Oncogene activation leads to cellular transformation by deregulation of biological processes such as proliferation and metabolism. Paradoxically, this can also sensitize cells to nutrient deprivation, potentially representing an Achilles' heel in early stage tumors. The mechanisms underlying this phenotype include loss of energetic and redox homeostasis as a result of metabolic reprogramming, favoring synthesis of macromolecules. Moreover, an emerging mechanism involving the deregulation of mRNA translation elongation through inhibition of eukaryotic elongation factor 2 kinase (eEF2K) is (...) presented. The potential consequences of eEF2K deregulation leading to cell death under nutrient depletion are discussed. Finally, the relevance of eEF2K as a master regulator of the response to nutrient deprivation in vivo, and its potential exploitation for therapeutic targeting of cancers, are elaborated. Overall, a better understanding of the adaptive mechanisms allowing tumors to circumvent oncogene‐induced hypersensitivity to nutrient deprivation is a promising avenue for uncovering novel therapeutic targets in cancers. (shrink)

How cell polarity is established and maintained is an important question in diverse biological contexts. Molecular mechanisms used to localize polarity proteins to distinct domains are likely context‐dependent and provide a feedback loop in order to maintain polarity. One such mechanism is the localized translation of mRNAs encoding polarity proteins, which will be the focus of this review and may play a more important role in the establishment and maintenance of polarity than is currently known. Localized translation of (...) mRNAs encoding polarity proteins can be used to establish polarity in response to an external signal, and to maintain polarity by local production of polarity determinants. The importance of this mechanism is illustrated by recent findings, including orb2‐dependent localized translation of aPKC mRNA at the apical end of elongating spermatid tails in the Drosophila testis, and the apical localization of stardust A mRNA in Drosophila follicle and embryonic epithelia. (shrink)

Translation of the genetic code occurs in a cycle where ribosomes engage mRNAs, synthesize protein, and then disengage in order to repeat the process again. The final part of this process—ribosome recycling, where ribosomes dissociate from mRNAs—involves a complex molecular choreography of specific protein factors to remove the large and small subunits of the ribosome in a coordinated fashion. Errors in this process can lead to the accumulation of ribosomes at stop codons or translation of downstream open reading (...) frames (ORFs). Ribosome recycling is also critical when a ribosome stalls during the elongation phase of translation and must be rescued to allow continued translation of the mRNA. Here we discuss the molecular interactions that drive ribosome recycling, and their regulation in the cell. We also examine the consequences of inefficient recycling with regards to disease, and its functional roles in synthesis of novel peptides, regulation of gene expression, and control of mRNA‐associated proteins. Alterations in ribosome recycling efficiency have the potential to impact many cellular functions but additional work is needed to understand how this regulatory power is utilized. (shrink)

The unfertilized sea urchin egg is a metabolically quiescent cell. Fertilization results in the activation of a variety of metabolic and biosynthetic pathways, including a 20‐ to 40‐fold increase in the rate of protein synthesis by 2 h after fertilization. This increase is regulated at a purely translational level without the need for new transcription. The greatest part of this increase is due to the translation of stored maternal mRNAs which were not translated in the egg. There is also (...) a 2‐to 3‐fold increase in the peptide elongation rate. The molecular and physiological mechanisms responsible for this activation process are beginning to be understood, and turn out to be much more complex than was anticipated. (shrink)

Localized mRNA translation is a biological process that allows mRNA to be translated on‐site, which is proposed to provide fine control in protein regulation, both spatially and temporally within a cell. We recently reported that Vasa, an RNA‐helicase, is a promising factor that appears to regulate this process on the spindle during the embryonic development of the sea urchin, yet the detailed roles and functional mechanisms of Vasa in this process are still largely unknown. In this review (...) article, to elucidate these remaining questions, we first summarize the prior knowledge and our recent findings in the area of Vasa research and further discuss how Vasa may function in localized mRNA translation, contributing to efficient protein regulation during rapid embryogenesis and cancer cell regulation. (shrink)

MYC is a transcription factor, which not only directly modulates multiple aspects of transcription and co‐transcriptional processing (e.g. RNA‐Polymerase II initiation, elongation, and mRNA capping), but also indirectly influences several steps of RNA metabolism, including both constitutive and alternative splicing, mRNA stability, and translation efficiency. As MYC is an oncoprotein whose expression is deregulated in multiple human cancers, identifying its critical downstream activities in tumors is of key importance for designing effective therapeutic strategies. With this knowledge (...) and recent technological advances, we now have multiple angles to reach the goal of targeting MYC in tumors, ranging from the direct reduction of MYC levels, to the dampening of selected house‐keeping functions in MYC‐overexpressing cells, to more targeted approaches based on MYC‐induced secondary effects. (shrink)

