Site-specific binding of viral plus single-stranded RNA to replicase-containing open virus-like particles of yeast

The Species-Specific Acquisition and Diversification of a K1-like Family of Killer Toxins in Budding Yeasts of the Saccharomycotina

Killer toxins are extracellular antifungal proteins that are produced by a wide variety of fungi, including Saccharomyces yeasts. Although many Saccharomyces killer toxins have been previously identified, their evolutionary origins remain uncertain given that many of these genes have been mobilized by double-stranded RNA (dsRNA) viruses. A survey of yeasts from the Saccharomyces genus has identified a novel killer toxin with a unique spectrum of activity produced by Saccharomyces paradoxus. The expression of this killer toxin is associated with the presence of a dsRNA totivirus and a satellite dsRNA. Genetic sequencing of the satellite dsRNA confirmed that it encodes a killer toxin with homology to the canonical ionophoric K1 toxin from Saccharomyces cerevisiae and has been named K1-like (K1L). Genomic homologs of K1L were identified in six non-Saccharomyces yeast species of the Saccharomycotina subphylum, predominantly in subtelomeric regions of the genome. When ectopically expressed in S. cerevisiae from cloned cDNAs, both K1L and its homologs can inhibit the growth of competing yeast species, confirming the discovery of a family of biologically active K1-like killer toxins. The sporadic distribution of these genes supports their acquisition by horizontal gene transfer followed by diversification. The phylogenetic relationship between K1L and its genomic homologs suggests a common ancestry and gene flow via dsRNAs and DNAs across taxonomic divisions. This appears to enable the acquisition of a diverse arsenal of killer toxins by different yeast species for potential use in niche competition.

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.

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.

Cap snatching of yeast L-A double-stranded RNA virus can operate in trans and requires viral polymerase actively engaging in transcription

Eukaryotic mRNA bears a cap structure (m(7)GpppX-) at the 5' terminus crucial for efficient translation and stability. The yeast L-A double-stranded RNA virus furnishes its mRNA with this structure by a novel cap-snatching mechanism in which the virus transfers an m(7)Gp moiety from host mRNA to the diphosphorylated 5' terminus of the viral transcript, thus forming on it an authentic cap structure (referred to as cap0) in the budding yeast. This capping reaction is essential for efficient viral expression. His-154 of the capsid protein Gag is involved in the cap transfer. Here we show that the virus can utilize an externally added viral transcript as acceptor in the capping reaction. The acceptor needs to be 5' diphosphorylated, consistent with the fact that the viral transcript bears diphosphate at the 5' terminus. A 5' triphosphorylated or monophosphorylated transcript does not function as acceptor. N7 methylation at the 5' cap guanine of mRNA is essential for cap donor activity. We also demonstrate that the capping reaction requires the viral polymerase actively engaging in transcription. Because the cap-snatching site of Gag is located at the cytoplasmic surface of the virion, whereas Pol is confined inside the virion, the result indicates coordination between the cap-snatching and polymerization sites. This will allow L-A virus to efficiently produce capsid proteins to form new virions when Pol is actively engaging in transcription. The coordination may also minimize the risk of accidental capping of nonviral RNA when Pol is dormant.

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.

Giardia lamblia: stable expression of green fluorescent protein mediated by giardiavirus

Giardia lamblia, an early diverging eukaryote that infects several species including humans and a major agent of water-borne diarrhea throughout the world, can be infected with a double-stranded RNA virus, giardiavirus (GLV). A chimeric GLV cDNA and green fluorescent protein (GFP) according to the cis-acting signals of the GLV genome required for expression of foreign gene was constructed and its in vitro transcript was electroporated into GLV-infected G. lamblia trophozoites, GFP was expressed transiently. pGDH5/NEO/GLV was constructed by combining the neomycin resistance cassette in which the neomycin phosphotransferase gene was flanked by Giardia glutamate dehydrogenase (GDH) uncoding regions and the transcription cassette in which the chimera of GLV cDNA and GFP was located downstream from GDH gene promoter on a single plasmid. This plasmid was electroporated into G. lamblia and the transfectants persistently expressed GFP under G418 selection. This stable transfection system should provide a valuable tool for genetic study of G. lamblia.

