Ribosomal frameshifting efficiency and gag/gag-pol ratio are critical for yeast M1 double-stranded RNA virus propagation

Expression of a coronavirus ribosomal frameshift signal in Escherichia coli: influence of tRNA anticodon modification on frameshifting

Eukaryotic ribosomal frameshift signals generally contain two elements, a heptanucleotide slippery sequence (XXXYYYN) and an RNA secondary structure, often an RNA pseudoknot, located downstream. Frameshifting takes place at the slippery sequence by simultaneous slippage of two ribosome-bound tRNAs. All of the tRNAs that are predicted to decode frameshift sites in the ribosomal A-site (XXXYYYN) possess a hypermodified base in the anticodon-loop and it is conceivable that these modifications play a role in the frameshift process. To test this, we expressed slippery sequence variants of the coronavirus IBV frameshift signal in strains of Escherichia coli unable to modify fully either tRNA(Lys) or tRNA(Asn). At the slippery sequences UUUAAAC and UUUAAAU (underlined codon decoded by tRNA(Asn), anticodon 5' QUU 3'), frameshifting was very inefficient (2 to 3%) and in strains deficient in the biosynthesis of Q base, was increased (AAU) or decreased (AAC) only two-fold. In E. coli, therefore, hypomodification of tRNA(Asn) had little effect on frameshifting. The situation with the efficient slippery sequences UUUAAAA (15%) and UUUAAAG (40%) (underlined codon decoded by tRNA(Lys), anticodon 5' mnm5s2UUU 3') was more complex, since the wobble base of tRNA(Lys) is modified at two positions. Of four available mutants, only trmE (s2UUU) had a marked influence on frameshifting, increasing the efficiency of the process at the slippery sequence UUUAAAA. No effect on frameshifting was seen in trmC1 (cmnm5s2UUU) or trmC2 (nm5s2UUU) strains and only a very small reduction (at UUUAAAG) was observed in an asuE (mnm5UUU) strain. The slipperiness of tRNA(Lys), therefore, cannot be ascribed to a single modification site on the base. However, the data support a role for the amino group of the mnm5 substitution in shaping the anticodon structure. Whether these conclusions can be extended to eukaryotic translation systems is uncertain. Although E. coli ribosomes changed frame at the IBV signal (UUUAAAG) with an efficiency similar to that measured in reticulocyte lysates (40%), there were important qualitative differences. Frameshifting of prokaryotic ribosomes was pseudoknot-independent (although secondary structure dependent) and appeared to require slippage of only a single tRNA.

Yeast killer systems

The killer phenomenon in yeasts has been revealed to be a multicentric model for molecular biologists, virologists, phytopathologists, epidemiologists, industrial and medical microbiologists, mycologists, and pharmacologists. The surprisingly widespread occurrence of the killer phenomenon among taxonomically unrelated microorganisms, including prokaryotic and eukaryotic pathogens, has engendered a new interest in its biological significance as well as its theoretical and practical applications. The search for therapeutic opportunities by using yeast killer systems has conceptually opened new avenues for the prevention and control of life-threatening fungal diseases through the idiotypic network that is apparently exploited by the immune system in the course of natural infections. In this review, the biology, ecology, epidemiology, therapeutics, serology, and idiotypy of yeast killer systems are discussed.

Peptidyl-transferase inhibitors have antiviral properties by altering programmed -1 ribosomal frameshifting efficiencies: development of model systems

The effects of two peptidyl-transferase inhibitors, anisomycin and sparsomycin, on ribosomal frameshifting efficiencies and the propagation of yeast double-stranded RNA viruses were examined. At sublethal doses in yeast cells these drugs specifically alter the efficiency of -1, but not of +1, ribosomal frameshifting. These compounds promote loss of the yeast L-A double-stranded RNA virus, which uses a programmed -1 ribosomal frameshift to produce its Gag-Pol fusion protein. Both of these drugs also change the efficiency of -1 ribosomal frameshifting in yeast and mammalian in vitro translation systems, suggesting that they may have applications to control the propagation of viruses of higher eukaryotes, which also use this translational regulatory mechanism. Our results offer a new set of antiviral agents that may potentially have a broad range of applications in the clinical, veterinary, and agricultural fields.