The expression of certain eukaryotic genes is – at least in part – controlled at the level of mRNA translation. The step of translational initiation represents the primary target for regulation. The regulation of the intracellular iron storage protein ferritin in response to iron levels provides a good example of translational control by a reversible RNA/protein interaction in the 5' untranslated region of an mRNA. We consider mechanisms by which mRNA/protein interactions may impede translation initiation (...) and discuss recent data suggesting that the ferritin example may represent the ‘tip of the iceberg’ of a more general theme for translational control. (shrink)

It is widely suggested that a eukaryotic mRNA typically contains one translation start site and encodes a single functional protein product. However, according to current points of view on translation initiation mechanisms, eukaryotic ribosomes can recognize several alternative translation start sites and the number of experimentally verified examples of alternative translation is growing rapidly. Also, the frequent occurrence of alternative translation events and their functional significance are supported by the results of computational evaluations. The (...) functional role of alternative translation and its contribution to eukaryotic proteome complexity are discussed. BioEssays 30:683–691, 2008. © 2008 Wiley Periodicals, Inc. (shrink)

The genetic code is a set of instructions that determine how the information in our genetic material is translated into amino acids. In general, it is universal for all organisms, from viruses and bacteria to humans. However, in the last few decades, exceptions to this rule have been identified both in pro‐ and eukaryotes. In this review, we discuss the 16 described alternative eukaryotic nuclear genetic codes and observe theories of their appearance in evolution. We consider possible molecular mechanisms that (...) allow codon reassignment. Most reassignments in nuclear genetic codes are observed for stop codons. Moreover, in several organisms, stop codons can simultaneously encode amino acids and serve as termination signals. In this case, the meaning of the codon is determined by the additional factors besides the triplets. A comprehensive review of various non‐standard coding events in the nuclear genomes provides a new insight into the translation mechanism in eukaryotes. (shrink)

Recent findings position the eukaryotic translation initiation factor eIF4E as a novel modulator of mRNA splicing, a process that impacts the form and function of resultant proteins. eIF4E physically interacts with the spliceosome and with some intron‐containing transcripts implying a direct role in some splicing events. Moreover, eIF4E drives the production of key components of the splicing machinery underpinning larger scale impacts on splicing. These drive eIF4E‐dependent reprogramming of the splicing signature. This work completes a series of studies (...) demonstrating eIF4E acts in all the major mRNA maturation steps whereby eIF4E drives production of the RNA processing machinery and escorts some transcripts through various maturation steps. In this way, eIF4E couples the mRNA processing‐export‐translation axis linking nuclear mRNA processing to cytoplasmic translation. eIF4E elevation is linked to worse outcomes in acute myeloid leukemia patients where these activities are dysregulated. Understanding these effects provides new insight into post‐transcriptional control and eIF4E‐driven cancers. (shrink)

Genotoxic stress leads to DNA damage which can be detrimental to the cell. A well‐orchestrated cellular response is mounted to manage and repair the genotoxic stress‐induced DNA damage. Our understanding of genotoxic stress response is derived mainly from studies focused on transcription, mRNA splicing, and protein turnover. Surprisingly not as much is understood about the role of mRNA translation and decay in genotoxic stress response. This is despite the fact that regulation of gene expression at the level (...) of mRNA translation and decay plays a critical role in a myriad of cellular processes. This review aims to summarize some of the known findings of the role of mRNA translation and decay by focusing on two categories of examples. We discuss examples of mRNA whose fates are regulated in the cytoplasm and RNA‐binding proteins that regulate mRNA fates in response to genotoxic stress. (shrink)

The poly(A)‐dependent translational regulation of maternal mRNAs is an important mechanism to execute stage‐specific programs of protein synthesis during early development. This control underlies many crucial developmental events including the meiotic maturation of oocytes and activation of the mitotic cell cycle at fertilization. A recent report(1) demonstrates that the 3′ untranslated region of the cyclin A1, B1, B2 and c‐mos mRNAs determines the timing and extent of their cytoplasmic polyadenylation and translational activation during Xenopus oocyte maturation. These studies further establish (...) that protein synthesis can be temporally and quantitatively controlled by developmentally regulated changes in the polyadenylation of maternal mRNAs. (shrink)

