SUP35 Gene Research Articles

Application of Flow Cytometry for Viability Assessment of Mutants for Translation Termination Factors in the Yeast Saccharomyces cerevisiae

Nonsense mutations in the essential SUP45 and SUP35 genes, encoding translation termination factors, affect the viability of Saccharomyces cerevisiae cells. Flow cytometry revealed that the viability of mutants was 3.5‒4 times lower compared to the wild-type. Moreover, the mutants were found to have higher sensitivity to ultrasonic treatment.

Transcription Factors Mcm1 and Sfp1 May Affect [PSI+] Prion Phenotype by Altering the Expression of the SUP35 Gene

Mcm1 is an essential Q/N-rich transcription factor. Q/N-rich proteins interact with each other, and many affect the [PSI+] prion formed by the translation termination factor Sup35 (eRF3). We found that transient MCM1 overexpression increased nonsense suppression in [PSI+] strains and SUP35 transcription. As we had discovered similar effects of another Q/N-rich transcription factor, Sfp1, here we focus on the roles of Mcm1 and Sfp1 in SUP35 expression, as well as on the effects of Sfp1 on the expression of the gene encoding another release factor, Sup45 (eRF1). Mutations in the SUP35 promoter showed that none of the potential Mcm1 binding sites affected the Sup35 protein level or nonsense suppression, even during MCM1 overexpression. Mcm1 itself neither formed aggregates in vivo nor affected Sup35 aggregation. In contrast, a mutation in the Sfp1-binding site decreased Sup35 production and [PSI+] toxicity of excess Sfp1. Mutation of the Sfp1 binding site in the SUP45 promoter lowered SUP45 expression and increased nonsense suppression even more drastically. Our data indicate that the mechanisms of Mcm1 and Sfp1 action differ. While Mcm1 seems unlikely to directly regulate SUP35 expression, Sfp1 appears to act through its binding sites and to directly activate SUP35 expression, which in turn may influence the [PSI+] prion phenotype and toxicity.

Gene Amplification as a Mechanism of Yeast Adaptation to Nonsense Mutations in Release Factor Genes.

Protein synthesis (translation) is one of the fundamental processes occurring in the cells of living organisms. Translation can be divided into three key steps: initiation, elongation, and termination. In the yeast Saccharomyces cerevisiae, there are two translation termination factors, eRF1 and eRF3. These factors are encoded by the SUP45 and SUP35 genes, which are essential; deletion of any of them leads to the death of yeast cells. However, viable strains with nonsense mutations in both the SUP35 and SUP45 genes were previously obtained in several groups. The survival of such mutants clearly involves feedback control of premature stop codon readthrough; however, the exact molecular basis of such feedback control remain unclear. To investigate the genetic factors supporting the viability of these SUP35 and SUP45 nonsense mutants, we performed whole-genome sequencing of strains carrying mutant sup35-n and sup45-n alleles; while no common SNPs or indels were found in these genomes, we discovered a systematic increase in the copy number of the plasmids carrying mutant sup35-n and sup45-n alleles. We used the qPCR method which confirmed the differences in the relative number of SUP35 and SUP45 gene copies between strains carrying wild-type or mutant alleles of SUP35 and SUP45 genes. Moreover, we compare the number of copies of the SUP35 and SUP45 genes in strains carrying different nonsense mutant variants of these genes as a single chromosomal copy. qPCR results indicate that the number of mutant gene copies is increased compared to the wild-type control. In case of several sup45-n alleles, this was due to a disomy of the entire chromosome II, while for the sup35-218 mutation we observed a local duplication of a segment of chromosome IV containing the SUP35 gene. Taken together, our results indicate that gene amplification is a common mechanism of adaptation to nonsense mutations in release factor genes in yeast.

Dangerous Stops: Nonsense Mutations Can Dramatically Increase Frequency of Prion Conversion.

