Proteins à la carte: riboproteogenomic exploration of bacterial N-terminal proteoform expression

In recent years, it has become evident that the true complexity of bacterial proteomes remains underestimated. Gene annotation tools are known to propagate biases and overlook certain classes of truly expressed proteins, particularly proteoforms-protein isoforms arising from a single gene. Recent (re-)annotation efforts heavily rely on ribosome profiling by providing a direct readout of translation to fully describe bacterial proteomes. In this study, we employ a robust riboproteogenomic pipeline to conduct a systematic census of expressed N-terminal proteoform pairs, representing two isoforms encoded by a single gene raised by annotated and alternative translation initiation, in Salmonella. Intriguingly, conditional-dependent changes in relative utilization of annotated and alternative translation initiation sites (TIS) were observed in several cases. This suggests that TIS selection is subject to regulatory control, adding yet another layer of complexity to our understanding of bacterial proteomes.

IMPORTANCE

With the emerging theme of genes within genes comprising the existence of alternative open reading frames (ORFs) generated by translation initiation at in-frame start codons, mechanisms that control the relative utilization of annotated and alternative TIS need to be unraveled and our molecular understanding of resulting proteoforms broadened. Utilizing complementary ribosome profiling strategies to map ORF boundaries, we uncovered dual-encoding ORFs generated by in-frame TIS usage in Salmonella. Besides demonstrating that alternative TIS usage may generate proteoforms with different characteristics, such as differential localization and specialized function, quantitative aspects of conditional retapamulin-assisted ribosome profiling (Ribo-RET) translation initiation maps offer unprecedented insights into the relative utilization of annotated and alternative TIS, enabling the exploration of gene regulatory mechanisms that control TIS usage and, consequently, the translation of N-terminal proteoform pairs.

dCas13-mediated translational repression for accurate gene silencing in mammalian cells

Current gene silencing tools based on RNA interference (RNAi) or, more recently, clustered regularly interspaced short palindromic repeats (CRISPR)‒Cas13 systems have critical drawbacks, such as off-target effects (RNAi) or collateral mRNA cleavage (CRISPR‒Cas13). Thus, a more specific method of gene knockdown is needed. Here, we develop CRISPRδ, an approach for translational silencing, harnessing catalytically inactive Cas13 proteins (dCas13). Owing to its tight association with mRNA, dCas13 serves as a physical roadblock for scanning ribosomes during translation initiation and does not affect mRNA stability. Guide RNAs covering the start codon lead to the highest efficacy regardless of the translation initiation mechanism: cap-dependent, internal ribosome entry site (IRES)-dependent, or repeat-associated non-AUG (RAN) translation. Strikingly, genome-wide ribosome profiling reveals the ultrahigh gene silencing specificity of CRISPRδ. Moreover, the fusion of a translational repressor to dCas13 further improves the performance. Our method provides a framework for translational repression-based gene silencing in eukaryotes.

The structure of a human translation initiation complex reveals two independent roles for the helicase eIF4A

Eukaryotic translation initiation involves recruitment of the 43S pre-initiation complex to the 5' end of mRNA by the cap-binding complex eIF4F, forming the 48S translation initiation complex (48S), which then scans along the mRNA until the start codon is recognized. We have previously shown that eIF4F binds near the mRNA exit channel of the 43S, leaving open the question of how mRNA secondary structure is removed as it enters the mRNA channel on the other side of the 40S subunit. Here we report the structure of a human 48S that shows that, in addition to the eIF4A that is part of eIF4F, there is a second eIF4A helicase bound at the mRNA entry site, which could unwind RNA secondary structures as they enter the 48S. The structure also reveals conserved interactions between eIF4F and the 43S, probaby explaining how eIF4F can promote mRNA recruitment in all eukaryotes.

The molecular basis of translation initiation and its regulation in eukaryotes

The regulation of gene expression is fundamental for life. Whereas the role of transcriptional regulation of gene expression has been studied for several decades, it has been clear over the past two decades that post-transcriptional regulation of gene expression, of which translation regulation is a major part, can be equally important. Translation can be divided into four main stages: initiation, elongation, termination and ribosome recycling. Translation is controlled mainly during its initiation, a process which culminates in a ribosome positioned with an initiator tRNA over the start codon and, thus, ready to begin elongation of the protein chain. mRNA translation has emerged as a powerful tool for the development of innovative therapies, yet the detailed mechanisms underlying the complex process of initiation remain unclear. Recent studies in yeast and mammals have started to shed light on some previously unclear aspects of this process. In this Review, we discuss the current state of knowledge on eukaryotic translation initiation and its regulation in health and disease. Specifically, we focus on recent advances in understanding the processes involved in assembling the 43S pre-initiation complex and its recruitment by the cap-binding complex eukaryotic translation initiation factor 4F (eIF4F) at the 5' end of mRNA. In addition, we discuss recent insights into ribosome scanning along the 5' untranslated region of mRNA and selection of the start codon, which culminates in joining of the 60S large subunit and formation of the 80S initiation complex.

The distinct translational landscapes of gram-negative Salmonella and gram-positive Listeria

Translational control in pathogenic bacteria is fundamental to gene expression and affects virulence and other infection phenotypes. We used an enhanced ribosome profiling protocol coupled with parallel transcriptomics to capture accurately the global translatome of two evolutionarily distant pathogenic bacteria-the Gram-negative bacterium Salmonella and the Gram-positive bacterium Listeria. We find that the two bacteria use different mechanisms to translationally regulate protein synthesis. In Salmonella, in addition to the expected correlation between translational efficiency and cis-regulatory features such as Shine-Dalgarno (SD) strength and RNA secondary structure around the initiation codon, our data reveal an effect of the 2nd and 3rd codons, where the presence of tandem lysine codons (AAA-AAA) enhances translation in both Salmonella and E. coli. Strikingly, none of these features are seen in efficiently translated Listeria transcripts. Instead, approximately 20% of efficiently translated Listeria genes exhibit 70 S footprints seven nt upstream of the authentic start codon, suggesting that these genes may be subject to a novel translational initiation mechanism. Our results show that SD strength is not a direct hallmark of translational efficiency in all bacteria. Instead, Listeria has evolved additional mechanisms to control gene expression level that are distinct from those utilised by Salmonella and E. coli.

Mycobacterium bovis BCG dodecin gene codes a functional protein despite of a start codon mutation

Dodecin is a dodecamer involved in flavin homeostasis, with interesting temperature and osmolarity endurance features in Mycobacterium tuberculosis. A single nucleotide polymorphism in the gene's start codon in BCG, converting ATG to ACG, is predicted to generate a N-terminal shorter isoform, lacking the first 7 amino acids. We previously reported that the shortened recombinant protein has reduced extremophilic features. Here we investigate if within the mycobacterial context dodecin can be produced from both alleles, carrying ATG and ACG start codons. Reporter gene assays using mcherry cloned downstream and in phase to both M.tb and BCG "upstream" regions confirms production of functional proteins. Complementation with both dod alleles similarly enhances M. smegmatis growth after entry into logarithmic phase and exposure to hydrogen peroxide, possibly implicating this protein in oxidative stress response mechanisms. Altogether these data indicate that BCG dodecin is indeed produced, notwithstanding in lower levels compared to M.tb, conferring similar phenotypes, even with the SNP altering the M.tb ATG start codon to the BCG ACG. This protein might be an interesting drug target for the development of new therapeutics against tuberculosis.

Start codon-associated ribosomal frameshifting mediates nutrient stress adaptation

A translating ribosome is typically thought to follow the reading frame defined by the selected start codon. Using super-resolution ribosome profiling, here we report pervasive out-of-frame translation immediately from the start codon. Start codon-associated ribosomal frameshifting (SCARF) stems from the slippage of ribosomes during the transition from initiation to elongation. Using a massively paralleled reporter assay, we uncovered sequence elements acting as SCARF enhancers or repressors, implying that start codon recognition is coupled with reading frame fidelity. This finding explains thousands of mass spectrometry spectra that are unannotated in the human proteome. Mechanistically, we find that the eukaryotic initiation factor 5B (eIF5B) maintains the reading frame fidelity by stabilizing initiating ribosomes. Intriguingly, amino acid starvation induces SCARF by proteasomal degradation of eIF5B. The stress-induced SCARF protects cells from starvation by enabling amino acid recycling and selective mRNA translation. Our findings illustrate a beneficial effect of translational 'noise' in nutrient stress adaptation.