The bifunctional DCOH protein binds to HNF1 independently of its 4-alpha-carbinolamine dehydratase activity

HNF1 is a liver enriched atypical homeoprotein isolated from vertebrates which is involved in the transcriptional activation of liver, kidney, intestine and pancreas specific genes. HNF1 contains an N-terminal dimerisation and a POU-like domain both essential together with the homeodomain for DNA specific recognition. Using the yeast two-hybrid system we searched for proteins interacting with HNF1. We repeatedly obtained cDNA clones encoding DCOH/4-alpha-carbinolamine dehydratase, an enzyme involved in the oxidation of aromatic amino acids that was shown to bind to and stabilise HNF1 dimers. Using the yeast system, we show that the enzymatic activity of DCOH is not essential for HNF1 binding and that the HNF1 dimerisation domain is sufficient for DCOH binding. Furthermore we demonstrate that both proteins co-localise in co-transfected cells.

Mof4-1 is an allele of the UPF1/IFS2 gene which affects both mRNA turnover and -1 ribosomal frameshifting efficiency

The mof4-1 (maintenance of frame) allele in the yeast Saccharomyces cerevisiae was isolated as a chromosomal mutation that increased the efficiency of -1 ribosomal frameshifting at the L-A virus frameshift site and caused loss of M1, the satellite virus of L-A. Here, we demonstrate that strains harboring the mof4-1 allele inactivated the nonsense-mediated mRNA decay pathway. The MOF4 gene was shown to be allelic to UPF1, a gene whose product is involved in the nonsense-mediated mRNA decay pathway. Although cells harboring the mof4-1 allele of the UPF1 gene lose the M1 virus, mutations in other UPF genes involved in nonsense-mediated mRNA decay maintain the M1 virus. The mof4-1 strain is more sensitive to the aminoglycoside antibiotic paromomycin than a upf1 delta strain, and frameshifting efficiency increases in a mof4-1 strain grown in the presence of this drug. Further, the ifs1 and ifs2 alleles previously identified as mutations that enhance frameshifting were shown to be allelic to the UPF2 and UPF1 genes, respectively, and both ifs strains maintained M1. These results indicate that mof4-1 is a unique allele of the UPF1 gene and that the gene product of the mof4-1 allele affects both -1 ribosomal frameshifting and mRNA turnover.

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.

Autogenous regulation of RNA translation and packaging by Rous sarcoma virus Pr76gag

Unspliced cytoplasmic retroviral RNA in chronically infected cells either is encapsidated by Gag proteins in the manufacture of virus or is used to direct synthesis of Gag proteins. Several models have been suggested to explain the sorting of viral RNA for these two purposes. Here we present evidence supporting a simple biochemical mechanism that accounts for the routing of retroviral RNA. Our results indicate that ribosomes compete with the Gag proteins to determine the fate of nascent retroviral RNA. Although the integrity of the entire Rous sarcoma virus leader sequence is important for retroviral packaging and translation, the RNA structure around the third small open reading frame, which neighbors the psi site required for packaging of the RNA, is particularly critical for maintenance of the balance between translation and packaging. These results support the hypothesis that Gag proteins autogenously regulate their synthesis and encapsidation of retroviral RNA and that an equilibrium exists between RNA destined for translation and packaging that is based on the intracellular levels of Gag proteins and ribosomes. To test the model, mRNAs with natural or mutated 5' leader sequences from Rous sarcoma virus were expressed in avian cells in the presence and absence of Pr76gag. We demonstrate that Pr76gag acts as a translational repressor of these mRNAs in a dose-dependent manner, supporting the hypothesis that Pr76gag can sort retroviral RNA for translation and encapsidation.

The retrotransposon Tf1 assembles virus-like particles that contain excess Gag relative to integrase because of a regulated degradation process

The retrotransposon Tf1, isolated from Schizosaccharomyces pombe, contains a single open reading frame with sequences encoding Gag, protease, reverse transcriptase, and integrase (IN). Tf1 has previously been shown to possess significant transposition activity. Although Tf1 proteins do assemble into virus-like particles, the assembly does not require readthrough of a translational reading frame shift or stop codon, common mechanisms used by retroelements to express Gag in molar excess of the polymerase proteins. This study was designed to determine if Tf1 particles contain equal amounts of Gag and polymerase proteins or whether they contain the typical molar excess of Gag. After using two separate methods to calibrate the strength of our antibodies, we found that both S. pombe extracts and partially purified Tf1 particles contained a 26-fold molar excess of Gag relative to IN. Knowing that Gag and IN are derived from the same Tf1 primary translation product, we concluded that the excess Gag most likely resulted from specific degradation of IN. We obtained evidence of regulated IN degradation in comparisons of Tf1 protein extracted from log-phase cells and that extracted from stationary-phase cells. The log-phase cells contained equal molar amounts of Gag and IN, whereas cells approaching stationary phase rapidly degraded IN, leaving an excess of Gag. Analysis of the reverse transcripts indicated that the bulk of reverse transcription occurred within the particles that possess a molar excess of Gag.