Amyloid formation is associated with many incurable diseases. For some of these, sporadic cases are much more common than familial ones. Some reports point to the role of somatic cell mosaicism in these cases via origination of amyloids in a limited number of cells, which can then spread through tissues. However, specific types of sporadic mutations responsible for such effects are unknown. In order to identify mutations capable of increasing the de novo appearance of amyloids, we searched for such mutants in the yeast prionogenic protein Sup35. We introduced to yeast cells an additional copy of the SUP35 gene with mutated amyloidogenic domain and observed that some nonsense mutations increased the incidence of prions by several orders of magnitude. This effect was related to exposure at the C-terminus of an internal amyloidogenic region of Sup35. We also discovered that SUP35 mRNA could undergo splicing, although inefficiently, causing appearance of a shortened Sup35 isoform lacking its functional domain, which was also highly prionogenic. Our data suggest that truncated forms of amyloidogenic proteins, resulting from nonsense mutations or alternative splicing in rare somatic cells, might initiate spontaneous localized formation of amyloids, which can then spread, resulting in sporadic amyloid disease.

Nonsense Mutations in the Yeast SUP35 Gene Affect the [PSI+] Prion Propagation.

The essential SUP35 gene encodes yeast translation termination factor eRF3. Previously, we isolated nonsense mutations sup35-n and proposed that the viability of such mutants can be explained by readthrough of the premature stop codon. Such mutations, as well as the prion [PSI+], can appear in natural yeast populations, and their combinations may have different effects on the cells. Here, we analyze the effects of the compatibility of sup35-n mutations with the [PSI+] prion in haploid and diploid cells. We demonstrated that sup35-n mutations are incompatible with the [PSI+] prion, leading to lethality of sup35-n [PSI+] haploid cells. In diploid cells the compatibility of [PSI+] with sup35-n depends on how the corresponding diploid was obtained. Nonsense mutations sup35-21, sup35-74, and sup35-218 are compatible with the [PSI+] prion in diploid strains, but affect [PSI+] properties and lead to the formation of new prion variant. The only mutation that could replace the SUP35 wild-type allele in both haploid and diploid [PSI+] strains, sup35-240, led to the prion loss. Possibly, short Sup351–55 protein, produced from the sup35-240 allele, is included in Sup35 aggregates and destabilize them. Alternatively, single molecules of Sup351–55 can stick to aggregate ends, and thus interrupt the fibril growth. Thus, we can conclude that sup35-240 mutation prevents [PSI+] propagation and can be considered as a new pnm mutation.

From past to future: suppressor mutations in yeast genes encoding translation termination factors

The study of the SUP45 and SUP35 genes of yeast Saccharomyces cerevisiae in the laboratory of Physiological Genetics of St. Petersburg State University began in 1964 when the first omnipotent nonsense suppressor mutations were obtained. During the following 55 years, a lot of information about these genes has been gained through the research efforts of various laboratories. Now we know that SUP45 and SUP35 encode translation termination factors eRF1 and eRF3, respectively. Both genes are essential, and sup45 and sup35 mutations lead not only to impaired translation but also to multiple pleiotropic effects. The aim of this review is to summarize known data about suppressor mutations in SUP45 or SUP35 genes.

Distinct mechanisms of phenotypic effects of inactivation and prionization of Swi1 protein in Saccharomyces cerevisiae

Prions are proteins that under the same conditions can exist in two or more conformations, and at least one of the conformations has infectious properties. The prionization of a protein is typically accompanied by its functional inactivation due to sequestration of monomers by the prion aggregates. The most of prions has been identified in the yeast Saccharomyces cerevisiae. One of them is [SWI+], a prion isoform of the Swi1 protein, which is a component of the evolutionarily conserved chromatin remodeling complex SWI/SNF. Earlier, it was shown that the prionization of [SWI+] induces a nonsense suppression, which leads to weak growth of the [SWI+] strains containing mutant variants of the SUP35 gene and the nonsense allele ade1-14UGA on selective medium without adenine. This effect occurs because of [SWI+] induction that causes a decrease in the amount of the SUP45 mRNA. Strains carrying the SWI1 deletion exhibit significantly higher suppression of the ade1-14UGA nonsense mutation than the [SWI+] strains. In the present study, we identified genes whose expression is altered in the background of the SWI1 deletion using RNA sequencing. We found that the ade1-14UGA suppression in the swi1Δ strains is caused by an increase in the expression of this mutant allele of the ADE1 gene. At the same time, the SUP45 expression level in the swi1Δ strains does not significantly differ from the expression level of this gene in the [swi-] strains. Thus, we have shown that the phenotypic effects of Swi1 prionization and deletion are mediated by different molecular mechanisms. Based on these data, we have concluded that the prionization of proteins is not only unequal to their inactivation, but also can lead to the acquisition of novel phenotypic effects and functions.