Increased levels of eIF2A inhibit translation by sequestering 40S ribosomal subunits

eIF2A was the first eukaryotic initiator tRNA carrier discovered but its exact function has remained enigmatic. Uncharacteristic of translation initiation factors, eIF2A is reported to be non-cytosolic in multiple human cancer cell lines. Attempts to study eIF2A mechanistically have been limited by the inability to achieve high yield of soluble recombinant protein. Here, we developed a purification paradigm that yields ∼360-fold and ∼6000-fold more recombinant human eIF2A from Escherichia coli and insect cells, respectively, than previous reports. Using a mammalian in vitro translation system, we found that increased levels of recombinant human eIF2A inhibit translation of multiple reporter mRNAs, including those that are translated by cognate and near-cognate start codons, and does so prior to start codon recognition. eIF2A also inhibited translation directed by all four types of cap-independent viral IRESs, including the CrPV IGR IRES that does not require initiation factors or initiator tRNA, suggesting excess eIF2A sequesters 40S subunits. Supplementation with additional 40S subunits prevented eIF2A-mediated inhibition and pull-down assays demonstrated direct binding between recombinant eIF2A and purified 40S subunits. These data support a model that eIF2A must be kept away from the translation machinery to avoid sequestering 40S ribosomal subunits.

Translation initiation at AUG and non-AUG triplets in plants

In plants and other eukaryotes, precise selection of translation initiation site (TIS) on mRNAs shapes the proteome in response to cellular events or environmental cues. The canonical translation of mRNAs initiates at a 5' proximal AUG codon in a favorable context. However, the coding and non-coding regions of plant genomes contain numerous unannotated alternative AUG and non-AUG TISs. Determining how and why these unexpected and prevalent TISs are activated in plants has emerged as an exciting research area. In this review, we focus on the selection of plant TISs and highlight studies that revealed previously unannotated TISs used in vivo via comparative genomics and genome-wide profiling of ribosome positioning and protein N-terminal ends. The biological signatures of non-AUG TIS-initiated open reading frames (ORFs) in plants are also discussed. We describe what is understood about cis-regulatory RNA elements and trans-acting eukaryotic initiation factors (eIFs) in the site selection for translation initiation by featuring the findings in plants along with supporting findings in non-plant species. The prevalent, unannotated TISs provide a hidden reservoir of ORFs that likely help reshape plant proteomes in response to developmental or environmental cues. These findings underscore the importance of understanding the mechanistic basis of TIS selection to functionally annotate plant genomes, especially for crops with large genomes.

Translational fidelity screens in mammalian cells reveal eIF3 and eIF4G2 as regulators of start codon selectivity

The translation initiation machinery and the ribosome orchestrate a highly dynamic scanning process to distinguish proper start codons from surrounding nucleotide sequences. Here, we performed genome-wide CRISPRi screens in human K562 cells to systematically identify modulators of the frequency of translation initiation at near-cognate start codons. We observed that depletion of any eIF3 core subunit promoted near-cognate start codon usage, though sensitivity thresholds of each subunit to sgRNA-mediated depletion varied considerably. Double sgRNA depletion experiments suggested that enhanced near-cognate usage in eIF3D depleted cells required canonical eIF4E cap-binding and was not driven by eIF2A or eIF2D-dependent leucine tRNA initiation. We further characterized the effects of eIF3D depletion and found that the N-terminus of eIF3D was strictly required for accurate start codon selection, whereas disruption of the cap-binding properties of eIF3D had no effect. Lastly, depletion of eIF3D activated TNFα signaling via NF-κB and the interferon gamma response. Similar transcriptional profiles were observed upon knockdown of eIF1A and eIF4G2, which also promoted near-cognate start codon usage, suggesting that enhanced near-cognate usage could potentially contribute to NF-κB activation. Our study thus provides new avenues to study the mechanisms and consequences of alternative start codon usage.

Interrogating the Function of Bicistronic Translational Control Elements to Improve Consistency of Gene Expression

Context independent gene expression is required for genetic circuits to maintain consistent and predicable behavior. Previous efforts to develop context independent translation have leveraged the helicase activity of translating ribosomes via bicistronic design translational control elements (BCDs) located within an efficiently translated leader peptide. We have developed a series of bicistronic translational control elements with strengths that span several orders of magnitude, maintain consistent expression levels across diverse sequence contexts, and are agnostic to common ligation sequences used in modular cloning systems. We have used this series of BCDs to investigate several features of this design, including the spacing of the start and stop codons, the nucleotide identity upstream of the start codon, and factors affecting translation of the leader peptide. To demonstrate the flexibility of this architecture and their value as a generic modular expression control cassette for synthetic biology, we have developed a set of robust BCDs for use in several Rhodococcus species.

PRRC2 proteins impact translation initiation by promoting leaky scanning

Roughly half of animal mRNAs contain upstream open reading frames (uORFs). These uORFs can represent an impediment to translation of the main ORF since ribosomes usually bind the mRNA cap at the 5' end and then scan for ORFs in a 5'-to-3' fashion. One way for ribosomes to bypass uORFs is via leaky scanning, whereby the ribosome disregards the uORF start codon. Hence leaky scanning is an important instance of post-transcriptional regulation that affects gene expression. Few molecular factors regulating or facilitating this process are known. Here we show that the PRRC2 proteins PRRC2A, PRRC2B and PRRC2C impact translation initiation. We find that they bind eukaryotic translation initiation factors and preinitiation complexes, and are enriched on ribosomes translating mRNAs with uORFs. We find that PRRC2 proteins promote leaky scanning past translation start codons, thereby promoting translation of mRNAs containing uORFs. Since PRRC2 proteins have been associated with cancer, this provides a mechanistic starting point for understanding their physiological and pathophysiological roles.

Elusive Trans-Acting Factors Which Operate with Type I (Poliovirus-like) IRES Elements

The phenomenon of internal initiation of translation was discovered in 1988 on poliovirus mRNA. The prototypic cis-acting element in the 5' untranslated region (5'UTR) of poliovirus mRNA, which is able to direct initiation at an internal start codon without the involvement of a cap structure, has been called an IRES (Internal Ribosome Entry Site or Segment). Despite its early discovery, poliovirus and other related IRES elements of type I are poorly characterized, and it is not yet clear which host proteins (a.k.a. IRES trans-acting factors, ITAFs) are required for their full activity in vivo. Here we discuss recent and old results devoted to type I IRESes and provide evidence that Poly(rC) binding protein 2 (PCBP2), Glycyl-tRNA synthetase (GARS), and Cold Shock Domain Containing E1 (CSDE1, also known as UNR) are major regulators of type I IRES activity.

Rapid 40S scanning and its regulation by mRNA structure during eukaryotic translation initiation

How the eukaryotic 43S preinitiation complex scans along the 5' untranslated region (5' UTR) of a capped mRNA to locate the correct start codon remains elusive. Here, we directly track yeast 43S-mRNA binding, scanning, and 60S subunit joining by real-time single-molecule fluorescence spectroscopy. 43S engagement with mRNA occurs through a slow, ATP-dependent process driven by multiple initiation factors including the helicase eIF4A. Once engaged, 43S scanning occurs rapidly and directionally at ∼100 nucleotides per second, independent of multiple cycles of ATP hydrolysis by RNA helicases post ribosomal loading. Scanning ribosomes can proceed through RNA secondary structures, but 5' UTR hairpin sequences near start codons drive scanning ribosomes at start codons backward in the 5' direction, requiring rescanning to arrive once more at a start codon. Direct observation of scanning ribosomes provides a mechanistic framework for translational regulation by 5' UTR structures and upstream near-cognate start codons.

