Identification of novel Arabidopsis thaliana upstream open reading frames that control expression of the main coding sequences in a peptide sequence-dependent manner

Phylogeny-linked occurrence of ribosome stalling on the mRNAs of Arabidopsis unfolded protein response factor bZIP60 orthologs in divergent plant species

The bZIP60, XBP1 and HAC1 mRNAs encode transcription factors that mediate the unfolded protein response (UPR) in plants, animals and yeasts, respectively. Upon UPR, these mRNAs undergo unconventional cytoplasmic splicing on the endoplasmic reticulum (ER) to produce active transcription factors. Although cytoplasmic splicing is conserved, the ER targeting mechanism differs between XBP1 and HAC1. The ER targeting of HAC1 mRNA occurs before translation, whereas that of XBP1 mRNA involves a ribosome-nascent chain complex that is stalled when a hydrophobic peptide emerges from the ribosome; the corresponding mechanism is unknown for bZIP60. Here, we analyzed ribosome stalling on bZIP60 orthologs of plants. Using a cell-free translation system, we detected nascent peptide-mediated ribosome stalling during the translation elongation of the mRNAs of Arabidopsis, rice and Physcomitrium (moss) orthologs, and the termination-step stalling in the Selaginella (lycopod) ortholog, all of which occurred ∼50 amino acids downstream of a hydrophobic region. Transfection experiments showed that ribosome stalling contributes to cytoplasmic splicing in bZIP60u orthologs of Arabidopsis and Selaginella. In contrast, ribosome stalling was undetectable for liverwort, Klebsormidium (basal land plant), and green algae orthologs. This study highlights the evolutionary diversity of ribosome stalling and its contribution to ER targeting in plants.

Upstream open reading frame-mediated upregulation of ANAC082 expression in response to nucleolar stress in Arabidopsis

Perturbations in ribosome biogenesis cause a type of cellular stress called nucleolar or ribosomal stress, which triggers adaptive responses in both animal and plant cells. The Arabidopsis ANAC082 transcription factor has been identified as a key mediator of the plant nucleolar stress response. The 5'-untranslated region (5'-UTR) of ANAC082 mRNA contains an upstream ORF (uORF) encoding an evolutionarily conserved amino acid sequence. Here, we report that this uORF mediates the upregulation of ANAC082 expression in response to nucleolar stress. When transgenic Arabidopsis plants containing a luciferase reporter gene under the control of the ANAC082 promoter and 5'-UTR were treated with reagents that induced nucleolar stress, expression of the reporter gene was enhanced in a uORF sequence-dependent manner. Additionally, we examined the effect of an endoplasmic reticulum (ER) stress-inducing reagent on reporter gene expression because the closest homolog of ANAC082 in Arabidopsis, ANAC103, is involved in the ER stress response. However, the ANAC082 uORF did not respond to ER stress. Interestingly, although ANAC103 has a uORF with an amino acid sequence similar to that of the ANAC082 uORF, the C-terminal sequence critical for regulation is not well conserved among ANAC103 homologs in Brassicaceae. Transient expression assays revealed that unlike the ANAC082 uORF, the ANAC103 uORF does not exert a sequence-dependent repressive effect. Altogether, our findings suggest that the ANAC082 uORF is important for the nucleolar stress response but not for the ER stress response, and that for this reason, the uORF sequence-dependent regulation was lost in ANAC103 during evolution.

Plant translational reprogramming for stress resilience

Organisms regulate gene expression to produce essential proteins for numerous biological processes, from growth and development to stress responses. Transcription and translation are the major processes of gene expression. Plants evolved various transcription factors and transcriptome reprogramming mechanisms to dramatically modulate transcription in response to environmental cues. However, even the genome-wide modulation of a gene's transcripts will not have a meaningful effect if the transcripts are not properly biosynthesized into proteins. Therefore, protein translation must also be carefully controlled. Biotic and abiotic stresses threaten global crop production, and these stresses are seriously deteriorating due to climate change. Several studies have demonstrated improved plant resistance to various stresses through modulation of protein translation regulation, which requires a deep understanding of translational control in response to environmental stresses. Here, we highlight the translation mechanisms modulated by biotic, hypoxia, heat, and drought stresses, which are becoming more serious due to climate change. This review provides a strategy to improve stress tolerance in crops by modulating translational regulation.

Genome-wide identification of Arabidopsis non-AUG-initiated upstream ORFs with evolutionarily conserved regulatory sequences that control protein expression levels

This study identified four novel regulatory non-AUG-initiated upstream ORFs (uORFs) with evolutionarily conserved sequences in Arabidopsis and elucidated the mechanism by which a non-AUG-initiated uORF promotes main ORF translation. Upstream open reading frames (uORFs) are short ORFs found in the 5'-untranslated regions (5'-UTRs) of eukaryotic transcripts and can influence the translation of protein-coding main ORFs (mORFs). Recent genome-wide ribosome profiling studies have revealed that hundreds or thousands of uORFs initiate translation at non-AUG start codons. However, the physiological significance of these non-AUG uORFs has so far been demonstrated for only a few of them. In this study, to identify physiologically important regulatory non-AUG uORFs in Arabidopsis, we took an approach that combined bioinformatics and experimental analysis. Since physiologically important non-AUG uORFs are likely to be conserved across species, we first searched the Arabidopsis genome for non-AUG-initiated uORFs with evolutionarily conserved sequences. Then, we examined the effects of the conserved non-AUG uORFs on the expression of the downstream mORFs using transient expression assays. As a result, three inhibitory and one promotive non-AUG uORFs were identified. Among the inhibitory non-AUG uORFs, two exerted repressive effects on mORF expression in an amino acid sequence-dependent manner. These two non-AUG uORFs are likely to encode regulatory peptides that cause ribosome stalling, thereby enhancing their repressive effects. In contrast, one of the identified regulatory non-AUG uORFs promoted mORF expression by alleviating the inhibitory effect of a downstream AUG-initiated uORF. These findings provide insights into the mechanisms that enable non-AUG uORFs to play regulatory roles despite their low translation initiation efficiencies.

Comparative analysis of wild-type accessions reveals novel determinants of Arabidopsis seed longevity

Understanding the genetic factors involved in seed longevity is of paramount importance in agricultural and ecological contexts. The polygenic nature of this trait suggests that many of them remain undiscovered. Here, we exploited the contrasting seed longevity found amongst Arabidopsis thaliana accessions to further understand this phenomenon. Concentrations of glutathione were higher in longer-lived than shorter-lived accessions, supporting that redox poise plays a prominent role in seed longevity. However, high seed permeability, normally associated with shorter longevity, is also present in long-lived accessions. Dry seed transcriptome analysis indicated that the contribution to longevity of stored messenger RNA (mRNAs) is complex, including mainly accession-specific mechanisms. The detrimental effect on longevity caused by other factors may be counterbalanced by higher levels of specific mRNAs stored in dry seeds, for instance those of heat-shock proteins. Indeed, loss-of-function mutant analysis demonstrated that heat-shock factors HSF1A and 1B contributed to longevity. Furthermore, mutants of the stress-granule zinc-finger protein TZF9 or the spliceosome subunits MOS4 or MAC3A/MAC3B, extended seed longevity, positioning RNA as a novel player in the regulation of seed viability. mRNAs of proteins with putative relevance to longevity were also abundant in shorter-lived accessions, reinforcing the idea that resistance to ageing is determined by multiple factors.