Protein synthesis inhibitors prolong the half‐lives of most mRNAs at least fourfold in the somatic cells of higher eukaryotes and in yeast cells. Some mRNAs are stabilized because the inhibitors affect mRNA‐specific regulatory factors; however, hundreds or thousands of other mRNAs are probably stabilized by a common mechanism. We propose that mRNA stabilization in cells treated with a translation inhibitor reflects a physiological process that occurs during each mitosis and is important for cell survival. Transcription and (...) class='Hi'>translation rates decline drastically during a 1–2 hour interval of mitosis. We hypothesize that translational repression during this interval somehow inactivates a critical component of the mRNA degradation machinery. As a result, mRNA half‐lives are prolonged during the interval when transcription is repressed. If labile mRNAs were not stabilized during mitosis they, and perhaps also the labile proteins they encode, would be depleted as the cell entered G1 phase, with deleterious consequences. Stabilization during mitosis, or in response to translation inhibitors, thus preserves the capacity of the cell to synthesize essential proteins as it enters G1 or recovers from inhibitor treatment. mRNA stabilization might serve a similar purpose during starvation or any stress negatively affecting translation. (shrink)

Despite a library full of literature on miRNA biology, core issues relating to miRNA target detection, biological effect, and mode of action remain controversial. This essay proposes that the predominant mechanism of direct miRNA action is translational inhibition, whereas the bulk of miRNA effects are mRNA based. It explores several issues confounding miRNA target detection, and discusses their impact on the dominance of “miRNA seed” dogma and the exploration of non‐canonical binding sites. Finally, it makes comparisons between miRNA target (...) prediction and transcription factor binding prediction, and questions the value of characterizing miRNA binding sites based on which miRNA nucleotides are paired with an mRNA. (shrink)

Six years ago, the Singer lab published a landmark paper which described how individual mRNA particles cross the nuclear pore complex in mammalian tissue culture cells. This involved the simultaneous imaging of mRNAs, each labeled by a large number of tethered fluorescent proteins and fluorescently tagged nuclear pore components. Now two groups have applied this technique to the budding yeast Saccharomyces cerevisiae. Their results indicate that in the course of nuclear export, mRNAs likely engage complexes that are present on (...) either side of the pore and that these interactions are modulated by proteins present in the messenger ribonucleoprotein (mRNP) complex. These findings lend support to the notion that just before and/or after the completion of nuclear export, mRNPs undergo one or more maturation steps that prepare the packaged mRNAs for translation. These results represent new and exciting insights into the mechanism of mRNA nuclear export. (shrink)

The coupling of protein synthesis and folding is a crucial yet poorly understood aspect of cellular protein folding. Over the past few years, it has become possible to experimentally follow and define protein folding on the ribosome, revealing principles that shape co‐translational folding and distinguish it from refolding in solution. Here, we highlight some of these recent findings from biochemical and biophysical studies and their potential significance for cellular protein biogenesis. In particular, we focus on nascent chain interactions with the (...) ribosome, interactions within the nascent protein, modulation of translation elongation rates, and the role of mechanical force that accompanies nascent protein folding. The ability to obtain mechanistic insight in molecular detail has set the stage for exploring the intricate process of nascent protein folding. We believe that the aspects discussed here will be generally important for understanding how protein synthesis and folding are coupled and regulated. (shrink)

Neuronal signalling involves multiple neuropeptides that are diverse in structure and function. Complex patterns of tissue‐specific expression arise from alternate RNA splicing of neuropeptide‐encoding gene transcripts. The pattern of expression and its role in cell signalling is diffecult to study at the level of single neurons in the complex vertebrate brain. However, in the model molluscan system, Lymnaea, it is possible to show that alternate mRNA expression of the FMRFamide gene is specific to single identified neurons. Two different transcripts (...) are expressed in a mutually exclusive manner in different neurons. Post‐translational processing of the two precursor proteins leads to completely distinct sets of neuropeptide transmitters. The function of these transmitter cocktails, resulting from alternate mRNA splicing, was studied physiologically in identified neurons forming part of a behaviourally important network regulating heartbeat. (shrink)