The effect of the nucleotides immediately upstream of the AUG start codon on the efficiency of translation initiation in sperm cells

It is widely known that an optimal nucleotide sequence context immediately upstream of the AUG start codon greatly improves the efficiency of translation initiation of mRNA in mammalian and plant somatic cells, which in turn increases protein levels. However, it is still unclear whether a similar regulatory mechanism is also present in highly differentiated cells. Here, we surveyed this issue in Arabidopsis thaliana sperm cells and found that the sequence context-mediated regulation of translation initiation in sperm cells is generally similar to that in somatic cells. A simple motif of four adenine nucleotides at positions - 1 to - 4 greatly improved the efficiency of translation initiation, and when the motif was present there, translation was even initiated at some non-AUG codons in sperm cells. However, unlike that in mammalian cells, a mainly effective nucleotide site to regulate the efficiency of translation initiation was not present at positions - 1 to - 4 in sperm cells. Meanwhile, different from somatic cells, sperm cells did not use eukaryotic translation initiation factor 1 to regulate the efficiency in a poor context consisting of the lowest frequency nucleotides. All these results contribute to our understanding of the cytoplasmic event of translation initiation in highly differentiated sperm cells.

The CYCLOPS Response Element in the NIN Promoter Is Important but Not Essential for Infection Thread Formation During Lotus japonicus-Rhizobia Symbiosis

The establishment of the legume-rhizobia symbiosis, termed the root-nodule symbiosis (RNS), requires elaborate interactions at the molecular level. The host plant-derived transcription factor NODULE INCEPTION (NIN) is known to be crucial for RNS, regulating associated processes such as alteration of root hair morphology, infection thread formation, and cell division during nodulation. This emphasizes the importance of the precise spatiotemporal regulation of NIN expression for the establishment of RNS; however, the detailed role of NIN promoter sequences in this process remains unclear. The daphne mutant, a nin mutant allele containing a chromosomal translocation approximately 7 kb upstream of the start codon, does not form nodules but does form infection threads, indicating that the region within 7 kb of the NIN start codon contributes to NIN expression during infection thread formation. CYCLOPS binds to a CYCLOPS response element (CYC-RE) in the NIN promoter, and cyclops mutants are defective in infection thread formation. Here, we performed complementation analysis in nin mutants, using various truncated forms of the NIN promoter, and found that the CYC-RE is important for infection thread formation. Additionally, the CYC-RE deletion mutant, generated through CRISPR/Cas9 technology, displayed a significant reduction in infection thread formation, indicating that the CYC-RE is important for the fine-tuning of NIN expression during this process. However, the fact that infection thread formation is not completely abolished in the CYC-RE deletion mutant suggests that cis and trans factors other than CYCLOPS and the CYC-RE may cooperatively regulate NIN expression for the induction of infection thread formation. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Pentatricopeptide repeat protein CNS1 regulates maize mitochondrial complex III assembly and seed development

Mitochondrial function relies on the assembly of electron transport chain complexes, which requires coordination between proteins encoded by the mitochondrion and those of the nucleus. Here, we cloned a maize (Zea mays) cytochrome c maturation FN stabilizer1 (CNS1) and found it encodes a pentatricopeptide repeat (PPR) protein. Members of the PPR family are widely distributed in plants and are associated with RNA metabolism in organelles. P-type PPR proteins play essential roles in stabilizing the 3'-end of RNA in mitochondria; whether a similar process exists for stabilizing the 5'-terminus of mitochondrial RNA remains unclear. The kernels of cns1 exhibited arrested embryo and endosperm development, whereas neither conventional splicing deficiency nor RNA editing difference in mitochondrial genes was observed. Instead, most of the ccmFN transcripts isolated from cns1 mutant plants were 5'-truncated and therefore lacked the start codon. Biochemical and molecular data demonstrated that CNS1 is a P-type PPR protein encoded by nuclear DNA and that it localizes to the mitochondrion. Also, one binding site of CNS1 located upstream of the start codon in the ccmFN transcript. Moreover, abnormal mitochondrial morphology and dramatic upregulation of alternative oxidase genes were observed in the mutant. Together, these results indicate that CNS1 is essential for reaching a suitable level of intact ccmFN transcripts through binding to the 5'-UTR of the RNAs and maintaining 5'-integrity, which is crucial for sustaining mitochondrial complex III function to ensure mitochondrial biogenesis and seed development in maize.

Free energy landscape of RNA binding dynamics in start codon recognition by eukaryotic ribosomal pre-initiation complex

Specific interaction between the start codon, 5’-AUG-3’, and the anticodon, 5’-CAU-3’, ensures accurate initiation of translation. Recent studies show that several near-cognate start codons (e.g. GUG and CUG) can play a role in initiating translation in eukaryotes. However, the mechanism allowing initiation through mismatched base-pairs at the ribosomal decoding site is still unclear at an atomic level. In this work, we propose an extended simulation-based method to evaluate free energy profiles, through computing the distance between each base-pair of the triplet interactions involved in recognition of start codons in eukaryotic translation pre-initiation complex. Our method provides not only the free energy penalty for mismatched start codons relative to the AUG start codon, but also the preferred pathways of transitions between bound and unbound states, which has not been described by previous studies. To verify the method, the binding dynamics of cognate (AUG) and near-cognate start codons (CUG and GUG) were simulated. Evaluated free energy profiles agree with experimentally observed changes in initiation frequencies from respective codons. This work proposes for the first time how a G:U mismatch at the first position of codon (GUG)-anticodon base-pairs destabilizes the accommodation in the initiating eukaryotic ribosome and how initiation at a CUG codon is nearly as strong as, or sometimes stronger than, that at a GUG codon. Our method is expected to be applied to study the affinity changes for various mismatched base-pairs.

Ribosomes synthesize proteins according to the sequence of nucleotides (A, U, G, and C) in mRNA, translating three nucleotides (codon) into an amino acid. If the reading frame is shifted, the resulting amino-acid sequence will be totally different. Hence, the translation should start at an exactly determined position of mRNA. This position is usually indicated by “AUG” (start codon), which is recognized by “CAU” anticodon in tRNA and then translated into methionine. However, translation sometimes initiates at another codon such as “GUG” or “CUG”, and its frequency varies depending on the codon. Then, what regulates the possibility of translation-initiation at such a mismatched codon? To answer the question, we employed computer simulation for the structural changes of RNAs and proteins involved in the process. Through the numerical analysis, we estimated how strongly the mRNA (codon) and the tRNA (anticodon) bind to each other and also inferred how they approach. The binding-strength correlates with the initiation frequency observed in experiments, and the approaching pathway could explain the difference. Our result shows the underlying mechanism for the fidelity of translation-initiation, and our method will be applied to the prediction and design of RNA-RNA interactions.

Relaxed initiation pausing of ribosomes drives oncogenic translation

Translation is a crucial process in cancer development and progression. Many oncogenic signaling pathways target the translation initiation stage to satisfy the increased anabolic demands of cancer cells. Using quantitative profiling of initiating ribosomes, we found that ribosomal pausing at the start codon serves as a "brake" to restrain the translational output. In response to oncogenic RAS signaling, the initiation pausing relaxes and contributes to the increased translational flux. Intriguingly, messenger RNA (mRNA) m⁶A modification in the vicinity of start codons influences the behavior of initiating ribosomes. Under oncogenic RAS signaling, the reduced mRNA methylation leads to relaxed initiation pausing, thereby promoting malignant transformation and tumor growth. Restored initiation pausing by inhibiting m⁶A demethylases suppresses RAS-mediated oncogenic translation and subsequent tumorigenesis. Our findings unveil a paradigm of translational control that is co-opted by RAS mutant cancer cells to drive malignant phenotypes.