Plants utilise ancient conserved peptide upstream open reading frames in stress-responsive translational regulation

The regulation of protein synthesis plays an important role in the growth and development of all organisms. Upstream open reading frames (uORFs) are commonly found in eukaryotic messenger RNA transcripts and typically attenuate the translation of associated downstream main ORFs (mORFs). Conserved peptide uORFs (CPuORFs) are a rare subset of uORFs, some of which have been shown to conditionally regulate translation by ribosome stalling. Here, we show that Arabidopsis CPuORF19, CPuORF46 and CPuORF47, which are ancient in origin, regulate translation of any downstream ORF, in response to the agriculturally significant environmental signals, heat stress and water limitation. Consequently, these CPuORFs represent a versatile toolkit for inducible gene expression with broad applications. Finally, we note that different classes of CPuORFs may operate during distinct phases of translation, which has implications for the bioengineering of these regulatory factors.

Translation and emerging functions of non-coding RNAs in inflammation and immunity

Regulatory non‐coding RNAs (ncRNAs) including small non‐coding RNAs (sRNAs), long non‐coding RNAs (lncRNAs), and circular RNAs (circRNAs) have gained considerable attention in the last few years. This is mainly due to their condition‐ and tissue‐specific expression and their various modes of action, which suggests them as promising biomarkers and therapeutic targets. One important mechanism of ncRNAs to regulate gene expression is through translation of short open reading frames (sORFs). These sORFs can be located in lncRNAs, in non‐translated regions of mRNAs where upstream ORFs (uORFs) represent the majority, or in circRNAs. Regulation of their translation can function as a quick way to adapt protein production to changing cellular or environmental cues, and can either depend solely on the initiation and elongation of translation, or on the roles of the produced functional peptides. Due to the experimental challenges to pinpoint translation events and to detect the produced peptides, translational regulation through regulatory RNAs is not well studied yet. In the case of circRNAs, they have only recently started to be recognized as regulatory molecules instead of mere artifacts of RNA biosynthesis. Of the many roles described for regulatory ncRNAs, we will focus here on their regulation during inflammation and in immunity.

Mapping the cellular response to electron transport chain inhibitors reveals selective signaling networks triggered by mitochondrial perturbation

Mitochondrial perturbation is a key event in chemical-induced organ toxicities that is incompletely understood. Here, we studied how electron transport chain (ETC) complex I, II, or III (CI, CII and CIII) inhibitors affect mitochondrial functionality, stress response activation, and cell viability using a combination of high-content imaging and TempO-Seq in HepG2 hepatocyte cells. CI and CIII inhibitors perturbed mitochondrial membrane potential (MMP) and mitochondrial and cellular ATP levels in a concentration- and time-dependent fashion and, under conditions preventing a switch to glycolysis attenuated cell viability, whereas CII inhibitors had no effect. TempO-Seq analysis of changes in mRNA expression pointed to a shared cellular response to CI and CIII inhibition. First, to define specific ETC inhibition responses, a gene set responsive toward ETC inhibition (and not to genotoxic, oxidative, or endoplasmic reticulum stress) was identified using targeted TempO-Seq in HepG2. Silencing of one of these genes, NOS3, exacerbated the impact of CI and CIII inhibitors on cell viability, indicating its functional implication in cellular responses to mitochondrial stress. Then by monitoring dynamic responses to ETC inhibition using a HepG2 GFP reporter panel for different classes of stress response pathways and applying pathway and gene network analysis to TempO-Seq data, we looked for downstream cellular events of ETC inhibition and identified the amino acid response (AAR) as being triggered in HepG2 by ETC inhibition. Through in silico approaches we provide evidence indicating that a similar AAR is associated with exposure to mitochondrial toxicants in primary human hepatocytes. Altogether, we (i) unravel quantitative, time- and concentration-resolved cellular responses to mitochondrial perturbation, (ii) identify a gene set associated with adaptation to exposure to active ETC inhibitors, and (iii) show that ER stress and an AAR accompany ETC inhibition in HepG2 and primary hepatocytes.

Understanding small ORF diversity through a comprehensive transcription feature classification

Small open reading frames (small ORFs/sORFs/smORFs) are potentially coding sequences smaller than 100 codons that have historically been considered junk DNA by gene prediction software and in annotation screening; however, the advent of next-generation sequencing has contributed to the deeper investigation of junk DNA regions and their transcription products, resulting in the emergence of smORFs as a new focus of interest in systems biology. Several smORF peptides were recently reported in noncanonical mRNAs as new players in numerous biological contexts; however, their relevance is still overlooked in coding potential analysis. Hence, this review proposes a smORF classification based on transcriptional features, discussing the most promising approaches to investigate smORFs based on their different characteristics. First, smORFs were divided into nonexpressed (intergenic) and expressed (genic) smORFs. Second, genic smORFs were classified as smORFs located in noncoding RNAs (ncRNAs) or canonical mRNAs. Finally, smORFs in ncRNAs were further subdivided into sequences located in small or long RNAs, whereas smORFs located in canonical mRNAs were subdivided into several specific classes depending on their localization along the gene. We hope that this review provides new insights into large-scale annotations and reinforces the role of smORFs as essential components of a hidden coding DNA world.

Poly(U)-specific endoribonuclease ENDOU promotes translation of human CHOP mRNA by releasing uORF element-mediated inhibition

Upstream open reading frames (uORFs) are known to negatively affect translation of the downstream ORF. The regulatory proteins involved in relieving this inhibition are however poorly characterized. In response to cellular stress, eIF2α phosphorylation leads to an inhibition of global protein synthesis, while translation of specific factors such as CHOP is induced. We analyzed a 105‐nt inhibitory uORF in the transcript of human CHOP (huORF^chop) and found that overexpression of the zebrafish or human ENDOU poly(U)‐endoribonuclease (Endouc or ENDOU‐1, respectively) increases CHOP mRNA translation also in the absence of stress. We also found that Endouc/ENDOU‐1 binds and cleaves the huORF^chop transcript at position 80G‐81U, which induces CHOP translation independently of phosphorylated eIF2α. However, both ENDOU and phospho‐eIF2α are nonetheless required for maximal translation of CHOP mRNA. Increased levels of ENDOU shift a huORF^chop reporter as well as endogenous CHOP transcripts from the monosome to polysome fraction, indicating an increase in translation. Furthermore, we found that the uncapped truncated huORF^chop‐69‐105‐nt transcript contains an internal ribosome entry site (IRES), facilitating translation of the cleaved transcript. Therefore, we propose a model where ENDOU‐mediated transcript cleavage positively regulates CHOP translation resulting in increased CHOP protein levels upon stress. Specifically, CHOP transcript cleavage changes the configuration of huORF^chop thereby releasing its inhibition and allowing the stalled ribosomes to resume translation of the downstream ORF.