The Zygosaccharomyces bailii antifungal virus toxin zygocin: cloning and expression in a heterologous fungal host

Zygocin, a monomeric protein toxin secreted by a virus-infected killer strain of the osmotolerant spoilage yeast Zygosaccharomyces bailii, kills a broad spectrum of human and phytopathogenic yeasts and filamentous fungi by disrupting cytoplasmic membrane function. The toxin is encoded by a double-stranded (ds)RNA killer virus (ZbV-M, for Z. bailii virus M) that stably persists within the yeast cell cytosol. In this study, the protein toxin was purified, its N-terminal amino acid sequence was determined, and a full-length cDNA copy of the 2.1 kb viral dsRNA genome was cloned and successfully expressed in a heterologous fungal system. Sequence analysis as well as zygocin expression in Schizosaccharomyces pombe indicated that the toxin is in vivo expressed as a 238-amino-acid preprotoxin precursor (pptox) consisting of a hydrophobic N-terminal secretion signal, followed by a potentially N-glycosylated pro-region and terminating in a classical Kex2p endopeptidase cleavage site that generates the N-terminus of the mature and biologically active protein toxin in a late Golgi compartment. Matrix-assisted laser desorption mass spectrometry further indicated that the secreted toxin is a monomeric 10.4 kDa protein lacking detectable post-translational modifications. Furthermore, we present additional evidence that in contrast with other viral antifungal toxins, zygocin immunity is not mediated by the toxin precursor itself and, therefore, heterologous pptox expression in a zygocin-sensitive host results in a suicidal phenotype. Final sequence comparisons emphasize the conserved pattern of functional elements present in dsRNA killer viruses that naturally infect phylogenetically distant hosts (Saccharomyces cerevisiae and Z. bailii) and reinforce models for the sequence elements that are in vivo required for viral RNA packaging and replication.

Recognition of RNA encapsidation signal by the yeast L-A double-stranded RNA virus

The encapsidation signal of the yeast L-A virus contains a 24-nucleotide stem-loop structure with a 5-nucleotide loop and an A bulged at the 5' side of the stem. The Pol part of the Gag-Pol fusion protein is responsible for encapsidation of viral RNA. Opened empty viral particles containing Gag-Pol specifically bind to this encapsidation signal in vitro. We found that binding to empty particles protected the bulged A and the flanking-two nucleotides from cleavage by Fe(II)-EDTA-generated hydroxyl radicals. The five nucleotides of the loop sequence ((4190)GAUCC(4194)) were not protected. However, T1 RNase protection and in vitro mutagenesis experiments indicated that G(4190) is essential for binding. Although the sequence of the other four nucleotides of the loop is not essential, data from RNase protection and chemical modification experiments suggested that C(4194) was also directly involved in binding to empty particles rather than indirectly through its potential base pairing with G(4190). These results suggest that the Pol domain of Gag-Pol contacts the encapsidation signal at two sites: one, the bulged A, and the other, G and C bases at the opening of the loop. These two sites are conserved in the encapsidation signal of M1, a satellite RNA of the L-A virus.

In vitro selective RNA synthesis with L-A virus nanoparticles

New in vitro RNA synthesis has been performed with an L-A virus nanoparticles, in which the gene and polymerase are integrated. The specific recognition sequence (packaging site) of L-A virus was inserted within a gene of interest. Based on the intrinsic replication cycle, the exogenous RNA with the packaging site was encapsulated by an empty L-A virus nanoparticle. The packaging site worked as a recognition site even for exogenous RNAs. The recognized RNA was replicated to dsRNA, and was then transcribed by empty L-A virus nanoparticles. These results indicate that empty L-A virus nanoparticles recognize an exogenous RNA with the packaging site and synthesize RNA in vitro.