Quantitative global studies reveal differential translational control by start codon context across the fungal kingdom

Eukaryotic protein synthesis generally initiates at a start codon defined by an AUG and its surrounding Kozak sequence context, but the quantitative importance of this context in different species is unclear. We tested this concept in two pathogenic Cryptococcus yeast species by genome-wide mapping of translation and of mRNA 5' and 3' ends. We observed thousands of AUG-initiated upstream open reading frames (uORFs) that are a major contributor to translation repression. uORF use depends on the Kozak sequence context of its start codon, and uORFs with strong contexts promote nonsense-mediated mRNA decay. Transcript leaders in Cryptococcus and other fungi are substantially longer and more AUG-dense than in Saccharomyces. Numerous Cryptococcus mRNAs encode predicted dual-localized proteins, including many aminoacyl-tRNA synthetases, in which a leaky AUG start codon is followed by a strong Kozak context in-frame AUG, separated by mitochondrial-targeting sequence. Analysis of other fungal species shows that such dual-localization is also predicted to be common in the ascomycete mould, Neurospora crassa. Kozak-controlled regulation is correlated with insertions in translational initiation factors in fidelity-determining regions that contact the initiator tRNA. Thus, start codon context is a signal that quantitatively programs both the expression and the structures of proteins in diverse fungi.

Temperature-dependent regulation of upstream open reading frame translation in S. cerevisiae

BACKGROUND

Translation of an mRNA in eukaryotes starts at an AUG codon in most cases, but near-cognate codons (NCCs) such as UUG, ACG, and AUU can also be used as start sites at low levels in Saccharomyces cerevisiae. Initiation from NCCs or AUGs in the 5'-untranslated regions (UTRs) of mRNAs can lead to translation of upstream open reading frames (uORFs) that might regulate expression of the main ORF (mORF). Although there is some circumstantial evidence that the translation of uORFs can be affected by environmental conditions, little is known about how it is affected by changes in growth temperature.

RESULTS

Using reporter assays, we found that changes in growth temperature can affect translation from NCC start sites in yeast cells, suggesting the possibility that gene expression could be regulated by temperature by altering use of different uORF start codons. Using ribosome profiling, we provide evidence that growth temperature regulates the efficiency of translation of nearly 200 uORFs in S. cerevisiae. Of these uORFs, most that start with an AUG codon have increased translational efficiency at 37 °C relative to 30 °C and decreased efficiency at 20 °C. For translationally regulated uORFs starting with NCCs, we did not observe a general trend for the direction of regulation as a function of temperature, suggesting mRNA-specific features can determine the mode of temperature-dependent regulation. Consistent with this conclusion, the position of the uORFs in the 5'-leader relative to the 5'-cap and the start codon of the main ORF correlates with the direction of temperature-dependent regulation of uORF translation. We have identified several novel cases in which changes in uORF translation are inversely correlated with changes in the translational efficiency of the downstream main ORF. Our data suggest that translation of these mRNAs is subject to temperature-dependent, uORF-mediated regulation.

CONCLUSIONS

Our data suggest that alterations in the translation of specific uORFs by temperature can regulate gene expression in S. cerevisiae.

A viral RNA motif involved in signaling the initiation of translation on non-AUG codons

Noncanonical translation, and particularly initiation on non-AUG codons, are frequently used by viral and cellular mRNAs during virus infection and disease. The Sindbis virus (SINV) subgenomic mRNA (sgRNA) constitutes a unique model system to analyze the translation of a capped viral mRNA without the participation of several initiation factors. Moreover, sgRNA can initiate translation even when the AUG initiation codon is replaced by other codons. Using SINV replicons, we examined the efficacy of different codons in place of AUG to direct the synthesis of the SINV capsid protein. The substitution of AUG by CUG was particularly efficient in promoting the incorporation of leucine or methionine in similar percentages at the amino terminus of the capsid protein. Additionally, valine could initiate translation when the AUG is replaced by GUG. The ability of sgRNA to initiate translation on non-AUG codons was dependent on the integrity of a downstream stable hairpin (DSH) structure located in the coding region. The structural requirements of this hairpin to signal the initiation site on the sgRNA were examined in detail. Of interest, a virus bearing CUG in place of AUG in the sgRNA was able to infect cells and synthesize significant amounts of capsid protein. This virus infects the human haploid cell line HAP1 and the double knockout variant that lacks eIF2A and eIF2D. Collectively, these findings indicate that leucine-tRNA or valine-tRNA can participate in the initiation of translation of sgRNA by a mechanism dependent on the DSH. This mechanism does not involve the action of eIF2, eIF2A, or eIF2D.

Comprehensive profiling of translation initiation in influenza virus infected cells

Translation can initiate at alternate, non-canonical start codons in response to stressful stimuli in mammalian cells. Recent studies suggest that viral infection and anti-viral responses alter sites of translation initiation, and in some cases, lead to production of novel immune epitopes. Here we systematically investigate the extent and impact of alternate translation initiation in cells infected with influenza virus. We perform evolutionary analyses that suggest selection against non-canonical initiation at CUG codons in influenza virus lineages that have adapted to mammalian hosts. We then use ribosome profiling with the initiation inhibitor lactimidomycin to experimentally delineate translation initiation sites in a human lung epithelial cell line infected with influenza virus. We identify several candidate sites of alternate initiation in influenza mRNAs, all of which occur at AUG codons that are downstream of canonical initiation codons. One of these candidate downstream start sites truncates 14 amino acids from the N-terminus of the N1 neuraminidase protein, resulting in loss of its cytoplasmic tail and a portion of the transmembrane domain. This truncated neuraminidase protein is expressed on the cell surface during influenza virus infection, is enzymatically active, and is conserved in most N1 viral lineages. We do not detect globally higher levels of alternate translation initiation on host transcripts upon influenza infection or during the anti-viral response, but the subset of host transcripts induced by the anti-viral response is enriched for alternate initiation sites. Together, our results systematically map the landscape of translation initiation during influenza virus infection, and shed light on the evolutionary forces shaping this landscape.

A common class of transcripts with 5'-intron depletion, distinct early coding sequence features, and N1-methyladenosine modification

Introns are found in 5' untranslated regions (5'UTRs) for 35% of all human transcripts. These 5'UTR introns are not randomly distributed: Genes that encode secreted, membrane-bound and mitochondrial proteins are less likely to have them. Curiously, transcripts lacking 5'UTR introns tend to harbor specific RNA sequence elements in their early coding regions. To model and understand the connection between coding-region sequence and 5'UTR intron status, we developed a classifier that can predict 5'UTR intron status with >80% accuracy using only sequence features in the early coding region. Thus, the classifier identifies transcripts with 5' proximal-intron-minus-like-coding regions ("5IM" transcripts). Unexpectedly, we found that the early coding sequence features defining 5IM transcripts are widespread, appearing in 21% of all human RefSeq transcripts. The 5IM class of transcripts is enriched for non-AUG start codons, more extensive secondary structure both preceding the start codon and near the 5' cap, greater dependence on eIF4E for translation, and association with ER-proximal ribosomes. 5IM transcripts are bound by the exon junction complex (EJC) at noncanonical 5' proximal positions. Finally, N¹-methyladenosines are specifically enriched in the early coding regions of 5IM transcripts. Taken together, our analyses point to the existence of a distinct 5IM class comprising ∼20% of human transcripts. This class is defined by depletion of 5' proximal introns, presence of specific RNA sequence features associated with low translation efficiency, N¹-methyladenosines in the early coding region, and enrichment for noncanonical binding by the EJC.

