structure Protects the 5'-end of an Uncapped Tombusvirus RNA Genome from Xrn Digestion

One of the many challenges faced by RNA viruses is the maintenance of their genomes during infections of host cells. Members of the family Tombusviridae are plus-strand RNA viruses with unmodified triphosphorylated genomic 5'-termini. The tombusvirus Carnation Italian ringspot virus was used to investigate how it protects its RNA genome from attack by 5'-end-targeting degradation enzymes. In vivo and in vitro assays were employed to determine the role of genomic RNA structure in conferring protection from the 5'-to-3' exoribonuclease Xrn. The results revealed that (i) the CIRV RNA genome is more resistant to Xrn than its sg mRNAs, (ii) the genomic 5'UTR folds into a compact RNA structure that effectively and independently prevents Xrn access, (iii) the RNA structure limiting 5'-access is formed by secondary and tertiary interactions that function cooperatively, (iv) the structure is also able to block access of RNA pyrophosphohydrolase to the genomic 5'-terminus, and (v) the RNA structure does not stall an actively digesting Xrn. Based on its proficiency at impeding Xrn 5'-access, we have termed this 5'-terminal structure an Xrn-evading RNA or xeRNA. These and other findings demonstrate that the 5'UTR of the CIRV RNA genome folds into a complex structural conformation that helps to protect its unmodified 5'-terminus from enzymatic decay during infections. IMPORTANCE The plus-strand RNA genomes of plant viruses in the large family Tombusviridae are not 5'-capped. Here we explored how a species in the type genus Tombusvirus protects its genomic 5'-end from cellular nuclease attack. Our results revealed that the 5'-terminal sequence of the CIRV genome folds into a complex RNA structure that limits access of the 5'-to-3' exoribonuclease Xrn, thereby protecting it from processive degradation. The RNA conformation also impeded access of RNA pyrophosphohydrolase, which converts 5'-triphosphorylated RNA termini into 5'-monophosphorylated forms, the preferred substrate for Xrn. This study represents the first report of a genome-encoded higher-order RNA structure independently conferring resistance to cellular 5'-end-attacking enzymes in an RNA plant virus.

Hadaka Virus 1: a Capsidless Eleven-Segmented Positive-Sense Single-Stranded RNA Virus from a Phytopathogenic Fungus, Fusarium oxysporum

Fungi collectively host various RNA viruses. Examples include encapsidated double-stranded RNA (dsRNA) viruses with diverse numbers of genomic segments (from 1 to 12) and capsidless viruses with nonsegmented (+)RNA genomes. Recently, viruses with unusual intermediate features of an infectious entity between encapsidated dsRNA viruses and capsidless (+)RNA viruses were found. They are called polymycoviruses, which typically have four to eight dsRNA genomic segments associated with one of the virus-encoded proteins and are phylogenetically distantly related to animal (+)RNA caliciviruses. Here, we identified a novel virus phylogenetically related to polymycoviruses, from the phytopathogenic fungus Fusarium oxysporum. The virus, termed Hadaka virus 1 (HadV1), has 11 (+)RNA genomic segments, the largest number in known (+)RNA viruses. Nevertheless, HadV1 lacked a typical structural protein of polymycoviruses and was not pelleted by standard ultracentrifugation, implying an unusual capsidless nature of HadV1. This study reveals a potential novel lifestyle of multisegmented RNA viruses.

