Alterations in ribosomal protein RPS28 can diversely affect translational accuracy in Saccharomyces cerevisiae

The Involvement of YNR069C in Protein Synthesis in the Baker's Yeast, Saccharomyces cerevisiae

Maintaining translation fidelity is a critical step within the process of gene expression. It requires the involvement of numerous regulatory elements to ensure the synthesis of functional proteins. The efficient termination of protein synthesis can play a crucial role in preserving this fidelity. Here, we report on investigating a protein of unknown function, YNR069C (also known as BSC5), for its activity in the process of translation. We observed a significant increase in the bypass of premature stop codons upon the deletion of YNR069C. Interestingly, the genomic arrangement of this ORF suggests a compatible mode of expression reliant on translational readthrough, incorporating the neighboring open reading frame. We also showed that the deletion of YNR069C results in an increase in the rate of translation. Based on our results, we propose that YNR069C may play a role in translation fidelity, impacting the overall quantity and quality of translation. Our genetic interaction analysis supports our hypothesis, associating the role of YNR069C to the regulation of protein synthesis.

Mutation at the Site of Hydroxylation in the Ribosomal Protein uL15 (RPL27a) Causes Specific Changes in the Repertoire of mRNAs Translated in Mammalian Cells

Ribosomal protein uL15 (RPL27a) carries a specific modification, hydroxylation, at the His39 residue, which neighbors the CCA terminus of the E-site-bound tRNA at the mammalian ribosome. Under hypoxia, the level of hydroxylation of this protein decreases. We transiently transfected HEK293T cells with constructs expressing wild-type uL15 or mutated uL15 (His39Ala) incapable of hydroxylation, and demonstrated that ribosomes containing both proteins are competent in translation. By applying RNA-seq to the total cellular and polysome-associated mRNAs, we identified differentially expressed genes (DEGs) in cells containing exogenous uL15 or its mutant form. Analyzing mRNA features of up- and down-regulated DEGs, we found an increase in the level of more abundant mRNAs and shorter CDSs in cells with uL15 mutant for both translated and total cellular mRNAs. The level of longer and rarer mRNAs, on the contrary, decreased. Our data show how ribosome heterogeneity can change the composition of the translatome and transcriptome, depending on the properties of the translated mRNAs.

Engineering and functional analysis of yeast with a monotypic 40S ribosome subunit

Ribosomes are not monolithic but dynamic machines composed of heterogeneous ribosomal protein (RP) paralogs with elusive functions. Isolation and characterization of monotypic ribosomes with homogeneous RP paralog compositions represent ideal approaches to understand the role of pervasive RP paralogs in customizing translation abilities but are largely hurdled by the complexity of the cellular ribosome pool (e.g., in Saccharomyces cerevisiae, 59 RP paralog pairs allow >10¹⁷ potential RP combinations). Here, we engineered a yeast with monotypic 40S ribosomes, including both defined and homogenous RP paralogs, and further functional studies revealed that duplicated RP paralogs impart robustness and phenotypic plasticity (such as paromomycin tolerance) through both gene dose amplification and paralog-specific regulation, paving a way for the study of monotypic ribosomes.

