Distinct stages of the translation elongation cycle revealed by sequencing ribosome-protected mRNA fragments

Non-AUG translation: a new start for protein synthesis in eukaryotes

This review by Kearse and Wilusz discusses the profound impact of non-AUG start codons in eukaryotic translation. It describes how misregulation of non-AUG initiation events contributes to multiple human diseases, including cancer and neurodegeneration, and how modulation of non-AUG usage may represent a novel therapeutic strategy.

Although it was long thought that eukaryotic translation almost always initiates at an AUG start codon, recent advancements in ribosome footprint mapping have revealed that non-AUG start codons are used at an astonishing frequency. These non-AUG initiation events are not simply errors but instead are used to generate or regulate proteins with key cellular functions; for example, during development or stress. Misregulation of non-AUG initiation events contributes to multiple human diseases, including cancer and neurodegeneration, and modulation of non-AUG usage may represent a novel therapeutic strategy. It is thus becoming increasingly clear that start codon selection is regulated by many trans-acting initiation factors as well as sequence/structural elements within messenger RNAs and that non-AUG translation has a profound impact on cellular states.

Codon usage regulates protein structure and function by affecting translation elongation speed in Drosophila cells

Codon usage biases are found in all eukaryotic and prokaryotic genomes and have been proposed to regulate different aspects of translation process. Codon optimality has been shown to regulate translation elongation speed in fungal systems, but its effect on translation elongation speed in animal systems is not clear. In this study, we used a Drosophila cell-free translation system to directly compare the velocity of mRNA translation elongation. Our results demonstrate that optimal synonymous codons speed up translation elongation while non-optimal codons slow down translation. In addition, codon usage regulates ribosome movement and stalling on mRNA during translation. Finally, we show that codon usage affects protein structure and function in vitro and in Drosophila cells. Together, these results suggest that the effect of codon usage on translation elongation speed is a conserved mechanism from fungi to animals that can affect protein folding in eukaryotic organisms.

Codon optimality, bias and usage in translation and mRNA decay

The advent of ribosome profiling and other tools to probe mRNA translation has revealed that codon bias - the uneven use of synonymous codons in the transcriptome - serves as a secondary genetic code: a code that guides the efficiency of protein production, the fidelity of translation and the metabolism of mRNAs. Recent advancements in our understanding of mRNA decay have revealed a tight coupling between ribosome dynamics and the stability of mRNA transcripts; this coupling integrates codon bias into the concept of codon optimality, or the effects that specific codons and tRNA concentrations have on the efficiency and fidelity of the translation machinery. In this Review, we first discuss the evidence for codon-dependent effects on translation, beginning with the basic mechanisms through which translation perturbation can affect translation efficiency, protein folding and transcript stability. We then discuss how codon effects are leveraged by the cell to tailor the proteome to maintain homeostasis, execute specific gene expression programmes of growth or differentiation and optimize the efficiency of protein production.

Regulated Ire1-dependent mRNA decay requires no-go mRNA degradation to maintain endoplasmic reticulum homeostasis in S. pombe

The unfolded protein response (UPR) monitors and adjusts the protein folding capacity of the endoplasmic reticulum (ER). In S. pombe, the ER membrane-resident kinase/endoribonuclease Ire1 utilizes a mechanism of selective degradation of ER-bound mRNAs (RIDD) to maintain homeostasis. We used a genetic screen to identify factors critical to the Ire1-mediated UPR and found several proteins, Dom34, Hbs1 and Ski complex subunits, previously implicated in ribosome rescue and mRNA no-go-decay (NGD). Ribosome profiling in ER-stressed cells lacking these factors revealed that Ire1-mediated cleavage of ER-associated mRNAs results in ribosome stalling and mRNA degradation. Stalled ribosomes iteratively served as a ruler to template precise, regularly spaced upstream mRNA cleavage events. This clear signature uncovered hundreds of novel target mRNAs. Our results reveal that the UPR in S. pombe executes RIDD in an intricate interplay between Ire1, translation, and the NGD pathway, and establish a critical role for NGD in maintaining ER homeostasis.

Most proteins need to fold into a specific shape in order to work properly. As such, cells have developed a number of ways to sense and respond to stressful conditions that cause their proteins to fold incorrectly. One place this happens is within a network of tubes inside the cell called the endoplasmic reticulum; this is where proteins that are destined for the cell surface or other compartments in the cell become folded. The endoplasmic reticulum has a limited capacity to fold proteins. When it is overwhelmed, the cell temporarily stops making the proteins that use up this capacity. This action makes up part of a larger set of responses collectively referred to as the “unfolded protein response”.

During the unfolded protein response, the production of some proteins is turned off when an enzyme called Ire1 cuts the transcript molecules that contain the instructions to build these proteins. Cutting these transcripts, however, creates a problem: it interrupts the translation of the transcript by the ribosome, the molecular machine that reads the genetic code to build proteins. Usually, a ribosome only comes off of a transcript when it arrives at a specific stop signal. Yet, ribosomes that run to the ends of broken transcripts never reach this signal and instead have to be rescued. If left without rescue, these stalled ribosomes could never be used again for translation of other transcripts, and the cell would lose the ability to make more proteins.

Guydosh, Kimmig et al. searched for new genes in the yeast Schizosaccharomyces pombe that are involved in the part of the unfolded protein response that occurs after the actions of the Ire1 enzyme. This search revealed that cells missing so-called ribosome rescue proteins (namely Dom34 and Hbs1) grow slowly under conditions that cause proteins to fold incorrectly. Guydosh, Kimmig et al. then looked to see where on the transcripts the ribosomes stall and remain un-rescued in the absence of these ribosome rescue proteins. These sites corresponded to places that were cut by Ire1, the majority of which were previously unknown. Together these findings indicate that ribosome rescue is a key part of the unfolded protein response in S. pombe because it removes ribosomes that stall at the broken ends of transcript molecules cut by the Ire1 enzyme.

An efficient and well-controlled response to conditions that cause proteins to fold incorrectly is important for human health. Loss of this control can lead to disorders as diverse as atherosclerosis, cancer and neurological diseases. By revealing that the unfolded protein response uses the ribosome rescue pathway, the findings improve our understanding of these health conditions and may open the door to new research and treatments.

Beyond Read-Counts: Ribo-seq Data Analysis to Understand the Functions of the Transcriptome

By mapping the positions of millions of translating ribosomes in the cell, ribosome profiling (Ribo-seq) has established its role as a powerful tool to study gene expression. Several laboratories have introduced modifications to the experimental protocol and expanded the repertoire of biochemical methods to study translation transcriptome-wide. However, the diversity of protocols highlights a need for standardization. At the same time, different computational analysis strategies have used Ribo-seq data to identify the set of translated sequences with high confidence. In this review we present an overview of such methodologies, outlining their assumptions, data requirements, and availability. At the interface between RNA and proteins, Ribo-seq can complement data from multiple omics approaches, zooming in on the central role of translation in the molecular cell.