Application of Upstream Open Reading Frames (uORFs) Editing for the Development of Stress-Tolerant Crops

Global population growth and climate change are posing increasing challenges to the production of a stable crop supply using current agricultural practices. The generation of genetically modified (GM) crops has contributed to improving crop stress tolerance and productivity; however, many regulations are still in place that limit their commercialization. Recently, alternative biotechnology-based strategies, such as gene-edited (GE) crops, have been in the spotlight. Gene-editing technology, based on the clustered regularly interspaced short palindromic repeats (CRISPR) platform, has emerged as a revolutionary tool for targeted gene mutation, and has received attention as a game changer in the global biotechnology market. Here, we briefly introduce the concept of upstream open reading frames (uORFs) editing, which allows for control of the translation of downstream ORFs, and outline the potential for enhancing target gene expression by mutating uORFs. We discuss the current status of developing stress-tolerant crops, and discuss uORF targets associated with salt stress-responsive genes in rice that have already been verified by transgenic research. Finally, we overview the strategy for developing GE crops using uORF editing via the CRISPR-Cas9 system. A case is therefore made that the mutation of uORFs represents an efficient method for developing GE crops and an expansion of the scope of application of genome editing technology.

Revisiting sORFs: overcoming challenges to identify and characterize functional microproteins

Short ORFs (sORFs), that is, occurrences of a start and stop codon within 100 codons or less, can be found in organisms of all domains of life, outnumbering annotated protein-coding ORFs by orders of magnitude. Even though functional proteins smaller than 100 amino acids are known, the coding potential of sORFs has often been overlooked, as it is not trivial to predict and test for functionality within the large number of sORFs. Recent advances in ribosome profiling and mass spectrometry approaches, together with refined bioinformatic predictions, have enabled a huge leap forward in this field and identified thousands of likely coding sORFs. A relatively low number of small proteins or microproteins produced from these sORFs have been characterized so far on the molecular, structural, and/or mechanistic level. These however display versatile and, in some cases, essential cellular functions, allowing for the exciting possibility that many more, previously unknown small proteins might be encoded in the genome, waiting to be discovered. This review will give an overview of the steadily growing microprotein field, focusing on eukaryotic small proteins. We will discuss emerging themes in the molecular action of microproteins, as well as advances and challenges in microprotein identification and characterization.

Insights Into Translatomics in the Nervous System

Most neurological disorders are caused by abnormal gene translation. Generally, dysregulation of elements involved in the translational process disrupts homeostasis in neurons and neuroglia. Better understanding of how the gene translation process occurs requires detailed analysis of transcriptomic and proteomic profile data. However, a lack of strictly direct correlations between mRNA and protein levels limits translational investigation by combining transcriptomic and proteomic profiling. The much better correlation between proteins and translated mRNAs than total mRNAs in abundance and insufficiently sensitive proteomics approach promote the requirement of advances in translatomics technology. Translatomics which capture and sequence the mRNAs associated with ribosomes has been effective in identifying translational changes by genetics or projections, ribosome stalling, local translation, and transcript isoforms in the nervous system. Here, we place emphasis on the main three translatomics methods currently used to profile mRNAs attached to ribosome-nascent chain complex (RNC-mRNA). Their prominent applications in neurological diseases including glioma, neuropathic pain, depression, fragile X syndrome (FXS), neurodegenerative disorders are outlined. The content reviewed here expands our understanding on the contributions of aberrant translation to neurological disease development.

Translational gene regulation in plants: A green new deal

The molecular machinery for protein synthesis is profoundly similar between plants and other eukaryotes. Mechanisms of translational gene regulation are embedded into the broader network of RNA-level processes including RNA quality control and RNA turnover. However, over eons of their separate history, plants acquired new components, dropped others, and generally evolved an alternate way of making the parts list of protein synthesis work. Research over the past 5 years has unveiled how plants utilize translational control to defend themselves against viruses, regulate translation in response to metabolites, and reversibly adjust translation to a wide variety of environmental parameters. Moreover, during seed and pollen development plants make use of RNA granules and other translational controls to underpin developmental transitions between quiescent and metabolically active stages. The economics of resource allocation over the daily light-dark cycle also include controls over cellular protein synthesis. Important new insights into translational control on cytosolic ribosomes continue to emerge from studies of translational control mechanisms in viruses. Finally, sketches of coherent signaling pathways that connect external stimuli with a translational response are emerging, anchored in part around TOR and GCN2 kinase signaling networks. These again reveal some mechanisms that are familiar and others that are different from other eukaryotes, motivating deeper studies on translational control in plants. This article is categorized under: Translation > Translation Regulation RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

uORFs: Important Cis-Regulatory Elements in Plants

Gene expression is regulated at many levels, including mRNA transcription, translation, and post-translational modification. Compared with transcriptional regulation, mRNA translational control is a more critical step in gene expression and allows for more rapid changes of encoded protein concentrations in cells. Translation is highly regulated by complex interactions between cis-acting elements and trans-acting factors. Initiation is not only the first phase of translation, but also the core of translational regulation, because it limits the rate of protein synthesis. As potent cis-regulatory elements in eukaryotic mRNAs, upstream open reading frames (uORFs) generally inhibit the translation initiation of downstream major ORFs (mORFs) through ribosome stalling. During the past few years, with the development of RNA-seq and ribosome profiling, functional uORFs have been identified and characterized in many organisms. Here, we review uORF identification, uORF classification, and uORF-mediated translation initiation. More importantly, we summarize the translational regulation of uORFs in plant metabolic pathways, morphogenesis, disease resistance, and nutrient absorption, which open up an avenue for precisely modulating the plant growth and development, as well as environmental adaption. Additionally, we also discuss prospective applications of uORFs in plant breeding.

First Come, First Served: Sui Generis Features of the First Intron

Most of the transcribed genes in eukaryotic cells are interrupted by intervening sequences called introns that are co-transcriptionally removed from nascent messenger RNA through the process of splicing. In Arabidopsis, 79% of genes contain introns and more than 60% of intron-containing genes undergo alternative splicing (AS), which ostensibly is considered to increase protein diversity as one of the intrinsic mechanisms for fitness to the varying environment or the internal developmental program. In addition, recent findings have prevailed in terms of overlooked intron functions. Here, we review recent progress in the underlying mechanisms of intron function, in particular by focusing on unique features of the first intron that is located in close proximity to the transcription start site. The distinct deposition of epigenetic marks and nucleosome density on the first intronic DNA sequence, the impact of the first intron on determining the transcription start site and elongation of its own expression (called intron-mediated enhancement, IME), translation control in 5′-UTR, and the new mechanism of the trans-acting function of the first intron in regulating gene expression at the post-transcriptional level are summarized.

uORF Shuffling Fine-Tunes Gene Expression at a Deep Level of the Process

Upstream open reading frames (uORFs) are present in the 5’ leader sequences (or 5’ untranslated regions) upstream of the protein-coding main ORFs (mORFs) in eukaryotic polycistronic mRNA. It is well known that a uORF negatively affects translation of the mORF. Emerging ribosome profiling approaches have revealed that uORFs themselves, as well as downstream mORFs, can be translated. However, it has also been revealed that plants can fine-tune gene expression by modulating uORF-mediated regulation in some situations. This article reviews several proposed mechanisms that enable genes to escape from uORF-mediated negative regulation and gives insight into the application of uORF-mediated regulation for precisely controlling gene expression.