Emerging evidence reveals that ribosomes are not monolithic but dynamic machines with heterogeneous protein compositions that can reshape ribosomal translational abilities and cellular adaptation to environmental changes. Duplications of ribosomal protein (RP) genes are ubiquitous among organisms and are believed to affect cell function through paralog-specific regulation (e.g., by generating heterogeneous ribosomes) and/or gene dose amplification. However, direct evaluations of their impacts on cell function remain elusive due to the highly heterogeneous cellular RP pool. Here, we engineered a yeast with homogeneous 40S RP paralog compositions, designated homo-40S, by deleting the entire set of alternative duplicated genes encoding yeast 40S RP paralogs. Homo-40S displayed mild growth defects along with high sensitivity to the translation inhibitor paromomycin and a significantly increased stop codon readthrough. Moreover, doubling of the remaining RP paralogous genes in homo-40S rescued these phenotypes markedly, although not fully, compared to the wild-type phenotype, indicating that the dose of 40S RP genes together with the heterogeneity of the contents was vital for maintaining normal translational functionalities and growth robustness. Additional experiments revealed that homo-40S improved paromomycin tolerance via acquisition of bypass mutations or evolved to be diploid to generate fast-growing derivatives, highlighting the mutational robustness of engineered yeast to accommodate environmental and genetic changes. In summary, our work demonstrated that duplicated RP paralogs impart robustness and phenotypic plasticity through both gene dose amplification and paralog-specific regulation, paving the way for the direct study of ribosome biology through monotypic ribosomes with a homogeneous composition of specific RP paralogs.

An age-downregulated ribosomal RpS28 protein variant regulates the muscle proteome

Recent evidence indicates that the composition of the ribosome is heterogeneous and that multiple types of specialized ribosomes regulate the synthesis of specific protein subsets. In Drosophila, we find that expression of the ribosomal RpS28 protein variants RpS28a and RpS28-like preferentially occurs in the germline, a tissue resistant to aging and that it significantly declines in skeletal muscle during aging. Muscle-specific overexpression of RpS28a at levels similar to those seen in the germline decreases early mortality and promotes the synthesis of a subset of proteins with known anti-aging roles, some of which have preferential expression in the germline. These findings indicate a contribution of specialized ribosomal proteins to the regulation of the muscle proteome during aging.

New 24-Membered Macrolactins Isolated from Marine Bacteria Bacillus siamensis as Potent Fungal Inhibitors against Sugarcane Smut

Sugarcane smut, caused by Sporisorium scitamineum, is one of the most devastating fungal diseases affecting sugarcane worldwide. To develop a potent sugarcane smut fungicide, secondary metabolites of marine-derived Bacillus siamensis were isolated and screened for inhibitory activities, which led to the discovery of five new 24-membered macrolactins, bamemacrolactins A-E (1-5), with 3 being the most potent inhibitor. The antifungal mechanism of 3 was studied by assessing its effects on mycelial morphology and the cell wall. Differential proteomics were used to analyze proteins in S. scitamineum upon treatment with bamemacrolactin C and to elucidate its antifungal mechanism. A total of 533 differentially expressed proteins were found. After the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses, eight target proteins were selected, and their functions were discussed. Six of the eight proteins were reported as antifungal targets. The target proteins are involved in the oxidative phosphorylation pathway. Therefore, the potent inhibition of S. scitamineum by compound 3 is most likely through oxidative phosphorylation and targeting a series of enzymes.

Role of the 40S beak ribosomal protein eS12 in ribosome biogenesis and function in Saccharomyces cerevisiae

In eukaryotes, the beak structure of 40S subunits is formed by the protrusion of the 18S rRNA helix 33 and three ribosomal proteins: eS10, eS12 and eS31. The exact role of these proteins in ribosome biogenesis is not well understood. While eS10 is an essential protein encoded by two paralogous genes in Saccharomyces cerevisiae, eS12 and eS31 are not essential proteins encoded by the single-copy genes RPS12 and UBI3, respectively. Here, we have analysed the contribution of yeast eS12 to ribosome biogenesis and compared it with that of eS31. Polysome analysis reveals that deletion of either RPS12 or UBI3 results in equivalent 40S deficits. Analysis of pre-rRNA processing indicates that eS12, akin to eS31, is required for efficient processing of 20S pre-rRNA to mature 18S rRNA. Moreover, we show that the 20S pre-rRNA accumulates within cytoplasmic pre-40S particles, as deduced from FISH experiments and the lack of nuclear retention of 40S subunit reporter proteins, in rps12∆ and ubi3∆ cells. However, these particles containing 20S pre-rRNA are not efficiently incorporated into polyribosomes. We also provide evidence for a genetic interaction between eS12 or eS31 and the late-acting 40S assembly factors Enp1 and Ltv1, which appears not to be linked to the dynamics of their association with or release from pre-40S particles in the absence of either eS12 or eS31. Finally, we show that eS12- and eS31-deficient ribosomes exhibit increased levels of translational misreading. Altogether, our data highlight distinct important roles of the beak region during ribosome assembly and function.

