Decreased peptidyltransferase activity correlates with increased programmed -1 ribosomal frameshifting and viral maintenance defects in the yeast Saccharomyces cerevisiae

Association of ScV-LA Virus with Host Protein Metabolism Determined by Proteomics Analysis and Cognate RNA Sequencing

Saccharomyces yeasts are highly dispersed in the environment and microbiota of higher organisms. The yeast killing phenotype, encoded by the viral system, was discovered to be a significant property for host survival. Minor alterations in transcription patterns underpin the reciprocal relationship between LA and M viruses and their hosts, suggesting the fine-tuning of the transcriptional landscape. To uncover the principal targets of both viruses, we performed proteomics analysis of virus-enriched subsets of host proteins in virus type-specific manner. The essential pathways of protein metabolism-from biosynthesis and folding to degradation-were found substantially enriched in virus-linked subsets. The fractionation of viruses allowed separation of virus-linked host RNAs, investigated by high-content RNA sequencing. Ribosomal RNA was found to be inherently associated with LA-lus virus, along with other RNAs essential for ribosome biogenesis. This study provides a unique portrayal of yeast virions through the characterization of the associated proteome and cognate RNAs, and offers a background for understanding ScV-LA viral infection persistency.

Adaptive Response of Saccharomyces Hosts to Totiviridae L-A dsRNA Viruses Is Achieved through Intrinsically Balanced Action of Targeted Transcription Factors

Totiviridae L-A virus is a widespread yeast dsRNA virus. The persistence of the L-A virus alone appears to be symptomless, but the concomitant presence of a satellite M virus provides a killer trait for the host cell. The presence of L-A dsRNA is common in laboratory, industrial, and wild yeasts, but little is known about the impact of the L-A virus on the host’s gene expression. In this work, based on high-throughput RNA sequencing data analysis, the impact of the L-A virus on whole-genome expression in three different Saccharomyces paradoxus and S. cerevisiae host strains was analyzed. In the presence of the L-A virus, moderate alterations in gene expression were detected, with the least impact on respiration-deficient cells. Remarkably, the transcriptional adaptation of essential genes was limited to genes involved in ribosome biogenesis. Transcriptional responses to L-A maintenance were, nevertheless, similar to those induced upon stress or nutrient availability. Based on these data, we further dissected yeast transcriptional regulators that, in turn, modulate the cellular L-A dsRNA levels. Our findings point to totivirus-driven fine-tuning of the transcriptional landscape in yeasts and uncover signaling pathways employed by dsRNA viruses to establish the stable, yet allegedly profitless, viral infection of fungi.

Impact of Genome Reduction in Microsporidia

Microsporidia represent an evolutionary outlier in the tree of life and occupy the extreme edge of the eukaryotic domain with some of their biological features. Many of these unicellular fungi-like organisms have reduced their genomic content to potentially the lowest limit. With some of the most compacted eukaryotic genomes, microsporidia are excellent model organisms to study reductive evolution and its functional consequences. While the growing number of sequenced microsporidian genomes have elucidated genome composition and organization, a recent increase in complementary post-genomic studies has started to shed light on the impacts of genome reduction in these unique pathogens. This chapter will discuss the biological framework enabling genome minimization and will use one of the most ancient and essential macromolecular complexes, the ribosome, to illustrate the effects of extreme genome reduction on a structural, molecular, and cellular level. We outline how reductive evolution in microsporidia has shaped DNA organization, the composition and function of the ribosome, and the complexity of the ribosome biogenesis process. Studying compacted mechanisms, processes, or macromolecular machines in microsporidia illuminates their unique lifestyle and provides valuable insights for comparative eukaryotic structural biology.

A functional connection between translation elongation and protein folding at the ribosome exit tunnel in Saccharomyces cerevisiae

Proteostasis needs to be tightly controlled to meet the cellular demand for correctly de novo folded proteins and to avoid protein aggregation. While a coupling between translation rate and co-translational folding, likely involving an interplay between the ribosome and its associated chaperones, clearly appears to exist, the underlying mechanisms and the contribution of ribosomal proteins remain to be explored. The ribosomal protein uL3 contains a long internal loop whose tip region is in close proximity to the ribosomal peptidyl transferase center. Intriguingly, the rpl3[W255C] allele, in which the residue making the closest contact to this catalytic site is mutated, affects diverse aspects of ribosome biogenesis and function. Here, we have uncovered, by performing a synthetic lethal screen with this allele, an unexpected link between translation and the folding of nascent proteins by the ribosome-associated Ssb-RAC chaperone system. Our results reveal that uL3 and Ssb-RAC cooperate to prevent 80S ribosomes from piling up within the 5' region of mRNAs early on during translation elongation. Together, our study provides compelling in vivo evidence for a functional connection between peptide bond formation at the peptidyl transferase center and chaperone-assisted de novo folding of nascent polypeptides at the solvent-side of the peptide exit tunnel.

Functional Interactions of Ribosomal Intersubunit Bridges in Saccharomyces cerevisiae

Ribosomes of Archaea and Eukarya share higher homology with each other than with bacterial ribosomes. For example, there is a set of 35 r-proteins that are specific only for archaeal and eukaryotic ribosomes. Three of these proteins-eL19, eL24, and eL41-participate in interactions between ribosomal subunits. The eukaryote-specific extensions of r-proteins eL19 and eL24 form two intersubunit bridges eB12 and eB13, which are present only in eukaryotic ribosomes. The third r-protein, eL41, forms bridge eB14. Notably, eL41 is found in all eukaryotes but only in some Archaea. It has been shown that bridges eB12 and eB13 are needed for efficient translation, while r-protein eL41 plays a minor role in ribosome function. Here, the functional interactions between intersubunit bridges were studied using budding yeast strains lacking different combinations of the abovementioned bridges/proteins. The growth phenotypes, levels of in vivo translation, ribosome-polysome profiles, and in vitro association of ribosomal subunits were analyzed. The results show a genetic interaction between r-protein eL41 and the eB12 bridge-forming region of eL19, and between r-proteins eL41 and eL24. It was possible to construct viable yeast strains with Archaea-like ribosomes lacking two or three eukaryote-specific bridges. These strains display slow growth and a poor translation phenotype. In addition, bridges eB12 and eB13 appear to cooperate during ribosome subunit association. These results indicate that nonessential structural elements of r-proteins become highly important in the context of disturbed subunit interactions. Therefore, eukaryote-specific bridges may contribute to the evolutionary success of eukaryotic translation machinery.