Interaction of Prions Causes Heritable Traits in Saccharomyces cerevisiae.

The concept of "protein-based inheritance" defines prions as epigenetic determinants that cause several heritable traits in eukaryotic microorganisms, such as Saccharomyces cerevisiae and Podospora anserina. Previously, we discovered a non-chromosomal factor, [NSI+], which possesses the main features of yeast prions, including cytoplasmic infectivity, reversible curability, dominance, and non-Mendelian inheritance in meiosis. This factor causes omnipotent suppression of nonsense mutations in strains of S. cerevisiae bearing a deleted or modified Sup35 N-terminal domain. In this work, we identified protein determinants of [NSI+] using an original method of proteomic screening for prions. The suppression of nonsense mutations in [NSI+] strains is determined by the interaction between [SWI+] and [PIN+] prions. Using genetic and biochemical methods, we showed that [SWI+] is the key determinant of this nonsense suppression, whereas [PIN+] does not cause nonsense suppression by itself but strongly enhances the effect of [SWI+]. We demonstrated that interaction of [SWI+] and [PIN+] causes inactivation of SUP45 gene that leads to nonsense suppression. Our data show that prion interactions may cause heritable traits in Saccharomyces cerevisiae.

Chromosome VIII disomy influences the nonsense suppression efficiency and transition metal tolerance of the yeast Saccharomyces cerevisiae.

The SUP35 gene of the yeast Saccharomyces cerevisiae encodes the translation termination factor eRF3. Mutations in this gene lead to the suppression of nonsense mutations and a number of other pleiotropic phenotypes, one of which is impaired chromosome segregation during cell division. Similar effects result from replacing the S. cerevisiae SUP35 gene with its orthologues. A number of genetic and epigenetic changes that occur in the sup35 background result in partial compensation for this suppressor effect. In this study we showed that in S. cerevisiae strains in which the SUP35 orthologue from the yeast Pichia methanolica replaces the S. cerevisiae SUP35 gene, chromosome VIII disomy results in decreased efficiency of nonsense suppression. This antisuppressor effect is not associated with decreased stop codon read-through. We identified SBP1, a gene that localizes to chromosome VIII, as a dosage-dependent antisuppressor that strongly contributes to the overall antisuppressor effect of chromosome VIII disomy. Disomy of chromosome VIII also leads to a change in the yeast strains' tolerance of a number of transition metal salts.

Locating folds of the in-register parallel β-sheet of the Sup35p prion domain infectious amyloid.

The [PSI+] prion is a self-propagating amyloid of the translation termination factor, Sup35p, of Saccharomyces cerevisiae. The N-terminal 253 residues (NM) of this 685-residue protein normally function in regulating mRNA turnover but spontaneously form infectious amyloid in vitro. We converted the three Ile residues in Sup35NM to Leu and then replaced 16 single residues with Ile, one by one, and prepared Ile-1-(13)C amyloid of each mutant, seeding with amyloid formed by the reference sequence Sup35NM. Using solid-state NMR, we showed that 10 of the residues examined, including six between residues 30 and 90, showed the ∼0.5-nm distance between labels diagnostic of the in-register parallel amyloid architecture. The five scattered N domain residues with wider spacing may be in turns or loops; one is a control at the C terminus of M. All mutants, except Q56I, showed little or no [PSI+] transmission barrier from the reference sequence, suggesting that they could assume a similar amyloid architecture in vitro when seeded with filaments of reference sequence Sup35NM. Infection of yeast cells expressing the reference SUP35 gene sequence with amyloid of several mutants produced [PSI+] transfectants with similar efficiency as did reference sequence Sup35NM amyloid. Our work provides a stringent demonstration that the Sup35 prion domain has the folded in-register parallel β-sheet architecture and suggests common locations of the folds. This architecture naturally suggests a mechanism of inheritance of conformation, the central mystery of prions.

A search for new factors that affect translation termination in yeast Saccharomyces cerevisiae

A search for proteins which interact with the eRF1 termination translation factor encoded by SUP45 gene in yeast Saccharomyces cerevisiae was carried out by screening yeast genes located on a multicopy plasmid. This approach allowed finding the ECM23 gene whose increased expression led to lower viability of the sup45 nonsense mutants. Overexpression ECM23 gene led also to an antisuppressor effect, although the amount of the eRF1protein was not changed. Possible mechanisms of how ECM23 can affect the viability of the sup45 nonsense mutants are discussed.