A Unique 5' Translation Element Discovered in Triticum Mosaic Virus

Several plant viruses encode elements at the 5' end of their RNAs, which, unlike most cellular mRNAs, can initiate translation in the absence of a 5' m7GpppG cap. Here, we describe an exceptionally long (739-nucleotide [nt]) leader sequence in triticum mosaic virus (TriMV), a recently emerged wheat pathogen that belongs to the Potyviridae family of positive-strand RNA viruses. We demonstrate that the TriMV 5' leader drives strong cap-independent translation in both wheat germ extract and oat protoplasts through a novel, noncanonical translation mechanism. Translation preferentially initiates at the 13th start codon within the leader sequence independently of eIF4E but involves eIF4G. We truncated the 5' leader to a 300-nucleotide sequence that drives cap-independent translation from the 5' end. We show that within this sequence, translation activity relies on a stem-loop structure identified at nucleotide positions 469 to 490. The disruption of the stem significantly impairs the function of the 5' untranslated region (UTR) in driving translation and competing against a capped RNA. Additionally, the TriMV 5' UTR can direct translation from an internal position of a bicistronic mRNA, and unlike cap-driven translation, it is unimpaired when the 5' end is blocked by a strong hairpin in a monocistronic reporter. However, the disruption of the identified stem structure eliminates such a translational advantage. Our results reveal a potent and uniquely controlled translation enhancer that may provide new insights into mechanisms of plant virus translational regulation.

IMPORTANCE

Many members of the Potyviridae family rely on their 5' end for translation. Here, we show that the 739-nucleotide-long triticum mosaic virus 5' leader bears a powerful translation element with features distinct from those described for other plant viruses. Despite the presence of 12 AUG start codons within the TriMV 5' UTR, translation initiates primarily at the 13th AUG codon. The TriMV 5' UTR is capable of driving cap-independent translation in vitro and in vivo, is independent of eIF4E, and can drive internal translation initiation. A hairpin structure at nucleotide positions 469 to 490 is required for the cap-independent translation and internal translation initiation abilities of the element and plays a role in the ability of the TriMV UTR to compete against a capped RNA in vitro. Our results reveal a novel translation enhancer that may provide new insights into the large diversity of plant virus translation mechanisms.

Initiation of mRNA translation in bacteria: structural and dynamic aspects

Initiation of mRNA translation is a major checkpoint for regulating level and fidelity of protein synthesis. Being rate limiting in protein synthesis, translation initiation also represents the target of many post-transcriptional mechanisms regulating gene expression. The process begins with the formation of an unstable 30S pre-initiation complex (30S pre-IC) containing initiation factors (IFs) IF1, IF2 and IF3, the translation initiation region of an mRNA and initiator fMet-tRNA whose codon and anticodon pair in the P-site following a first-order rearrangement of the 30S pre-IC produces a locked 30S initiation complex (30SIC); this is docked by the 50S subunit to form a 70S complex that, following several conformational changes, positional readjustments of its ligands and ejection of the IFs, becomes a 70S initiation complex productive in initiation dipeptide formation. The first EF-G-dependent translocation marks the beginning of the elongation phase of translation. Here, we review structural, mechanistic and dynamical aspects of this process.

Polyamine stimulation of eEF1A synthesis based on the unusual position of a complementary sequence to 18S rRNA in eEF1A mRNA

It is thought that Shine-Dalgarno-like sequences, which exhibit complementarity to the nucleotide sequences at the 3'-end of 18S rRNA, are not present in eukaryotic mRNAs. However, complementary sequences consisting of more than 5 nucleotides to the 3'-end of 18S rRNA, i.e., a CR sequence, are present at -17 to -32 upstream from the initiation codon AUG in 18 mRNAs involved in protein synthesis except eEF1A mRNA. Thus, effects of the CR sequence in mRNAs and polyamines on protein synthesis were examined using control and polyamine-reduced FM3A and NIH3T3 cells. Polyamines did not stimulate protein synthesis encoded by 18 mRNAs possessing a normal CR sequence. When the CR sequence was deleted, protein synthetic activities decreased to less than 70% of intact mRNAs. In eEF1A mRNA, the CR sequence was located at -33 to -39 upstream from the initiation codon AUG, and polyamines stimulated eEF1A synthesis about threefold. When the CR sequence was shifted to -22 to -28 upstream from the AUG, eEF1A synthesis increased in polyamine-reduced cells and the degree of polyamine stimulation decreased greatly. The results indicate that the CR sequence exists in many eukaryotic mRNAs, and the location of a CR sequence in mRNAs influences polyamine stimulation of protein synthesis.

Start codon targeted (SCoT) polymorphism in toxic and non-toxic accessions of Jatropha curcas L. and development of a codominant SCAR marker

Thirty six start codon targeted (SCoT) primers were used for characterization of 48 accessions of Jatropha curcas from different countries and include material with genetic variation for levels of phorbol esters, yield, seed oil content, test weight and plant type. SCoT analysis revealed high polymorphism and 74% of the primers generated polymorphic profiles. The SCoT6 primer discriminated edible and toxic accessions in a single reaction while the SCoT26 and 27 primers produced amplicons specific to toxic and non-toxic accessions, respectively. The polymorphic SCoT markers obtained with these three primers were converted to sequence characterized amplicon regions (SCARs) which resulted in codominant SCARs with SCoT6 primer and dominant SCARs with SCoT 26 and 27 primers. The codominant nature of SCoT6 primer and the resultant SCAR6 primer were validated on intraspecific hybrids derived from a cross between non-toxic and toxic accessions. The accession JP38 from Madagascar was found to be distinct and showed accession specific bands with 9 different SCoT primers. Sequence analysis of polymorphic amplicons obtained with SCoT6 primer showed a 65 bp deletion in accessions with low/zero phorbol esters. Diversity analysis separated the toxic and non-toxic accessions into two groups and the accessions JP29 and JP48 from Mexico formed a third cluster.

Cell-cycle and DNA damage regulation of the DNA mismatch repair protein Msh2 occurs at the transcriptional and post-transcriptional level

DNA mismatch repair during replication is a conserved process essential for maintaining genomic stability. Mismatch repair is also implicated in cell-cycle arrest and apoptosis after DNA damage. Because yeast and human mismatch repair systems are well conserved, we have employed the budding yeast Saccharomyces cerevisiae to understand the regulation and function of the mismatch repair gene MSH2. Using a luciferase-based transcriptional reporter, we defined a 218-bp region upstream of MSH2 that contains cell-cycle and DNA damage responsive elements. The 5' end of the MSH2 transcript was mapped by primer extension and was found to encode a small upstream open reading frame (uORF). Mutagenesis of the uORF start codon or of the uORF stop codon, which creates a continuous reading frame with MSH2, increased Msh2 steady-state protein levels ∼2-fold. Furthermore, we found that the cell-cycle transcription factors Swi6, Swi4, and Mbp1-along with SCB/MCB cell-cycle binding sites upstream of MSH2-are all required for full basal expression of MSH2. Mutagenesis of the cell-cycle boxes resulted in a minor reduction in basal Msh2 levels and a 3-fold defect in mismatch repair. Disruption of the cell-cycle boxes also affected growth in a DNA polymerase-defective strain background where mismatch repair is essential, particularly in the presence of the DNA damaging agent methyl methane sulfonate (MMS). Promoter replacements conferring constitutive expression of MSH2 revealed that the transcriptional induction in response to MMS is required to maintain induced levels of Msh2. Turnover experiments confirmed an elevated rate of degradation in the presence of MMS. Taken together, the data show that the DNA damage regulation of Msh2 occurs at the transcriptional and post-transcriptional levels. The transcriptional and translational control elements identified are conserved in mammalian cells, underscoring the use of yeast as a model system to examine the regulation of MSH2.