Ribosomal protein eL24, involved in two intersubunit bridges, stimulates translation initiation and elongation

Interactions between subunits in the Saccharomyces cerevisiae ribosome are mediated by universal and eukaryote-specific intersubunit bridges. Universal bridges are positioned close to the ribosomal functional centers, while eukaryote-specific bridges are mainly located on the periphery of the ribosome. Two bridges, eB13 and B6, are formed by the ribosomal protein eL24. The eukaryotic eL24 is composed of an N-terminal domain, a linker region and a C-terminal α-helix. Here, the functions of different domains of eL24 in the S. cerevisiae ribosome were evaluated. The C-terminal domain and the linker region of the eL24 form eukaryote-specific eB13 bridge. Phenotypic characterization of the eL24 deletion mutants indicated that the functional integrity of the eB13 bridge mainly depends on the protein-protein contacts between eL24 and eS6. Further investigation showed importance of the eB13 bridge in the subunit joining in vivo and in vitro. In vitro translation assay demonstrated the role of the eB13 bridge in both initiation and elongation steps of translation. Intriguingly, results of in vitro translation experiment suggest involvement of the N-terminal domain of eL24 in the translation initiation. Therefore, eL24 performs number of tasks required for the optimal ribosome functionality.

Atomic resolution snapshot of Leishmania ribosome inhibition by the aminoglycoside paromomycin

Leishmania is a single-celled eukaryotic parasite afflicting millions of humans worldwide, with current therapies limited to a poor selection of drugs that mostly target elements in the parasite's cell envelope. Here we determined the atomic resolution electron cryo-microscopy (cryo-EM) structure of the Leishmania ribosome in complex with paromomycin (PAR), a highly potent compound recently approved for treatment of the fatal visceral leishmaniasis (VL). The structure reveals the mechanism by which the drug induces its deleterious effects on the parasite. We further show that PAR interferes with several aspects of cytosolic translation, thus highlighting the cytosolic rather than the mitochondrial ribosome as the primary drug target. The results also highlight unique as well as conserved elements in the PAR-binding pocket that can serve as hotspots for the development of novel therapeutics.

The ATPase Fap7 Tests the Ability to Carry Out Translocation-like Conformational Changes and Releases Dim1 during 40S Ribosome Maturation

Late in their maturation, nascent small (40S) ribosomal subunits bind 60S subunits to produce 80S-like ribosomes. Because of the analogy of this translation-like cycle to actual translation, and because 80S-like ribosomes do not produce any protein, it has been suggested that this represents a quality control mechanism for subunit functionality. Here we use genetic and biochemical experiments to show that the essential ATPase Fap7 promotes formation of the rotated state, a key intermediate in translocation, thereby releasing the essential assembly factor Dim1 from pre-40S subunits. Bypassing this quality control step produces defects in reading frame maintenance. These results show how progress in the maturation cascade is linked to a test for a key functionality of 40S ribosomes: their ability to translocate the mRNA⋅tRNA pair. Furthermore, our data demonstrate for the first time that the translation-like cycle is a quality control mechanism that ensures the fidelity of the cellular ribosome pool.

Methylation of yeast ribosomal protein Rpl3 promotes translational elongation fidelity

Rpl3, a highly conserved ribosomal protein, is methylated at histidine 243 by the Hpm1 methyltransferase in Saccharomyces cerevisiae. Histidine 243 lies close to the peptidyl transferase center in a functionally important region of Rpl3 designated as the basic thumb that coordinates the decoding, peptidyl transfer, and translocation steps of translation elongation. Hpm1 was recently implicated in ribosome biogenesis and translation. However, the biological role of methylation of its Rpl3 substrate has not been identified. Here we interrogate the role of Rpl3 methylation at H243 by investigating the functional impact of mutating this histidine residue to alanine (rpl3-H243A). Akin to Hpm1-deficient cells, rpl3-H243A cells accumulate 35S and 23S pre-rRNA precursors to a similar extent, confirming an important role for histidine methylation in pre-rRNA processing. In contrast, Hpm1-deficient cells but not rpl3-H243A mutants show perturbed levels of ribosomal subunits. We show that Hpm1 has multiple substrates in different subcellular fractions, suggesting that methylation of proteins other than Rpl3 may be important for controlling ribosomal subunit levels. Finally, translational fidelity assays demonstrate that like Hpm1-deficient cells, rpl3-H243A mutants have defects in translation elongation resulting in decreased translational accuracy. These data suggest that Rpl3 methylation at H243 is playing a significant role in translation elongation, likely via the basic thumb, but has little impact on ribosomal subunit levels. Hpm1 is therefore a multifunctional methyltransferase with independent roles in ribosome biogenesis and translation.

The Functional Role of eL19 and eB12 Intersubunit Bridge in the Eukaryotic Ribosome

During translation, the two eukaryotic ribosomal subunits remain associated through 17 intersubunit bridges, five of which are eukaryote specific. These are mainly localized to the peripheral regions and are believed to stabilize the structure of the ribosome. The functional importance of these bridges remains largely unknown. Here, the essentiality of the eukaryote-specific bridge eB12 has been investigated. The main component of this bridge is ribosomal protein eL19 that is composed of an N-terminal globular domain, a middle region, and a long C-terminal α-helix. The analysis of deletion mutants demonstrated that the globular domain and middle region of eL19 are essential for cell viability, most likely functioning in ribosome assembly. The eB12 bridge, formed by contacts between the C-terminal α-helix of eL19 and 18S rRNA in concert with additional stabilizing interactions involving either eS7 or uS17, is dispensable for viability. Nevertheless, eL19 mutants impaired in eB12 bridge formation displayed slow growth phenotypes, altered sensitivity/resistance to translational inhibitors, and enhanced hyperosmotic stress tolerance. Biochemical analyses determined that the eB12 bridge contributes to the stability of ribosome subunit interactions in vitro. 60S subunits containing eL19 variants defective in eB12 bridge formation failed to form 80S ribosomes regardless of Mg(2+) concentration. The reassociation of 40S and mutant 60S subunits was markedly improved in the presence of deacetylated tRNA, emphasizing the importance of tRNAs during the subunit association. We propose that the eB12 bridge plays an important role in subunit joining and in optimizing ribosome functionality.

Crystal Structures of the uL3 Mutant Ribosome: Illustration of the Importance of Ribosomal Proteins for Translation Efficiency

The ribosome has been described as a ribozyme in which ribosomal RNA is responsible for peptidyl-transferase reaction catalysis. The W255C mutation of the universally conserved ribosomal protein uL3 has diverse effects on ribosome function (e.g., increased affinities for transfer RNAs, decreased rates of peptidyl-transfer), and cells harboring this mutation are resistant to peptidyl-transferase inhibitors (e.g., anisomycin). These observations beg the question of how a single amino acid mutation may have such wide ranging consequences. Here, we report the structure of the vacant yeast uL3 W255C mutant ribosome by X-ray crystallography, showing a disruption of the A-site side of the peptidyl-transferase center (PTC). An additional X-ray crystallographic structure of the anisomycin-containing mutant ribosome shows that high concentrations of this inhibitor restore a "WT-like" configuration to this region of the PTC, providing insight into the resistance mechanism of the mutant. Globally, our data demonstrate that ribosomal protein uL3 is structurally essential to ensure an optimal and catalytically efficient organization of the PTC, highlighting the importance of proteins in the RNA-centered ribosome.