Modification of [PSI +] prion properties by combining amino acid changes in N-terminal domain of Sup35 protein

The prion [PSI +] is an amyloid isoform of the release factor eRF3 encoded by the SUP35 gene in Saccharomyces cerevisiae yeast. Naturally occurring amyloid complexes have been studied for a long time, yet their structural organization is still not well understood. The formation of amyloid forms of the wild-type Sup35 protein (Sup35p) is directed by its N-terminal portion, which forms a superpleated β-sheet structure. We previously constructed five mutants, each of which carried a replacement in two consecutive amino acids, one in each of the oligopeptide repeats (OR) and in the Sup35p N-terminal region. Mutations sup35-M1 (YQ46-47KK) and sup35-M2 (QQ61-62KK) lead to the compete loss of prion conformation. Three other mutants, i.e., sup35-M3 (QQ70-71KK), sup35-M4 (QQ80-81KK), and sup35-M5 (QQ89-90KK), formed functional prions. In the current study, we investigated the contribution of each mutant peptide to the stability of the prion and aggregation properties, and compared the effects of single mutants and combinations of different mutant alleles. Studies were carried out in yeast strains designed to carry single or a combination of different SUP35 alleles. Based on our analysis, we propose a model that clarifies the 3D organization of the β-sheet within the prion. We also provide evidence that sup35-M2 and sup35-M4 mutations change the 3D structure of prion complexes. We propose that the destabilization of prion complexes in these mutants is due to the decreased efficiency of the fragmentation of the prion aggregates by chaperone complexes.

Идентификация геновSaccharomyces cerevisiae, влияющих на синтетическую летальность приона [PSI+] с мутациями в генеSUP45

Previously, we proposed a test system allowing to perform search for genes that influence the properties of the Sup35 and Sup45 protein. This test is based on the phenomenon of lethality of diploids that combine mutations in SUP45 gene with [PSI+] prion. Lethality of this combination depends on the type of sup45 mutation, and the properties of the prion. [PSI+] variant, which is a strong suppressor ([PSI+]s), showing synthetic lethality with all the nonsense mutations and some missense sup45 mutations in the heterozygote state. The presence of extra copies of a gene under test that affects the phenotypic manifestation of prion [PSI+] or translation termination factors properties, leads to the increase or decrease in diploid lethality. Gene library screening using this test system allowed us to establish the effect of ten fragments of genomic DNA of yeast on synthetic lethality. Deletion analysis of these regions has led to the identification of the HLJ1 and TEF2 genes, as affecting Sup35 protein prionization and/or the efficiency of translation termination.

Regulation of release factor expression using a translational negative feedback loop: A systems analysis

The essential eukaryote release factor eRF1, encoded by the yeast SUP45 gene, recognizes stop codons during ribosomal translation. SUP45 nonsense alleles are, however, viable due to the establishment of feedback-regulated readthrough of the premature termination codon; reductions in full-length eRF1 promote tRNA-mediated stop codon readthrough, which, in turn, drives partial production of full-length eRF1. A deterministic mathematical model of this eRF1 feedback loop was developed using a staged increase in model complexity. Model predictions matched the experimental observation that strains carrying the mutant SUQ5 tRNA (a weak UAA suppressor) in combination with any of the tested sup45(UAA) nonsense alleles exhibit threefold more stop codon readthrough than that of an SUQ5 yeast strain. The model also successfully predicted that eRF1 feedback control in an SUQ5 sup45(UAA) mutant would resist, but not completely prevent, imposed changes in eRF1 expression. In these experiments, the introduction of a plasmid-borne SUQ5 copy into a sup45(UAA) SUQ5 mutant directed additional readthrough and full-length eRF1 expression, despite feedback. Secondly, induction of additional sup45(UAA) mRNA expression in a sup45(UAA) SUQ5 strain also directed increased full-length eRF1 expression. The autogenous sup45 control mechanism therefore acts not to precisely control eRF1 expression, but rather as a damping mechanism that only partially resists changes in release factor expression level. The validated model predicts that the degree of feedback damping (i.e., control precision) is proportional to eRF1 affinity for the premature stop codon. The validated model represents an important tool to analyze this and other translational negative feedback loops.