The search for viruses infecting fungi, or mycoviruses, has extended our knowledge about the diversity of RNA viruses, as exemplified by the discovery of polymycoviruses, a phylogenetic group of multisegmented RNA viruses with unusual forms. The genomic RNAs of known polymycoviruses, which show a phylogenetic affinity for animal positive-sense single-stranded RNA [(+)RNA] viruses such as caliciviruses, are comprised of four conserved segments with an additional zero to four segments. The double-stranded form of polymycovirus genomic RNA is assumed to be associated with a virally encoded protein (proline-alanine-serine-rich protein [PASrp]) in either of two manners: a capsidless colloidal form or a filamentous encapsidated form. Detailed molecular characterizations of polymycoviruses, however, have been conducted for only a few strains. Here, a novel polymyco-related virus named Hadaka virus 1 (HadV1), from the phytopathogenic fungus Fusarium oxysporum, was characterized. The genomic RNA of HadV1 consisted of an 11-segmented positive-sense RNA with highly conserved terminal nucleotide sequences. HadV1 shared the three conserved segments with known polymycoviruses but lacked the PASrp-encoding segment. Unlike the known polymycoviruses and encapsidated viruses, HadV1 was not pelleted by conventional ultracentrifugation, possibly due to the lack of PASrp. This result implied that HadV1 exists only as a soluble form with naked RNA. Nevertheless, the 11 genomic segments of HadV1 have been stably maintained through host subculturing and conidiation. Taken together, the results of this study revealed a virus with a potential novel virus lifestyle, carrying many genomic segments without typical capsids or PASrp-associated forms.

A case for a negative-strand coding sequence in a group of positive-sense RNA viruses

Positive-sense single-stranded RNA viruses form the largest and most diverse group of eukaryote-infecting viruses. Their genomes comprise one or more segments of coding-sense RNA that function directly as messenger RNAs upon release into the cytoplasm of infected cells. Positive-sense RNA viruses are generally accepted to encode proteins solely on the positive strand. However, we previously identified a surprisingly long (∼1,000-codon) open reading frame (ORF) on the negative strand of some members of the family Narnaviridae which, together with RNA bacteriophages of the family Leviviridae, form a sister group to all other positive-sense RNA viruses. Here, we completed the genomes of three mosquito-associated narnaviruses, all of which have the long reverse-frame ORF. We systematically identified narnaviral sequences in public data sets from a wide range of sources, including arthropod, fungal, and plant transcriptomic data sets. Long reverse-frame ORFs are widespread in one clade of narnaviruses, where they frequently occupy >95 per cent of the genome. The reverse-frame ORFs correspond to a specific avoidance of CUA, UUA, and UCA codons (i.e. stop codon reverse complements) in the forward-frame RNA-dependent RNA polymerase ORF. However, absence of these codons cannot be explained by other factors such as inability to decode these codons or GC3 bias. Together with other analyses, we provide the strongest evidence yet of coding capacity on the negative strand of a positive-sense RNA virus. As these ORFs comprise some of the longest known overlapping genes, their study may be of broad relevance to understanding overlapping gene evolution and de novo origin of genes.

The exoribonuclease Xrn1 is a post-transcriptional negative regulator of autophagy

Macroautophagy/autophagy is a conserved catabolic process that promotes survival during stress. Autophagic dysfunction is associated with pathologies such as cancer and neurodegenerative diseases. Thus, autophagy must be strictly modulated at multiple levels (transcriptional, post-transcriptional, translational and post-translational) to prevent deregulation. Relatively little is known about the post-transcriptional control of autophagy. Here we report that the exoribonuclease Xrn1/XRN1 functions as a negative autophagy factor in the yeast Saccharomyces cerevisiae and in mammalian cells. In yeast, chromosomal deletion of XRN1 enhances autophagy and the frequency of autophagosome formation. Loss of Xrn1 results in the upregulation of autophagy-related (ATG) transcripts under nutrient-replete conditions, and this effect is dependent on the ribonuclease activity of Xrn1. Xrn1 expression is regulated by the yeast transcription factor Ash1 in rich conditions. In mammalian cells, siRNA depletion of XRN1 enhances autophagy and the replication of 2 picornaviruses. This work provides insight into the role of the RNA decay factor Xrn1/XRN1 as a post-transcriptional regulator of autophagy.

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.

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.