Ribosomal protein methyltransferases in the yeast Saccharomyces cerevisiae: Roles in ribosome biogenesis and translation

A significant percentage of the methyltransferasome in Saccharomyces cerevisiae and higher eukaryotes is devoted to methylation of the translational machinery. Methylation of the RNA components of the translational machinery has been studied extensively and is important for structure stability, ribosome biogenesis, and translational fidelity. However, the functional effects of ribosomal protein methylation by their cognate methyltransferases are still largely unknown. Previous work has shown that the ribosomal protein Rpl3 methyltransferase, histidine protein methyltransferase 1 (Hpm1), is important for ribosome biogenesis and translation elongation fidelity. In this study, yeast strains deficient in each of the ten ribosomal protein methyltransferases in S. cerevisiae were examined for potential defects in ribosome biogenesis and translation. Like Hpm1-deficient cells, loss of four of the nine other ribosomal protein methyltransferases resulted in defects in ribosomal subunit synthesis. All of the mutant strains exhibited resistance to the ribosome inhibitors anisomycin and/or cycloheximide in plate assays, but not in liquid culture. Translational fidelity assays measuring stop codon readthrough, amino acid misincorporation, and programmed -1 ribosomal frameshifting, revealed that eight of the ten enzymes are important for translation elongation fidelity and the remaining two are necessary for translation termination efficiency. Altogether, these results demonstrate that ribosomal protein methyltransferases in S. cerevisiae play important roles in ribosome biogenesis and translation.

Final pre-40S maturation depends on the functional integrity of the 60S subunit ribosomal protein L3

Ribosomal protein L3 is an evolutionarily conserved protein that participates in the assembly of early pre-60S particles. We report that the rpl3[W255C] allele, which affects the affinity and function of translation elongation factors, impairs cytoplasmic maturation of 20S pre-rRNA. This was not seen for other mutations in or depletion of L3 or other 60S ribosomal proteins. Surprisingly, pre-40S particles containing 20S pre-rRNA form translation-competent 80S ribosomes, and translation inhibition partially suppresses 20S pre-rRNA accumulation. The GTP-dependent translation initiation factor Fun12 (yeast eIF5B) shows similar in vivo binding to ribosomal particles from wild-type and rpl3[W255C] cells. However, the GTPase activity of eIF5B failed to stimulate processing of 20S pre-rRNA when assayed with ribosomal particles purified from rpl3[W255C] cells. We conclude that L3 plays an important role in the function of eIF5B in stimulating 3′ end processing of 18S rRNA in the context of 80S ribosomes that have not yet engaged in translation. These findings indicate that the correct conformation of the GTPase activation region is assessed in a quality control step during maturation of cytoplasmic pre-ribosomal particles.

Recent progress has provided us with detailed knowledge of the structure and function of eukaryotic ribosomes. However, our understanding of the intricate processes of pre-ribosome assembly and the transition to translation-competent ribosomal subunits remains incomplete. The early and intermediate steps of ribosome assembly occur successively in the nucleolus and nucleoplasm. The pre-ribosomal subunits are then exported to the cytoplasm where final maturation steps, notably including D site cleavage of the 20S pre-rRNA to mature 18S rRNA, confer subunit joining and translation competence. Recent evidence indicates that pre-40S subunits are subject to a quality control step involving the GTP-dependent translation initiation factor eIF5B/Fun12, in the context of 80S-like ribosomes. Here, we demonstrate the involvement of 60S subunits in promoting 20S pre-rRNA cleavage. In particular, we show that a specific point mutation in the 60S subunit ribosomal protein L3 (rpl3[W255C]) leads to the accumulation of pre-40S particles that contain the 20S pre-rRNA but are translation-competent. Notably, this mutation prevents the stimulation of the GTPase activity of eIF5B/Fun12, which is also required for site D cleavage. We conclude that L3 plays an important role in regulating the function of eIF5B/Fun12 during 3′ end processing of 18S rRNA at site D, in the context of 80S ribosomes that have not yet engaged in translation.

Yeast telomere maintenance is globally controlled by programmed ribosomal frameshifting and the nonsense-mediated mRNA decay pathway

We have previously shown that ~10% of all eukaryotic mRNAs contain potential programmed -1 ribosomal frameshifting (-1 PRF) signals and that some function as mRNA destabilizing elements through the Nonsense-Mediated mRNA Decay (NMD) pathway by directing translating ribosomes to premature termination codons. Here, the connection between -1 PRF, NMD and telomere end maintenance are explored. Functional -1 PRF signals were identified in the mRNAs encoding two components of yeast telomerase, EST1 and EST2, and in mRNAs encoding proteins involved in recruiting telomerase to chromosome ends, STN1 and CDC13. All of these elements responded to mutants and drugs previously known to stimulate or inhibit -1 PRF, further supporting the hypothesis that they promote -1 PRF through the canonical mechanism. All affected the steady-state abundance of a reporter mRNA and the wide range of -1 PRF efficiencies promoted by these elements enabled the determination of an inverse logarithmic relationship between -1 PRF efficiency and mRNA accumulation. Steady-state abundances of the endogenous EST1, EST2, STN1 and CDC13 mRNAs were similarly inversely proportional to changes in -1 PRF efficiency promoted by mutants and drugs, supporting the hypothesis that expression of these genes is post-transcriptionally controlled by -1 PRF under native conditions. Overexpression of EST2 by ablation of -1 PRF signals or inhibition of NMD promoted formation of shorter telomeres and accumulation of large budded cells at the G2/M boundary. A model  is presented describing how limitation and maintenance of correct stoichiometries of telomerase components by -1 PRF is used to maintain yeast telomere length.

Regulation of gene expression by macrolide-induced ribosomal frameshifting

The expression of many genes is controlled by upstream ORFs (uORFs). Typically, the progression of the ribosome through a regulatory uORF, which depends on the physiological state of the cell, influences the expression of the downstream gene. In the classic mechanism of induction of macrolide resistance genes, antibiotics promote translation arrest within the uORF, and the static ribosome induces a conformational change in mRNA, resulting in the activation of translation of the resistance cistron. We show that ketolide antibiotics, which do not induce ribosome stalling at the uORF of the ermC resistance gene, trigger its expression via a unique mechanism. Ketolides promote frameshifting at the uORF, allowing the translating ribosome to invade the intergenic spacer. The dynamic unfolding of the mRNA structure leads to the activation of resistance. Conceptually similar mechanisms may control other cellular genes. The identified property of ketolides to reduce the fidelity of reading frame maintenance may have medical implications.