In vitro selection of packaging sites in a double-stranded RNA virus

The Saccharomyces cerevisiae double-stranded RNA virus ScVL1 recognizes a small sequence in the viral plus strand for both packaging and replication. Viral particles will bind to this viral binding sequence (VBS) with high affinity in vitro. An in vitro selection procedure has been used to optimize binding, and the sequences isolated have been analyzed for packaging and replication in vivo. The selected sequence consists of a stem with a bulged A residue topped by a loop of several bases. Four residues of the 18 bases are absolutely conserved for tight binding. These all fall in regions that appear to be single stranded. Eight more residues have preferred identities, and six of these are in the stem. The VBS is similar to the R17 bacteriophage coat protein binding site. Packaging and replication require tight binding to viral particles.

Prions and RNA viruses of Saccharomyces cerevisiae

Saccharomyces cerevisiae is host to the dsRNA viruses L-A (including its killer toxin-encoding satellite, M) and L-BC, the 20S and 23S ssRNA replicons, and the putative prions, [URE3] and [PSI]. review the genetic and biochemical evidence indicating that [URE3] and [PSI] are prion forms of Ure2p and Sup35p, respectively. Each has an N-terminal domain involved in propagation or generation of the prion state and a C-terminal domain responsible for the protein's normal function, nitrogen regulation, or translation termination, respectively. The L-A dsRNA virus expression, replication, and RNA packaging are reviewed. L-A uses a -1 ribosomal frameshift to produce a Gag-Pol fusion protein. The host SK12, SK13 and SK18 proteins block translation of nonpoly(A) mRNAs (such as viral mRNA). Mutants deficient in 60S ribosomal subunits replicate L-A poorly, but not if cells are also ski-. Interaction of 60S subunits with the 3' polyA is suggested. SKI1/XRN1 is a 5'--> 3' exoribonuclease that degrades uncapped mRNAs. The viral Gag protein decapitates cellular mRNAs apparently to decoy this enzyme from working on viral mRNA.

Saccharomyces cerevisiae L-BC double-stranded RNA virus replicase recognizes the L-A positive-strand RNA 3' end

L-A and L-BC are two double-stranded RNA viruses present in almost all strains of Saccharomyces cerevisiae. L-A, the major species, has been extensively characterized with in vitro systems established, but little is known about L-BC. Here we report in vitro template-dependent transcription, replication, and RNA recognition activities of L-BC. The L-BC replicase activity converts positive, single-stranded RNA to double-stranded RNA by synthesis of the complementary RNA strand. Although L-A and L-BC do not interact in vivo, in vitro L-BC virions can replicate the positive, single-stranded RNA of L-A and its satellite, M1, with the same 3' end sequence and stem-loop requirements shown by L-A virions for its own template. However, the L-BC virions do not recognize the internal replication enhancer of the L-A positive strand. In a direct comparison of L-A and L-BC virions, each preferentially recognizes its own RNA for binding, replication, and transcription. These results suggest a close evolutionary relation of these two viruses, consistent with their RNA-dependent RNA polymerase sequence similarities.

Characterization of double-stranded RNA satellites associated with the Trichomonas vaginalis virus

Three small and distinct satellite double-stranded RNAs (dsRNAs) denoted s1, s1', and s2 were recently described for a Trichomonas vaginalis isolate harboring a dsRNA virus. Since characterization of these satellite dsRNAs might provide insight into the virus replication cycle and virus-host interactions, full-length cDNAs to s1 and s1' dsRNAs were synthesized and sequenced. s1 dsRNA has 688 bp, and s1' dsRNA has 616 bp. A 228-bp open reading frame that begins at nucleotide 37 was detected on a putative sense strand of s1. All satellite RNAs were found associated with RNA-dependent RNA polymerase (RDRP) activity that banded on CsCl gradients. Within carrier trichomonads, satellite RNAs synthesized single-stranded replicative intermediates. An in vitro assay was established to assess replication of satellite RNAs. Transcripts generated from s1 cDNA, for example, served as a template for the viral RDRP. These templates had a polarity similar to that of the replicative intermediate found in the satellite-harboring parasites. Importantly, the recognition of s1 RNA was shown to be specific, since unrelated RNAs did not serve as templates for RDRP under the same experimental conditions. The data indicate that the cDNA of s1 has a specific and essential sequence needed for recognition by the viral RDRP and for subsequent RNA synthesis. Both s1 and s1' have conserved domains, albeit of unproven function, but which may be required for replication.