Comprehensive genome-wide identification of angiosperm upstream ORFs with peptide sequences conserved in various taxonomic ranges using a novel pipeline, ESUCA

Background

Upstream open reading frames (uORFs) in the 5′-untranslated regions (5′-UTRs) of certain eukaryotic mRNAs encode evolutionarily conserved functional peptides, such as cis-acting regulatory peptides that control translation of downstream main ORFs (mORFs). For genome-wide searches for uORFs with conserved peptide sequences (CPuORFs), comparative genomic studies have been conducted, in which uORF sequences were compared between selected species. To increase chances of identifying CPuORFs, we previously developed an approach in which uORF sequences were compared using BLAST between Arabidopsis and any other plant species with available transcript sequence databases. If this approach is applied to multiple plant species belonging to phylogenetically distant clades, it is expected to further comprehensively identify CPuORFs conserved in various plant lineages, including those conserved among relatively small taxonomic groups.

Results

To efficiently compare uORF sequences among many species and efficiently identify CPuORFs conserved in various taxonomic lineages, we developed a novel pipeline, ESUCA. We applied ESUCA to the genomes of five angiosperm species, which belong to phylogenetically distant clades, and selected CPuORFs conserved among at least three different orders. Through these analyses, we identified 89 novel CPuORF families. As expected, ESUCA analysis of each of the five angiosperm genomes identified many CPuORFs that were not identified from ESUCA analyses of the other four species. However, unexpectedly, these CPuORFs include those conserved across wide taxonomic ranges, indicating that the approach used here is useful not only for comprehensive identification of narrowly conserved CPuORFs but also for that of widely conserved CPuORFs. Examination of the effects of 11 selected CPuORFs on mORF translation revealed that CPuORFs conserved only in relatively narrow taxonomic ranges can have sequence-dependent regulatory effects, suggesting that most of the identified CPuORFs are conserved because of functional constraints of their encoded peptides.

Conclusions

This study demonstrates that ESUCA is capable of efficiently identifying CPuORFs likely to be conserved because of the functional importance of their encoded peptides. Furthermore, our data show that the approach in which uORF sequences from multiple species are compared with those of many other species, using ESUCA, is highly effective in comprehensively identifying CPuORFs conserved in various taxonomic ranges.

Translational Landscape of Protein-Coding and Non-Protein-Coding RNAs upon Light Exposure in Arabidopsis

Light is one of the most essential environmental clues for plant growth and morphogenesis. Exposure to blue monochromatic light from darkness is a turning point for plant biological activity, and as a result dramatic changes in gene expression occur. To understand the translational impacts of blue light, we have performed ribosome profiling analysis and called translated open reading frames (ORFs) de novo within not only mRNAs but also non-coding RNAs (ncRNAs). Translation efficiency of 3,823 protein-coding ORFs, such as nuclear chloroplast-related genes, was up-regulated by blue light exposure. Moreover, the translational activation of the microRNA biogenesis-related genes, DCL1 and HYL1, was induced by blue light. Considering the 3-nucleotide codon periodicity of ribosome footprints, a few hundred short ORFs lying on ncRNAs and upstream ORFs (uORFs) on mRNAs were found that had differential translation status between blue light and dark. uORFs are known to have a negative effect on the expression of the main ORFs (mORFs) on the same mRNAs. Our analysis suggests that the translation of uORFs is likely to be more stimulated than that of the corresponding mORFs, and uORF-mediated translational repression of the mORFs in five genes was alleviated by blue light exposure. With data-based annotation of the ORFs, our analysis provides insights into the translatome in response to environmental changes, such as those involving light.

Translational Inhibition of α-Neurexin 2

Neurexins are extensively investigated presynaptic cell-adhesion molecules which play important roles in transmitting signals and processing information at synapses that connect neurons into a vast network of cellular communications. Synaptic transmission of information is a fast and dynamic process which relies on rapid and tight regulation of synaptic protein expression. However, the mechanism underlying those regulation is still not fully understood. Therefore, we explore how the expression of NRXN2α, one of encoding genes for neurexins, is regulated at the translational level. NRXN2α transcript has a long and conserved 5'-untranslated region (5'UTR) suggestive of the rapid regulation of protein expression at the translational level. We first demonstrate that the 5'UTR has negative effects on the expression of the NRXN2α and find a critical subregion responsible for the major inhibitory function. Then we identify a particular secondary structure of G-quadruplex in the 5'UTR. Moreover, we find that the synergistic roles of G-quadruplex and upstream AUGs are responsible for most of NRXN2α-5'UTR inhibitory effects. In conclusion, we uncovered 5' UTR of neurexin2 potentially inhibits neurexin2 translation by multiple mechanisms. In addition, this study underscores the importance of direct protein quantitation in experiments rather than using mRNA as an indirect estimate of protein expression.

uORFlight: a vehicle toward uORF-mediated translational regulation mechanisms in eukaryotes

Upstream open reading frames (uORFs) are prevalent in eukaryotic mRNAs. They act as a translational control element for precisely tuning the expression of the downstream major open reading frame (mORF). uORF variation has been clearly associated with several human diseases. In contrast, natural uORF variants in plants have not ever been identified or linked with any phenotypic changes. The paucity of such evidence encouraged us to generate this database-uORFlight (http://uorflight.whu.edu.cn). It facilitates the exploration of uORF variation among different splicing models of Arabidopsis and rice genes. Most importantly, users can evaluate uORF frequency among different accessions at the population scale and find out the causal single nucleotide polymorphism (SNP) or insertion/deletion (INDEL), which can be associated with phenotypic variation through database mining or simple experiments. Such information will help to make hypothesis of uORF function in plant development or adaption to changing environments on the basis of the cognate mORF function. This database also curates plant uORF relevant literature into distinct groups. To be broadly interesting, our database expands uORF annotation into more species of fungus (Botrytis cinerea and Saccharomyces cerevisiae), plant (Brassica napus, Glycine max, Gossypium raimondii, Medicago truncatula, Solanum lycopersicum, Solanum tuberosum, Triticum aestivum and Zea mays), metazoan (Caenorhabditis elegans and Drosophila melanogaster) and vertebrate (Homo sapiens, Mus musculus and Danio rerio). Therefore, uORFlight will light up the runway toward how uORF genetic variation determines phenotypic diversity and advance our understanding of translational control mechanisms in eukaryotes.

Metabolite Control of Translation by Conserved Peptide uORFs: The Ribosome as a Metabolite Multisensor

Ribosomes translate the mRNA code into protein, and this process can be controlled by metabolites that bind to the translating ribosome in interaction with the nascent protein.