Genetic elements of plant viruses as tools for genetic engineering

Viruses have developed successful strategies for propagation at the expense of their host cells. Efficient gene expression, genome multiplication, and invasion of the host are enabled by virus-encoded genetic elements, many of which are well characterized. Sequences derived from plant DNA and RNA viruses can be used to control expression of other genes in vivo. The main groups of plant virus genetic elements useful in genetic engineering are reviewed, including the signals for DNA-dependent and RNA-dependent RNA synthesis, sequences on the virus mRNAs that enable translational control, and sequences that control processing and intracellular sorting of virus proteins. Use of plant viruses as extrachromosomal expression vectors is also discussed, along with the issue of their stability.

Ribosomal frameshifting in yeast viruses

Proper maintenance of translational reading frame by ribosomes is essential for cell growth and viability. In the last 10 years it has been shown that a number of viruses induce ribosomes to shift reading frame in order to regulate the expression of gene products having enzymatic functions. Studies on ribosomal frameshifting in viruses of yeast have been particularly enlightening. The roles of viral mRNA sequences and secondary structures have been elucidated and a picture of how these interact with host chromosomal gene products is beginning to emerge. The efficiency of ribosomal frameshifting is important for viral particle assembly, and has identified ribosomal frameshifting as a potential target for antiviral agents. The availability of mutants of host chromosomal gene products involved in maintaining the efficiency of ribosomal frameshifting bodes well for the use of yeast in future studies of ribosomal frameshifting.

5 S rRNA is involved in fidelity of translational reading frame

Chromosomal mutants (maintenance of frame = mof) in which the efficiency of -1 ribosomal frameshifting is increased can be isolated using constructs in which lacZ expression is dependent upon a -1 shift of reading frame. We isolate a new mof mutation, mof9, in Saccharomyces cerevisiae and show that it is complemented by both single and multi-copy 5 S rDNA clones. Two independent insertion mutations in the rDNA locus (rDNA::LEU2 and rDNA::URA3) also display the Mof- phenotype and are also complemented by single and multi-copy 5 S rDNA clones. Mutant 5 S rRNAs expressed from a plasmid as 20-50% of total 5 S rRNA in a wild-type host also induced the Mof- phenotype. The increase in frameshifting is greatest when the lacZ reporter gene is expressed on a high copy, episomal vector. No differences were found in 5 S rRNA copy number or electrophoretic mobilities in mof9 strains. Both mof9 and rDNA::LEU2 increase the efficiency of +1 frameshifting as well but have no effect on readthrough of UAG or UAA termination codons, indicating that not all translational specificity is affected. These data suggest a role for 5 S rRNA in the maintenance of frame in translation.

Yeast virus propagation depends critically on free 60S ribosomal subunit concentration

Over 30 MAK (maintenance of killer) genes are necessary for propagation of the killer toxin-encoding M1 satellite double-stranded RNA of the L-A virus. Sequence analysis revealed that MAK7 is RPL4A, one of the two genes encoding ribosomal protein L4 of the 60S subunit. We further found that mutants with mutations in 18 MAK genes (including mak1 [top1], mak7 [rpl4A], mak8 [rpl3], mak11, and mak16) had decreased free 60S subunits. Mutants with another three mak mutations had half-mer polysomes, indicative of poor association of 60S and 40S subunits. The rest of the mak mutants, including the mak3 (N-acetyltransferase) mutant, showed a normal profile. The free 60S subunits, L-A copy number, and the amount of L-A coat protein in the mak1, mak7, mak11, and mak16 mutants were raised to the normal level by the respective normal single-copy gene. Our data suggest that most mak mutations affect M1 propagation by their effects on the supply of proteins from the L-A virus and that the translation of the non-poly(A) L-A mRNA depends critically on the amount of free 60S ribosomal subunits, probably because 60S association with the 40S subunit waiting at the initiator AUG is facilitated by the 3' poly(A).