Virus-mediated expression of firefly luciferase in the parasitic protozoan Giardia lamblia

Giardia lamblia, a prevalent human pathogen and one of the lineages that branched earliest from prokaryotes, can be infected with a double-stranded RNA virus, giardiavirus (GLV). The 6,277-bp viral genome has been previously cloned (A.L. Wang, H.-M. Yang, K.A. Shen, and C.C. Wang, Proc. Natl. Acad. Sci. USA 90:8595-8599, 1993; C.-H. Wu, C.C. Wang, H.M. Yang, and A.L. Wang, Gene, in press) and was converted to a transfection vector for G. lamblia in the present study. By flanking the firefly luciferase gene with the 5' and 3' untranslated regions (UTRs) of the GLV genome, transcript of the construct was synthesized in vitro with T7 polymerase and used to transfect G. lamblia WB trophozoites already infected with GLV (WBI). Optimal electroporation conditions used for the transfection were set at 1,000 V/cm and 500 microF, which resulted in expression of significant luciferase activity up to 120 h after electroporation. Furthermore, the mRNA and the antisense RNA of the luciferase gene were both detected by reverse transcription and PCR from 6 to 120 h postelectroporation, whereas no antisense RNA of luciferase was observed in the electroporated virus-free Giardia WB trophozoites. The mRNA of luciferase was detectable in the virus-free trophozoites by reverse transcription and PCR only up to 20 h after the electroporation, indicating that the introduced mRNA was replicated only by the viral RNA-dependent RNA polymerase inside the WBI cells. This expression of luciferase was dependent on the presence of UTRs on both ends of the viral genome transcript, including a putative packaging site that was apparently indispensable for luciferase expression. This is the first time that a viral vector in the form of mRNA URTs has been successfully used in transfecting a protozoan.

Packaging in a yeast double-stranded RNA virus

The yeast virus ScV-L1 has only two genes, cap and pol, which encode the capsid polypeptide and the viral polymerase, respectively. The second gene is translated only as a cap-pol fusion protein. This fusion protein is responsible for recognition of a specific small stem and loop region of the viral plus strands, of 19 to 31 bases in length, ensuring packaging specificity. We have used a related virus, ScV-La, which has about 29% codon identity with ScV-L1 in the most conserved region of the pol gene, to map the region in pol that is responsible for packaging L1. Characterization of a number of chimeric viral proteins that recognize L1 but have the La capsid region delimits the region necessary for recognition of L1 to a 76- to 82-codon portion of pol. In addition, we show that overproduction of the La capsid polypeptide results in curing of the ScV-La virus, analogous to the production of plants resistant to RNA viruses by virtue of systemic production of viral coat protein.

Translational maintenance of frame: mutants of Saccharomyces cerevisiae with altered -1 ribosomal frameshifting efficiencies

A special site on the (+) strand of the L-A dsRNA virus induces about 2% of ribosomes translating the gag open reading frame to execute a -1 frameshift and thus produce the viral gag-pol fusion protein. Using constructs in which a -1 ribosomal frameshift at this site was necessary for expression of lacZ we isolated chromosomal mutants in which the efficiency of frameshifting was increased. These mutants comprise eight genes, named mof (maintenance of frame). The mof1-1, mof2-1, mof4-1, mof5-1 and mof6-1 strains cannot maintain M1 dsRNA at 30 degrees, but, paradoxically, do not lose L-A. The mof2-1, mof5-1 and mof6-1 strains are temperature sensitive for growth at 37 degrees, and all three show striking cell cycle phenotypes. The mof2-1 strains arrest with mother and daughter cells almost equal in size, mof5-1 arrests with multiple buds and mof6-1 arrests as single large unbudded cells. mof2-1 and mof5-1 strains are also Pet-. The mof mutations show differential effects on various frameshifting signals.