Nonsense-Mediated mRNA Decay Deficiency Affects the Auxin Response and Shoot Regeneration in Arabidopsis

Plants generally possess a strong ability to regenerate organs; for example, in tissue culture, shoots can regenerate from callus, a clump of actively proliferating, undifferentiated cells. Processing of pre-mRNA and ribosomal RNAs is important for callus formation and shoot regeneration. However, our knowledge of the roles of RNA quality control via the nonsense-mediated mRNA decay (NMD) pathway in shoot regeneration is limited. Here, we examined the shoot regeneration phenotypes of the low-beta-amylase1 (lba1)/upstream frame shift1-1 (upf1-1) and upf3-1 mutants, in which the core NMD components UPF1 and UPF3 are defective. These mutants formed callus from hypocotyl explants normally, but this callus behaved abnormally during shoot regeneration: the mutant callus generated numerous adventitious root structures instead of adventitious shoots in an auxin-dependent manner. Quantitative RT-PCR and microarray analyses showed that the upf mutations had widespread effects during culture on shoot-induction medium. In particular, the expression patterns of early auxin response genes, including those encoding AUXIN/INDOLE ACETIC ACID (AUX/IAA) family members, were significantly affected in the upf mutants. Also, the upregulation of shoot apical meristem-related transcription factor genes, such as CUP-SHAPED COTYLEDON1 (CUC1) and CUC2, was inhibited in the mutants. Taken together, these results indicate that NMD-mediated transcriptomic regulation modulates the auxin response in plants and thus plays crucial roles in the early stages of shoot regeneration.

Computational and Experimental Tools to Monitor the Changes in Translation Efficiency of Plant mRNA on a Genome-Wide Scale: Advantages, Limitations, and Solutions

The control of translation in the course of gene expression regulation plays a crucial role in plants' cellular events and, particularly, in responses to environmental factors. The paradox of the great variance between levels of mRNAs and their protein products in eukaryotic cells, including plants, requires thorough investigation of the regulatory mechanisms of translation. A wide and amazingly complex network of mechanisms decoding the plant genome into proteome challenges researchers to design new methods for genome-wide analysis of translational control, develop computational algorithms detecting regulatory mRNA contexts, and to establish rules underlying differential translation. The aims of this review are to (i) describe the experimental approaches for investigation of differential translation in plants on a genome-wide scale; (ii) summarize the current data on computational algorithms for detection of specific structure⁻function features and key determinants in plant mRNAs and their correlation with translation efficiency; (iii) highlight the methods for experimental verification of existed and theoretically predicted features within plant mRNAs important for their differential translation; and finally (iv) to discuss the perspectives of discovering the specific structural features of plant mRNA that mediate differential translation control by the combination of computational and experimental approaches.

Translation Termination and Ribosome Recycling in Eukaryotes

Termination of mRNA translation occurs when a stop codon enters the A site of the ribosome, and in eukaryotes is mediated by release factors eRF1 and eRF3, which form a ternary eRF1/eRF3-guanosine triphosphate (GTP) complex. eRF1 recognizes the stop codon, and after hydrolysis of GTP by eRF3, mediates release of the nascent peptide. The post-termination complex is then disassembled, enabling its constituents to participate in further rounds of translation. Ribosome recycling involves splitting of the 80S ribosome by the ATP-binding cassette protein ABCE1 to release the 60S subunit. Subsequent dissociation of deacylated transfer RNA (tRNA) and messenger RNA (mRNA) from the 40S subunit may be mediated by initiation factors (priming the 40S subunit for initiation), by ligatin (eIF2D) or by density-regulated protein (DENR) and multiple copies in T-cell lymphoma-1 (MCT1). These events may be subverted by suppression of termination (yielding carboxy-terminally extended read-through polypeptides) or by interruption of recycling, leading to reinitiation of translation near the stop codon.

Craniofrontonasal Syndrome Caused by Introduction of a Novel uATG in the 5'UTR of EFNB1

Craniofrontonasal syndrome (CFNS) is an X-linked disorder caused by EFNB1 mutations in which females are more severely affected than males. Severe male phenotypes are associated with mosaicism, supporting cellular interference for sex bias in this disease. Although many variants have been found in the coding region of EFNB1, only 2 pathogenic variants have been identified in the same nucleotide in 5'UTR, disrupting the stop codon of an upstream open reading frame (uORF). uORFs are known to be part of a wide range of post-transcriptional regulation processes, and just recently, their association with human diseases has come to light. In the present study, we analyzed EFNB1 in a female patient with typical features of CFNS. We identified a variant, located at c.-411, creating a new upstream ATG (uATG) in the 5'UTR of EFNB1, which is predicted to alter an existing uORF. Dual-luciferase reporter assays showed significant reduction in protein translation, but no difference in the mRNA levels. Our study demonstrates, for the first time, the regulatory impact of uATG formation on EFNB1 levels and suggests that this should be the target region in molecular diagnosis of CFNS cases without pathogenic variants in the coding and splice sites regions of EFNB1.

Transcripts from downstream alternative transcription start sites evade uORF-mediated inhibition of gene expression in Arabidopsis

The upstream ORFs (uORFs) in the 5′UTRs of mRNA often function as repressors of main ORF translation or triggers of nonsense-mediated mRNA decay. In this study, we report on transcription start site (TSS) selection when etiolated Arabidopsis seedlings are exposed to blue light, and reveal that transcription from uORF-avoiding TSSs is induced throughout the genome. It is possible that transcripts arising from TSSs downstream of uORFs evade uORF-mediated inhibition of gene expression. Thus, uORF-avoiding transcription starts are an important mechanism of gene expression regulation during a plant’s response to environmental changes.

Plants adapt to alterations in light conditions by controlling their gene expression profiles. Expression of light-inducible genes is transcriptionally induced by transcription factors such as HY5. However, few detailed analyses have been carried out on the control of transcription start sites (TSSs). Of the various wavelengths of light, it is blue light (BL) that regulates physiological responses such as hypocotyl elongation and flowering time. To understand how gene expression is controlled not only by transcript abundance but also by TSS selection, we examined genome-wide TSS profiles in Arabidopsis seedlings after exposure to BL irradiation following initial growth in the dark. Thousands of genes use multiple TSSs, and some transcripts have upstream ORFs (uORFs) that take precedence over the main ORF (mORF) encoding proteins. The uORFs often function as translation inhibitors of the mORF or as triggers of nonsense-mediated mRNA decay (NMD). Transcription from TSSs located downstream of the uORFs in 220 genes is enhanced by BL exposure. This type of regulation is found in HY5 and HYH, major regulators of light-dependent gene expression. Translation efficiencies of the genes showing enhanced usage of these TSSs increased upon BL exposure. We also show that transcripts from TSSs upstream of uORFs in 45 of the 220 genes, including HY5, accumulated in a mutant of NMD. These results suggest that BL controls gene expression not only by enhancing transcriptions but also by choosing the TSS, and transcripts from downstream TSSs evade uORF-mediated inhibition to ensure high expression of light-regulated genes.