Effects of cycloheximide on the interpretation of ribosome profiling experiments in Schizosaccharomyces pombe

Stress conditions lead to global and gene-specific changes in RNA translation. Ribosome profiling experiments have identified genome-wide alterations in the distribution of ribosomes along mRNAs. However, it is contentious whether these changes reflect real responses, or whether they are artefacts caused by the use of inhibitors of translation (notably cycloheximide). To address this issue we performed ribosome profiling with the fission yeast Schizosaccharomyces pombe under conditions of exponential growth (unstressed) and nitrogen starvation (nutritional stress), and both in the presence and absence of cycloheximide. We examined several aspects of the translational response, including density of ribosomal footprints on coding sequences, 5′ leader ribosomal densities, distribution of ribosomes along coding sequences, and ribosome codon occupancies. Cycloheximide had minor effects on overall ribosome density, which affected mostly mRNAs encoding ribosomal proteins. Nitrogen starvation caused an accumulation of ribosomes on 5′ leaders in both cycloheximide-treated and untreated cells. By contrast, stress-induced ribosome accumulation on the 5′ side of coding sequences was cycloheximide-dependent. Finally, codon occupancy showed strong positive correlations in cycloheximide-treated and untreated cells. Our results demonstrate that cycloheximide does influence some of the results of ribosome profiling experiments, although it is not clear if this effect is always artefactual.

Mining for Micropeptides

Advances in computational biology and large-scale transcriptome analyses have revealed that a much larger portion of the genome is transcribed than was previously recognized, resulting in the production of a diverse population of RNA molecules with both protein-coding and noncoding potential. Emerging evidence indicates that several RNA molecules have been mis-annotated as noncoding and in fact harbor short open reading frames (sORFs) that encode functional peptides and that have evaded detection until now due to their small size. sORF-encoded peptides (SEPs), or micropeptides, have been shown to have important roles in fundamental biological processes and in the maintenance of cellular homeostasis. These small proteins can act independently, for example as ligands or signaling molecules, or they can exert their biological functions by engaging with and modulating larger regulatory proteins. Given their small size, micropeptides may be uniquely suited to fine-tune complex biological systems.

Transcriptome-wide measurement of translation by ribosome profiling

Translation is one of the fundamental processes of life. It comprises the assembly of polypeptides whose amino acid sequence corresponds to the codon sequence of an mRNA's ORF. Translation is performed by the ribosome; therefore, in order to understand translation and its regulation we must be able to determine the numbers and locations of ribosomes on mRNAs in vivo. Furthermore, we must be able to examine their redistribution in different physiological contexts and in response to experimental manipulations. The ribosome profiling method provides us with an opportunity to learn these locations, by sequencing a cDNA library derived from the short fragments of mRNA covered by the ribosome. Since its original description, the ribosome profiling method has undergone continuing development; in this article we describe the method's current state. Important improvements include: the incorporation of sample barcodes to enable library multiplexing, the incorporation of unique molecular identifiers to enable to removal of duplicated sequences, and the replacement of a gel-purification step with the enzymatic degradation of unligated linker.

Ubiquitination of stalled ribosome triggers ribosome-associated quality control

Translation arrest by polybasic sequences induces ribosome stalling, and the arrest product is degraded by the ribosome-mediated quality control (RQC) system. Here we report that ubiquitination of the 40S ribosomal protein uS10 by the E3 ubiquitin ligase Hel2 (or RQT1) is required for RQC. We identify a RQC-trigger (RQT) subcomplex composed of the RNA helicase-family protein Slh1/Rqt2, the ubiquitin-binding protein Cue3/Rqt3, and yKR023W/Rqt4 that is required for RQC. The defects in RQC of the RQT mutants correlate with sensitivity to anisomycin, which stalls ribosome at the rotated form. Cryo-electron microscopy analysis reveals that Hel2-bound ribosome are dominantly the rotated form with hybrid tRNAs. Ribosome profiling reveals that ribosomes stalled at the rotated state with specific pairs of codons at P-A sites serve as RQC substrates. Rqt1 specifically ubiquitinates these arrested ribosomes to target them to the RQT complex, allowing subsequent RQC reactions including dissociation of the stalled ribosome into subunits.Several protein quality control mechanisms are in place to trigger the rapid degradation of aberrant polypeptides and mRNAs. Here the authors describe a mechanism of ribosome-mediated quality control that involves the ubiquitination of ribosomal proteins by the E3 ubiquitin ligase Hel2/RQT1.

Prediction of ribosome footprint profile shapes from transcript sequences

MOTIVATION

Ribosome profiling is a useful technique for studying translational dynamics and quantifying protein synthesis. Applications of this technique have shown that ribosomes are not uniformly distributed along mRNA transcripts. Understanding how each transcript-specific distribution arises is important for unraveling the translation mechanism.

RESULTS

Here, we apply kernel smoothing to construct predictive features and build a sparse model to predict the shape of ribosome footprint profiles from transcript sequences alone. Our results on Saccharomyces cerevisiae data show that the marginal ribosome densities can be predicted with high accuracy. The proposed novel method has a wide range of applications, including inferring isoform-specific ribosome footprints, designing transcripts with fast translation speeds and discovering unknown modulation during translation.

AVAILABILITY AND IMPLEMENTATION

A software package called riboShape is freely available at https://sourceforge.net/projects/riboshape

CONTACT

yss@berkeley.edu.

Ribosomal Stalling During Translation: Providing Substrates for Ribosome-Associated Protein Quality Control

Cells of all organisms survey problems during translation elongation, which may happen as a consequence of mRNA aberrations, inefficient decoding, or other sources. In eukaryotes, ribosome-associated quality control (RQC) senses elongation-stalled ribosomes and promotes dissociation of ribosomal subunits. This so-called ribosomal rescue releases the mRNA for degradation and allows 40S subunits to be recycled for new rounds of translation. However, the nascent polypeptide chains remain linked to tRNA and associated with the rescued 60S subunits. As a final critical step in this pathway, the Ltn1/Listerin E3 ligase subunit of the RQC complex (RQCc) ubiquitylates the nascent chain, which promotes clearance of the 60S subunit while simultaneously marking the nascent chain for elimination. Here we review the ribosomal stalling and rescue steps upstream of the RQCc, where one witnesses intersection with cellular machineries implicated in translation elongation, translation termination, ribosomal subunit recycling, and mRNA quality control. We emphasize both recent progress and future directions in this area, as well as examples linking ribosomal rescue with the production of Ltn1-RQCc substrates.