Exploring IRES region accessibility by interference of foot-and-mouth disease virus infectivity

Translation initiation of picornavirus RNA is driven by an internal ribosome entry site (IRES) element located upstream of the initiator codon. RNA structure organization as well as RNA-protein interaction plays a fundamental role in internal initiation. IRES activity has been mainly analyzed in the context of reporter genes, lacking regions of the viral genome potentially affecting translation efficiency. With the aim to understand the vulnerability of the IRES and translation start region to small molecules in the context of the viral genome, we designed a set of customized RNase-resistant 2'O-methyl antisense oligoribonucleotides (2'OMe AONs) based on RNA structure data. These AONs were then used to monitor their capacity to interfere viral RNA translation, and thus, to inhibit virus yield. Foot-and-mouth disease virus (FMDV) RNA translation can be initiated at two in-frame AUG codons. We show here that a 2'OMe AON complementary to AUG2 inhibited viral multiplication more efficiently than the one that targeted AUG1. Furthermore, the response of the viral RNA to AONs targeting the IRES region denoted important differences between tissue culture cells and cell-free systems, reinforcing the need to analyze viral RNA response in living cells. Importantly, we have identified four specific motifs within the IRES element that are targets for viral inhibitors both in tissue culture cells and in cell-free systems. The identified targets define accessible regions to small molecules, which disturb either the RNA structural organization or the RNA-protein interactions needed to initiate translation in FMDV RNA.

Role of helix 44 of 16S rRNA in the fidelity of translation initiation

The molecular mechanisms that govern translation initiation to ensure accuracy remain unclear. Here, we provide evidence that the subunit-joining step of initiation is controlled in part by a conformational change in the 1408 region of helix h44. First, chemical probing of 30S initiation complexes formed with either a cognate (AUG) or near-cognate (AUC) start codon shows that an IF1-dependent enhancement at A1408 is reduced in the presence of AUG. This change in reactivity is due to a conformational change rather than loss of IF1, because other portions of the IF1 footprint are unchanged and high concentrations of IF1 fail to diminish the reactivity difference seen at A1408. Second, mutations in h44 such as A1413C stimulate 50S docking and cause reduced reactivity at A1408. Third, streptomycin, which has been shown by Rodnina and coworkers to stimulate 50S docking by reversing the inhibitory effects of IF1, also causes reduced reactivity at A1408. Collectively, these data support a model in which IF1 alters the A1408 region of h44 in a way that makes 50S docking unfavorable, and canonical codon-anticodon pairing in the P site restores h44 to a docking-favorable conformation. We also find that, in the absence of factors, the cognate 30S•AUG•fMet-tRNA ternary complex is >1000-fold more stable than the near-cognate 30S•AUC•fMet-tRNA complex. Hence, the selectivity of ternary complex formation is inherently high, exceeding that of initiation in vivo by more than 10-fold.

HCV IRES domain IIb affects the configuration of coding RNA in the 40S subunit's decoding groove

Hepatitis C virus (HCV) uses a structured internal ribosome entry site (IRES) RNA to recruit the translation machinery to the viral RNA and begin protein synthesis without the ribosomal scanning process required for canonical translation initiation. Different IRES structural domains are used in this process, which begins with direct binding of the 40S ribosomal subunit to the IRES RNA and involves specific manipulation of the translational machinery. We have found that upon initial 40S subunit binding, the stem-loop domain of the IRES that contains the start codon unwinds and adopts a stable configuration within the subunit's decoding groove. This configuration depends on the sequence and structure of a different stem-loop domain (domain IIb) located far from the start codon in sequence, but spatially proximal in the IRES•40S complex. Mutation of domain IIb results in misconfiguration of the HCV RNA in the decoding groove that includes changes in the placement of the AUG start codon, and a substantial decrease in the ability of the IRES to initiate translation. Our results show that two distal regions of the IRES are structurally communicating at the initial step of 40S subunit binding and suggest that this is an important step in driving protein synthesis.

Upstream open reading frames: molecular switches in (patho)physiology

Conserved upstream open reading frames (uORFs) are found within many eukaryotic transcripts and are known to regulate protein translation. Evidence from genetic and bioinformatic studies implicates disturbed uORF-mediated translational control in the etiology of human diseases. A genetic mouse model has recently provided proof-of-principle support for the physiological relevance of uORF-mediated translational control in mammals. The targeted disruption of the uORF initiation codon within the transcription factor CCAAT/enhancer binding protein β (C/EBPβ) gene resulted in deregulated C/EBPβ protein isoform expression, associated with defective liver regeneration and impaired osteoclast differentiation. The high prevalence of uORFs in the human transcriptome suggests that intensified search for mutations within 5' RNA leader regions may reveal a multitude of alterations affecting uORFs, causing pathogenic deregulation of protein expression.

The HCV IRES pseudoknot positions the initiation codon on the 40S ribosomal subunit

The hepatitis C virus (HCV) genomic RNA contains an internal ribosome entry site (IRES) in its 5' untranslated region, the structure of which is essential for viral protein translation. The IRES includes a predicted pseudoknot interaction near the AUG start codon, but the results of previous studies of its structure have been conflicting. Using mutational analysis coupled with activity and functional assays, we verified the importance of pseudoknot base pairings for IRES-mediated translation and, using 35 mutants, conducted a comprehensive study of the structural tolerance and functional contributions of the pseudoknot. Ribosomal toeprinting experiments show that the entirety of the pseudoknot element positions the initiation codon in the mRNA binding cleft of the 40S ribosomal subunit. Optimal spacing between the pseudoknot and the start site AUG resembles that between the Shine-Dalgarno sequence and the initiation codon in bacterial mRNAs. Finally, we validated the HCV IRES pseudoknot as a potential drug target using antisense 2'-OMe oligonucleotides.

Kinetic and thermodynamic analysis of the role of start codon/anticodon base pairing during eukaryotic translation initiation

Start codon recognition is a crucial event in the initiation of protein synthesis. To gain insight into the mechanism of start codon recognition in eukaryotes, we used a yeast reconstituted initiation system to isolate the step of Met-tRNA(i)*eIF2*GTP ternary complex (TC) binding to the 40S subunit. We examined the kinetics and thermodynamics of this step in the presence of base changes in the mRNA start codon and initiator methionyl tRNA anticodon, in order to investigate the effects of base pairing and sequence on the stability of the resulting 43S*mRNA complex. We observed that the formation of three base pairs, rather than their identities, was the key determinant of stability of TC binding, indicating that nothing is inherently special about the sequence AUG for this step. Surprisingly, the rate constant for TC binding to the 40S subunit was strongly codon dependent, whereas the rate constant for TC dissociation from the 43S*mRNA complex was not. The data suggest a model in which, after the initial diffusion-limited encounter of TC with the 40S subunit, the formation of three matching start codon/anticodon base pairs triggers a conformational change that locks the complex into a stable state. This induced-fit mechanism supports the proposal that initiation codon recognition by the 43S complex induces a conformational change from an open state to a closed one that arrests movement along the mRNA.

XPC initiation codon mutation in xeroderma pigmentosum patients with and without neurological symptoms

Two unrelated xeroderma pigmentosum (XP) patients, with and without neurological abnormalities, respectively, had identical defects in the XPC DNA nucleotide excision repair (NER) gene. Patient XP21BE, a 27-year-old woman, had developmental delay and early onset of sensorineural hearing loss. In contrast, patient XP329BE, a 13-year-old boy, had a normal neurological examination. Both patients had marked lentiginous hyperpigmentation and multiple skin cancers at an early age. Their cultured fibroblasts showed similar hypersensitivity to killing by UV and reduced repair of DNA photoproducts. Cells from both patients had a homozygous c.2T>G mutation in the XPC gene which changed the ATG initiation codon to arginine (AGG). Both had low levels of XPC message and no detectable XPC protein on Western blotting. There was no functional XPC activity in both as revealed by the failure of localization of XPC and other NER proteins at the sites of UV-induced DNA damage in a sensitive in vivo immunofluorescence assay. XPC cDNA containing the initiation codon mutation was functionally inactive in a post-UV host cell reactivation (HCR) assay. Microsatellite markers flanking the XPC gene showed only a small region of identity ( approximately 30kBP), indicating that the patients were not closely related. Thus, the initiation codon mutation resulted in DNA repair deficiency in cells from both patients and greatly increased cancer susceptibility. The neurological abnormalities in patient XP21BE may be related to close consanguinity and simultaneous inheritance of other recessive genes or other gene modifying effects rather than the influence of XPC gene itself.