Boron-Dependent Translational Suppression of the Borate Exporter BOR1 Contributes to the Avoidance of Boron Toxicity

Boron (B) is an essential element for plants; however, as high B concentrations are toxic, B transport must be tightly regulated. BOR1 is a borate exporter in Arabidopsis (Arabidopsis thaliana) that facilitates B translocation into shoots under B deficiency conditions. When the B supply is sufficient, BOR1 expression is down-regulated by selective degradation of BOR1 protein, while additional BOR1 regulatory mechanisms are proposed to exist. In this study, we identified a novel B-dependent BOR1 translational suppression mechanism. In vivo and in vitro reporter assays demonstrated that BOR1 translation was reduced in a B-dependent manner and that the 5'-untranslated region was both necessary and sufficient for this process. Mutational analysis revealed that multiple upstream open reading frames in the 5'-untranslated region were required for BOR1 translational suppression, and this process depended on the efficiency of translational reinitiation at the BOR1 open reading frame after translation of the upstream open reading frames. To understand the physiological significance of BOR1 regulation, we characterized transgenic plants defective in either one or both of the BOR1 regulation mechanisms. BOR1 translational suppression was induced at higher B concentrations than those triggering BOR1 degradation. Plants lacking both regulation mechanisms exhibited more severe shoot growth reduction under high-B conditions than did plants lacking BOR1 degradation alone, thus demonstrating the importance of BOR1 translational suppression. This study demonstrates that two mechanisms of posttranscriptional BOR1 regulation, each induced under different B concentrations, contribute to the avoidance of B toxicity in plants.

Recognition of the polycistronic nature of human genes is critical to understanding the genotype-phenotype relationship

Technological advances promise unprecedented opportunities for whole exome sequencing and proteomic analyses of populations. Currently, data from genome and exome sequencing or proteomic studies are searched against reference genome annotations. This provides the foundation for research and clinical screening for genetic causes of pathologies. However, current genome annotations substantially underestimate the proteomic information encoded within a gene. Numerous studies have now demonstrated the expression and function of alternative (mainly small, sometimes overlapping) ORFs within mature gene transcripts. This has important consequences for the correlation of phenotypes and genotypes. Most alternative ORFs are not yet annotated because of a lack of evidence, and this absence from databases precludes their detection by standard proteomic methods, such as mass spectrometry. Here, we demonstrate how current approaches tend to overlook alternative ORFs, hindering the discovery of new genetic drivers and fundamental research. We discuss available tools and techniques to improve identification of proteins from alternative ORFs and finally suggest a novel annotation system to permit a more complete representation of the transcriptomic and proteomic information contained within a gene. Given the crucial challenge of distinguishing functional ORFs from random ones, the suggested pipeline emphasizes both experimental data and conservation signatures. The addition of alternative ORFs in databases will render identification less serendipitous and advance the pace of research and genomic knowledge. This review highlights the urgent medical and research need to incorporate alternative ORFs in current genome annotations and thus permit their inclusion in hypotheses and models, which relate phenotypes and genotypes.

Plant Nucleolar Stress Response, a New Face in the NAC-Dependent Cellular Stress Responses

The nucleolus is the most prominent nuclear domain, where the core processes of ribosome biogenesis occur vigorously. All these processes are finely orchestrated by many nucleolar factors to build precisely ribosome particles. In animal cells, perturbations of ribosome biogenesis, mostly accompanied by structural disorders of the nucleolus, cause a kind of cellular stress to induce cell cycle arrest, senescence, or apoptosis, which is called nucleolar stress response. The best-characterized pathway of this stress response involves p53 and MDM2 as key players. p53 is a crucial transcription factor that functions in response to not only nucleolar stress but also other cellular stresses such as DNA damage stress. These cellular stresses release p53 from the inhibition by MDM2, an E3 ubiquitin ligase targeting p53, in various ways, which leads to p53-dependent activation of a set of genes. In plants, genetic impairments of ribosome biogenesis factors or ribosome components have been shown to cause characteristic phenotypes, including a narrow and pointed leaf shape, implying a common signaling pathway connecting ribosomal perturbations and certain aspects of growth and development. Unlike animals, however, plants have neither p53 nor MDM2 family proteins. Then the question arises whether plant cells have a nucleolar stress response pathway. In recent years, it has been reported that several members of the plant-specific transcription factor family NAC play critical roles in the pathways responsive to various cellular stresses. In this mini review, we outline the plant cellular stress response pathways involving NAC transcription factors with reference to the p53-MDM2-dependent pathways of animal cells, and discuss the possible involvement of a plant-unique, NAC-mediated pathway in the nucleolar stress response in plants.

A uORF Represses the Transcription Factor AtHB1 in Aerial Tissues to Avoid a Deleterious Phenotype

AtHB1 is an Arabidopsis (Arabidopsis thaliana) homeodomain-leucine zipper transcription factor that participates in hypocotyl elongation under short-day conditions. Here, we show that its expression is posttranscriptionally regulated by an upstream open reading frame (uORF) located in its 5' untranslated region. This uORF encodes a highly conserved peptide (CPuORF) that is present in varied monocot and dicot species. The Arabidopsis uORF and its maize (Zea mays) homolog repressed the translation of the main open reading frame in cis, independent of the sequence of the latter. Published ribosome footprinting results and the analysis of a frame-shifted uORF, in which the repression capability was lost, indicated that the uORF causes ribosome stalling. The regulation exerted by the CPuORF was tissue specific and did not act in the absence of light. Moreover, a photosynthetic signal is needed for the CPuORF action, since plants with uncoupled chloroplasts did not show uORF-dependent repression. Plants transformed with the native AtHB1 promoter driving AtHB1 expression did not show differential phenotypes, whereas those transformed with a construct in which the uORF was mutated exhibited serrated leaves, compact rosettes, and, most significantly, short nondehiscent anthers and siliques containing fewer or no seeds. Thus, we propose that the uncontrolled expression of AtHB1 is deleterious for the plant and, hence, finely repressed by a translational mechanism.

Small but Mighty: Functional Peptides Encoded by Small ORFs in Plants

Peptides encoded by small open reading frames (sORFs, usually <100 codons) play critical regulatory roles in plant development and environmental responses. Despite their importance, only a small number of these peptides have been identified and characterized. Genomic studies have revealed that many plant genomes contain thousands of possible sORFs, which could potentially encode small peptides. The challenge is to distinguish translated sORFs from nontranslated ones. Here, we highlight advances in methodologies for identifying these hidden sORFs in plant genomes, including ribosome profiling and proteomics. We also examine the evidence for new peptides arising from sORFs and discuss their functions in plant development, environmental responses, and translational control.