Hydroxylation of protein constituents of the human translation system: structural aspects and functional assignments

During the current decade, data on the post-translational hydroxylation of specific amino acid residues of some ribosomal proteins and translation factors in both eukaryotes and eubacteria have accumulated. The reaction is catalyzed by dedicated oxygenases (so-called ribosomal oxygenases), whose action is impaired under hypoxia conditions. The modification occurs at amino acid residues directly involved in the formation of the main functional sites of ribosomes and factors. This review summarizes currently available data on the specific hydroxylation of protein constituents of eukaryotic and eubacterial translation systems with a special emphasis on the human system, as well as on the links between hypoxia impacts on the operation of ribosomal oxygenases, the functioning of the translational apparatus and human health problems.

Ribosomal Proteins Regulate MHC Class I Peptide Generation for Immunosurveillance

The MHC class I antigen presentation system enables T cell immunosurveillance of cancers and viruses. A substantial fraction of the immunopeptidome derives from rapidly degraded nascent polypeptides (DRiPs). By knocking down each of the 80 ribosomal proteins, we identified proteins that modulate peptide generation without altering source protein expression. We show that 60S ribosomal proteins L6 (RPL6) and RPL28, which are adjacent on the ribosome, play opposite roles in generating an influenza A virus-encoded peptide. Depleting RPL6 decreases ubiquitin-dependent peptide presentation, whereas depleting RPL28 increases ubiquitin-dependent and -independent peptide presentation. 40S ribosomal protein S28 (RPS28) knockdown increases total peptide supply in uninfected cells by increasing DRiP synthesis from non-canonical translation of "untranslated" regions and non-AUG start codons and sensitizes tumor cells for T cell targeting. Our findings raise the possibility of modulating immunosurveillance by pharmaceutical targeting ribosomes.

Codon-specific effects of tRNA anticodon loop modifications on translational misreading errors in the yeast Saccharomyces cerevisiae

Protein synthesis requires both high speed and accuracy to ensure a healthy cellular environment. Estimates of errors during protein synthesis in Saccharomyces cerevisiae have varied from 10-3 to 10-4 errors per codon. Here, we show that errors made by ${\rm{tRNA}}^{\rm Glu}_{\rm UUC}$ in yeast can vary 100-fold, from 10-6 to 10-4 errors per codon. The most frequent errors require a G•U mismatch at the second position for the near cognate codon GGA (Gly). We also show, contrary to our previous results, that yeast tRNAs can make errors involving mismatches at the wobble position but with low efficiency. We have also assessed the effect on misreading frequency of post-transcriptional modifications of tRNAs, which are known to regulate cognate codon decoding in yeast. We tested the roles of mcm5s2U34 and t6A37 and show that their effects depend on details of the codon anticodon interaction including the position of the modification with respect to the base mismatch and the nature of that mismatch. Both mcm5 and s2 modification of wobble uridine strongly stabilizes G2•U35 mismatches when ${\rm{tRNA}}^{\rm Glu}_{\rm UUC}$ misreads the GGA Gly codon but has weaker effects on other mismatches. By contrast, t6A37 destabilizes U1•U36 mismatches when ${\rm{tRNA}}^{\rm Lys}_{\rm UUU}$ misreads UAA or UAG but stabilizes mismatches at the second and wobble positions.