Adequate reprogramming of cellular metabolism in response to stresses or suboptimal growth conditions involves a myriad of coordinated changes that serve to promote cell survival. As protein synthesis is an energetically expensive process, its regulation under stress is of critical importance. Reprogramming of messenger RNA (mRNA) translation involves well‐understood stress‐activated kinases that target components of translation initiation machinery, resulting in the robust inhibition of general translation and promotion of the translation of stress‐responsive proteins. Translational arrest (...) of mRNAs also results in the accumulation of transcripts in cytoplasmic foci called stress granules. Recent studies focus on the key roles of transfer RNA (tRNA) in stress‐induced translational reprogramming. These include stress‐specific regulation of tRNA pools, codon‐biased translation influenced by tRNA modifications, tRNA miscoding, and tRNA cleavage. In combination, signal transduction pathways and tRNA metabolism changes regulate translation during stress, resulting in adaptation and cell survival. This review examines molecular mechanisms that regulate protein synthesis in response to stress. (shrink)

All eukaryotic cellular mRNAs, and most viral mRNAs, are blocked at their 5′ ends with a cap structure (m7GpppX, where × is any nucleotide). Poliovirus, along with a small number of other animal and plant viral mRNAs, does not contain a 5′ cap structure. Since the cap structure functions to facilitate ribosome binding to mRNA, translation of poliovirus must proceed by a cap‐independent mechanism. Consistent with this, recent studies have shown that ribosomes can bind to an internal region (...) within the long 5′ noncoding sequence of poliovirus RNA. Possible mechanisms for cap‐independent translation are discussed. Cap‐independent translation of poliovirus RNA is of major importance to the mechanism of shut‐off of host protein synthesis after infection. Moreover, it is likely to play a role in determining poliovirus neurovirulence and attenuation. (shrink)

It is well established that microRNAs (miRNAs) induce mRNA degradation by binding to 3′ untranslated regions (UTRs). The functionality of sites in the coding domain sequence (CDS), on the other hand, remains under discussion. Such sites have limited impact on target mRNA abundance and recent work suggests that miRNAs bind in the CDS to inhibit translation. What then could be the regulatory benefits of translation inhibition through CDS targeting compared to mRNA degradation following 3′ UTR (...) binding? We propose that these domain‐dependent effects serve to diversify the functional repertoire of post‐transcriptional gene expression control. Possible regulatory benefits may include tuning the time‐scale and magnitude of post‐transcriptional regulation, regulating protein abundance depending on or independently of the cellular state, and regulation of the protein abundance of alternative splice variants. Finally, we review emerging evidence that these ideas may generalize to RNA‐binding proteins beyond miRNAs and Argonaute proteins. (shrink)

Complexes of two or more proteins form many, if not most, of the intracellular “machines” that execute physical and chemical work, and transmit information. Complexes can form from stochastic post‐translational interactions of fully formed proteins, but recent attention has shifted to co‐translational interactions in which the most common mechanism involves binding of a mature constituent to an incomplete polypeptide emerging from a translating ribosome. Studies in yeast have revealed co‐translational interactions during formation of multiple major complexes, and together with recent (...) mammalian cell studies, suggest widespread utilization of the mechanism. These translation‐dependent interactions can involve a single or multiple mRNA templates, can be uni‐ or bi‐directional, and can use multi‐protein sub‐complexes as a binding component. Here, we discuss benefits of co‐translational complex assembly including accuracy and efficiency, overcoming hidden interfaces, localized and hierarchical assembly, and reduction of orphan protein degradation, toxicity, and dominant‐negative pathogenesis, all serving to improve cell fitness. (shrink)

Animal, protist and viral messenger RNAs (mRNAs) are most prominently modified at the beginning by methylation of cap‐adjacent nucleotides at the 2′‐O‐position of the ribose (cOMe) by dedicated cap methyltransferases (CMTrs). If the first nucleotide of an mRNA is an adenosine, PCIF1 can methylate at the N6‐position (m6A), while internally the Mettl3/14 writer complex can methylate. These modifications are introduced co‐transcriptionally to affect many aspects of gene expression including localisation to synapses and local translation. Of particular interest, transcription (...) start sites of many genes are heterogeneous leading to sequence diversity at the beginning of mRNAs, which together with cOMe and m6Am could constitute an extensive novel layer of gene expression control. Given the role of cOMe and m6A in local gene expression at synapses and higher brain functions including learning and memory, such code could be implemented at the transcriptional level for lasting memories through local gene expression at synapses. (shrink)

Pre‐existing but untranslated or ‘poised’ mRNA exists as a means to rapidly induce the production of specific proteins in response to stimuli and as a safeguard to limit the actions of these proteins. The translation of poised mRNA enables immune cells to express quickly genes that enhance immune responses. The molecular mechanisms that repress the translation of poised mRNA and, upon stimulation, enable translation have yet to be elucidated. They likely reflect intrinsic properties of (...) the mRNAs and their interactions with trans‐acting factors that direct poised mRNAs away from or into the ribosome. Here, I discuss mechanisms by which this might be regulated. (shrink)