A rapid, inexpensive yeast-based dual-fluorescence assay of programmed--1 ribosomal frameshifting for high-throughput screening

Programmed −1 ribosomal frameshifting (−1 PRF) is a mechanism that directs elongating ribosomes to shift-reading frame by 1 base in the 5′ direction that is utilized by many RNA viruses. Importantly, rates of −1 PRF are fine-tuned by viruses, including Retroviruses, Coronaviruses, Flavivriuses and in two endogenous viruses of the yeast Saccharomyces cerevisiae, to deliver the correct ratios of different viral proteins for efficient replication. Thus, −1 PRF presents a novel target for antiviral therapeutics. The underlying molecular mechanism of −1 PRF is conserved from yeast to mammals, enabling yeast to be used as a logical platform for high-throughput screens. Our understanding of the strengths and pitfalls of assays to monitor −1 PRF have evolved since the initial discovery of −1 PRF. These include controlling for the effects of drugs on protein expression and mRNA stability, as well as minimizing costs and the requirement for multiple processing steps. Here we describe the development of an automated yeast-based dual fluorescence assay of −1 PRF that provides a rapid, inexpensive automated pipeline to screen for compounds that alter rates of −1 PRF which will help to pave the way toward the discovery and development of novel antiviral therapeutics.

The central core region of yeast ribosomal protein L11 is important for subunit joining and translational fidelity

Yeast ribosomal protein L11 is positioned at the intersubunit cleft of the large subunit central protuberance, forming an intersubunit bridge with the small subunit protein S18. Mutants were engineered in the central core region of L11 which interacts with Helix 84 of the 25S rRNA. Numerous mutants in this region conferred 60S subunit biogenesis defects. Specifically, many mutations of F96 and the A66D mutant promoted formation of halfmers as assayed by sucrose density ultracentrifugation. Halfmer formation was not due to deficiency in 60S subunit production, suggesting that the mutants affected subunit-joining. Chemical modification analyses indicated that the A66D mutant, but not the F96 mutants, promoted changes in 25S rRNA structure, suggesting at least two modalities for subunit joining defects. 25S rRNA structural changes were located both adjacent to A66D (in H84), and more distant (in H96-7). While none of the mutants significantly affected ribosome/tRNA binding constants, they did have strong effects on cellular growth at both high and low temperatures, in the presence of translational inhibitors, and promoted changes in translational fidelity. Two distinct mechanisms are proposed by which L11 mutants may affect subunit joining, and identification of the amino acids associated with each of these processes are presented. These findings may have implications for our understanding of multifaceted diseases such as Diamond--Blackfan anemia which have been linked in part with mutations in L11.

The many paths to frameshifting: kinetic modelling and analysis of the effects of different elongation steps on programmed -1 ribosomal frameshifting

Several important viruses including the human immunodeficiency virus type 1 (HIV-1) and the SARS-associated Coronavirus (SARS-CoV) employ programmed −1 ribosomal frameshifting (PRF) for their protein expression. Here, a kinetic framework is developed to describe −1 PRF. The model reveals three kinetic pathways to −1 PRF that yield two possible frameshift products: those incorporating zero frame encoded A-site tRNAs in the recoding site, and products incorporating −1 frame encoded A-site tRNAs. Using known kinetic rate constants, the individual contributions of different steps of the translation elongation cycle to −1 PRF and the ratio between two types of frameshift products were evaluated. A dual fluorescence reporter was employed in Escherichia coli to empirically test the model. Additionally, the study applied a novel mass spectrometry approach to quantify the ratios of the two frameshift products. A more detailed understanding of the mechanisms underlying −1 PRF may provide insight into developing antiviral therapeutics.

A molecular clamp ensures allosteric coordination of peptidyltransfer and ligand binding to the ribosomal A-site

Although the ribosome is mainly comprised of rRNA and many of its critical functions occur through RNA–RNA interactions, distinct domains of ribosomal proteins also participate in switching the ribosome between different conformational/functional states. Prior studies demonstrated that two extended domains of ribosomal protein L3 form an allosteric switch between the pre- and post-translocational states. Missing was an explanation for how the movements of these domains are communicated among the ribosome's functional centers. Here, a third domain of L3 called the basic thumb, that protrudes roughly perpendicular from the W-finger and is nestled in the center of a cagelike structure formed by elements from three separate domains of the large subunit rRNA is investigated. Mutagenesis of basically charged amino acids of the basic thumb to alanines followed by detailed analyses suggests that it acts as a molecular clamp, playing a role in allosterically communicating the ribosome's tRNA occupancy status to the elongation factor binding region and the peptidyltransferase center, facilitating coordination of their functions through the elongation cycle. The observation that these mutations affected translational fidelity, virus propagation and cell growth demonstrates how small structural changes at the atomic scale can propagate outward to broadly impact the biology of cell.

Achieving a golden mean: mechanisms by which coronaviruses ensure synthesis of the correct stoichiometric ratios of viral proteins

In retroviruses and the double-stranded RNA totiviruses, the efficiency of programmed -1 ribosomal frameshifting is critical for ensuring the proper ratios of upstream-encoded capsid proteins to downstream-encoded replicase enzymes. The genomic organizations of many other frameshifting viruses, including the coronaviruses, are very different, in that their upstream open reading frames encode nonstructural proteins, the frameshift-dependent downstream open reading frames encode enzymes involved in transcription and replication, and their structural proteins are encoded by subgenomic mRNAs. The biological significance of frameshifting efficiency and how the relative ratios of proteins encoded by the upstream and downstream open reading frames affect virus propagation has not been explored before. Here, three different strategies were employed to test the hypothesis that the -1 PRF signals of coronaviruses have evolved to produce the correct ratios of upstream- to downstream-encoded proteins. Specifically, infectious clones of the severe acute respiratory syndrome (SARS)-associated coronavirus harboring mutations that lower frameshift efficiency decreased infectivity by >4 orders of magnitude. Second, a series of frameshift-promoting mRNA pseudoknot mutants was employed to demonstrate that the frameshift signals of the SARS-associated coronavirus and mouse hepatitis virus have evolved to promote optimal frameshift efficiencies. Finally, we show that a previously described frameshift attenuator element does not actually affect frameshifting per se but rather serves to limit the fraction of ribosomes available for frameshifting. The findings of these analyses all support a "golden mean" model in which viruses use both programmed ribosomal frameshifting and translational attenuation to control the relative ratios of their encoded proteins.