eIF5A facilitates translation termination globally and promotes the elongation of many non polyproline-specific tripeptide sequences

eIF5A is an essential protein involved in protein synthesis, cell proliferation and animal development. High eIF5A expression is observed in many tumor types and has been linked to cancer metastasis. Recent studies have shown that eIF5A facilitates the translation elongation of stretches of consecutive prolines. Activated eIF5A binds to the empty E-site of stalled ribosomes, where it is thought to interact with the peptidyl-tRNA situated at the P-site. Here, we report a genome-wide analysis of ribosome stalling in Saccharomyces cerevisiae eIF5A depleted cells using 5Pseq. We confirm that, in the absence of eIF5A, ribosomes stall at proline stretches, and extend previous studies by identifying eIF5A-dependent ribosome pauses at termination and at >200 tripeptide motifs. We show that presence of proline, glycine and charged amino acids at the peptidyl transferase center and at the beginning of the peptide exit tunnel arrest ribosomes in eIF5A-depleted cells. Lack of eIF5A also renders ribosome accumulation at the stop codons. Our data indicate specific protein functional groups under the control of eIF5A, including ER-coupled translation and GTPases in yeast and cytoskeleton organization, collagen metabolism and cell differentiation in humans. Our results support a broad mRNA-specific role of eIF5A in translation and identify the conserved motifs that affect translation elongation from yeast to humans.

Accurate Recovery of Ribosome Positions Reveals Slow Translation of Wobble-Pairing Codons in Yeast

Ribosome profiling quantitatively captures ribosome locations during translation. The resulting profiles of ribosome locations are widely used to study translational speed. However, an accurate estimation of the ribosome location depends on identifying the A-site from ribosome profiling reads, a problem that was previously unsolved. Here, we propose a novel method to estimate the ribosome A-site positions from high-coverage ribosome profiling reads. Our model allows more reads to be used, accurately explains the 3-nt periodicity of ribosome profiling reads from various lengths, and recovers consistent ribosome positions across different lengths. Our recovered ribosome positions are correctly highly skewed toward a single frame within a codon. They retain subcodon resolution and enable detection of off-frame translational events, such as frameshifts. Our method improves the correlation with other estimates of codon decoding time. Furthermore, the refined profiles show that yeast wobble-pairing codons are translated slower than their synonymous Watson-Crick-pairing codons. These results provide evidence that protein synthetic rate can be tuned by codon usage bias.

The Growing Toolbox for Protein Synthesis Studies

Protein synthesis stands at the last stage of the central dogma of molecular biology, providing a final regulatory layer for gene expression. Reacting to environmental cues and internal signals, the translation machinery can quickly tune the translatome from a pre-existing pool of RNAs, before the transcriptome changes. Although the translation reaction itself has been known since the 1950s, the quantitative or even qualitative measurement of its efficacy in cells has posed experimental and analytic hurdles. In this review, we outline the array of state-of-the-art methods that have emerged to tackle the hidden aspects of translational control.

Protein charge distribution in proteomes and its impact on translation

As proteins are synthesized, the nascent polypeptide must pass through a negatively charged exit tunnel. During this stage, positively charged stretches can interact with the ribosome walls and slow the translation. Therefore, charged polypeptides may be important factors that affect protein expression. To determine the frequency and distribution of positively and negatively charged stretches in different proteomes, the net charge was calculated for every 30 consecutive amino acid residues, which corresponds to the length of the ribosome exit tunnel. The following annotated and reviewed proteins in the UniProt database (Swiss-Prot) were analyzed: 551,705 proteins from different organisms and a total of 180 million protein segments. We observed that there were more negative than positive stretches and that super-charged positive sequences (i.e., net charges ≥ 14) were underrepresented in the proteomes. Overall, the proteins were more positively charged at their N-termini and C-termini, and this feature was present in most organisms and subcellular localizations. To investigate whether the N-terminal charges affect the elongation rates, previously published ribosomal profiling data obtained from S. cerevisiae, without translation-interfering drugs, were analyzed. We observed a nonlinear effect of the charge on the ribosome occupancy in which values ≥ +5 and ≤ -6 showed increased and reduced ribosome densities, respectively. These groups also showed different distributions across 80S monosomes and polysomes. Basic polypeptides are more common within short proteins that are translated by monosomes, whereas negative stretches are more abundant in polysome-translated proteins. These findings suggest that the nascent peptide charge impacts translation and can be one of the factors that regulate translation efficiency and protein expression.

Which factors shape the sequence of amino acids that will form a protein? The biochemical features of amino acids, such as their charge and hydrophobicity, are important drivers of protein tridimensional folding, which creates interaction sites for binding other molecules and directs proteins to specific cellular compartments. These features all impact the activity of the proteins after they are produced. Another less obvious factor that influences the protein’s primary structure may be how efficiently a given amino acid sequence is produced by the ribosome. It is known that a repetitive stretch of positively charged amino acids may interact with the negative charges in the ribosome exit tunnel, slowing, or even halting, translation. By analyzing the charge of protein stretches in different organisms, we observed that proteins tend to present positively charged stretches at their extremities, and high charge values can slow (for positive charges) or speed (for negative charges) translation. An interesting consequence of this trend is that proteins that are translated in high quantities by several ribosomes at the same RNA (polysomes) tend to have more negatively charged stretches than proteins that are translated by a single ribosome per RNA (monosomes).

Mining for Micropeptides

Advances in computational biology and large-scale transcriptome analyses have revealed that a much larger portion of the genome is transcribed than was previously recognized, resulting in the production of a diverse population of RNA molecules with both protein-coding and noncoding potential. Emerging evidence indicates that several RNA molecules have been mis-annotated as noncoding and in fact harbor short open reading frames (sORFs) that encode functional peptides and that have evaded detection until now due to their small size. sORF-encoded peptides (SEPs), or micropeptides, have been shown to have important roles in fundamental biological processes and in the maintenance of cellular homeostasis. These small proteins can act independently, for example as ligands or signaling molecules, or they can exert their biological functions by engaging with and modulating larger regulatory proteins. Given their small size, micropeptides may be uniquely suited to fine-tune complex biological systems.

Alteration of protein function by a silent polymorphism linked to tRNA abundance

Synonymous single nucleotide polymorphisms (sSNPs) are considered neutral for protein function, as by definition they exchange only codons, not amino acids. We identified an sSNP that modifies the local translation speed of the cystic fibrosis transmembrane conductance regulator (CFTR), leading to detrimental changes to protein stability and function. This sSNP introduces a codon pairing to a low-abundance tRNA that is particularly rare in human bronchial epithelia, but not in other human tissues, suggesting tissue-specific effects of this sSNP. Up-regulation of the tRNA cognate to the mutated codon counteracts the effects of the sSNP and rescues protein conformation and function. Our results highlight the wide-ranging impact of sSNPs, which invert the programmed local speed of mRNA translation and provide direct evidence for the central role of cellular tRNA levels in mediating the actions of sSNPs in a tissue-specific manner.