New developments in fungal virology

Although viruses are widely distributed in fungi, their biological significance to their hosts is still poorly understood. A large number of fungal viruses are associated with latent infections of their hosts. With the exception of the killer-immune character in the yeasts, smuts, and hypovirulence in the chestnut blight fungus, fungal properties that can specifically be related to virus infection are not well defined. Mycoviruses are not known to have natural vectors; they are transmitted in nature intracellularly by hyphal anastomosis and heterokaryosis, and are disseminated via spores. Because fungi have a potential for plasmogamy and cytoplasmic exchange during extended periods of their life cycles and because they produce many types of propagules (sexual and asexual spores), often in great profusion, mycoviruses have them accessible to highly efficient means for transmission and spread. It is no surprise, therefore, that fungal viruses are not known to have an extracellular phase to their life cycles. Although extracellular transmission of a few fungal viruses have been demonstrated, using fungal protoplasts, the lack of conventional methods for experimental transmission of these viruses have been, and remains, an obstacle to understanding their biology. The recent application of molecular biological approaches to the study of mycoviral dsRNAs and the improvements in DNA-mediated fungal transformation systems, have allowed a clearer understanding of the molecular biology of mycoviruses to emerge. Considerable progress has been made in elucidating the genome organization and expression strategies of the yeast L-A virus and the unencapsidated RNA virus associated with hypovirulence in the chestnut blight fungus. These recent advances in the biochemical and molecular characterization of the genomes of fungal viruses and associated satellite dsRNAs, as they relate to the biological properties of these viruses and to their interactions with their hosts are the focus of this chapter.

Elimination of L-A double-stranded RNA virus of Saccharomyces cerevisiae by expression of gag and gag-pol from an L-A cDNA clone

We report that expression of a nearly full-length cDNA clone of the L-A double-stranded RNA virus causes virus loss in a wild-type strain of Saccharomyces cerevisiae. We show that in this system exclusion of the L-A virus is independent of the presence of the packaging site or of cis sites for replication and transcription and completely dependent on expression of functional recombinant gag and gag-pol fusion protein. Thus, this exclusion is not explained in terms of overexpression of packaging signals. Mutation of the chromosomal SKI2 gene, known to repress the copy number of double-stranded RNA cytoplasmic replicons of S. cerevisiae, nearly eliminates the exclusion. We suggest that exclusion is due to competition by proteins expressed from the plasmid for a possibly limiting cellular factor. Our hypotheses on exclusion of L-A proteins may also apply to resistance to plant viruses produced by expression of viral replicases in transgenic plants.

Movement of the guide sequence during RNA catalysis by a group I ribozyme

Ribozymes derived from the self-splicing pre-ribosomal RNA of Tetrahymena act as sequence-specific endonucleases. The reaction involves binding an RNA or DNA substrate by base pairing to the internal guide sequence (IGS) to form helix P1. Site-specific photo-crosslinking localized the 5' end of the IGS in helix P1 to the vicinity of conserved bases between helices P4 and P5, supporting a major feature of the Michel-Westhof three-dimensional structure model. The crosslinked ribozyme retained catalytic activity. When not base-paired, the IGS was still specifically crosslinked, but the major site was 37 A distant from the reactive site in the experimentally supported three-dimensional model. The data indicate that a substantial induced-fit conformational change accompanies P1 formation, and they provide a physical basis for understanding the transport of oligonucleotides to the catalytic core of the ribozyme. The ability of RNA to orchestrate large-scale conformational changes may help explain why the ribosome and the spliceosome are RNA-based machines.