Identification of genes and pathways in the synovia of women with osteoarthritis by bioinformatics analysis

Osteoarthritis (OA) has a high prevalence in female patients and sex may be a key factor affecting the progression of OA. The aim of the present study was to identify genetic signatures in the synovial membranes of female patients with OA and to elucidate the potential associated molecular mechanisms. The gene expression profiles of the GSE55457 and GSE55584 datasets were obtained from the Gene Expression Omnibus database. Data of two synovial membranes from normal female individuals (GSM1337306 and GSM1337310) and two synovial membranes from female patients affected by OA (GSM1337327 and GSM1337330) were obtained from the dataset GSE55457, and those of three synovial membranes from female patients affected by OA (GSM1339628, GSM1339629 and GSM1339632) were obtained from the dataset GSE55584. Differentially expressed genes (DEGs) were identified by using Morpheus software. Protein-protein interaction (PPI) networks of the DEGs were constructed by using Cytoscape software. Subsequently, Gene Ontology (GO) function and Kyoto Encyclopaedia of Genes and Genomes (KEGG) pathway enrichment analyses of the top module of the PPI network were performed by using ClueGo. A total of 377 DEGs were identified in the synovial membranes of OA patients compared with those of normal individuals, including 164 upregulated and 213 downregulated genes. The top 10 hub genes were ubiquitin (UB)C, ribosomal protein (RP) L23A, mammalian target of rapamycin, heat shock protein 90 α family class A member 1, RPS28, RPL37A, RPS24, RPS4X, RPS18 and UBB. The results of the GO analysis indicated that the DEGs included in the top module of the PPI were mainly enriched in the terms ‘nuclear-transcribed mRNA catabolic process’, ‘nonsense mediated decay’, and ‘cytoplasmic translation and ribosomal small subunit biogenesis’. KEGG pathway analysis indicated that the DEGs included in the top one module were mainly enriched in the ‘ribosome’ pathway. The present study provides a systematic, molecular-level understanding of the degeneration of the synovial membrane in the progression of OA in female patients. The hub genes and molecules associated with the synovial membrane may be used as biomarkers and therapeutic targets for the treatment of OA in female patients with OA.

A Ribosomopathy Reveals Decoding Defective Ribosomes Driving Human Dysmorphism

Ribosomal protein (RP) gene mutations, mostly associated with inherited or acquired bone marrow failure, are believed to drive disease by slowing the rate of protein synthesis. Here de novo missense mutations in the RPS23 gene, which codes for uS12, are reported in two unrelated individuals with microcephaly, hearing loss, and overlapping dysmorphic features. One individual additionally presents with intellectual disability and autism spectrum disorder. The amino acid substitutions lie in two highly conserved loop regions of uS12 with known roles in maintaining the accuracy of mRNA codon translation. Primary cells revealed one substitution severely impaired OGFOD1-dependent hydroxylation of a neighboring proline residue resulting in 40S ribosomal subunits that were blocked from polysome formation. The other disrupted a predicted pi-pi stacking interaction between two phenylalanine residues leading to a destabilized uS12 that was poorly tolerated in 40S subunit biogenesis. Despite no evidence of a reduction in the rate of mRNA translation, these uS12 variants impaired the accuracy of mRNA translation and rendered cells highly sensitive to oxidative stress. These discoveries describe a ribosomopathy linked to uS12 and reveal mechanistic distinctions between RP gene mutations driving hematopoietic disease and those resulting in developmental disorders.

The eukaryote-specific N-terminal extension of ribosomal protein S31 contributes to the assembly and function of 40S ribosomal subunits