Synonymous single nucleotide polymorphisms (sSNPs) occur at high frequency in the human genome and are associated with ~50 diseases in humans; the responsible molecular mechanisms remain enigmatic. Here, we investigate the impact of the common sSNP, T2562G, on cystic fibrosis transmembrane conductance regulator (CFTR). Although this sSNP, by itself, does not cause cystic fibrosis (CF), it is prevalent in patients with CFTR-related disorders. T2562G sSNP modifies the local translation speed at the Thr854 codon, leading to changes in CFTR stability and channel function. This sSNP introduces a codon pairing to a low-abundance tRNA, which is particularly rare in human bronchial epithelia, but not in other human tissues, suggesting a tissue-specific effect of this sSNP. Enhancement of the cellular concentration of the tRNA cognate to the mutant ACG codon rescues the stability and conduction defects of T2562G-CFTR. These findings reveal an unanticipated mechanism—inverting the programmed local speed of mRNA translation in a tRNA-dependent manner—for sSNP-associated diseases.

Kinetic modeling predicts a stimulatory role for ribosome collisions at elongation stall sites in bacteria

Ribosome stalling on mRNAs can decrease protein expression. To decipher ribosome kinetics at stall sites, we induced ribosome stalling at specific codons by starving the bacterium Escherichia coli for the cognate amino acid. We measured protein synthesis rates from a reporter library of over 100 variants that encoded systematic perturbations of translation initiation rate, the number of stall sites, and the distance between stall sites. Our measurements are quantitatively inconsistent with two widely-used kinetic models for stalled ribosomes: ribosome traffic jams that block initiation, and abortive (premature) termination of stalled ribosomes. Rather, our measurements support a model in which collision with a trailing ribosome causes abortive termination of the stalled ribosome. In our computational analysis, ribosome collisions selectively stimulate abortive termination without fine-tuning of kinetic rate parameters at ribosome stall sites. We propose that ribosome collisions serve as a robust timer for translational quality control pathways to recognize stalled ribosomes.

Beyond the Triplet Code: Context Cues Transform Translation

The elucidation of the genetic code remains among the most influential discoveries in biology. While innumerable studies have validated the general universality of the code and its value in predicting and analyzing protein coding sequences, established and emerging work has also suggested that full genome decryption may benefit from a greater consideration of a codon's neighborhood within an mRNA than has been broadly applied. This Review examines the evidence for context cues in translation, with a focus on several recent studies that reveal broad roles for mRNA context in programming translation start sites, the rate of translation elongation, and stop codon identity.

Ribosome profiling and dynamic regulation of translation in mammals

Protein synthesis is an energy-demanding cellular process. Consequently, a well-timed, fine-tuned and plastic regulation of translation is needed to adjust and maintain cell states under dynamically changing environments. Genome-wide monitoring of translation was recently facilitated by ribosome profiling, which uncovered key features of translation regulation. In this review, we summarize recent ribosome profiling studies in mammals providing novel insight in dynamic translation regulation, notably related to circadian rhythms, diurnal feeding/fasting cycles, cell cycle progression, stress responses, and tRNA landscapes. In particular, recent results show that regulating translation initiation and elongation represent important mechanisms used in mammalian cells to rapidly modulate protein expression in dynamically changing environments.

Translation of CircRNAs

Circular RNAs (circRNAs) are abundant and evolutionarily conserved RNAs of largely unknown function. Here, we show that a subset of circRNAs is translated in vivo. By performing ribosome footprinting from fly heads, we demonstrate that a group of circRNAs is associated with translating ribosomes. Many of these ribo-circRNAs use the start codon of the hosting mRNA, are bound by membrane-associated ribosomes, and have evolutionarily conserved termination codons. In addition, we found that a circRNA generated from the muscleblind locus encodes a protein, which we detected in fly head extracts by mass spectrometry. Next, by performing in vivo and in vitro translation assays, we show that UTRs of ribo-circRNAs (cUTRs) allow cap-independent translation. Moreover, we found that starvation and FOXO likely regulate the translation of a circMbl isoform. Altogether, our study provides strong evidence for translation of circRNAs, revealing the existence of an unexplored layer of gene activity.

Selective stalling of human translation through small-molecule engagement of the ribosome nascent chain

Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a key role in regulating the levels of plasma low-density lipoprotein cholesterol (LDL-C). Here, we demonstrate that the compound PF-06446846 inhibits translation of PCSK9 by inducing the ribosome to stall around codon 34, mediated by the sequence of the nascent chain within the exit tunnel. We further show that PF-06446846 reduces plasma PCSK9 and total cholesterol levels in rats following oral dosing. Using ribosome profiling, we demonstrate that PF-06446846 is highly selective for the inhibition of PCSK9 translation. The mechanism of action employed by PF-06446846 reveals a previously unexpected tunability of the human ribosome that allows small molecules to specifically block translation of individual transcripts.

Many disease-mediating proteins have proven difficult to target with traditional small-molecule pharmaceuticals. In this paper, we report that a small molecule, PF-06446846, directly inhibits translation of one such protein, proprotein convertase subtilisin/kexin type 9 (PCSK9), by acting on the translating human ribosome. PF-06446846 causes the translating ribosome to stall soon after translating the PCSK9 signal sequence. We further show that PF-06446846 activity is dependent on the amino acid sequence of the nascent chain inside the ribosome exit tunnel. In a rat safety study, we observe decreases in plasma PCSK9, total cholesterol, and low-density lipoprotein (LDL) cholesterol. Using mass spectrometry in cell culture and ribosome profiling, we demonstrate that despite acting on the ribosome, which synthesizes every protein in the cell, PF-06446846 displays a high level of selectivity for PCSK9. This unexpected potential for small molecules to selectively inhibit the human ribosome opens the possibility for future development of small molecules targeting disease-mediating proteins that were previously thought to be undruggable.

Synonymous Codons: Choose Wisely for Expression

The genetic code, which defines the amino acid sequence of a protein, also contains information that influences the rate and efficiency of translation. Neither the mechanisms nor functions of codon-mediated regulation were well understood. The prevailing model was that the slow translation of codons decoded by rare tRNAs reduces efficiency. Recent genome-wide analyses have clarified several issues. Specific codons and codon combinations modulate ribosome speed and facilitate protein folding. However, tRNA availability is not the sole determinant of rate; rather, interactions between adjacent codons and wobble base pairing are key. One mechanism linking translation efficiency and codon use is that slower decoding is coupled to reduced mRNA stability. Changes in tRNA supply mediate biological regulationfor instance,, changes in tRNA amounts facilitate cancer metastasis.