SPE1 and SPE2: two essential genes in the biosynthesis of polyamines that modulate +1 ribosomal frameshifting in Saccharomyces cerevisiae

We previously showed that a mutant of Saccharomyces cerevisiae, which cannot make spermidine as a result of a deletion in the SPE2 gene (spe2 delta), exhibits a marked elevation in +1 ribosomal frameshifting efficiency in response to the Ty1 frameshift sequence, CUU AGG C. In the present study, we found that spermidine deprivation alone does not result in increased +1 ribosomal frameshifting efficiency. The high level of +1 ribosomal frameshifting efficiency in spe2 delta cells is the result of the combined effects of both spermidine deprivation and the large increase in the level of intracellular putrescine resulting from the derepression of the gene for ornithine decarboxylase (SPE1) in spermidine-deficient strains.

The sequences of and distance between two cis-acting signals determine the efficiency of ribosomal frameshifting in human immunodeficiency virus type 1 and human T-cell leukemia virus type II in vivo

We have analyzed in cell culture the sequence elements that control the level of ribosomal frameshifting in the human T-cell leukemia virus type II (HTLV-2) gag-pro junction. The slippery sequence of HTLV-2 is sufficient to dictate a basal level of frameshifting. This level is enhanced by its upstream sequence context and by the downstream stem-loop structure which is located at an optimal distance of 7 bases. Frameshifting in human immunodeficiency virus gag-pol is similar to that of HTLV-2 gag-pro. However, experiments using hybrid cassettes of HTLV-2 and human immunodeficiency virus type 1 frameshift elements show that while the slippery sequence of HTLV-2 is less efficient, the stem-loop structure is a more efficient enhancer.

Identification and analysis of the pseudoknot-containing gag-pro ribosomal frameshift signal of simian retrovirus-1

The pro and pol genes of simian retrovirus-1 (SRV-1) are expressed as parts of a fusion protein generated by -1 ribosomal frameshifting. To investigate the requirements for frameshifting at the gag-pro overlap, we have inserted a stretch of 58 nucleotides containing the proposed frameshift signal into a plasmid that allows monitoring of translation in all three reading frames. In vitro translation of mRNAs derived from this plasmid indicated that the 58 nucleotides from the SRV-1 gag-pro overlap were sufficient to induce an efficient -1 shift in a heterologous context. Mutational analysis demonstrated that the slip site is formed at the heptanucleotide G GGA AAC. The frameshift efficiency of the wild type sequence in rabbit reticulocyte lysate was 23%. A second component of the frameshift signal is formed by a pseudoknot seven bases downstream of the slip site. The presence of this pseudoknot was confirmed by mutational analysis, employing complementary and compensatory base changes, and by probing the structure of short RNA transcripts containing the frameshift signal. Adding increasing amounts of an SRV-1 pseudoknot containing RNA transcript to a translation reaction programmed with an SRV-1 frameshift reporter mRNA had no effect on the frameshift efficiency, arguing against the role of a specific pseudoknot-recognising factor in the frameshifting process.

Translational frameshifting at the gag-pol junction of human immunodeficiency virus type 1 is not increased in infected T-lymphoid cells

A frameshift event is necessary for expression of the products of the pol gene in a number of retroviruses, including human immunodeficiency virus type 1 (HIV-1). The basic signals necessary for frameshifting consist of a shifty sequence in which the ribosome slips and a downstream stimulatory structure which can be either a stem-loop or a pseudoknot. In HIV-1, much attention has been paid to the frameshift site itself, and only recently has the role of the downstream structure been examined. Here we used a luciferase-based experimental system to analyze in vivo the cis and trans factors potentially involved in controlling frameshifting efficiency at the gag-pol junction of HIV-1. We demonstrated that high-level frameshifting is dependent on the presence of a palindromic region located downstream of the site where the frameshift event takes place. Frameshifting efficiencies were found to be identical in mouse fibroblasts and the natural host cells of the virus, i.e., CD4+ human lymphoid cells. Furthermore, no increase in frameshifting was observed upon virus infection. Previous observations have shown that viral infection leads to specific alteration of tRNAs involved in translation of shifty sites (D. Hatfield, Y.-X. Feng, B.J. Lee, A. Rein, J.G. Levin, and S. Oroszlan, Virology 173:736-742, 1989). The results presented here strongly suggest that these modifications do not affect frameshifting efficiency.