Sequences of three dsRNAs associated with La France disease of the cultivated mushroom (Agaricus bisporus)

La France disease of the cultivated mushroom, Agaricus bisporus, is known to be associated with the presence of a number of dsRNA segments. The nucleotide sequences of the dsRNAs M2 (1.3 kb), M1 (1.55 kb) and L3 (2.8 kb), invariably associated with the disease, were determined. Putative coding sequences for proteins with molecular weights of 38, 40 and 87 kDa were found for M2, M1 and L3 dsRNAs, respectively. The average G + C content of these dsRNAs was 43%, close to that of A. bisporus nuclear DNA. The nucleotide sequences, as well as the amino acid sequences, appear to be unique, as no matching sequences could be found among databases. S3 dsRNA (0.39 kb), which is occasionally found in large amounts in diseased mushrooms, is an internally deleted variant of M2 dsRNA and is largely composed of the non-coding ends of that dsRNA.

In vivo mapping of a sequence required for interference with the yeast killer virus

The Saccharomyces cerevisiae viruses are noninfectious double-stranded RNA viruses whose segments are separately encapsidated. A large viral double-stranded RNA (L1; 4580 base pairs) encodes all required viral functions. M1, a double-stranded RNA of 1.9 kilobases, encodes an extracellular toxin (killer toxin) and cellular immunity to that toxin. Some strains contain smaller, S, double-stranded RNAs, derived from M1 by internal deletion. Particles containing these defective interfering RNAs can displace M1 particles by faster replication and thus convert the host strain to a nonkiller phenotype. In this work, we report the development of an assay in which the expression of S plus-strand from an inducible plasmid causes the loss of M1 particles. This assay provides a convenient method for identifying in vivo cis-acting sequences important in viral replication and packaging. We have mapped the sequence involved in interference to a region of 132 base pairs that includes two sequences similar to the viral binding site sequence previously identified in L1 by in vitro experiments.

Transfection of the Giardia lamblia double-stranded RNA virus into giardia lamblia by electroporation of a single-stranded RNA copy of the viral genome

The development of a genetic vector for protozoan parasites is a major hurdle yet to be crossed in the study of the molecular and cellular biology of these parasites. We have identified and isolated a double-stranded RNA virus (G. lamblia virus [GLV]) from certain strains of the intestinal parasitic protozoan Giardia lamblia (A. L. Wang and C. C. Wang, Mol. Biochem. Parasitol. 21:269-276, 1986), which is capable of infecting other virus-free strains of G. lamblia (R. L. Miller, A. L. Wang, and C. C. Wang, Exp. Parasitol. 66:118-123, 1988). Here we demonstrate that G. lamblia can be infected with GLV by electroporating uninfected cells with purified single-stranded RNA (E. S. Furfine, T. C. White, A. L. Wang, and C. C. Wang, Nucleic Acids Res. 17:7453-7467, 1989) representing a full-length copy of one strand of the GLV double-stranded RNA genome. To the best of our knowledge, this is the first demonstration in vivo that a single-stranded RNA is a competent replicative intermediate for this class of double-stranded RNA virus. In addition, this result represents the first long-term transfection of a protozoan by a single species of RNA and will hopefully expedite the development of GLV as a genetic transfecting vector.