Differentiation of ncRNAs from small mRNAs in Escherichia coli O157:H7 EDL933 (EHEC) by combined RNAseq and RIBOseq - ryhB encodes the regulatory RNA RyhB and a peptide, RyhP

Background

While NGS allows rapid global detection of transcripts, it remains difficult to distinguish ncRNAs from short mRNAs. To detect potentially translated RNAs, we developed an improved protocol for bacterial ribosomal footprinting (RIBOseq). This allowed distinguishing ncRNA from mRNA in EHEC. A high ratio of ribosomal footprints per transcript (ribosomal coverage value, RCV) is expected to indicate a translated RNA, while a low RCV should point to a non-translated RNA.

Results

Based on their low RCV, 150 novel non-translated EHEC transcripts were identified as putative ncRNAs, representing both antisense and intergenic transcripts, 74 of which had expressed homologs in E. coli MG1655. Bioinformatics analysis predicted statistically significant target regulons for 15 of the intergenic transcripts; experimental analysis revealed 4-fold or higher differential expression of 46 novel ncRNA in different growth media. Out of 329 annotated EHEC ncRNAs, 52 showed an RCV similar to protein-coding genes, of those, 16 had RIBOseq patterns matching annotated genes in other enterobacteriaceae, and 11 seem to possess a Shine-Dalgarno sequence, suggesting that such ncRNAs may encode small proteins instead of being solely non-coding. To support that the RIBOseq signals are reflecting translation, we tested the ribosomal-footprint covered ORF of ryhB and found a phenotype for the encoded peptide in iron-limiting condition.

Conclusion

Determination of the RCV is a useful approach for a rapid first-step differentiation between bacterial ncRNAs and small mRNAs. Further, many known ncRNAs may encode proteins as well.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3586-9) contains supplementary material, which is available to authorized users.

Mechanism and Regulation of Protein Synthesis in Saccharomyces cerevisiae

In this review, we provide an overview of protein synthesis in the yeast Saccharomyces cerevisiae The mechanism of protein synthesis is well conserved between yeast and other eukaryotes, and molecular genetic studies in budding yeast have provided critical insights into the fundamental process of translation as well as its regulation. The review focuses on the initiation and elongation phases of protein synthesis with descriptions of the roles of translation initiation and elongation factors that assist the ribosome in binding the messenger RNA (mRNA), selecting the start codon, and synthesizing the polypeptide. We also examine mechanisms of translational control highlighting the mRNA cap-binding proteins and the regulation of GCN4 and CPA1 mRNAs.

Insights into the mechanisms of eukaryotic translation gained with ribosome profiling

The development of Ribosome Profiling (RiboSeq) has revolutionized functional genomics. RiboSeq is based on capturing and sequencing of the mRNA fragments enclosed within the translating ribosome and it thereby provides a ‘snapshot’ of ribosome positions at the transcriptome wide level. Although the method is predominantly used for analysis of differential gene expression and discovery of novel translated ORFs, the RiboSeq data can also be a rich source of information about molecular mechanisms of polypeptide synthesis and translational control. This review will focus on how recent findings made with RiboSeq have revealed important details of the molecular mechanisms of translation in eukaryotes. These include mRNA translation sensitivity to drugs affecting translation initiation and elongation, the roles of upstream ORFs in response to stress, the dynamics of elongation and termination as well as details of intrinsic ribosome behavior on the mRNA after translation termination. As the RiboSeq method is still at a relatively early stage we will also discuss the implications of RiboSeq artifacts on data interpretation.

SPECtre: a spectral coherence--based classifier of actively translated transcripts from ribosome profiling sequence data

Background

Active protein translation can be assessed and measured using ribosome profiling sequencing strategies. Prevailing analytical approaches applied to this technology make use of sequence fragment length profiling or reading frame occupancy enrichment to differentiate between active translation and background noise, however they do not consider additional characteristics inherent to the technology which limits their overall accuracy.

Results

Here, we present an analytical tool that models the overall tri­nucleotide periodicity of ribosomal occupancy using a classifier based on spectral coherence. Our software, SPECtre, examines the relationship of normalized ribosome profiling read coverage over a rolling series of windows along a transcript relative to an idealized reference signal without the matched requirement of mRNA-Seq.

Conclusions

A comparison of SPECtre against previously published methods on existing data shows a marked improvement in accuracy for detecting active translation and exhibits overall high accuracy at a low false discovery rate. In addition, SPECtre performs comparably to a recently published method similarly based on spectral coherence, however with reduced runtime and memory requirements. SPECtre is available as an open source software package at https://github.com/mills-lab/spectreok.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-016-1355-4) contains supplementary material, which is available to authorized users.

Comparative survey of the relative impact of mRNA features on local ribosome profiling read density

Ribosome profiling (Ribo-seq), a promising technology for exploring ribosome decoding rates, is characterized by the presence of infrequent high peaks in ribosome footprint density and by long alignment gaps. Here, to reduce the impact of data heterogeneity we introduce a simple normalization method, Ribo-seq Unit Step Transformation (RUST). RUST is robust and outperforms other normalization techniques in the presence of heterogeneous noise. We illustrate how RUST can be used for identifying mRNA sequence features that affect ribosome footprint densities globally. We show that a few parameters extracted with RUST are sufficient for predicting experimental densities with high accuracy. Importantly the application of RUST to 30 publicly available Ribo-seq data sets revealed a substantial variation in sequence determinants of ribosome footprint frequencies, questioning the reliability of Ribo-seq as an accurate representation of local ribosome densities without prior quality control. This emphasizes our incomplete understanding of how protocol parameters affect ribosome footprint densities.

Ribosome profiling data can suffer from uneven coverage which hampers estimation of elongation rates. Connor et al. present an enhanced data smoothing method for Ribo-seq data and highlight significant variability in sequence determinants of ribosome density in publicly available data sets.

Global Analysis of Truncated RNA Ends Reveals New Insights into Ribosome Stalling in Plants

High-throughput approaches for profiling the 5' ends of RNA degradation intermediates on a genome-wide scale are frequently applied to analyze and validate cleavage sites guided by microRNAs (miRNAs). However, the complexity of the RNA degradome other than miRNA targets is currently largely uncharacterized, and this limits the application of RNA degradome studies. We conducted a global analysis of 5'-truncated mRNA ends that mapped to coding sequences (CDSs) of Arabidopsis thaliana, rice (Oryza sativa), and soybean (Glycine max). Based on this analysis, we provide multiple lines of evidence to show that the plant RNA degradome contains in vivo ribosome-protected mRNA fragments. We observed a 3-nucleotide periodicity in the position of free 5' RNA ends and a bias toward the translational frame. By examining conserved peptide upstream open reading frames (uORFs) of Arabidopsis and rice, we found a predominance of 5' termini of RNA degradation intermediates that were separated by a length equal to a ribosome-protected mRNA fragment. Through the analysis of RNA degradome data, we discovered uORFs and CDS regions potentially associated with stacked ribosomes in Arabidopsis. Furthermore, our analysis of RNA degradome data suggested that the binding of Arabidopsis ARGONAUTE7 to a noncleavable target site of miR390 might directly hinder ribosome movement. This work demonstrates an alternative use of RNA degradome data in the study of ribosome stalling.