The archaea-/eukaryote-specific 40S-ribosomal-subunit protein S31 is expressed as an ubiquitin fusion protein in eukaryotes and consists of a conserved body and a eukaryote-specific N-terminal extension. In yeast, S31 is a practically essential protein, which is required for cytoplasmic 20S pre-rRNA maturation. Here, we have studied the role of the N-terminal extension of the yeast S31 protein. We show that deletion of this extension partially impairs cell growth and 40S subunit biogenesis and confers hypersensitivity to aminoglycoside antibiotics. Moreover, the extension harbours a nuclear localization signal that promotes active nuclear import of S31, which associates with pre-ribosomal particles in the nucleus. In the absence of the extension, truncated S31 inefficiently assembles into pre-40S particles and two subpopulations of mature small subunits, one lacking and another one containing truncated S31, can be identified. Plasmid-driven overexpression of truncated S31 partially suppresses the growth and ribosome biogenesis defects but, conversely, slightly enhances the hypersensitivity to aminoglycosides. Altogether, these results indicate that the N-terminal extension facilitates the assembly of S31 into pre-40S particles and contributes to the optimal translational activity of mature 40S subunits but has only a minor role in cytoplasmic cleavage of 20S pre-rRNA at site D.

Modulation of efficiency of translation termination in Saccharomyces cerevisiae

Nonsense suppression is a readthrough of premature termination codons. It typically occurs either due to the recognition of stop codons by tRNAs with mutant anticodons, or due to a decrease in the fidelity of translation termination. In the latter case, suppressors usually promote the readthrough of different types of nonsense codons and are thus called omnipotent nonsense suppressors. Omnipotent nonsense suppressors were identified in yeast Saccharomyces cerevisiae in 1960s, and most of subsequent studies were performed in this model organism. Initially, omnipotent suppressors were localized by genetic analysis to different protein- and RNA-encoding genes, mostly the components of translational machinery. Later, nonsense suppression was found to be caused not only by genomic mutations, but also by epigenetic elements, prions. Prions are self-perpetuating protein conformations usually manifested by infectious protein aggregates. Modulation of translational accuracy by prions reflects changes in the activity of their structural proteins involved in different aspects of protein synthesis. Overall, nonsense suppression can be seen as a "phenotypic mirror" of events affecting the accuracy of the translational machine. However, the range of proteins participating in the modulation of translation termination fidelity is not fully elucidated. Recently, the list has been expanded significantly by findings that revealed a number of weak genetic and epigenetic nonsense suppressors, the effect of which can be detected only in specific genetic backgrounds. This review summarizes the data on the nonsense suppressors decreasing the fidelity of translation termination in S. cerevisiae, and discusses the functional significance of the modulation of translational accuracy.

OGFOD1 catalyzes prolyl hydroxylation of RPS23 and is involved in translation control and stress granule formation

2-Oxoglutarate (2OG) and Fe(II)-dependent oxygenase domain-containing protein 1 (OGFOD1) is predicted to be a conserved 2OG oxygenase, the catalytic domain of which is related to hypoxia-inducible factor prolyl hydroxylases. OGFOD1 homologs in yeast are implicated in diverse cellular functions ranging from oxygen-dependent regulation of sterol response genes (Ofd1, Schizosaccharomyces pombe) to translation termination/mRNA polyadenylation (Tpa1p, Saccharomyces cerevisiae). However, neither the biochemical activity of OGFOD1 nor the identity of its substrate has been defined. Here we show that OGFOD1 is a prolyl hydroxylase that catalyzes the posttranslational hydroxylation of a highly conserved residue (Pro-62) in the small ribosomal protein S23 (RPS23). Unusually OGFOD1 retained a high affinity for, and forms a stable complex with, the hydroxylated RPS23 substrate. Knockdown or inactivation of OGFOD1 caused a cell type-dependent induction of stress granules, translational arrest, and growth impairment in a manner complemented by wild-type but not inactive OGFOD1. The work identifies a human prolyl hydroxylase with a role in translational regulation.