Identification of Arabidopsis thaliana upstream open reading frames encoding peptide sequences that cause ribosomal arrest

Specific sequences of certain nascent peptides cause programmed ribosomal arrest during mRNA translation to control gene expression. In eukaryotes, most known regulatory arrest peptides are encoded by upstream open reading frames (uORFs) present in the 5′-untranslated region of mRNAs. However, to date, a limited number of eukaryotic uORFs encoding arrest peptides have been reported. Here, we searched for arrest peptide-encoding uORFs among Arabidopsis thaliana uORFs with evolutionarily conserved peptide sequences. Analysis of in vitro translation products of 22 conserved uORFs identified three novel uORFs causing ribosomal arrest in a peptide sequence-dependent manner. Stop codon-scanning mutagenesis, in which the effect of changing the uORF stop codon position on the ribosomal arrest was examined, and toeprint analysis revealed that two of the three uORFs cause ribosomal arrest during translation elongation, whereas the other one causes ribosomal arrest during translation termination. Transient expression assays showed that the newly identified arrest-causing uORFs exerted a strong sequence-dependent repressive effect on the expression of the downstream reporter gene in A. thaliana protoplasts. These results suggest that the peptide sequences of the three uORFs identified in this study cause ribosomal arrest in the uORFs, thereby repressing the expression of proteins encoded by the main ORFs.

Evidence for a Role of ANAC082 as a Ribosomal Stress Response Mediator Leading to Growth Defects and Developmental Alterations in Arabidopsis

Ribosome-related mutants in Arabidopsis thaliana share several notable characteristics regarding growth and development, which implies the existence of a common pathway that responds to disorders in ribosome biogenesis. As a first step to explore this pathway genetically, we screened a mutagenized population of root initiation defective2 (rid2), a temperature-sensitive mutant that is impaired in pre-rRNA processing, and isolated suppressor of root initiation defective two1 (sriw1), a suppressor mutant in which the defects of cell proliferation observed in rid2 at the restrictive temperature was markedly rescued. sriw1 was identified as a missense mutation of the NAC transcription factor gene ANAC082 The sriw1 mutation greatly alleviated the developmental abnormalities of rid2 and four other tested ribosome-related mutants, including rid3 However, the impaired pre-rRNA processing in rid2 and rid3 was not relieved by sriw1 Expression of ANAC082 was localized to regions where phenotypic effects of ribosome-related mutations are readily evident and was elevated in rid2 and rid3 compared with the wild type. These findings suggest that ANAC082 acts downstream of perturbation of biogenesis of the ribosome and may mediate a set of stress responses leading to developmental alterations and cell proliferation defects.

In Search of Lost Small Peptides

A large body of evidence indicates that genome annotation pipelines have biased our view of coding sequences because they generally undersample small proteins and peptides. The recent development of genome-wide translation profiling reveals the prevalence of small/short open reading frames (smORFs or sORFs), which are scattered over all classes of transcripts, including both mRNAs and presumptive long noncoding RNAs. Proteomic approaches further confirm an unexpected variety of smORF-encoded peptides (SEPs), representing an overlooked reservoir of bioactive molecules. Indeed, functional studies in a broad range of species from yeast to humans demonstrate that SEPs can harbor key activities for the control of development, differentiation, and physiology. Here we summarize recent advances in the discovery and functional characterization of smORF/SEPs and discuss why these small players can no longer be ignored with regard to genome function.

Arabidopsis Polyamine oxidase-2 uORF is required for downstream translational regulation

In eukaryotic mRNAs, small upstream open reading frames (uORFs) located in the 5'-untranslated region control the translation of the downstream main ORF. Polyamine oxidase (PAO) enzymes catalyze the oxidation of higher polyamines such as spermidine and spermine, and therefore contribute to the maintenance of intracellular polyamine content and to the regulation of physiological processes through their catabolic products. Recently, we reported that the Arabidopsis thaliana Polyamine Oxidase 2 (AtPAO2) is post-transcriptionally regulated by its 5'-UTR region through an uORF. In the present study, we analyzed whether the translation of the uORF is needed for the translational repression of the main ORF, and whether the inactivation of the uORF had an effect on the translational control mediated by polyamines. To this aim, we generated diverse single mutations in the uORF sequence; these mutant 5'-UTRs were fused to the GUS reporter gene, and tested in onion monolayer cells and A. thaliana transgenic seedlings. Removal of the start codon or introduction of a premature stop codon in the AtPAO2 uORF sequence abolished the negative regulation of the GUS expression exerted by the wild-type AtPAO2 uORF. An artificial uORF (32 amino acids in length) generated by the addition of a single nucleotide in AtPAO2 uORF proved to be less repressive than the wild-type uORF. Thus, our findings suggest that translation of the AtPAO2 uORF is necessary for the translational repression of the main ORF.

The Minimum Open Reading Frame, AUG-Stop, Induces Boron-Dependent Ribosome Stalling and mRNA Degradation

Upstream open reading frames (uORFs) are often translated ahead of the main ORF of a gene and regulate gene expression, sometimes in a condition-dependent manner, but such a role for the minimum uORF (hereafter referred to as AUG-stop) in living organisms is currently unclear. Here, we show that AUG-stop plays an important role in the boron (B)-dependent regulation of NIP5;1, encoding a boric acid channel required for normal growth under low B conditions in Arabidopsis thaliana High B enhanced ribosome stalling at AUG-stop, which was accompanied by the suppression of translation and mRNA degradation. This mRNA degradation was promoted by an upstream conserved sequence present near the 5'-edge of the stalled ribosome. Once ribosomes translate a uORF, reinitiation of translation must take place in order for the downstream ORF to be translated. Our results suggest that reinitiation of translation at the downstream NIP5;1 ORF is enhanced under low B conditions. A genome-wide analysis identified two additional B-responsive genes, SKU5 and the transcription factor gene ABS/NGAL1, which were regulated by B-dependent ribosome stalling through AUG-stop. This regulation was reproduced in both plant and animal transient expression and cell-free translation systems. These findings suggest that B-dependent AUG-stop-mediated regulation is common in eukaryotes.

hnRNPs and ELAVL1 cooperate with uORFs to inhibit protein translation

Most of our knowledge about translation regulatory mechanisms comes from studies on lower organisms. However, the translation control system of higher organisms is less understood. Here we find that in 5΄ untranslated region (5΄UTR) of human Annexin II receptor (AXIIR) mRNA, there are two upstream open reading frames (uORFs) acting in a fail-safe manner to inhibit the translation from the main AUG. These uORFs are unfavorable for re-initiation after termination of uORF translation. Heterogeneous nuclear ribonucleoprotein A2B1 (hnRNPA2B1), hnRNPA0 and ELAV like RNA binding protein 1 (ELAVL1) bind to the 5΄UTR of AXIIR mRNA. They focus the translation of uORFs on uORF1 and attenuate leaky scanning that bypasses uORFs. The cooperation between the two uORFs and the three proteins formed a multiple fail-safe system that tightly inhibits the translation of downstream AXIIR. Such cooperation between multiple molecules and elements reflects that higher organism develops a complex translation regulatory system to achieve accurate and flexible gene expression control.