XRN1 Is a Species-Specific Virus Restriction Factor in Yeasts

In eukaryotes, the degradation of cellular mRNAs is accomplished by Xrn1 and the cytoplasmic exosome. Because viral RNAs often lack canonical caps or poly-A tails, they can also be vulnerable to degradation by these host exonucleases. Yeast lack sophisticated mechanisms of innate and adaptive immunity, but do use RNA degradation as an antiviral defense mechanism. One model is that the RNA of yeast viruses is subject to degradation simply as a side effect of the intrinsic exonuclease activity of proteins involved in RNA metabolism. Contrary to this model, we find a highly refined, species-specific relationship between Xrn1p and the "L-A" totiviruses of different Saccharomyces yeast species. We show that the gene XRN1 has evolved rapidly under positive natural selection in Saccharomyces yeast, resulting in high levels of Xrn1p protein sequence divergence from one yeast species to the next. We also show that these sequence differences translate to differential interactions with the L-A virus, where Xrn1p from S. cerevisiae is most efficient at controlling the L-A virus that chronically infects S. cerevisiae, and Xrn1p from S. kudriavzevii is most efficient at controlling the L-A-like virus that we have discovered within S. kudriavzevii. All Xrn1p orthologs are equivalent in their interaction with another virus-like parasite, the Ty1 retrotransposon. Thus, the activity of Xrn1p against totiviruses is not simply an incidental consequence of the enzymatic activity of Xrn1p, but rather Xrn1p co-evolves with totiviruses to maintain its potent antiviral activity and limit viral propagation in Saccharomyces yeasts. Consistent with this, we demonstrated that Xrn1p physically interacts with the Gag protein encoded by the L-A virus, suggesting a host-virus interaction that is more complicated than just Xrn1p-mediated nucleolytic digestion of viral RNAs.

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.

Pr77 and L1TcRz: A dual system within the 5'-end of L1Tc retrotransposon, internal promoter and HDV-like ribozyme

The sequence corresponding to the first 77 nucleotides of the L1Tc and NARTc non-LTR retrotransposons from Trypanosoma cruzi is an internal promoter (Pr77) that generates abundant, although poorly translatable, un-spliced transcripts. It has been recently described that L1TcRz, an HDV-like ribozyme, resides within the 5'-end of the RNA from the L1Tc and NARTc retrotransposons. Remarkably, the same first 77 nucleotides of L1Tc/NARTc elements comprise both the Pr77 internal promoter and the HDV-like L1TcRz. The L1TcRz cleaves on the 5'-side of the +1 nucleotide of the L1Tc element insuring that the promoter and the ribozyme functions travel with the transposon during retrotransposition. The ribozyme activity would prevent the mobilization of upstream sequences and insure the individuality of the L1Tc/NARTc copies transcribed from associated tandems. The Pr77/L1TcRz sequence is also found in other trypanosomatid's non-LTR retrotransposons and degenerated retroposons. The possible conservation of the ribozyme activity in a widely degenerated retrotransposon, as the Leishmania SIDERs, could indicate that the presence of this element and the catalytic activity could play some favorable genetic regulation. The functional implications of the Pr77/L1TcRz dual system in the regulation of the L1Tc/NARTc retrotransposons and in the gene expression of trypanosomatids are also discussed in this paper.

Efficient HIV-1 inhibition by a 16 nt-long RNA aptamer designed by combining in vitro selection and in silico optimisation strategies

The human immunodeficiency virus type-1 (HIV-1) genome contains multiple, highly conserved structural RNA domains that play key roles in essential viral processes. Interference with the function of these RNA domains either by disrupting their structures or by blocking their interaction with viral or cellular factors may seriously compromise HIV-1 viability. RNA aptamers are amongst the most promising synthetic molecules able to interact with structural domains of viral genomes. However, aptamer shortening up to their minimal active domain is usually necessary for scaling up production, what requires very time-consuming, trial-and-error approaches. Here we report on the in vitro selection of 64 nt-long specific aptamers against the complete 5′-untranslated region of HIV-1 genome, which inhibit more than 75% of HIV-1 production in a human cell line. The analysis of the selected sequences and structures allowed for the identification of a highly conserved 16 nt-long stem-loop motif containing a common 8 nt-long apical loop. Based on this result, an in silico designed 16 nt-long RNA aptamer, termed RNApt16, was synthesized, with sequence 5′-CCCCGGCAAGGAGGGG-3′. The HIV-1 inhibition efficiency of such an aptamer was close to 85%, thus constituting the shortest RNA molecule so far described that efficiently interferes with HIV-1 replication.