Early development in many animals is programmed by maternally inherited messenger RNAs. Many of these mRNAs are translationally dormant in immature oocytes, but are recruited onto polysomes during meiotic maturation, fertilization, or early embryogenesis. In contrast, other mRNAs that are translated in oocytes are released from polysomes during these later stages of development. Recent studies have begun to define the cis and trans elements that regulate both translational repression and translational induction of maternal mRNA. The inhibition of translation (...) of some mRNAs during early development is controlled by discrete sequences residing in the 3′ and 5′ untranslated regions, respectively. The translation of other RNAs is due to polyadenylation which, at least in oocytes of the frog Xenopus laevis, is regulated by a U‐rich cytoplasmic polyadenylation element (CPE). Although similar, the CPE sequences of various mRNAs are sufficiently different to be bound by different proteins. Two of these proteins and their interactions are described here. (shrink)

The translocation of tRNAs through the ribosome proceeds through numerous small steps in which tRNAs gradually shift their positions on the small and large ribosomal subunits. The most urgent questions are: (i) whether these intermediates are important; (ii) how the ribosomal translocase, the GTPase elongation factor G (EF‐G), promotes directed movement; and (iii) how the energy of GTP hydrolysis is coupled to movement. In the light of recent advances in biophysical and structural studies, we argue that intermediate states of (...) translocation are snapshots of dynamic fluctuations that guide the movement. In contrast to current models of stepwise translocation, kinetic evidence shows that the tRNAs move synchronously on the two ribosomal subunits in a rapid reaction orchestrated by EF‐G and GTP hydrolysis. EF‐G combines the energy regimes of a GTPase and a motor protein and facilitates tRNA movement by a combination of directed Brownian ratchet and power stroke mechanisms. (shrink)

Abstract3′ untranslated regions (3′ UTRs) of mRNAs have many functions, including mRNA processing and transport, translational regulation, and mRNA degradation and stability. These different functions require cis‐elements in 3′ UTRs that can be either sequence motifs or RNA structures. Here we review the role of secondary structures in the functioning of 3′ UTRs and discuss some of the trans‐acting factors that interact with these secondary structures in eukaryotic organisms. We propose potential participation of 3′‐UTR secondary structures in cytoplasmic (...) polyadenylation in the model organism Drosophila melanogaster. Because the secondary structures of 3′ UTRs are essential for post‐transcriptional regulation of gene expression, their disruption leads to a wide range of disorders, including cancer and cardiovascular diseases. Trans‐acting factors, such as STAU1 and nucleolin, which interact with 3′‐UTR secondary structures of target transcripts, influence the pathogenesis of neurodegenerative diseases and tumor metastasis, suggesting that they are possible therapeutic targets. (shrink)

Degradation of eukaryotic RNAs that contain premature termination codons (PTC) during nonsense‐mediated mRNA decay (NMD) is initiated by RNA decapping or endonucleolytic cleavage driven by conserved factors. Models for NMD mechanisms, including recognition of PTCs or the timing and role of protein phosphorylation for RNA degradation are challenged by new results. For example, the depletion of the SMG5/7 heterodimer, thought to activate RNA degradation by decapping, leads to a phenotype showing a defect of endonucleolytic activity of NMD complexes. This (...) phenotype is not correlated to a decreased binding of the endonuclease SMG6 with the core NMD factor UPF1, suggesting that it is the result of an imbalance between active (e.g., in polysomes) and inactive (e.g., in RNA‐protein condensates) states of NMD complexes. Such imbalance between multiple complexes is not restricted to NMD and should be taken into account when establishing causal links between gene function perturbation and observed phenotypes. (shrink)

The c‐myc proto‐oncogene is believed to be involved in the regulation of cell growth and differentiation. Deregulation of this gene, resulting in an inappropriate increase of gene product, can contribute to cancer formation. One of the ways in which the expression of the c‐myc gene can be deregulated is by the stabilization of the labile c‐myc mRNA. The rapid degradation of the c‐myc transcript appears to be mediated by at least two distinct regions in the mRNA. One lies (...) in the 3' untranslated region, and presumably consists of (A+U)‐rich sequences. The other lies in the C‐terminal part of the coding region and colocalizes with sequences encoding protein‐dimerization motifs. The exact mechanism by which the destabilizing elements function is not yet clear. Shortening of the poly(A) tail of the c‐myc message appears to precede degradation of the transcript. When translation is blocked, this shortening is slowed down and the mRNA is stabilized. This suggests that deadenylation is required before degradation of the mRNA body can take place. (shrink)