Transfection of the Giardia lamblia double-stranded RNA virus into giardia lamblia by electroporation of a single-stranded RNA copy of the viral genome

The development of a genetic vector for protozoan parasites is a major hurdle yet to be crossed in the study of the molecular and cellular biology of these parasites. We have identified and isolated a double-stranded RNA virus (G. lamblia virus [GLV]) from certain strains of the intestinal parasitic protozoan Giardia lamblia (A. L. Wang and C. C. Wang, Mol. Biochem. Parasitol. 21:269-276, 1986), which is capable of infecting other virus-free strains of G. lamblia (R. L. Miller, A. L. Wang, and C. C. Wang, Exp. Parasitol. 66:118-123, 1988). Here we demonstrate that G. lamblia can be infected with GLV by electroporating uninfected cells with purified single-stranded RNA (E. S. Furfine, T. C. White, A. L. Wang, and C. C. Wang, Nucleic Acids Res. 17:7453-7467, 1989) representing a full-length copy of one strand of the GLV double-stranded RNA genome. To the best of our knowledge, this is the first demonstration in vivo that a single-stranded RNA is a competent replicative intermediate for this class of double-stranded RNA virus. In addition, this result represents the first long-term transfection of a protozoan by a single species of RNA and will hopefully expedite the development of GLV as a genetic transfecting vector.

Overproduction of yeast viruslike particles by strains deficient in a mitochondrial nuclease

Saccharomyces cerevisiae strains are often host to several types of cytoplasmic double-stranded RNA (dsRNA) genomes, some of which are encapsidated by the L-A dsRNA product, an 86,000-dalton coat protein. Here we present the finding that nuclear recessive mutations in the NUC1 gene, which encodes the major nonspecific nuclease of yeast mitochondria, resulted in at least a 10-fold increase in amounts of the L-A dsRNA and its encoded coat protein. The effect of nuc1 mutations on L-A abundance was completely suppressed in strains that also hosted the killer-toxin-encoding M dsRNA. Both NUC1 and nuc1 strains containing the L-A genome exhibited an increase in coat protein abundance and a concomitant increase in L-A dsRNA when the cells were grown on a nonfermentable carbon source rather than on glucose, an effect independent of the increase in coat protein due to nuc1 mutations or to the absence of M. The increase in L-A expression in nuc1 strains was similar to that observed in strains with mutations in the nuclear gene encoding the most abundant outer mitochondrial membrane protein, porin. nuc1 mutations did not affect the level of porin in the mitochondrial outer membrane. Since the effect of mutations in nuc1 was to alter the copy number of the L-A coat protein genome rather than to change the level of the M toxin genome (as do mak and ski mutations), these mutations define a new class of nuclear genes affecting yeast dsRNA abundance.

A single-stranded RNA copy of the Giardia lamblia virus double-stranded RNA genome is present in the infected Giardia lamblia

An isolate of Giardia lamblia infected with the double-stranded RNA virus (GLV) has two major species of RNA that are not present in an uninfected isolate. One of these species is the previously characterized double-stranded RNA genome of GLV (1). The second species of RNA appears to be a full length copy of one strand of the double-stranded RNA genome. This full length single-stranded RNA is not present in viral particles isolated from the growth medium. The cellular concentration of the single-stranded RNA changes during exponential and stationary phases of cell growth in a fashion consistent with a viral replicative intermediate or mRNA. The single-stranded species does not appear to be polyadenylated.

Overlapping genes in a yeast double-stranded RNA virus

The Saccharomyces cerevisiae viruses have a large viral double-stranded RNA which encodes the major viral capsid polypeptide. We have previously shown that this RNA (L1) also encodes a putative viral RNA-dependent RNA polymerase (D. F. Pietras, M. E. Diamond, and J. A. Bruenn, Nucleic Acids Res., 16:6226, 1988). The organization and expression of the viral genome is similar to that of the gag-pol region of the retroviruses. The complete sequence of L1 demonstrates two large open reading frames on the plus strand which overlap by 129 bases. The first is the gene for the capsid polypeptide, and the second is the gene for the putative RNA polymerase. One of the products of in vitro translation of the denatured viral double-stranded RNA is a polypeptide of the size expected of a capsid-polymerase fusion protein, resulting from a -1 frameshift within the overlapping region. A polypeptide of the size expected for a capsid-polymerase fusion product was found in virions, and it was recognized in Western blots (immunoblots) by antibodies to a synthetic peptide derived from the predicted polymerase sequence.