Characterization of the termination-reinitiation strategy employed in the expression of influenza B virus BM2 protein

Coupled expression of the M1 and BM2 open-reading frames (ORFs) of influenza B from the dicistronic segment 7 mRNA occurs by a process of termination-dependent reinitiation. The AUG start codon of the BM2 ORF overlaps the stop codon of the upstream M1 ORF in the pentanucleotide UAAUG, and BM2 synthesis is dependent upon translation of the M1 ORF and termination at the stop codon. Here, we have investigated the mRNA sequence requirements for BM2 expression. Termination-reinitiation is dependent upon 45 nucleotide (nt) of RNA immediately upstream of the UAAUG pentanucleotide, which includes an essential stretch complementary to 18S rRNA helix 26. Thus, similar to the caliciviruses, base-pairing between mRNA and rRNA is likely to play a role in tethering the 40S subunit to the mRNA following termination at the M1 stop codon. Consistent with this, repositioning of the M1 stop codon more than 24 nt downstream from the BM2 start codon inhibited BM2 expression. RNA structure probing revealed that the RNA upstream of the UAAUG overlap is not highly structured, but upon encountering the M1 stop codon by the ribosome, a stem-loop may form immediately 5' of the ribosome, with the 18S rRNA complementary region in the apical loop and in close proximity to helix 26. Mutational analysis reveals that the normal requirements for start site selection in BM2 expression are suspended, with little effect of initiation codon context and efficient use of noncanonical initiation codons. This suggests that the full complement of initiation factors is not required for the reinitiation process.

Ribosomes bind leaderless mRNA in Escherichia coli through recognition of their 5'-terminal AUG

Leaderless mRNAs are translated in the absence of upstream signals that normally contribute to ribosome binding and translation efficiency. In order to identify ribosomal components that interact with leaderless mRNA, a fragment of leaderless cI mRNA from bacteriophage lambda, with a 4-thiouridine (4(S)-U) substituted at the +2 position of the AUG start codon, was used to form cross-links to Escherichia coli ribosomes during binary (mRNA+ribosome) and ternary (mRNA+ribosome+initiator tRNA) complex formation. Ribosome binding assays (i.e., toeprints) demonstrated tRNA-dependent binding of leaderless mRNA to ribosomes; however, cross-links between the start codon and 30S subunit rRNA and r-proteins formed independent of initiator tRNA. Toeprints revealed that a leaderless mRNA's 5'-AUG is required for stable binding. Furthermore, the addition of a 5'-terminal AUG triplet to a random RNA fragment can make it both competent and competitive for ribosome binding, suggesting that a leaderless mRNA's start codon is a major feature for ribosome interaction. Cross-linking assays indicate that a subset of 30S subunit r-proteins, located at either end of the mRNA tunnel, contribute to tRNA-independent contacts and/or interactions with a leaderless mRNA's start codon. The interaction of leaderless mRNA with ribosomes may reveal features of mRNA binding and AUG recognition that are distinct from known signals but are important for translation initiation of all mRNAs.

[Start codon in the serine proteinase gene from Bacillus intermedius]

The translation initiation site in the extracellular serine subtilisin-like proteinase gene from Bacillus intermedius (aprBi) (AN AY754946) secreting at the stationary growth phase was established. The analysis of aprBi open reading frame revealed three putative translation start sites (TTG, GTG, ATG). Using SignalP online freeware program we have determined the functional activity probability of each of them. To identify the translation start point the modified subtilisin-like protease genes carrying nucleotide replacements in supposed start codons were developed using oligonucleotide-directed mutagenesis. We have investigated the expression of these genetic constructions in protease-deficient strain B. subtilis AJ73. According our results it was concluded that the translation in aprBi gene starts from GTG kodon.

Translational regulation of human methionine synthase by upstream open reading frames

Methionine synthase is a key enzyme poised at the intersection of folate and sulfur metabolism and functions to reclaim homocysteine to the methionine cycle. The 5' leader sequence in human MS is 394 nucleotides long and harbors two open reading frames (uORFs). In this study, regulation of the main open reading frame by the uORFs has been elucidated. Both uORFs downregulate translation as demonstrated by mutation of the upstream AUG codons (uAUG) either singly or simultaneously. The uAUGs are capable of recruiting the 40S ribosomal complex as revealed by their ability to drive reporter expression in constructs in which the luciferase is fused to the uORFs. uORF2, which is predicted to encode a 30 amino acid long polypeptide, has a clustering of rare codons encoding arginine and proline. Mutation of a tandemly repeated rare codon for arginine at positions 3 and 4 in uORF2 to either common codons for the same amino acid or common codons for alanine results in complete alleviation of translation inhibition. This suggests a mechanism for ribosome stalling and demonstrates that the cis-effects on translation by uORF2 is dependent on the nucleotide sequence but is apparently independent of the sequence of the encoded peptide. This study reveals complex regulation of the essential housekeeping gene, methionine synthase, by the uORFs in its leader sequence.

Leaky scanning and scanning-independent ribosome migration on the tricistronic S1 mRNA of avian reovirus

The S1 genome segments of avian and Nelson Bay reovirus encode tricistronic mRNAs containing three sequential partially overlapping open reading frames (ORFs). The translation start site of the 3'-proximal ORF encoding the sigmaC protein lies downstream of two ORFs encoding the unrelated p10 and p17 proteins and more than 600 nucleotides distal from the 5'-end of the mRNA. It is unclear how translation of this remarkable tricistronic mRNA is regulated. We now show that the p10 and p17 ORFs are coordinately expressed by leaky scanning. Translation initiation events at these 5'-proximal ORFs, however, have little to no effect on translation of the 3'-proximal sigmaC ORF. Northern blotting, insertion of upstream stop codons or optimized translation start sites, 5'-truncation analysis, and poliovirus 2A protease-mediated cleavage of eIF4G indicated sigmaC translation derives from a full-length tricistronic mRNA using a mechanism that is eIF4G-dependent but leaky scanning- and translation reinitiation-independent. Further analysis of artificial bicistronic mRNAs failed to provide any evidence that sigmaC translation derives from an internal ribosome entry site. Additional features of the S1 mRNA and the mechanism of sigmaC translation also differ from current models of ribosomal shunting. Translation of the tricistronic reovirus S1 mRNA, therefore, is dependent both on leaky scanning and on a novel scanning-independent mechanism that allows translation initiation complexes to efficiently bypass two functional upstream ORFs.

Cloning and characterization of the rat lysyl oxidase gene promoter: identification of core promoter elements and functional nuclear factor I-binding sites

Lysyl oxidase (LO) stabilizes the extracellular matrix by cross-linking collagen and elastin. To assess the transcriptional regulation of LO, we cloned the 5'-flanking region with 3,979 bp of the rat LO gene. LO transcription started at multiple sites clustered at the region from -78 to -51 upstream of ATG. The downstream core promoter element functionally independent of the initiator predominantly activated the TATA-less LO gene. 5' Deletion assays illustrated a sequence of 804 bp upstream of ATG sufficient for eliciting the maximal promoter activity and the region -709/-598 exhibiting strongly enhancing effects on the reporter gene expression in transiently transfected RFL6 cells. DNase I footprinting assays showed a protected pattern existing in the fragment -612/-580, which contains a nuclear factor I (NFI)-binding site at the region -594/-580 confirmed by electrophoretic mobility supershift assays. Mutations on this acting site decreased both NFI binding affinity in gel shift assays and stimulation of SV40 promoter activities in cells transfected with the NFI-binding site-SV40 promoter chimeric construct. Furthermore, at least two functional NFI-binding sites, including another one located at -147/-133, were identified in the LO promoter region -804/-1. Only NFI-A and NFI-B were expressed in rat lung fibroblasts, and their interaction with the LO gene was sensitively modulated by exogenous stimuli such as cigarette smoke condensate. In conclusion, the isolated rat LO gene promoter contains functionally independent initiator and downstream core promoter elements, and the conserved NFI-binding sites play a critical role in the LO gene activation.