Pseudoknot-Dependent Programmed —1 Ribosomal Frameshifting: Structures, Mechanisms and Models

Programmed —1 ribosomal frameshifting is a translational recoding strategy that takes place during the elongation phase of protein biosynthesis. Frameshifting occurs in response to specific signals in the mRNA; a slippery sequence, where the ribosome changes frame, and a stimulatory RNA secondary structure, usually a pseudoknot, located immediately downstream. During the frameshift the ribosome slips backwards by a single nucleotide (in the 5′-wards/—1 direction) and continues translation in the new, overlapping reading frame, generating a fusion protein composed of the products of both the original and the —1 frame coding regions. In eukaryotes, frameshifting is largely a phenomenon of virus gene expression and associated predominantly with the expression of viral replicases. Research on frameshifting impacts upon diverse topics, including the ribosomal elongation cycle, RNA structure and function, tRNA modification, virus replication, antiviral intervention, evolution and bioinformatics. This chapter focuses on the structure and function of frameshift-stimulatory RNA pseudoknots and mechanistic aspects of ribosomal frameshifting. A variety of models of the frameshifting process are discussed in the light of recent advances in our understanding of ribosome structure and the elongation cycle.

Ribosomal protein L3 functions as a 'rocker switch' to aid in coordinating of large subunit-associated functions in eukaryotes and Archaea

Although ribosomal RNAs (rRNAs) comprise the bulk of the ribosome and carry out its main functions, ribosomal proteins also appear to play important structural and functional roles. Many ribosomal proteins contain long, nonglobular domains that extend deep into the rRNA cores. In eukaryotes and Archaea, ribosomal protein L3 contains two such extended domains tethered to a common globular hub, thus providing an excellent model to address basic questions relating to ribosomal protein structure/function relationships. Previous work in our laboratory identified the central ‘W-finger’ extension of yeast L3 in helping to coordinate ribosomal functions. New studies on the ‘N-terminal’ extension in yeast suggest that it works with the W-finger to coordinate opening and closing of the corridor through which the 3′ end of aa-tRNA moves during the process of accommodation. Additionally, the effect of one of the L3 N-terminal extension mutants on the interaction between C75 of the aa-tRNA and G2921 (Escherichia coli G2553) of 25S rRNA provides the first evidence of the effect of a ribosomal protein on aa-tRNA positioning and peptidyltransfer, possibly through the induced fit model. A model is presented describing how all three domains of L3 may function together as a ‘rocker switch’ to coordinate the stepwise processes of translation elongation.

Ribosomal frameshifting in response to hypomodified tRNAs in Xenopus oocytes

We used Xenopus oocytes as an intracellular system to study ribosomal frameshifting. Microinjection of oocytes with a construct encoding the naturally occurring UUU or AAC codon at the frameshift site demonstrated that the level of frameshifting was similar or lower than found normally in retroviral frameshifting in mammalian cells, suggesting that oocytes are a reliable system to study this event. Phenylalanine (Phe) or asparagine (Asn) tRNAs with and without the highly modified wyebutoxine (Y) or queuosine (Q) base, respectively, were microinjected to assess their ability to promote frameshifting. tRNAPhe+Y inhibited the level of frameshifting, while tRNAPhe-Y promoted frameshifting providing evidence that the hypomodified form does not act only to enhance frameshifting, but is an essential requirement. Both tRNAAsn+Q and tRNAAsn-Q were used indiscriminately in frameshifting, whether the frameshift site contained the wild-type AAC, or the mutant AAU codon, suggesting that Q base modification status does not influence this process.

Differences in the expression of the hepatitis C virus core+1 open reading frame between a nuclear and a cytoplasmic expression system

The hepatitis C virus (HCV) genome possesses an open reading frame (ORF) overlapping the core gene at +1 nucleotide (core+1 ORF). Initial in vitro studies suggested that the core+1 ORF is translated by a ribosomal -2/+1 frameshift mechanism during elongation of the viral polyprotein. Recent studies, however, based on transfection of mammalian cells with reporter constructs have shown that translation of the core+1 ORF is mediated from internal core+1 codons. To resolve the apparent discrepancies associated with the mechanism of core+1 translation, we examined the expression of the HCV-1 and HCV-1a (H) core+1 ORF in a cytoplasmic transcription system based on Huh-7/T7 cells that constitutively synthesize the T7 RNA polymerase in comparison to that in Huh-7 cells. We showed that the efficiency of both the -2/+1 and -1/+2 frameshift events operating at the HCV-1 core codons 8-11 is significantly enhanced in the Huh-7/T7 cytoplasmic transcription system and is dependent on the presence of the consecutive adenine (A) residues within core codons 8-11. In contrast, internal translation initiation at core+1 codons 85/87 occurs in both the nuclear and cytoplasmic transcription systems and is not repressed by the ribosomal frameshifting event. Finally, although core+1 codons 85/87 is the most efficient site for internal initiation of core+1 translation, it may not be unique, as additional internal core+1 codon(s) appear to drive translation at low levels.

Structure/function analysis of yeast ribosomal protein L2

Ribosomal protein L2 is a core element of the large subunit that is highly conserved among all three kingdoms. L2 contacts almost every domain of the large subunit rRNA and participates in an intersubunit bridge with the small subunit rRNA. It contains a solvent-accessible globular domain that interfaces with the solvent accessible side of the large subunit that is linked through a bridge to an extension domain that approaches the peptidyltransferase center. Here, screening of randomly generated library of yeast RPL2A alleles identified three translationally defective mutants, which could be grouped into two classes. The V48D and L125Q mutants map to the globular domain. They strongly affect ribosomal A-site associated functions, peptidyltransferase activity and subunit joining. H215Y, located at the tip of the extended domain interacts with Helix 93. This mutant specifically affects peptidyl–tRNA binding and peptidyltransferase activity. Both classes affect rRNA structure far away from the protein in the A-site of the peptidyltransferase center. These findings suggest that defective interactions with Helix 55 and with the Helix 65–66 structure may indicate a certain degree of flexibility in L2 in the neck region between the two other domains, and that this might help to coordinate tRNA–ribosome interactions.