Resting complexes of the persistent yeast 20S RNA Narnavirus consist solely of the 20S RNA viral genome and its RNA polymerase p91

The positive strand 20S RNA narnavirus persistently infects Saccharomyces cerevisiae. The 20S RNA genome has a single gene that encodes the RNA-dependent RNA polymerase (p91). 20S RNA forms ribonucleoprotein resting complexes (RNPs) with p91 and resides in the cytoplasm. Here we found no host proteins stoichiometrically associated with the RNP by pull-down experiments. Furthermore, 20S RNA, when expressed from a vector in Escherichia coli, formed RNPs with p91 in the absence of yeast proteins. This interaction required the 3' cis signal for complex formation. Moreover, when 23S RNA, the genome of another narnavirus, was expressed in E. coli, it also formed RNPs with its RNA polymerase p104. Finally, when both RNAs were expressed in the same E. coli cell, they formed RNPs only with their cognate RNA polymerases. These results altogether indicate that narnaviruses RNPs consist of only the viral genomes and their cognate RNA polymerases. Because the copy number of the RNPs can be induced almost equivalent to those of rRNAs in some yeast strains, the absence of host proteins may alleviate the burden on the host by not sequestering proteins into the RNPs. It may also contribute to the persistent infection of narnaviruses by decreasing their visibility.

Viruses and prions of Saccharomyces cerevisiae

Saccharomyces cerevisiae has been a key experimental organism for the study of infectious diseases, including dsRNA viruses, ssRNA viruses, and prions. Studies of the mechanisms of virus and prion replication, virus structure, and structure of the amyloid filaments that are the basis of yeast prions have been at the forefront of such studies in these classes of infectious entities. Yeast has been particularly useful in defining the interactions of the infectious elements with cellular components: chromosomally encoded proteins necessary for blocking the propagation of the viruses and prions, and proteins involved in the expression of viral components. Here, we emphasize the L-A dsRNA virus and its killer-toxin-encoding satellites, the 20S and 23S ssRNA naked viruses, and the several infectious proteins (prions) of yeast.

Exon B of human surfactant protein A2 mRNA, alone or within its surrounding sequences, interacts with 14-3-3; role of cis-elements and secondary structure

Human surfactant protein A, an innate immunity molecule, is encoded by two genes: SFTPA1 (SP-A1) and SFTPA2 (SP-A2). The 5' untranslated (5'UTR) splice variant of SP-A2 (ABD), but not of SP-A1 (AD), contains exon B (eB), which is an enhancer for transcription and translation. We investigated whether eB contains cis-regulatory elements that bind trans-acting factors in a sequence-specific manner as well as the role of the eB mRNA secondary structure. Binding of cytoplasmic NCI-H441 proteins to wild-type eB, eB mutant, AD, and ABD 5'UTR mRNAs were studied by RNA electromobility shift assays (REMSAs). The bound proteins were identified by mass spectroscopy and specific antibodies (Abs). We found that 1) proteins bind eB mRNA in a sequence-specific manner, with two cis-elements identified within eB to be important; 2) eB secondary structure is necessary for binding; 3) mass spectroscopy and specific Abs in REMSAs identified 14-3-3 proteins to bind (directly or indirectly) eB and the natural SP-A2 (ABD) splice variant but not the SP-A1 (AD) splice variant; 4) other ribosomal and cytoskeletal proteins, and translation factors, are also present in the eB mRNA-protein complex; 5) knockdown of 14-3-3 β/α isoform resulted in a downregulation of SP-A2 expression. In conclusion, proteins including the 14-3-3 family bind two cis-elements within eB of hSP-A2 mRNA in a sequence- and secondary structure-specific manner. Differential regulation of SP-A1 and SP-A2 is mediated by the 14-3-3 protein family as well as by a number of other proteins that bind UTRs with or without eB mRNA.

Manipulation of cellular processing bodies and their constituents by viruses

The processing bodies (PBs) are a form of cytoplasmic aggregates that house the cellular RNA decay machinery as well as many RNA-binding proteins and mRNAs. The PBs are constitutively present in eukaryotic cells and are involved in maintaining cellular homeostasis by regulating RNA metabolism, cell signaling, and survival. Virus infections result in modification of the PBs and their constituents. Many viruses induce compositionally altered PBs, while many others use specific components of the PBs for their replication. PB constituents are also known to restrict virus replication by a variety of mechanisms. Further, continuing studies in this rapidly emerging field of PB-virus interactions will undoubtedly provide important clues to the understanding of the role of PBs in cellular homeostasis as well as their role in virus infections and innate immune signaling.