Spermidine deficiency increases +1 ribosomal frameshifting efficiency and inhibits Ty1 retrotransposition in Saccharomyces cerevisiae

Polyamines have been implicated in nucleic acid-related functions and in protein biosynthesis. RNA sequences that specifically direct ribosomes to shift reading frame in the -1 and +1 directions may be used to probe the mechanisms controlling translational fidelity. We examined the effects of spermidine on translational fidelity by an in vivo assay in which changes in beta-galactosidase activity are dependent on yeast retrovirus Ty +1 and yeast double-stranded RNA virus L-A -1 ribosomal frameshifting signals. In spe2 delta mutants of Saccharomyces cerevisiae, which cannot make spermidine as a result of a deletion in the SPE2 gene, there is a marked elevation in +1 but no change in -1 ribosomal frameshifting. The increase in +1 ribosomal frameshifting efficiency is accompanied by a striking decrease in Ty1 retrotransposition.

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.

Host control of yeast dsRNA virus propagation and expression

Yeast controls propagation of the L-A dsRNA virus, and thus pathogenicity, by partially blocking translation of viral mRNA. L-A makes a Gag-Pol fusion protein by a -1 ribosomal frameshift, regulated by the host but critical for satellite RNA propagation. Discovery of the KEX proteases, by their requirement for killer toxin expression from a satellite dsRNA of L-A, led to the identification of mammalian prohormone processing proteases.

A rare tRNA-Arg(CCU) that regulates Ty1 element ribosomal frameshifting is essential for Ty1 retrotransposition in Saccharomyces cerevisiae

Translation of the yeast retrotransposon Ty1 TYA1(gag)-TYB1(pol) gene occurs by a +1 ribosomal frameshifting event at the sequence CUU AGG C. Because overexpression of a low abundance tRNA-Arg(CCU) encoded by the HSX1 gene resulted in a reduction in Ty1 frameshifting, it was suggested that a translational pause at the AGG-Arg codon is required for optimum frameshifting. The present work shows that the absence of tRNA-Arg(CCU) affects Ty1 transposition, translational frameshifting, and accumulation of mature TYB1 proteins. Transposition of genetically tagged Ty1 elements decreases at least 50-fold and translational frameshifting increases 3-17-fold in cells lacking tRNA-Arg(CCU). Accumulation of Ty1-integrase and Ty1-reverse transcriptase/ribonuclease H is defective in an hsx1 mutant. The defect in Ty1 transposition is complemented by the wild-type HSX1 gene or a mutant tRNA-Arg(UCU) gene containing a C for T substitution in the first position of the anticodon. Overexpression of TYA1 stimulates Ty1 transposition 50-fold above wild-type levels when the level of transposition is compared in isogenic hsx1 and HSX1 strains. Thus, the HSX1 gene determines the ratio of the TYA1 to TYA1-TYB1 precursors required for protein processing or stability, and keeps expression of TYB1 a rate-limiting step in the retrotransposition cycle.

A heptanucleotide sequence mediates ribosomal frameshifting in mammalian cells

Ribosomal frameshifting is an essential requirement for replication of many viruses and retrovirus-like elements. It is regarded as a potential target for antiretroviral therapy. It has been shown that the frameshifting event takes place in the -1 direction within a sequence, the slippery sequence, which is usually followed by structured RNA. To distinguish between the basic sequence requirements and the modulating elements in intact cells, we have established a sensitive assay system for quantitative determination of ribosomal frameshifting in mammalian cell culture. In this assay system, the gag and pol genes of human immunodeficiency virus type 1 are replaced by the genes for the functional enzymes beta-galactosidase and luciferase, respectively. The sensitivity of the test system allows us to demonstrate for the first time that the slippery sequence, a heptanucleotide, is sufficient to mediate a basal level of ribosomal frameshifting independent of its position within a gene. The stem-loop sequence serves only as a positive modulator. These data indicate that frameshifting could also occur during translation of cellular genes in which a slippery sequence is present within the reading frame. The resulting putative transframe proteins might have a functional importance for cellular processes.

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.

Yeast MAK3 N-acetyltransferase recognizes the N-terminal four amino acids of the major coat protein (gag) of the L-A double-stranded RNA virus

The MAK3 gene of Saccharomyces cerevisiae encodes an N-acetyltransferase whose acetylation of the N terminus of the L-A double-stranded RNA virus major coat protein (gag) is necessary for viral assembly. We show that the first 4 amino acids of the L-A gag protein sequence, MLRF, are a portable signal for N-terminal acetylation by MAK3. Amino acids 2, 3, and 4 are each important for acetylation by the MAK3 enzyme. In yeast cells, only three mitochondrial proteins are known to have the MAK3 acetylation signal, suggesting an explanation for the slow growth of mak3 mutants on nonfermentable carbon sources.