Ribosomal position and contacts of mRNA in eukaryotic translation initiation complexes

The position of mRNA on 40S ribosomal subunits in eukaryotic initiation complexes was determined by UV crosslinking using mRNAs containing uniquely positioned 4-thiouridines. Crosslinking of mRNA positions (+)11 to ribosomal protein (rp) rpS2(S5p) and rpS3(S3p), and (+)9-(+)11 and (+)8-(+)9 to h18 and h34 of 18S rRNA, respectively, indicated that mRNA enters the mRNA-binding channel through the same layers of rRNA and proteins as in prokaryotes. Upstream of the P-site, the proximity of positions (-)3/(-)4 to rpS5(S7p) and h23b, (-)6/(-)7 to rpS14(S11p), and (-)8-(-)11 to the 3'-terminus of 18S rRNA (mRNA/rRNA elements forming the bacterial Shine-Dalgarno duplex) also resembles elements of the bacterial mRNA path. In addition to these striking parallels, differences between mRNA paths included the proximity in eukaryotic initiation complexes of positions (+)7/(+)8 to the central region of h28, (+)4/(+)5 to rpS15(S19p), and (-)6 and (-)7/(-)10 to eukaryote-specific rpS26 and rpS28, respectively. Moreover, we previously determined that eukaryotic initiation factor2alpha (eIF2alpha) contacts position (-)3, and now report that eIF3 interacts with positions (-)8-(-)17, forming an extension of the mRNA-binding channel that likely contributes to unique aspects of eukaryotic initiation.

Polypeptide chain termination and stop codon readthrough on eukaryotic ribosomes

During protein translation, a variety of quality control checks ensure that the resulting polypeptides deviate minimally from their genetic encoding template. Translational fidelity is central in order to preserve the function and integrity of each cell. Correct termination is an important aspect of translational fidelity, and a multitude of mechanisms and players participate in this exquisitely regulated process. This review explores our current understanding of eukaryotic termination by highlighting the roles of the different ribosomal components as well as termination factors and ribosome-associated proteins, such as chaperones.

Biochemical evidence of translational infidelity and decreased peptidyltransferase activity by a sarcin/ricin domain mutation of yeast 25S rRNA

A C-->U mutation (rdn5) in the conserved sarcin/ricin domain of yeast 25S rRNA has been shown to cause translational suppression and paromomycin resistance. It also separates the killing from the misreading effect of this antibiotic. We confirm these findings and provide in vitro evidence that rdn5 causes a 3-fold increase in translational errors and resistance to paromomycin. The role of this 25S rRNA domain in ribosome's decoding function was further demonstrated when 60S subunits from rdn5 cells were combined with 40S subunits from cells carrying an error-prone mutation in the eukaryotic accuracy center ribosomal protein S23, an homologue of Escherichia coli S12. These hybrids exhibited an error frequency similar to that of rdn5 alone, despite the error-prone mutation in S23. This was accompanied by extreme resistance to paromomycin, unlike the effects of the individual mutations. Furthermore, rdn5 lowers peptidyltransferase activity measured as a second-order rate constant (kcat/K(s)) corresponding to the rate of peptide bond formation. This mutation was also found to affect translocation. Elongation factor 2 (EF2)-dependent translocation of Ac-Phe-tRNA from the A- to P-site was achieved at an EF2 concentration 3.5 times lower than in wild type. In conclusion, the sarcin/ricin domain of 25S rRNA influences decoding, peptide bond formation and translocation.

Translational suppressors and antisuppressors alter the efficiency of the Ty1 programmed translational frameshift

Certain viruses, transposons, and cellular genes have evolved specific sequences that induce high levels of specific translational errors. Such "programmed misreading" can result in levels of frameshifting or nonsense codon readthrough that are up to 1,000-fold higher than normal. Here we determine how a number of mutations in yeast affect the programmed misreading used by the yeast Ty retrotransposons. These mutations have previously been shown to affect the general accuracy of translational termination. We find that among four nonsense suppressor ribosomal mutations tested, one (a ribosomal protein mutation) enhanced the efficiency of the Tyl frameshifting, another (an rRNA mutation) reduced frameshifting, and two others (another ribosomal protein mutation and another rRNA mutation) had no effect. Three antisuppressor rRNA mutations all reduced Tyl frameshifting; however the antisuppressor mutation in the ribosomal protein did not show any effect. Among nonribosomal mutations, the allosuppressor protein phosphatase mutation enhanced Tyl frameshifting, whereas the partially inactive prion form of the release factor eRF3 caused a slight decrease, if any effect. A mutant form of the other release factor, eRF1, also had no effect on frameshifting. Our data suggest that Ty frameshifting is under the control of the cellular translational machinery. Surprisingly we find that translational suppressors can affect Ty frameshifting in either direction, whereas antisuppressors have either no effect or cause a decrease.