Ribonuclease selection for ribosome profiling

Ribosome profiling has emerged as a powerful method to assess global gene translation, but methodological and analytical challenges often lead to inconsistencies across labs and model organisms. A critical issue in ribosome profiling is nuclease treatment of ribosome-mRNA complexes, as it is important to ensure both stability of ribosomal particles and complete conversion of polysomes to monosomes. We performed comparative ribosome profiling in yeast and mice with various ribonucleases including I, A, S7 and T1, characterized their cutting preferences, trinucleotide periodicity patterns and coverage similarities across coding sequences, and showed that they yield comparable estimations of gene expression when ribosome integrity is not compromised. However, ribosome coverage patterns of individual transcripts had little in common between the ribonucleases. We further examined their potency at converting polysomes to monosomes across other commonly used model organisms, including bacteria, nematodes and fruit flies. In some cases, ribonuclease treatment completely degraded ribosome populations. Ribonuclease T1 was the only enzyme that preserved ribosomal integrity while thoroughly converting polysomes to monosomes in all examined species. This study provides a guide for ribonuclease selection in ribosome profiling experiments across most common model systems.

Ribosome Footprint Profiling of Translation throughout the Genome

Ribosome profiling has emerged as a technique for measuring translation comprehensively and quantitatively by deep sequencing of ribosome-protected mRNA fragments. By identifying the precise positions of ribosomes, footprinting experiments have unveiled key insights into the composition and regulation of the expressed proteome, including delineating potentially functional micropeptides, revealing pervasive translation on cytosolic RNAs, and identifying differences in elongation rates driven by codon usage or other factors. This Primer looks at important experimental and analytical concerns for executing ribosome profiling experiments and surveys recent examples where the approach was developed to explore protein biogenesis and homeostasis.

Dynamics of ribosome scanning and recycling revealed by translation complex profiling

Regulation of messenger RNA translation is central to eukaryotic gene expression control. Regulatory inputs are specified by them RNA untranslated regions (UTRs) and often target translation initiation. Initiation involves binding of the 40S ribosomal small subunit (SSU) and associated eukaryotic initiation factors (eIFs)near the mRNA 5′ cap; the SSU then scans in the 3′ direction until it detects the start codon and is joined by the 60S ribosomal large subunit (LSU) to form the 80S ribosome. Scanning and other dynamic aspects of the initiation model have remained as conjectures because methods to trap early intermediates were lacking. Here we uncover the dynamics of the complete translation cycle in live yeast cells using translation complex profile sequencing (TCP-seq), a method developed from the ribosome profiling approach. We document scanning by observing SSU footprints along 5′ UTRs. Scanning SSU have 5′-extended footprints (up to~75 nucleotides), indicative of additional interactions with mRNA emerging from the exit channel, promoting forward movement. We visualized changes in initiation complex conformation as SSU footprints coalesced into three major sizes at start codons (19, 29 and 37 nucleotides). These share the same 5′ start site but differ at the 3′ end, reflecting successive changes at the entry channel from an open to a closed state following start codon recognition. We also observe SSU 'lingering' at stop codons after LSU departure. Our results underpin mechanistic models of translation initiation and termination, built on decades of biochemical and structural investigation, with direct genome-wide in vivo evidence. Our approach captures ribosomal complexes at all phases of translation and will aid in studying translation dynamics in diverse cellular contexts. Dysregulation of translation is common in disease and, for example, SSU scanning is a target of anti-cancer drug development. TCP-seq will prove useful in discerning differences in mRNA-specific initiation in pathologies and their response to treatment.

Dual randomization of oligonucleotides to reduce the bias in ribosome-profiling libraries

Protein translation is at the heart of cellular metabolism and its in-depth characterization is key for many lines of research. Recently, ribosome profiling became the state-of-the-art method to quantitatively characterize translation dynamics at a transcriptome-wide level. However, the strategy of library generation affects its outcomes. Here, we present a modified ribosome-profiling protocol starting from yeast, human cells and vertebrate brain tissue. We use a DNA linker carrying four randomized positions at its 5′ end and a reverse-transcription (RT) primer with three randomized positions to reduce artifacts during library preparation. The use of seven randomized nucleotides allows to efficiently detect library-generation artifacts. We find that the effect of polymerase chain reaction (PCR) artifacts is relatively small for global analyses when sufficient input material is used. However, when input material is limiting, our strategy improves the sensitivity of gene-specific analyses. Furthermore, randomized nucleotides alleviate the skewed frequency of specific sequences at the 3′ end of ribosome-protected fragments (RPFs) likely resulting from ligase specificity. Finally, strategies that rely on dual ligation show a high degree of gene-coverage variation. Taken together, our approach helps to remedy two of the main problems associated with ribosome-profiling data. This will facilitate the analysis of translational dynamics and increase our understanding of the influence of RNA modifications on translation.

Dynamics of Chloroplast Translation during Chloroplast Differentiation in Maize

Chloroplast genomes in land plants contain approximately 100 genes, the majority of which reside in polycistronic transcription units derived from cyanobacterial operons. The expression of chloroplast genes is integrated into developmental programs underlying the differentiation of photosynthetic cells from non-photosynthetic progenitors. In C4 plants, the partitioning of photosynthesis between two cell types, bundle sheath and mesophyll, adds an additional layer of complexity. We used ribosome profiling and RNA-seq to generate a comprehensive description of chloroplast gene expression at four stages of chloroplast differentiation, as displayed along the maize seedling leaf blade. The rate of protein output of most genes increases early in development and declines once the photosynthetic apparatus is mature. The developmental dynamics of protein output fall into several patterns. Programmed changes in mRNA abundance make a strong contribution to the developmental shifts in protein output, but output is further adjusted by changes in translational efficiency. RNAs with prioritized translation early in development are largely involved in chloroplast gene expression, whereas those with prioritized translation in photosynthetic tissues are generally involved in photosynthesis. Differential gene expression in bundle sheath and mesophyll chloroplasts results primarily from differences in mRNA abundance, but differences in translational efficiency amplify mRNA-level effects in some instances. In most cases, rates of protein output approximate steady-state protein stoichiometries, implying a limited role for proteolysis in eliminating unassembled or damaged proteins under non-stress conditions. Tuned protein output results from gene-specific trade-offs between translational efficiency and mRNA abundance, both of which span a large dynamic range. Analysis of ribosome footprints at sites of RNA editing showed that the chloroplast translation machinery does not generally discriminate between edited and unedited RNAs. However, editing of ACG to AUG at the rpl2 start codon is essential for translation initiation, demonstrating that ACG does not serve as a start codon in maize chloroplasts.