L-A-lus, a new variant of the L-A totivirus found in wine yeasts with Klus killer toxin-encoding Mlus double-stranded RNA: possible role of killer toxin-encoding satellite RNAs in the evolution of their helper viruses

Yeast killer viruses are widely distributed in nature. Several toxins encoded in double-stranded RNA (dsRNA) satellites of the L-A totivirus have been described, including K1, K2, K28, and Klus. The 4.6-kb L-A genome encodes the Gag major structural protein that forms a 39-nm icosahedral virion and Gag-Pol, a minor fusion protein. Gag-Pol has transcriptase and replicase activities responsible for maintenance of L-A (or its satellite RNAs). Recently we reported a new killer toxin, Klus. The L-A virus in Klus strains showed poor hybridization to known L-A probes, suggesting substantial differences in their sequences. Here we report the characterization of this new L-A variant named L-A-lus. At the nucleotide level, L-A and L-A-lus showed only 73% identity, a value that increases to 86% in the amino acid composition of Gag or Gag-Pol. Two regions in their genomes, however, the frameshifting region between Gag and Pol and the encapsidation signal, are 100% identical, implying the importance of these two cis signals in the virus life cycle. L-A-lus shows higher resistance than L-A to growth at high temperature or to in vivo expression of endo- or exonucleases. L-A-lus also has wider helper activity, being able to maintain not only Mlus but also M1 or a satellite RNA of L-A called X. In a screening of 31 wine strains, we found that none of them had L-A; they carried either L-A-lus or a different L-A variant in K2 strains. Our data show that distinct M killer viruses are specifically associated with L-As with different nucleotide compositions, suggesting coevolution.

The family narnaviridae: simplest of RNA viruses

Members of the virus family Narnaviridae contain the simplest genomes of any RNA virus, ranging from 2.3 to 3.6 kb and encoding only a single polypeptide that has an RNA-dependent RNA polymerase domain. The family is subdivided into two genera based on subcellular location: members of the genus Narnavirus have been found in the yeast Saccharomyces cerevisiae and in the oomycete Phytophthora infestans and are confined to the cytosol, while members of the genus Mitovirus have been found only in filamentous fungi and are found in mitochondria. None identified thus far encodes a capsid protein; like several other RNA viruses of lower eukaryotes, their genomes are confined within lipid vesicles. As more family members are discovered, their importance as genetic elements is becoming evident. The unique association of the genus Mitovirus with mitochondria renders them potentially valuable tools to study biology of lower eukaryotes.

Expression profile of genes involved in pathogenesis of pediatric Crohn's disease

BACKGROUND AND AIM

Expression profiling of genes specific to pediatric Crohn's Disease (CD) patients was performed to elucidate the molecular mechanisms underlying disease cause and pathogenesis at disease onset.

METHODS

We used suppressive subtractive hybridization (SSH) and differential screening analysis to profile the mRNA expression patterns of children with CD and age- and sex-matched controls without inflammatory bowel disease (IBD).

RESULTS

Sequence analysis of 1000 clones enriched by SSH identified 75 functionally annotated human genes, represented by 430 clones. The 75 genes have potential involvement in gene networks, such as antigen presentation, inflammation, infection mechanism, connective tissue development, cell cycle and cancer. Twenty-eight genes were previously described in association with CD, while 47 were new genes not previously reported in the context of IBD. Additionally, 29 of the 75 genes have been previously implicated in bacterial and viral infections. Quantitative real-time reverse transcription polymerase chain reaction performed on ileal-derived RNA from 13 CD and nine non-IBD patients confirmed the upregulation of extracellular matrix gene MMP2 (P = 0.001), and cell proliferation gene REG1A (P = 0.063) in our pediatric CD cohort.