Polyglutamine Toxicity Is Controlled by Prion Composition and Gene Dosage in Yeast

Polyglutamine expansion causes diseases in humans and other mammals. One example is Huntington's disease. Fragments of human huntingtin protein having an expanded polyglutamine stretch form aggregates and cause cytotoxicity in yeast cells bearing endogenous QN-rich proteins in the aggregated (prion) form. Attachment of the proline(P)-rich region targets polyglutamines to the large perinuclear deposit (aggresome). Aggresome formation ameliorates polyglutamine cytotoxicity in cells containing only the prion form of Rnq1 protein. Here we show that expanded polyglutamines both with (poly-QP) or without (poly-Q) a P-rich stretch remain toxic in the presence of the prion form of translation termination (release) factor Sup35 (eRF3). A Sup35 derivative that lacks the QN-rich domain and is unable to be incorporated into aggregates counteracts cytotoxicity, suggesting that toxicity is due to Sup35 sequestration. Increase in the levels of another release factor, Sup45 (eRF1), due to either disomy by chromosome II containing the SUP45 gene or to introduction of the SUP45-bearing plasmid counteracts poly-Q or poly-QP toxicity in the presence of the Sup35 prion. Protein analysis confirms that polyglutamines alter aggregation patterns of Sup35 and promote aggregation of Sup45, while excess Sup45 counteracts these effects. Our data show that one and the same mode of polyglutamine aggregation could be cytoprotective or cytotoxic, depending on the composition of other aggregates in a eukaryotic cell, and demonstrate that other aggregates expand the range of proteins that are susceptible to sequestration by polyglutamines.

NSI +] determinant has a pleiotropic phenotypic manifestation that is modulated by SUP35, SUP45, and VTS1 genes

We recently discovered the novel non-chromosomal determinant in Saccharomyces cerevisiae [NSI(+)] (nonsense suppression inducer), which causes omnipotent nonsense suppression in strains where the Sup35 N-terminal domain is deleted. [NSI(+)] possesses yeast prion features and does not correspond to previously identified yeast prion determinants. Here, we show that [NSI(+)] enhances nonsense codon read-through and inhibits vegetative growth in S. cerevisiae. Using a large-scale overexpression screen to identify genes that impact the phenotypic effects of [NSI(+)], we found that the SUP35 and SUP45 genes encoding the translation termination factors eRF3 and eRF1, respectively, modulate nonsense suppression in [NSI(+)] strains. The VTS1 gene encodes an NQ-enriched RNA-binding protein that enhances nonsense suppression in [NSI(+)] and [nsi(-)] strains. We demonstrate that VTS1 overexpression, like [NSI(+)] induction, causes translational read-through and growth defects in S. cerevisiae.

Search for genes encoding potentially amyloidogenic proteins involved in regulation of nonsense -suppresion in Sacharom yces cerevisiae

Previously, the deletion of SUP35N has been shown to create the genetic background for identification of the novel genes and epigenetic determinants controlling the nonsense-suppression. Here, using a genomic overexpression screen, we have found several genes encoding potentially amyloidogenic proteins, whose overexpression affects the suppressor phenotype in the strain producing the chimeric protein Aβ-Sup35MC on the background of the deletion of SUP35 gene encoding releasing factor eRF 3. It has been demonstrated the NAB2, NAB3 and VTS1 genes participate in the regulation of nonsense-suppression in S. cerevisiae.

Identification of genes influencing synthetic lethality of genetic and epigenetic alterations in translation termination factors in yeast