Role of a tRNA base modification and its precursors in frameshifting in eukaryotes

Little is known about the role of specific base modifications of transfer RNAs. Wyosine bases are tRNA(Phe)-specific modifications that are distinguished by differentiated, lateral side chains and base methylations appended to the core ring structure of a universally conserved G37, adjacent to the anticodon of Phe tRNAs. Based on previous data, we hypothesized that this modification was needed for -1 frameshifting. Using a reporter system incorporating a SCV-LA yeast virus slippery site for detecting -1 frameshifts in vivo, yeast strains were created that enabled chemical-genetic dissection of the role of different functional groups of wyebutosine that are added in a three-step post-transcriptional set of reactions. With this system, hypomodification increased Phe-specific frameshifting, with incremental changes in frameshift efficiency after specific intermediates in the progression of wyebutosine synthesis. These data combined with investigations of wild-type and hypomodified tRNA binding to ribosomes suggest that frameshift efficiency is kinetically and not thermodynamically controlled. The progressive nature of frameshift efficiency with the stage of modification is consistent with a stepwise evolution and tuning of frameshift potential. The stepwise tuning of frameshift efficiency could explain why tRNA(Phe) in some eukaryotes is not fully modified but, rather, hypomodified to capture a specific frameshift potential.

Functional analysis of Saccharomyces cerevisiae ribosomal protein Rpl3p in ribosome synthesis

Ribosome synthesis in eukaryotes requires a multitude of trans-acting factors. These factors act at many steps as the pre-ribosomal particles travel from the nucleolus to the cytoplasm. In contrast to the well-studied trans-acting factors, little is known about the contribution of the ribosomal proteins to ribosome biogenesis. Herein, we have analysed the role of ribosomal protein Rpl3p in 60S ribosomal subunit biogenesis. In vivo depletion of Rpl3p results in a deficit in 60S ribosomal subunits and the appearance of half-mer polysomes. This phenotype is likely due to the instability of early and intermediate pre-ribosomal particles, as evidenced by the low steady-state levels of 27SA₃, 27SB_S and 7S_L/S precursors. Furthermore, depletion of Rpl3p impairs the nucleocytoplasmic export of pre-60S ribosomal particles. Interestingly, flow cytometry analysis indicates that Rpl3p-depleted cells arrest in the G1 phase. Altogether, we suggest that upon depletion of Rpl3p, early assembly of 60S ribosomal subunits is aborted and subsequent steps during their maturation and export prevented.

Ribosomal protein L3: gatekeeper to the A site

Ribosomal protein L3 (L3) is an essential and indispensable component for formation of the peptidyltransferase center. Atomic resolution ribosome structures reveal two extensions of L3 protruding deep into the core of the large subunit. The central extension of L3 in Saccharomyces cerevisiae was investigated using a combination of molecular genetic, biochemical, chemical probing, and molecular modeling methods. A reciprocal relationship between ribosomal affinity for eEF-1A stimulated binding of aa-tRNA and for eEF2 suggests that the central extension of L3 may function as an allosteric switch in coordinating binding of the elongation factors. Opening of the aa-tRNA accommodation corridor promoted resistance to the A site-specific translational inhibitor anisomycin, suggesting a competitive model for anisomycin resistance. These changes were also found to inhibit peptidyltransferase activity, stimulating programmed -1 ribosomal frameshifting and promoting virus propagation defects. These studies provide a basis for deeper insight into rational design of small molecule antiviral therapeutics.

Specific effects of ribosome-tethered molecular chaperones on programmed -1 ribosomal frameshifting

The ribosome-associated molecular chaperone complexes RAC (Ssz1p/Zuo1p) and Ssb1p/Ssb2p expose a link between protein folding and translation. Disruption of the conserved nascent peptide-associated complex results in cell growth and translation fidelity defects. To better understand the consequences of deletion of either RAC or Ssb1p/2p, experiments relating to cell growth and programmed ribosomal frameshifting (PRF) were assayed. Genetic analyses revealed that deletion of Ssb1p/Ssb2p or of Ssz1p/Zuo1p resulted in specific inhibition of -1 PRF and defects in Killer virus maintenance, while no effects were observed on +1 PRF. These factors may provide a new set of targets to exploit against viruses that use -1 PRF. Quantitative measurements of growth profiles of isogenic wild-type and mutant cells showed that translational inhibitors exacerbate underlying growth defects in these mutants. Previous studies have identified -1 PRF signals in yeast chromosomal genes and have demonstrated an inverse relationship between -1 PRF efficiency and mRNA stability. Analysis of published DNA microarray experiments reveals conditions under which Ssb1, Ssb2, Ssz1, and Zuo1 transcript levels are regulated independently of those of genes encoding ribosomal proteins. Thus, the findings presented here suggest that these trans-acting factors could be used by cells to posttranscriptionally regulate gene expression through -1 PRF.

Pokeweed antiviral protein depurinates the sarcin/ricin loop of the rRNA prior to binding of aminoacyl-tRNA to the ribosomal A-site

Ribosome-inactivating proteins, such as the pokeweed antiviral protein (PAP), inhibit translation by depurinating the conserved sarcin/ricin loop of the large ribosomal RNA. Depurinated ribosomes are unable to bind elongation factor 2, and, thus, the translocation step of the elongation cycle is inhibited. Though the consequences of depurination are well characterized, the ribosome conformation required for depurination to take place has not been described. In this report, we correlate biochemical and genetic data to conclude that pokeweed antiviral protein depurinates the sarcin/ricin loop when the A-site of the ribosomal peptidyl-transferase center is unoccupied. We show that prior incubation of ribosomes with puromycin, an analog of the 3'-terminus of aminoacyl-tRNA, inhibits both binding and depurination by PAP in a concentration-dependent manner. Expression of PAP in the yeast strain mak8-1 results in little depurination unless the cells are lysed, a process that would promote loss of aminoacyl-tRNA from the ribosome. The mak8-1 strain is known to exhibit a higher affinity for aminoacyl-tRNA compared with wild-type cells, and therefore, its ribosomes are more resistant to PAP in vivo. These data contribute to the mechanism of action of pokeweed antiviral protein; specifically, they have uncovered the ribosomal conformation required for depurination that leads to subsequent translation inhibition.

Eukaryotic ribosomal proteins lacking a eubacterial counterpart: important players in ribosomal function

The ribosome is a macromolecular machine responsible for protein synthesis in all organisms. Despite the enormous progress in studies on the structure and function of prokaryotic ribosomes, the respective molecular details of the mechanism by which the eukaryotic ribosome and associated factors construct a polypeptide accurately and rapidly still remain largely unexplored. Eukaryotic ribosomes possess more RNA and a higher number of proteins than eubacterial ribosomes. As the tertiary structure and basic function of the ribosomes are conserved, what is the contribution of these additional elements? Elucidation of the role of these components should provide clues to the mechanisms of translation in eukaryotes and help unravel the molecular mechanisms underlying the differences between eukaryotic and eubacterial ribosomes. This article focuses on a class of eukaryotic ribosomal proteins that do not have a eubacterial homologue. These proteins play substantial roles in ribosomal structure and function, and in mRNA binding and nascent peptide folding. The role of these proteins in human diseases and viral expression, as well as their potential use as targets for antiviral agents is discussed.