CONCLUSION

The retrieval of 28 genes previously reported in association with adult CD emphasizes the importance of these genes in the pediatric setting. The observed upregulation of REG1A and MMP2, and their known impact on cell proliferation and extracellular matrix remodeling, agrees with the clinical behavior of the disease. Moreover, the expressions of bacterial- and virus-related genes in our CD-patient tissues support the concept that microbial agents are important in the etiopathogenesis of CD.

Beet necrotic yellow vein virus subgenomic RNA3 is a cleavage product leading to stable non-coding RNA required for long-distance movement

Beet necrotic yellow vein virus (BNYVV) is a multipartite RNA virus. BNYVV RNA3 does not accumulate in non-host transgenic Arabidopsis thaliana plants when expressed using a 35S promoter. However, a 3'-derivative species has been detected in transgenic plants and in transient expression assays conducted in Nicotiana benthamiana and Beta macrocarpa. The 3'-derivative species is similar to the previously reported subgenomic RNA3 produced during virus infection. 5' RACE revealed that the truncated forms had identical 5' ends. The 5' termini carried the coremin motif also present on BNYVV RNA5, beet soil-borne mosaic virus RNA3 and 4, and cucumber mosaic virus group 2 RNAs. This RNA3 species lacks a m(7)Gppp at the 5' end of the cleavage products, whether expressed transiently or virally. Mutagenesis revealed the importance of the coremin sequence for both long-distance movement and stabilization of the cleavage product in vivo and in vitro. The isolation of various RNA3 5'-end products suggests the existence of a cleavage between nt 212 and 1234 and subsequent exonucleolytic degradation, leading to the accumulation of a non-coding RNA. When RNA3 was incubated in wheatgerm extracts, truncated forms appeared rapidly and their appearance was protein- and divalent ion-dependent.

Yeast RNA viruses as indicators of exosome activity: human exosome hCsl4p participates in RNA degradation in Saccharomyces cerevisiae'

The exosome is an evolutionarily conserved 10-mer complex involved in RNA metabolism, located in both the nucleus and the cytoplasm. The cytoplasmic exosome plays an important role in mRNA turnover through its 3'→5' exonucleolytic activity. The superkiller (SKI) phenotype of yeast was originally identified as an increase of killer toxin production due to elevated levels of the L-A double-stranded RNA (dsRNA) Totivirus and its satellite toxin-encoding M dsRNA. Most SKI genes were later shown to be either components of the exosome or modulators of its activity. Variations in the amount of Totivirus are, thus, good indicators of yeast exosome activity, and can be used to analyse its components. Furthermore, if exosome proteins of higher eukaryotes were functional in S. cerevisiae, these viruses would provide a simple tool to analyse their function. In this work, we have found that hCSL4, the human orthologue of SKI4 in the yeast exosome, rescues the null phenotype of the deletion mutant. hCsl4p shares with Ski4p conserved S1 RNA-binding domains, but lacks the N-terminal third of Ski4p. Nevertheless, it interacts with the Dis3p exonuclease of yeast exosome, and partially complements the superkiller phenotype of ski4-1 mutation. The elimination of the N-terminal third of Ski4p does not affect its activity, indicating that it is dispensable for RNA degradation. We have also identified the point mutation G152E in hCSL4, equivalent to the ski4-1 mutation G253E, which impairs the activity of the protein, thus validating our approach of using yeast RNA virus to analyse the exosome of higher eukaryotes.

An RNA pseudoknot is required for production of yellow fever virus subgenomic RNA by the host nuclease XRN1