Overproduction of yeast viruslike particles by strains deficient in a mitochondrial nuclease

Saccharomyces cerevisiae strains are often host to several types of cytoplasmic double-stranded RNA (dsRNA) genomes, some of which are encapsidated by the L-A dsRNA product, an 86,000-dalton coat protein. Here we present the finding that nuclear recessive mutations in the NUC1 gene, which encodes the major nonspecific nuclease of yeast mitochondria, resulted in at least a 10-fold increase in amounts of the L-A dsRNA and its encoded coat protein. The effect of nuc1 mutations on L-A abundance was completely suppressed in strains that also hosted the killer-toxin-encoding M dsRNA. Both NUC1 and nuc1 strains containing the L-A genome exhibited an increase in coat protein abundance and a concomitant increase in L-A dsRNA when the cells were grown on a nonfermentable carbon source rather than on glucose, an effect independent of the increase in coat protein due to nuc1 mutations or to the absence of M. The increase in L-A expression in nuc1 strains was similar to that observed in strains with mutations in the nuclear gene encoding the most abundant outer mitochondrial membrane protein, porin. nuc1 mutations did not affect the level of porin in the mitochondrial outer membrane. Since the effect of mutations in nuc1 was to alter the copy number of the L-A coat protein genome rather than to change the level of the M toxin genome (as do mak and ski mutations), these mutations define a new class of nuclear genes affecting yeast dsRNA abundance.

Accumulation of viruslike particles in a yeast mutant lacking a mitochondrial pore protein

The lack of mitochondrial porin is not lethal in Saccharomyces cerevisiae, but it impairs some respiratory functions and, therefore, growth on nonfermentable carbon sources such as glycerol. However, after a lag phase porinless mutant cells adapt to growth on glycerol, accumulating large amounts of an 86-kilodalton (kDa) protein (M. Dihanich, K. Suda, and G. Schatz, EMBO J. 6:723-728, 1987) and of a 5-kilobase RNA. Immunogold labeling localized the 86 kDa-protein exclusively to the cytosol fraction, although most of it cosedimented with the microsome fraction in earlier cell fractionations. This discrepancy was resolved when the 86-kDa protein was identified as the major coat protein in viruslike particles (VLPs) which is encoded by a double-stranded RNA (L-A RNA). Elimination of VLPs in the original porinless strain by introduction of the mak10 or the mak3 mutation increased the respiratory defect and prolonged its lag phase on nonfermentable carbon sources. The fact that the simultaneous loss of VLPs and respiratory functions are the introduction of mak10 or mak3 occurred even in some porin-containing wild-type strains suggests that there is a link between VLP and mitochondrial functions.

Accumulation of viruslike particles in a yeast mutant lacking a mitochondrial pore protein

The lack of mitochondrial porin is not lethal in Saccharomyces cerevisiae, but it impairs some respiratory functions and, therefore, growth on nonfermentable carbon sources such as glycerol. However, after a lag phase porinless mutant cells adapt to growth on glycerol, accumulating large amounts of an 86-kilodalton (kDa) protein (M. Dihanich, K. Suda, and G. Schatz, EMBO J. 6:723-728, 1987) and of a 5-kilobase RNA. Immunogold labeling localized the 86 kDa-protein exclusively to the cytosol fraction, although most of it cosedimented with the microsome fraction in earlier cell fractionations. This discrepancy was resolved when the 86-kDa protein was identified as the major coat protein in viruslike particles (VLPs) which is encoded by a double-stranded RNA (L-A RNA). Elimination of VLPs in the original porinless strain by introduction of the mak10 or the mak3 mutation increased the respiratory defect and prolonged its lag phase on nonfermentable carbon sources. The fact that the simultaneous loss of VLPs and respiratory functions are the introduction of mak10 or mak3 occurred even in some porin-containing wild-type strains suggests that there is a link between VLP and mitochondrial functions.