Identification of functionally important amino acids of ribosomal protein L3 by saturation mutagenesis

There is accumulating evidence that many ribosomal proteins are involved in shaping rRNA into their functionally correct conformations through RNA-protein interactions. Moreover, although rRNA seems to play the central role in all aspects of ribosome function, ribosomal proteins may be involved in facilitating communication between different functional regions in ribosome, as well as between the ribosome and cellular factors. In an effort to more fully understand how ribosomal proteins may influence ribosome function, we undertook large-scale mutational analysis of ribosomal protein L3, a core protein of the large subunit that has been implicated in numerous ribosome-associated functions in the past. A total of 98 different rpl3 alleles were genetically characterized with regard to their effects on killer virus maintenance, programmed -1 ribosomal frameshifting, resistance/hypersensitivity to the translational inhibitor anisomycin and, in specific cases, the ability to enhance translation of a reporter mRNA lacking the 5' (7)mGppp cap structure and 3' poly(A) tail. Biochemical studies reveal a correlation between an increased affinity for aminoacyl-tRNA and the extent of anisomycin resistance and a decreased peptidyltransferase activity and increased frameshifting efficiency. Immunoblot analyses reveal that the superkiller phenotype is not due to a defect in the ability of ribosomes to recruit the Ski-complex, suggesting that the defect lies in a reduced ability of mutant ribosomes to distinguish between cap(+)/poly(A)(+) and cap(-)/poly(A)(-) mRNAs. The results of these analyses are discussed with regard to how protein-rRNA interactions may affect ribosome function.

Structural and functional analysis of 5S rRNA in Saccharomyces cerevisiae

5S rRNA extends from the central protuberance of the large ribosomal subunit, through the A-site finger, and down to the GTPase-associated center. Here, we present a structure-function analysis of seven 5S rRNA alleles which are sufficient for viability in the yeast Saccharomyces cerevisiae when expressed in the absence of wild-type 5S rRNAs, and extend this analysis using a large bank of mutant alleles that show semi-dominant phenotypes in the presence of wild-type 5S rRNA. This analysis supports the hypothesis that 5S rRNA serves to link together several different functional centers of the ribosome. Data are also presented which suggest that in eukaryotic genomes selection has favored the maintenance of multiple alleles of 5S rRNA, and that these may provide cells with a mechanism to post-transcriptionally regulate gene expression.

Expression of a truncated form of ribosomal protein L3 confers resistance to pokeweed antiviral protein and the Fusarium mycotoxin deoxynivalenol

The contamination of important agricultural products such as wheat, barley, or maize with the trichothecene mycotoxin deoxynivalenol (DON) due to infection with Fusarium species is a worldwide problem. Trichothecenes inhibit protein synthesis by targeting ribosomal protein L3. Pokeweed antiviral protein (PAP), a ribosome-inactivating protein binds to L3 to depurinate the alpha-sarcin/loop of the large rRNA. Plants transformed with the wild-type PAP show lesions and express very low levels of PAP because PAP autoregulates its expression by destabilizing its own mRNA. We show here that transgenic tobacco plants expressing both the wild-type PAP and a truncated form of yeast L3 (L3delta) are phenotypically normal. PAP mRNA and protein levels are very high in these plants, indicating that L3delta suppresses the autoregulation of PAP mRNA expression. Ribosomes are not depurinated in the transgenic plants expressing PAP and L3delta, even though PAP is associated with ribosomes. The expression of the endogenous tobacco ribosomal protein L3 is up-regulated in these plants and they are resistant to the Fusarium mycotoxin DON. These results demonstrate that expression of an N-terminal fragment of yeast L3 leads to trans-dominant resistance to PAP and the trichothecene mycotoxin DON, providing evidence that both toxins target L3 by a common mechanism.

A three-stemmed mRNA pseudoknot in the SARS coronavirus frameshift signal

A wide range of RNA viruses use programmed -1 ribosomal frameshifting for the production of viral fusion proteins. Inspection of the overlap regions between ORF1a and ORF1b of the SARS-CoV genome revealed that, similar to all coronaviruses, a programmed -1 ribosomal frameshift could be used by the virus to produce a fusion protein. Computational analyses of the frameshift signal predicted the presence of an mRNA pseudoknot containing three double-stranded RNA stem structures rather than two. Phylogenetic analyses showed the conservation of potential three-stemmed pseudoknots in the frameshift signals of all other coronaviruses in the GenBank database. Though the presence of the three-stemmed structure is supported by nuclease mapping and two-dimensional nuclear magnetic resonance studies, our findings suggest that interactions between the stem structures may result in local distortions in the A-form RNA. These distortions are particularly evident in the vicinity of predicted A-bulges in stems 2 and 3. In vitro and in vivo frameshifting assays showed that the SARS-CoV frameshift signal is functionally similar to other viral frameshift signals: it promotes efficient frameshifting in all of the standard assay systems, and it is sensitive to a drug and a genetic mutation that are known to affect frameshifting efficiency of a yeast virus. Mutagenesis studies reveal that both the specific sequences and structures of stems 2 and 3 are important for efficient frameshifting. We have identified a new RNA structural motif that is capable of promoting efficient programmed ribosomal frameshifting. The high degree of conservation of three-stemmed mRNA pseudoknot structures among the coronaviruses suggests that this presents a novel target for antiviral therapeutics.

Factors affecting translation at the programmed -1 ribosomal frameshifting site of Cocksfoot mottle virus RNA in vivo

The ratio between proteins P27 and replicase of Cocksfoot mottle virus (CfMV) is regulated via a -1 programmed ribosomal frameshift (-1 PRF). A minimal frameshift signal with a slippery U UUA AAC heptamer and a downstream stem-loop structure was inserted into a dual reporter vector and directed -1 PRF with an efficiency of 14.4 +/- 1.9% in yeast and 2.4 +/- 0.7% in bacteria. P27-encoding CfMV sequence flanking the minimal frameshift signal caused approximately 2-fold increase in the -1 PRF efficiencies both in yeast and in bacteria. In addition to the expected fusion proteins, termination products ending putatively at the frameshift site were found in yeast cells. We propose that the amount of premature translation termination from control mRNAs played a role in determining the calculated -1PRF efficiency. Co-expression of CfMV P27 with the dual reporter vector containing the minimal frameshift signal reduced the production of the downstream reporter, whereas replicase co-expression had no pronounced effect. This finding allows us to propose that CfMV protein P27 may influence translation at the frameshift site but the mechanism needs to be elucidated.