Modelling in Escherichia coli of mutations in mitoribosomal protein S12: novel mutant phenotypes of rpsL

The rpsL gene of Escherichia coli encodes the highly conserved rps12 protein of the ribosomal accuracy centre. We have used the E. coli gene to model the phenotypic effects of specific substitutions found in the mitochondrial gene for rps12. Variants created by in vitro mutagenesis were tested in two different plasmid vector systems, in both streptomycin-sensitive and streptomycin-resistant hosts. A substitution with respect to eubacterial rps12 (K87-->Q), found in all metazoan and fungal mitochondrial orthologues thus far studied, is associated with low-level resistance to streptomycin and a modest (15%) drop in translational elongation rate, but without significant effects on translational accuracy. An amino-acid replacement at a highly conserved leucine residue (L56-->H), associated with the phenotype of sensitivity to mechanical vibration and hemizygous female lethality in Drosophila, creates a functionally inactive but structurally stable protein that is not assembled into ribosomes. The presence in the cell of the mutant, but not wild-type, rpsL greatly downregulates the level of a prominent polypeptide of approximately 50 kDa. These results indicate novel structure-function relationships in rps12 genes affecting translational function, ribosome assembly and drug sensitivity, and indicate a novel regulatory pathway that may influence ribosome biogenesis.

Mutations in yeast ribosomal proteins S28 and S4 affect the accuracy of translation and alter the sensitivity of the ribosomes to paromomycin

Ribosomal proteins S12, S5 and S4 of Escherichia coli are essential for the control of translational accuracy. Their yeast equivalents, i.e., S28, S4 and S13, have also been implicated in this process. Using a poly(U)-dependent cell-free translation system, we determined the accuracy of translation and the sensitivity to antibiotic paromomycin of yeast ribosomes carrying mutant ribosomal proteins S28 and/or S4. Our results confirm by quantitative biochemical methods previous genetic data showing that proteins S28 and S4 are involved in the decoding activity of the ribosome and interact to control translational accuracy. We find that the suppressor mutation SUP44 in yeast S4, decreased the accuracy of translation. To examine the effect of mutant S28, we disrupted RPS28B and introduced in RPS28A the same substitutions that cause hyperaccurate translation or antibiotic resistance in bacteria. Three of these substitutions (Lys-62-->Asn, Thr or Gln) similarly increased translational accuracy in vitro or antibiotic resistance. In the presence of the SUP44 mutation, these substitutions partially reversed the decrease of translational accuracy caused by SUP44. However, the Lys-62-->Arg substitution decreased translational accuracy and caused antibiotic sensitivity both in nonsuppressor and in SUP44 haploids. These results establish the role of Lys-62 of S28 in optimizing translational accuracy and provide a more precise view of the functional role of two important ribosomal proteins.

The accuracy center of a eukaryotic ribosome

Mutations in yeast ribosomal proteins and ribosomal RNAs have been shown to affect translational fidelity. These mutations include: proteins homologous to Escherichia coli's S4, S5, and S12; a eukaryote specific ribosomal protein; yeast ribosomal rRNA alterations at positions corresponding to 517, 912, and 1054 in 16S E. coli rRNA and to 2658 in the sarcin-ricin domain of 23S E. coli rRNA. Overall there appears to be a remarkable conservation of the accuracy center throughout evolution.