117 Translation termination in eukaryotic cells is deter mined by proteins Sup35 (eRF3) and Sup45 (eRF1) [1], which interact with a large number of partners [2]. In yeast Saccharomyces cerevisiae, protein Sup35 can form an aggregating epigenetically inherited con former (prion) [PSI+] [3]. This prion is carried through the cytoplasm and causes disturbances in translation termination, which are phenotypically identified as the dominant omnipotent nonsense sup pression. [PSI+] variants with different properties (nonsense suppression efficiency and transmission stability in mitosis) can be obtained in the same yeast strain. The presence of prion [PSI+] leads to lethality in the haploid yeast strain carrying mutations in the gene encoding another termination factor, Sup45 [4]. We have shown that the combination in the diploid strain of some mutant alleles of the SUP45 gene in the heterozygous state with prion [PSI+] entails the death of the hybrid [5]. The synthetic lethality of prion [PSI+] and mutant allele of the sup45 gene depends both on the type of mutant allele and the prion variant. Variant [PSI+], which is a strong suppressor (“strong” [PSI+], or [PSI+]S), causes synthetic lethality with all nonsense mutations and some missense mutations sup45 in the heterozygote. Our data indicate that the lethality of hybrids is correlated with a decreased activity of the Sup45 protein in the cell in case of sup45 mutations. This paper describes a test system that allows identification of proteins that affect the stability of prion [PSI+] and/or the efficiency of translation ter mination by their effect on the synthetic lethality of the prion conformer Sup35 and mutant alleles of SUP45. This test system is suitable to search for pro teins that affect the translation termination efficiency and/or prion maintenance in yeast cells. Gene library screening using this test system allowed us to identify the CUR1 gene, whose influence on another prion, [URE3], was shown earlier but the effect on transla tion termination factors was not known. This test system is based on qualitative assessment of the viability of a diploid strain containing simulta neously (1) prion [PSI+], (2) sup45 mutation in the heterozygote, and (3) test gene(s) in a multicopy plas mid (Fig. 1). Synthetic lethality is manifested as the absence of growth of diploids with genotype SUP45/sup45 [PSI+] (Fig. 2, mutations 101–107, 111, and 116 are incompatible with the “strong” [PSI+]S). If the efficiency of translation termination increases due to the presence of additional copies of the test gene (Х), formation of viable hybrids with genotype +/sup45 [PSI+] [gene Х]n is observed. Conversely, a decrease in the translation termination efficiency in Identification of Genes Influencing Synthetic Lethality of Genetic and Epigenetic Alterations in Translation Termination Factors in Yeast

Yeast H2A.Z, FACT complex and RSC regulate transcription of tRNA gene through differential dynamics of flanking nucleosomes

FACT complex is involved in elongation and ensures fidelity in the initiation step of transcription by RNA polymerase (pol) II. Histone variant H2A.Z is found in nucleosomes at the 5′-end of many genes. We report here H2A.Z-chaperone activity of the yeast FACT complex on the short, nucleosome-free, non-coding, pol III-transcribed yeast tRNA genes. On a prototype gene, yeast SUP4, chromatin remodeler RSC and FACT regulate its transcription through novel mechanisms, wherein the two gene-flanking nucleosomes containing H2A.Z, play different roles. Nhp6, which ensures transcription fidelity and helps load yFACT onto the gene flanking nucleosomes, has inhibitory role. RSC maintains a nucleosome abutting the gene terminator downstream, which results in reduced transcription rate in active state while H2A.Z probably helps RSC in keeping the gene nucleosome-free and serves as stress-sensor. All these factors maintain an epigenetic state which allows the gene to return quickly from repressed to active state and tones down the expression from the active SUP4 gene, required probably to maintain the balance in cellular tRNA pool.

NSI +]: a novel non-Mendelian nonsense suppressor determinant in Saccharomyces cerevisiae

Non-Mendelian determinants that control heritable traits in yeast are subdivided into two major groups-one that includes DNA- or RNA-based elements and another that comprises protein-based factors that are analogous to mammalian prion. All yeast non-Mendelian determinants show dominant inheritance, and some of them demonstrate cytoplasmic infectivity. Only prions, however, harbor-specific features, such as high frequency of induction following overproduction of prion-encoding protein, loss of the protein's normal function, and reversible curability. Here, we describe a novel nonchromosomal determinant that, in addition to [PSI (+)] and [ISP (+)], is involved in epigenetic control of nonsense suppression. This determinant, which we have designated [NSI (+)], causes nonsense suppression in the strains bearing the N-terminal-deleted or -modified SUP35 gene, but has no manifestation in the strains with the intact copy of SUP35. [NSI (+)] shows dominant non-Mendelian inheritance, reversible curability and may be transmitted by cytoduction, albeit with low frequency. Similar to yeast prions, this determinant can be cured by deletion or mutational inactivation of Hsp104. We have shown that [NSI (+)] does not correspond to the already identified yeast prions. Based on the data obtained, we hypothesize that [NSI (+)] is a novel prion factor involved in epigenetic control of nonsense suppression.