Characterization of the sequence element directing translation reinitiation in RNA of the calicivirus rabbit hemorrhagic disease virus

The calicivirus minor capsid protein VP2 is expressed via reinitiation of protein synthesis after termination of translation of the preceding VP1 gene. A sequence element of about 80 nucleotides denoted "termination upstream ribosomal binding site" (TURBS) (25) is crucial for reinitiation. Deletion mapping in the TURBS of a rabbit calicivirus identified two short sequence motifs that were crucial for VP2 expression. Motif 1 is conserved among caliciviruses and is complementary to a sequence in the 18S rRNA. Single-residue exchanges in this motif severely impaired reinitiation when they affected the putative rRNA binding, whereas an exchange preserving complementarity had only a minor effect. Single exchanges in motif 2 were rather well tolerated, but the introduction of double exchanges almost blocked VP2 expression. In contrast, the deletion analyses showed that the RNA between the two motifs is of minor importance. The distance between motif 2 and the start site was found to be important, since deletions of increasing length in this sequence or upstream positioning of the start codon reduced VP2 expression stepwise to low levels, whereas multiple-nucleotide exchanges in this region were tolerated. The low flexibility of the arrangement of TURBS motif 2 and the start codon stand in marked contrast to the requirements with regard to the location of the stop codon of the preceding VP1 gene, which could be moved far downstream with continuous reduction, but without loss, of VP2 translation. The sequence mapping resulted in a refined model of the reinitiation mechanism leading to VP2 expression.

Post-transcriptional control of the MCT-1-associated protein DENR/DRP by RNA-binding protein AUF1

BACKGROUND

There is often a poor correlation observed between protein and RNA in eukaryotic systems, supporting the emerging pardigm that many of the abnormalities in a cancer cell's proteome may be achieved by differential recruitment of mRNAs to polysomes referred to as the translational profile. The MCT-1 oncogene product has recently been shown to interact with the cap complex and to modulate the translational profile of cell lines when MCT-1 was highly expressed. The MCT-1 protein modifies mRNA translational profiles through its interaction with DENR/DRP, a protein containing an SUI1 domain involved in recognition of the translation initiation codon. It has been shown previously that the protein levels of DENR/DRP go up in parallel with increasing cell density, however the mechanism(s) underlying this increase is poorly understood at present. The 3'-untranslated region (3'UTR) of DENR/DRP was found to have a high number of uracyl (U)- and adenine (A)-rich sequences (AREs). Many RNA-binding proteins (RBPs) have been shown to recognize and bind to mRNAs that contains AREs generally present in the 3'UTR of mRNAs. RBPs binding to AREs such as AUF1, BRF1, KSRP, and TTP are known to regulate mRNA turnover, while TIAR and TIA-1 influence mRNA translation.

MATERIALS AND METHODS

We assessed the association of several ARE binding proteins with DENR/DRP mRNA by reverse transcription of the RNA obtained after immunoprecipitation of cell lysates from HEK 293 cells growing at varying levels of cell density. HEK 293 cells were transfected with an AUF1 silencing vector (shRNA), and protein levels of DENR/DRP were analyzed by Western blotting.

RESULTS

We demonstrated that both HuR and AUF1 bind to discrete regions of DENR/DRP mRNA and that AUF1 silencing increases DENR/DRP protein levels.

CONCLUSION

Our data established a cell density-dependent interaction of AUF1 protein with DENR/DRP mRNA that modulates DENR/DRP protein levels.

Non-AUG translation initiation of a fungal RING finger repressor involved in photocarotenogenesis

The RING finger protein CrgA acts as a negative regulator of light-induced carotene biosynthesis in the fungus Mucor circinelloides. Sequence analysis of the crgA coding region upstream of the first AUG codon revealed the existence of an additional non-canonical RING finger domain at the most N-terminal end of the protein. The newly identified RING finger domain is required for CrgA to regulate photocarotenogenesis, as deduced from site-directed mutagenesis experiments. The role of both RING finger domains in the stability of CrgA has been investigated in a yeast system. Wild type CrgA, but not the RING finger deleted forms, is highly unstable and is stabilized by inhibition of the proteasome function, which suggests that native CrgA is degraded by the proteasome and that active RING finger domains are required for proteasome-mediated CrgA degradation. To identify the translation start of CrgA, a mutational analysis of putative initiation codons in the 5' region of the crgA gene was accomplished. We demonstrated that a GUG codon located upstream of the first AUG is the sole initiator of CrgA translation. To our knowledge, this is the first report of a naturally occurring non-AUG start codon for a RING finger regulatory protein. A combination of suboptimal translation initiation and proteasome degradation may help to maintain the low cellular levels of CrgA observed in wild type cells, which is probably required for accurate regulation of photocarotenogenesis.

Influences on gene expression in vivo by a Shine-Dalgarno sequence

The Shine-Dalgarno (SD+: 5'-AAGGAGG-3') sequence anchors the mRNA by base pairing to the 16S rRNA in the small ribosomal subunit during translation initiation. We have here compared how an SD+ sequence influences gene expression, if located upstream or downstream of an initiation codon. The positive effect of an upstream SD+ is confirmed. A downstream SD+ gives decreased gene expression. This effect is also valid for appropriately modified natural Escherichia coli genes. If an SD+ is placed between two potential initiation codons, initiation takes place predominantly at the second start site. The first start site is activated if the distance between this site and the downstream SD+ is enlarged and/or if the second start site is weakened. Upstream initiation is eliminated if a stable stem-loop structure is placed between this SD+ and the upstream start site. The results suggest that the two start sites compete for ribosomes that bind to an SD+ located between them. A minor positive contribution to upstream initiation resulting from 3' to 5' ribosomal diffusion along the mRNA is suggested. Analysis of the E. coli K12 genome suggests that the SD+ or SD-like sequences are systematically avoided in the early coding region suggesting an evolutionary significance.

Unfolding of mRNA secondary structure by the bacterial translation initiation complex

Translation initiation is a key step for regulating the level of numerous proteins within the cell. In bacteria, the 30S initiation complex directly binds to the translation initiation region (TIR) of the mRNA. How the ribosomal 30S subunit assembles on highly structured TIR is not known. Using fluorescence-based experiments, we assayed 12 different mRNAs that form secondary structures with various stabilities and contain Shine-Dalgarno (SD) sequences of different strengths. A strong correlation was observed between the stability of the mRNA structure and the association and dissociation rate constants. Interestingly, in the presence of initiation factors and initiator tRNA, the association kinetics of structured mRNAs showed two distinct phases. The second phase was found to be important for unfolding structured mRNAs to form a stable 30S initiation complex. We show that unfolding of structured mRNAs requires a SD sequence, the start codon, fMet-tRNA(fMet), and the GTP bound form of initiation factor 2 bound to the 30S subunit.

Identification and expression analysis of alternative splice variants of the rat Hax-1 gene

Hax-1 protein, which has been studied in mice and humans, shows a potent anti-apoptotic activity and is involved in regulation of cell motility. Cloning of the rat Hax-1 cDNA has revealed seven alternative transcripts, which differ mostly in their 5' region. Alternative splicing concerns exon 1, skipped in 5 transcripts, intron 1 which is partially retained in these transcripts, exon 2, which can be partially skipped, and intron 2, retained in one variant. The existence of different splicing variants was confirmed by exon-junction-specific RT-PCR and RNase protection assay. Analysis of expression indicates that overall Hax-1 mRNA level is relatively low in most tissues and very high in testes, and that the expression pattern of the variants is similar in different tissues. Presence of different transcripts implies the existence of several protein isoforms, with three putative start codons. The existence of at least three protein isoforms was confirmed by Western blot. Interestingly, high mRNA level in testes does not translate into high protein level, suggesting the existence of tissue-specific translational regulation or regulated protein degradation.