Ligation-free ribosome profiling of cell type-specific translation in the brain

Ribosome profiling has emerged as a powerful tool for genome-wide measurements of translation, but library construction requires multiple ligation steps and remains cumbersome relative to more conventional deep-sequencing experiments. We report a new, ligation-free approach to ribosome profiling that does not require ligation. Library construction for ligation-free ribosome profiling can be completed in one day with as little as 1 ng of purified RNA footprints. We apply ligation-free ribosome profiling to mouse brain tissue to identify new patterns of cell type-specific translation and test its ability to identify translational targets of mTOR signaling in the brain.

Adjacent Codons Act in Concert to Modulate Translation Efficiency in Yeast

Translation elongation efficiency is largely thought of as the sum of decoding efficiencies for individual codons. Here, we find that adjacent codon pairs modulate translation efficiency. Deploying an approach in Saccharomyces cerevisiae that scored the expression of over 35,000 GFP variants in which three adjacent codons were randomized, we have identified 17 pairs of adjacent codons associated with reduced expression. For many pairs, codon order is obligatory for inhibition, implying a more complex interaction than a simple additive effect. Inhibition mediated by adjacent codons occurs during translation itself as GFP expression is restored by increased tRNA levels or by non-native tRNAs with exact-matching anticodons. Inhibition operates in endogenous genes, based on analysis of ribosome profiling data. Our findings suggest translation efficiency is modulated by an interplay between tRNAs at adjacent sites in the ribosome and that this concerted effect needs to be considered in predicting the functional consequences of codon choice.

Ribosome-associated protein quality control

Protein synthesis by the ribosome can fail for numerous reasons including faulty mRNA, insufficient availability of charged tRNAs and genetic errors. All organisms have evolved mechanisms to recognize stalled ribosomes and initiate pathways for recycling, quality control and stress signaling. Here we review the discovery and molecular dissection of the eukaryotic ribosome-associated quality-control pathway for degradation of nascent polypeptides arising from interrupted translation.

A Regression-Based Analysis of Ribosome-Profiling Data Reveals a Conserved Complexity to Mammalian Translation

A fundamental goal of genomics is to identify the complete set of expressed proteins. Automated annotation strategies rely on assumptions about protein-coding sequences (CDSs), e.g., they are conserved, do not overlap, and exceed a minimum length. However, an increasing number of newly discovered proteins violate these rules. Here we present an experimental and analytical framework, based on ribosome profiling and linear regression, for systematic identification and quantification of translation. Application of this approach to lipopolysaccharide-stimulated mouse dendritic cells and HCMV-infected human fibroblasts identifies thousands of novel CDSs, including micropeptides and variants of known proteins, that bear the hallmarks of canonical translation and exhibit translation levels and dynamics comparable to that of annotated CDSs. Remarkably, many translation events are identified in both mouse and human cells even when the peptide sequence is not conserved. Our work thus reveals an unexpected complexity to mammalian translation suited to provide both conserved regulatory or protein-based functions.

Increased ribosome density associated to positively charged residues is evident in ribosome profiling experiments performed in the absence of translation inhibitors

It has been proposed that polybasic peptides cause slower movement of ribosomes through an electrostatic interaction with the highly negative ribosome exit tunnel. Ribosome profiling data-the sequencing of short ribosome-bound fragments of mRNA-is a powerful tool for the analysis of mRNA translation. Using the yeast Saccharomyces cerevisiae as a model, we showed that reduced translation efficiency associated with polybasic protein sequences could be inferred from ribosome profiling. However, an increase in ribosome density at polybasic sequences was evident only when the commonly used translational inhibitors cycloheximide and anisomycin were omitted during mRNA isolation. Since ribosome profiling performed without inhibitors agrees with experimental evidence obtained by other methods, we conclude that cycloheximide and anisomycin must be avoided in ribosome profiling experiments.

nanoCAGE reveals 5' UTR features that define specific modes of translation of functionally related MTOR-sensitive mRNAs

The diversity of MTOR-regulated mRNA translation remains unresolved. Whereas ribosome-profiling suggested that MTOR almost exclusively stimulates translation of the TOP (terminal oligopyrimidine motif) and TOP-like mRNAs, polysome-profiling indicated that MTOR also modulates translation of mRNAs without the 5' TOP motif (non-TOP mRNAs). We demonstrate that in ribosome-profiling studies, detection of MTOR-dependent changes in non-TOP mRNA translation was obscured by low sensitivity and methodology biases. Transcription start site profiling using nano-cap analysis of gene expression (nanoCAGE) revealed that not only do many MTOR-sensitive mRNAs lack the 5' TOP motif but that 5' UTR features distinguish two functionally and translationally distinct subsets of MTOR-sensitive mRNAs: (1) mRNAs with short 5' UTRs enriched for mitochondrial functions, which require EIF4E but are less EIF4A1-sensitive; and (2) long 5' UTR mRNAs encoding proliferation- and survival-promoting proteins, which are both EIF4E- and EIF4A1-sensitive. Selective inhibition of translation of mRNAs harboring long 5' UTRs via EIF4A1 suppression leads to sustained expression of proteins involved in respiration but concomitant loss of those protecting mitochondrial structural integrity, resulting in apoptosis. Conversely, simultaneous suppression of translation of both long and short 5' UTR mRNAs by MTOR inhibitors results in metabolic dormancy and a predominantly cytostatic effect. Thus, 5' UTR features define different modes of MTOR-sensitive translation of functionally distinct subsets of mRNAs, which may explain the diverse impact of MTOR and EIF4A inhibitors on neoplastic cells.

Improved Ribosome-Footprint and mRNA Measurements Provide Insights into Dynamics and Regulation of Yeast Translation

Ribosome-footprint profiling provides genome-wide snapshots of translation, but technical challenges can confound its analysis. Here, we use improved methods to obtain ribosome-footprint profiles and mRNA abundances that more faithfully reflect gene expression in Saccharomyces cerevisiae. Our results support proposals that both the beginning of coding regions and codons matching rare tRNAs are more slowly translated. They also indicate that emergent polypeptides with as few as three basic residues within a ten-residue window tend to slow translation. With the improved mRNA measurements, the variation attributable to translational control in exponentially growing yeast was less than previously reported, and most of this variation could be predicted with a simple model that considered mRNA abundance, upstream open reading frames, cap-proximal structure and nucleotide composition, and lengths of the coding and 5' UTRs. Collectively, our results provide a framework for executing and interpreting ribosome-profiling studies and reveal key features of translational control in yeast.