Mutations in ribosomal protein L3 and 23S ribosomal RNA at the peptidyl transferase centre are associated with reduced susceptibility to tiamulin in Brachyspira spp. isolates

The pleuromutilin antibiotic tiamulin binds to the ribosomal peptidyl transferase centre. Three groups of Brachyspira spp. isolates with reduced tiamulin susceptibility were analysed to define resistance mechanisms to the drug. Mutations were identified in genes encoding ribosomal protein L3 and 23S rRNA at positions proximal to the peptidyl transferase centre. In two groups of laboratory-selected mutants, mutations were found at nucleotide positions 2032, 2055, 2447, 2499, 2504 and 2572 of 23S rRNA (Escherichia coli numbering) and at amino acid positions 148 and 149 of ribosomal protein L3 (Brachyspira pilosicoli numbering). In a third group of clinical B. hyodysenteriae isolates, only a single mutation at amino acid 148 of ribosomal protein L3 was detected. Chemical footprinting experiments show a reduced binding of tiamulin to ribosomal subunits from mutants with decreased susceptibility to the drug. This reduction in drug binding is likely the resistance mechanism for these strains. Hence, the identified mutations located near the tiamulin binding site are predicted to be responsible for the resistance phenotype. The positions of the mutated residues relative to the bound drug advocate a model where the mutations affect tiamulin binding indirectly through perturbation of nucleotide U2504.

Systematic analysis of bicistronic reporter assay data

Bicistronic reporter assay systems have become a mainstay of molecular biology. While the assays themselves encompass a broad range of diverse and unrelated experimental protocols, the numerical data garnered from these experiments often have similar statistical properties. In general, a primary dataset measures the paired expression of two internally controlled reporter genes. The expression ratio of these two genes is then normalized to an external control reporter. The end result is a ‘ratio of ratios’ that is inherently sensitive to propagation of the error contributed by each of the respective numerical components. The statistical analysis of this data therefore requires careful handling in order to control for the propagation of error and its potentially misleading effects. A careful survey of the literature found no consistent method for the statistical analysis of data generated from these important and informative assay systems. In this report, we present a detailed statistical framework for the systematic analysis of data obtained from bicistronic reporter assay systems. Specifically, a dual luciferase reporter assay was employed to measure the efficiency of four programmed −1 frameshift signals. These frameshift signals originate from the L-A virus, the SARS-associated Coronavirus and computationally identified frameshift signals from two Saccharomyces cerevisiae genes. Furthermore, these statistical methods were applied to prove that the effects of anisomycin on programmed −1 frameshifting are statistically significant. A set of Microsoft Excel spreadsheets, which can be used as templates for data generated by dual reporter assay systems, and an online tutorial are available at our website (http://dinmanlab.umd.edu/statistics). These spreadsheets could be easily adapted to any bicistronic reporter assay system.

A reassessment of the response of the bacterial ribosome to the frameshift stimulatory signal of the human immunodeficiency virus type 1

HIV-1 uses a programmed -1 ribosomal frameshift to produce the precursor of its enzymes. This frameshift occurs at a specific slippery sequence followed by a stimulatory signal, which was recently shown to be a two-stem helix, for which a three-purine bulge separates the upper and lower stems. In the present study, we investigated the response of the bacterial ribosome to this signal, using a translation system specialized for the expression of a firefly luciferase reporter. The HIV-1 frameshift region was inserted at the beginning of the coding sequence of the luciferase gene, such that its expression requires a -1 frameshift. Mutations that disrupt the upper or the lower stem of the frameshift stimulatory signal or replace the purine bulge with pyrimidines decreased the frameshift efficiency, whereas compensatory mutations that re-form both stems restored the frame-shift efficiency to near wild-type level. These mutations had the same effect in a eukaryotic translation system, which shows that the bacterial ribosome responds like the eukaryote ribosome to the HIV-1 frameshift stimulatory signal. Also, we observed, in contrast to a previous report, that a stop codon immediately 3' to the slippery sequence does not decrease the frameshift efficiency, ruling out a proposal that the frameshift involves the deacylated-tRNA and the peptidyl-tRNA in the E and P sites of the ribosome, rather than the peptidyl-tRNA and the aminoacyl-tRNA in the P and A sites, as commonly assumed. Finally, mutations in 16S ribosomal RNA that facilitate the accommodation of the incoming aminoacyl-tRNA in the A site decreased the frameshift efficiency, which supports a previous suggestion that the frameshift occurs when the aminoacyl-tRNA occupies the A/T entry site.

Ribosomal protein L3: influence on ribosome structure and function

Early studies demonstrated roles for ribosomal protein L3 in peptidyltransferase center formation and the ability of cells to propagate viruses. More recent studies have linked these two processes via the effects of mutants and drugs on programmed -1 ribosomal frameshifting. Here, we show that mutant forms of L3 result in ribosomes having increased affinities for both aminoacyl- and peptidyl-tRNAs. These defects potentiate the effects of sparsomycin, which promotes increased aminoalcyl-tRNA binding at the P-site, while antagonizing the effects anisomycin, a drug that promotes decreased peptidyl-tRNA binding at the A-site. The changes in ribosome affinities for tRNAs also correlate with decreased peptidyltransferase activities of mutant ribosomes, and with decreased rates of cell growth and protein synthesis. In vivo dimethylsulfate (DMS) protection studies reveal that small changes in L3 primary sequence also have significant effects on rRNA structure as far away as 100 A, supporting an allosteric model of ribosome function.

Delayed rRNA processing results in significant ribosome biogenesis and functional defects

mof6-1 was originally isolated as a recessive mutation in Saccharomyces cerevisiae which promoted increased efficiencies of programmed -1 ribosomal frameshifting and rendered cells unable to maintain the killer virus. Here, we demonstrate that mof6-1 is a unique allele of the histone deacetylase RPD3, that the deacetylase function of Rpd3p is required for controlling wild-type levels of frameshifting and virus maintenance, and that the closest human homolog can fully complement these defects. Loss of the Rpd3p-associated histone deacetylase function, either by mutants of rpd3 or loss of the associated gene product Sin3p or Sap30p, results in a delay in rRNA processing rather than in an rRNA transcriptional defect. This results in production of ribosomes having lower affinities for aminoacyl-tRNA and diminished peptidyltransferase activities. We hypothesize that decreased rates of peptidyl transfer allow ribosomes with both A and P sites occupied by tRNAs to pause for longer periods of time at -1 frameshift signals, promoting increased programmed -1 ribosomal frameshifting efficiencies and subsequent loss of the killer virus. The frameshifting defect is accentuated when the demand for ribosomes is highest, suggesting that rRNA posttranscriptional modification is the bottleneck in ribosome biogenesis.