Super-resolution ribosome profiling reveals unannotated translation events in Arabidopsis

Deep sequencing of ribosome footprints (ribosome profiling) maps and quantifies mRNA translation. Because ribosomes decode mRNA every 3 nt, the periodic property of ribosome footprints could be used to identify novel translated ORFs. However, due to the limited resolution of existing methods, the 3-nt periodicity is observed mostly in a global analysis, but not in individual transcripts. Here, we report a protocol applied to Arabidopsis that maps over 90% of the footprints to the main reading frame and thus offers super-resolution profiles for individual transcripts to precisely define translated regions. The resulting data not only support many annotated and predicted noncanonical translation events but also uncover small ORFs in annotated noncoding RNAs and pseudogenes. A substantial number of these unannotated ORFs are evolutionarily conserved, and some produce stable proteins. Thus, our study provides a valuable resource for plant genomics and an efficient optimization strategy for ribosome profiling in other organisms.

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.

Decoding sORF translation - from small proteins to gene regulation

Translation is best known as the fundamental mechanism by which the ribosome converts a sequence of nucleotides into a string of amino acids. Extensive research over many years has elucidated the key principles of translation, and the majority of translated regions were thought to be known. The recent discovery of wide-spread translation outside of annotated protein-coding open reading frames (ORFs) came therefore as a surprise, raising the intriguing possibility that these newly discovered translated regions might have unrecognized protein-coding or gene-regulatory functions. Here, we highlight recent findings that provide evidence that some of these newly discovered translated short ORFs (sORFs) encode functional, previously missed small proteins, while others have regulatory roles. Based on known examples we will also speculate about putative additional roles and the potentially much wider impact that these translated regions might have on cellular homeostasis and gene regulation.

Upstream ORFs are prevalent translational repressors in vertebrates

Regulation of gene expression is fundamental in establishing cellular diversity and a target of natural selection. Untranslated mRNA regions (UTRs) are key mediators of post-transcriptional regulation. Previous studies have predicted thousands of ORFs in 5'UTRs, the vast majority of which have unknown function. Here, we present a systematic analysis of the translation and function of upstream open reading frames (uORFs) across vertebrates. Using high-resolution ribosome footprinting, we find that (i)uORFs are prevalent within vertebrate transcriptomes, (ii) the majority show signatures of active translation, and (iii)uORFs act as potent regulators of translation and RNA levels, with a similar magnitude to miRNAs. Reporter experiments reveal clear repression of downstream translation by uORFs/oORFs. uORF number, intercistronic distance, overlap with the CDS, and initiation context most strongly influence translation. Evolution has targeted these features to favor uORFs amenable to regulation over constitutively repressive uORFs/oORFs. Finally, we observe that the regulatory potential of uORFs on individual genes is conserved across species. These results provide insight into the regulatory code within mRNA leader sequences and their capacity to modulate translation across vertebrates.

Genome-Wide Search for Translated Upstream Open Reading Frames in Arabidopsis Thaliana

Upstream open reading frames (uORFs) are open reading frames that occur within the 5' UTR of an mRNA. uORFs have been found in many organisms. They play an important role in gene regulation, cell development, and in various metabolic processes. It is believed that translated uORFs reduce the translational efficiency of the main coding region. However, only few uORFs are experimentally characterized. In this paper, we use ribosome footprinting together with a semi-supervised approach based on stacking classification models to identify translated uORFs in Arabidopsis thaliana. Our approach identified 5360 potentially translated uORFs in 2051 genes. GO terms enriched in genes with translated uORFs include catalytic activity, binding, transferase activity, phosphotransferase activity, kinase activity, and transcription regulator activity. The reported uORFs occur with a higher frequency in multi-isoform genes, and some uORFs are affected by alternative transcript start sites or alternative splicing events. Association rule mining revealed sequence features associated with the translation status of the uORFs. We hypothesize that uORF translation is a complex process that might be regulated by multiple factors. The identified uORFs are available online at:https://www.dropbox.com/sh/zdutupedxafhly8/AABFsdNR5zDfiozB7B4igFcja?dl=0. This paper is the extended version of our research presented at ISBRA 2015.

Ribosome profiling reveals dynamic translational landscape in maize seedlings under drought stress

Plants can respond to environmental changes with various mechanisms occurred at transcriptional and translational levels. Thus far, there have been relatively extensive understandings of stress responses of plants on transcriptional level, while little information is known about that on translational level. To uncover the landscape of translation in plants in response to drought stress, we performed the recently developed ribosome profiling assay with maize seedlings growing under normal and drought conditions. Comparative analysis of the ribosome profiling data and the RNA-seq data showed that the fold changes of gene expression at transcriptional level were moderately correlated with that of translational level globally (R(2) = 0.69). However, less than half of the responsive genes were shared by transcription and translation under drought condition, suggesting that drought stress can introduce transcriptional and translational responses independently. We found that the translational efficiencies of 931 genes were changed significantly in response to drought stress. Further analysis revealed that the translational efficiencies of genes were highly influenced by their sequence features including GC content, length of coding sequences and normalized minimal free energy. In addition, we detected potential translation of 3063 upstream open reading frames (uORFs) on 2558 genes and these uORFs may affect the translational efficiency of downstream main open reading frames (ORFs). Our study indicates that plant can respond to drought stress with highly dynamic translational mechanism, that acting synergistically with that of transcription.

The Plant Peptidome: An Expanding Repertoire of Structural Features and Biological Functions

Peptides fulfill a plethora of functions in plant growth, development, and stress responses. They act as key components of cell-to-cell communication, interfere with signaling and response pathways, or display antimicrobial activity. Strikingly, both the diversity and amount of plant peptides have been largely underestimated. Most characterized plant peptides to date acting as small signaling peptides or antimicrobial peptides are derived from nonfunctional precursor proteins. However, evidence is emerging on peptides derived from a functional protein, directly translated from small open reading frames (without the involvement of a precursor) or even encoded by primary transcripts of microRNAs. These novel types of peptides further add to the complexity of the plant peptidome, even though their number is still limited and functional characterization as well as translational evidence are often controversial. Here, we provide a comprehensive overview of the reported types of plant peptides, including their described functional and structural properties. We propose a novel, unifying peptide classification system to emphasize the enormous diversity in peptide synthesis and consequent complexity of the still expanding knowledge on the plant peptidome.

Transcriptome-wide measurement of ribosomal occupancy by ribosome profiling

Gene expression profiling provides a tool to analyze the internal states of cells or organisms, and their responses to perturbations. While global measurements of mRNA levels have thus been widely used for many years, it is only through the recent development of the ribosome profiling technique that an analogous examination of global mRNA translation programs has become possible. Ribosome profiling reveals which RNAs are being translated to what extent and where the translated open reading frames are located. In addition, different modes of translation regulation can be distinguished and characterized. Here, we present an optimized, step-by-step protocol for ribosome profiling. Although established in Caenorhabditis elegans, our protocol and optimization approaches should be equally usable for other model organisms or cell culture with little adaptation. Next to providing a protocol, we compare two different methods for isolation of single ribosomes and two different library preparations, and describe strategies to optimize the RNase digest and to reduce ribosomal RNA contamination in the libraries. Moreover, we discuss bioinformatic strategies to evaluate the quality of the data and explain how the data can be analyzed for different applications. In sum, this article seeks to facilitate the understanding, execution, and optimization of ribosome profiling experiments.