Integrated in vivo and in vitro nascent chain profiling reveals widespread translational pausing

Although the importance of the nonuniform progression of elongation in translation is well recognized, there have been few attempts to explore this process by directly profiling nascent polypeptides, the relevant intermediates of translation. Such approaches will be essential to complement other approaches, including ribosome profiling, which is extremely powerful but indirect with respect to the actual translation processes. Here, we use the nascent polypeptide's chemical trait of having a covalently attached tRNA moiety to detect translation intermediates. In a case study, Escherichia coli SecA was shown to undergo nascent polypeptide-dependent translational pauses. We then carried out integrated in vivo and in vitro nascent chain profiling (iNP) to characterize 1,038 proteome members of E. coli that were encoded by the first quarter of the chromosome with respect to their propensities to accumulate polypeptidyl-tRNA intermediates. A majority of them indeed undergo single or multiple pauses, some occurring only in vitro, some occurring only in vivo, and some occurring both in vivo and in vitro. Thus, translational pausing can be intrinsically robust, subject to in vivo alleviation, or require in vivo reinforcement. Cytosolic and membrane proteins tend to experience different classes of pauses; membrane proteins often pause multiple times in vivo. We also note that the solubility of cytosolic proteins correlates with certain categories of pausing. Translational pausing is widespread and diverse in nature.

Genome-wide quantification of 5'-phosphorylated mRNA degradation intermediates for analysis of ribosome dynamics

Co-translational mRNA degradation is a widespread process in which 5'-3' exonucleolytic degradation follows the last translating ribosome, thus producing an in vivo ribosomal footprint that delimits the 5' position of the mRNA molecule within the ribosome. To study this degradation process and ribosome dynamics, we developed 5PSeq, which is a method that profiles the genome-wide abundance of mRNA degradation intermediates by virtue of their 5'-phosphorylated (5'P) ends. The approach involves targeted ligation of an oligonucleotide to the 5'P end of mRNA degradation intermediates, followed by depletion of rRNA molecules, reverse transcription of 5'P mRNAs and Illumina high-throughput sequencing. 5PSeq can identify translational pauses at rare codons that are often masked when using alternative methods. This approach can be applied to previously extracted RNA samples, and it is straightforward and does not require polyribosome purification or in vitro RNA footprinting. The protocol we describe here can be applied to Saccharomyces cerevisiae and potentially to other eukaryotic organisms. Three days are required to generate 5PSeq libraries.

Exploring Ribosome Positioning on Translating Transcripts with Ribosome Profiling

Recent technological advances (e.g., microarrays and massively parallel sequencing) have facilitated genome-wide measurement of many aspects of gene regulation. Ribosome profiling is a high-throughput sequencing method used to measure gene expression at the level of translation. This is accomplished by quantifying both the number of translating ribosomes and their locations on mRNA transcripts. The inventors of this approach have published several methods papers detailing its implementation and addressing the basics of ribosome profiling data analysis. Here we describe our lab's procedure, which differs in some respects from those published previously. In addition, we describe a data analysis pipeline, Ribomap, for ribosome profiling data. Ribomap allocates sequence reads to alternative mRNA isoforms, normalizes sequencing bias along transcripts using RNA-seq data, and outputs count vectors of per-codon ribosome occupancy for each transcript.

Detecting actively translated open reading frames in ribosome profiling data

RNA-sequencing protocols can quantify gene expression regulation from transcription to protein synthesis. Ribosome profiling (Ribo-seq) maps the positions of translating ribosomes over the entire transcriptome. We have developed RiboTaper (available at https://ohlerlab.mdc-berlin.de/software/), a rigorous statistical approach that identifies translated regions on the basis of the characteristic three-nucleotide periodicity of Ribo-seq data. We used RiboTaper with deep Ribo-seq data from HEK293 cells to derive an extensive map of translation that covered open reading frame (ORF) annotations for more than 11,000 protein-coding genes. We also found distinct ribosomal signatures for several hundred upstream ORFs and ORFs in annotated noncoding genes (ncORFs). Mass spectrometry data confirmed that RiboTaper achieved excellent coverage of the cellular proteome. Although dozens of novel peptide products were validated in this manner, few of the currently annotated long noncoding RNAs appeared to encode stable polypeptides. RiboTaper is a powerful method for comprehensive de novo identification of actively used ORFs from Ribo-seq data.

Understanding Biases in Ribosome Profiling Experiments Reveals Signatures of Translation Dynamics in Yeast

Ribosome profiling produces snapshots of the locations of actively translating ribosomes on messenger RNAs. These snapshots can be used to make inferences about translation dynamics. Recent ribosome profiling studies in yeast, however, have reached contradictory conclusions regarding the average translation rate of each codon. Some experiments have used cycloheximide (CHX) to stabilize ribosomes before measuring their positions, and these studies all counterintuitively report a weak negative correlation between the translation rate of a codon and the abundance of its cognate tRNA. In contrast, some experiments performed without CHX report strong positive correlations. To explain this contradiction, we identify unexpected patterns in ribosome density downstream of each type of codon in experiments that use CHX. These patterns are evidence that elongation continues to occur in the presence of CHX but with dramatically altered codon-specific elongation rates. The measured positions of ribosomes in these experiments therefore do not reflect the amounts of time ribosomes spend at each position in vivo. These results suggest that conclusions from experiments in yeast using CHX may need reexamination. In particular, we show that in all such experiments, codons decoded by less abundant tRNAs were in fact being translated more slowly before the addition of CHX disrupted these dynamics.

Ribosome profiling measures the precise locations of millions of actively translating ribosomes on mRNAs. In theory, the frequency with which ribosomes are observed positioned over each type of codon can be used to quantify the speed with which each codon is translated. In practice, ribosome profiling experiments in yeast that use translation inhibitors to arrest translation before measuring the positions of ribosomes report very different apparent translation speeds for each codon than experiments that do not use inhibitors. To explain this inconsistency, we show that a previously unappreciated mechanism causes experiments using translation inhibitors to not measure ribosomes at each position on mRNAs in proportion to the actual amount of time spent there in vivo. Understanding this mechanism reveals that experiments without inhibitors more accurately measure translation dynamics and provides guidance for the design and interpretation of future ribosome profiling experiments.

Ribosome profiling reveals the what, when, where and how of protein synthesis

Ribosome profiling, which involves the deep sequencing of ribosome-protected mRNA fragments, is a powerful tool for globally monitoring translation in vivo. The method has facilitated discovery of the regulation of gene expression underlying diverse and complex biological processes, of important aspects of the mechanism of protein synthesis, and even of new proteins, by providing a systematic approach for experimental annotation of coding regions. Here, we introduce the methodology of ribosome profiling and discuss examples in which this approach has been a key factor in guiding biological discovery, including its prominent role in identifying thousands of novel translated short open reading frames and alternative translation products.