Cells and mice infected with arthropod-borne flaviviruses produce a small subgenomic RNA that is colinear with the distal part of the viral 3'-untranslated region (UTR). This small subgenomic flavivirus RNA (sfRNA) results from the incomplete degradation of the viral genome by the host 5'-3' exonuclease XRN1. Production of the sfRNA is important for the pathogenicity of the virus. This study not only presents a detailed description of the yellow fever virus (YFV) sfRNA but, more importantly, describes for the first time the molecular characteristics of the stalling site for XRN1 in the flavivirus genome. Similar to the case for West Nile virus, the YFV sfRNA was produced by XRN1. However, in contrast to the case for other arthropod-borne flaviviruses, not one but two sfRNAs were detected in YFV-infected mammalian cells. The smaller of these two sfRNAs was not observed in infected mosquito cells. The larger sfRNA could also be produced in vitro by incubation with purified XRN1. These two YFV sfRNAs formed a 5'-nested set. The 5' ends of the YFV sfRNAs were found to be just upstream of the previously predicted RNA pseudoknot PSK3. RNA structure probing and mutagenesis studies provided strong evidence that this pseudoknot structure was formed and served as the molecular signal to stall XRN1. The sequence involved in PSK3 formation was cloned into the Sinrep5 expression vector and shown to direct the production of an sfRNA-like RNA. These results underscore the importance of the RNA pseudoknot in stalling XRN1 and also demonstrate that it is the sole viral requirement for sfRNA production.

Yeast double-stranded RNA virus L-A deliberately synthesizes RNA transcripts with 5'-diphosphate

L-A is a persistent double-stranded RNA virus commonly found in the yeast Saccharomyces cerevisiae. Isolated L-A virus synthesizes positive strand transcripts in vitro. We found that the 5' termini of the transcripts are diphosphorylated. The 5'-terminal nucleotide is G, and GDP was the best substrate among those examined to prime the reaction. When GTP was used, the triphosphate of GTP incorporated into the 5'-end was converted to diphosphate. This activity was not dependent on host CTL1 RNA triphosphatase. The 5'-end of the GMP-primed transcript also was converted to diphosphate, the beta-phosphate of which was derived from the gamma-phosphate of ATP present in the polymerization reaction. These results demonstrate that L-A virus commands elaborate enzymatic systems to ensure its transcript to be 5'-diphosphorylated. Transcripts of M1, a satellite RNA of L-A virus, also had diphosphate at the 5' termini. Because viral transcripts are released from the virion into the cytoplasm to be translated and encapsidated into a new viral particle, a stage most vulnerable to degradation in the virus replication cycle, our results suggest that the 5'-diphosphate status is important for transcript stability. Consistent with this, L-A transcripts made in vitro are resistant to the affinity-purified Ski1p 5'-exonuclease. We also discuss the implication of these findings on translation of viral RNA. Because the viral transcript has no conventional 5'-cap structure, this work may shed light on the metabolism of non-self-RNA in yeast.

Viruses and the cellular RNA decay machinery

The ability to control cellular and viral gene expression, either globally or selectively, is central to a successful viral infection, and it is also crucial for the host to respond and eradicate pathogens. In eukaryotes, regulation of message stability contributes significantly to the control of gene expression and plays a prominent role in the normal physiology of a cell as well as in its response to environmental and pathogenic stresses. Not surprisingly, emerging evidence indicates that there are significant interactions between the eukaryotic RNA turnover machinery and a wide variety of viruses. Interestingly, in many cases viruses have evolved mechanisms not only to evade eradication by these pathways, but also to manipulate them for enhanced viral replication and gene expression. Given our incomplete understanding of how many of these pathways are normally regulated, viruses should be powerful tools to help deconstruct the complex networks and events governing eukaryotic RNA stability. Copyright © 2010 John Wiley & Sons, Ltd.

This article is categorized under: 1

RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms

2

RNA in Disease and Development > RNA in Disease

A highly structured, nuclease-resistant, noncoding RNA produced by flaviviruses is required for pathogenicity

Viral noncoding RNAs have been shown to play an important role in virus-host interplay to facilitate virus replication. We report that members of the genus Flavivirus, a large group of medically important encephalitic RNA viruses, produce a unique and highly structured noncoding RNA of 0.3-0.5 kb derived from the 3' untranslated region of the viral genome. Using West Nile virus as a model, we show that this subgenomic RNA is a product of incomplete degradation of viral genomic RNA by cellular ribonucleases. Highly conserved RNA structures located at the beginning of the 3' untranslated region render this RNA resistant to nucleases, and the resulting subgenomic RNA product is essential for virus-induced cytopathicity and pathogenicity. Thus, flaviviruses evolved a unique strategy to generate a noncoding RNA product that allows them to kill the host more efficiently.