The complexity of organisms is not simply determined by the number of their genes, but to a large extent by how gene expression is controlled. In addition to transcriptional regulation, this involves several layers of post‐transcriptional control, such as translational repression, microRNA‐mediated mRNA degradation and translational inhibition, alternative splicing, and the regulated generation of functionally distinct gene products from a single mRNA through alternative use of translation initiation sites. Much progress has been made in describing the molecular (...) basis for these gene regulatory mechanisms. However, it is now a major challenge to translate this knowledge into deeper understanding of the physiological processes, both normal and pathological, that they govern. Using the C/EBP family of transcription factors as an example, the present review describes recent genetic experiments addressing this general problem and discusses how the physiological importance of newly discovered regulatory mechanisms might be determined. (shrink)

It is a long-standing view that global translation varies during the cell cycle and is much lower in mitosis than in other cell-cycle phases. However, the central papers in the literature are not in agreement about the extent of downregulation in mitosis, ranging from a dramatic decrease to only a marginal reduction. Herein, it is argued that the discrepancy derives from technical challenges. Cell-cycle-dependent variations are most conveniently studied in synchronized cells, but the synchronization methods by themselves often evoke (...) stress responses that, in turn, affect translation rates. Further, it is argued that previously reported cell-cycle-dependent changes in the global translation rate to a large extent reflect responses to the synchronization methods. Recent findings strongly suggest that the global translation rate is not regulated in a cell-cycle-dependent manner. Novel techniques allowing a genome-wide analysis of translational profiles suggest that the extent and importance of selective translational regulation associated with cell-cycle transitions have been underestimated. Therefore, the main question is which messenger RNAs (mRNAs) are translated, rather than whether the global translation rate is decreased. (shrink)

Cells contain a myriad of membraneless ribonucleoprotein (RNP) condensates with distinct compositions of proteins and RNAs. RNP condensates participate in different cellular activities, including RNA storage, mRNA translation or decay, stress response, etc. RNP condensates are assembled via liquid‐liquid phase separation (LLPS) driven by multivalent interactions. Transition of RNP condensates into bodies with abnormal material properties, such as solid‐like amyloid structures, is associated with the pathogenesis of various diseases. In this review, we focus on how RNAs regulate multiple (...) aspects of RNP condensates, such as dynamic assembly and/or disassembly and biophysical properties. RNA properties – including concentration, sequence, length and structure – also determine the phase behaviors of RNP condensates. RNA is also involved in specifying autophagic degradation of RNP condensates. Unraveling the role of RNA in RNPs provides novel insights into pathological accumulation of RNPs in various diseases. This new understanding can potentially be harnessed to develop therapeutic strategies. (shrink)

Cytochrome P450IIE1 is involved in the metabolic activation of many xenobiotics involved with human toxicity. In particular, cellular concentrations of P450IIE1 are significantly induced by the most widely abused drug in our society today, alcohol. As a result, the synthesis and degradation of this form of P450 has significant health consequences. The regulation of the steady‐state concentration of P450IIE1 is an extremely complex process. The enzyme is regulated by transcriptional activation, mRNA stabilization, increased mRNA translatability and decreased protein (...) degradation. The principal mechanism which controls the induction process depends on the chemical nature of the inducer, the age, and the nutritional and hormonal status of the animal. There also appear to be significant sex differences in the expression of P450IIE1. It is entirely possible that the regulation of the enzyme concentration under any given set of conditions will involve all of the mechanisms to different extents. (shrink)

Over one hundred genes have been isolated from the human genome and shown to be causally related to specific human genetic diseases. Studies with gene‐specific probes have demonstrated that the mutations resulting in a particular phenotype are highly heterogeneous as a group, ranging from alterations in transcription or RNA processing in the nucleus, through to errors in mRNA translation in the cytoplasm. Even where the gene‐specific probe is not available, defects have been localized to chromosomal regions by family (...) studies. Recently developed methods for moving along the chromosome from a linked marker to the mutation are resulting in rapid advances in the understanding of many monogenic disorders. (shrink)

The present review highlights an association between autism, Alzheimer disease , and fragile X syndrome . We propose a conceptual framework involving the amyloid-beta peptide , Abeta precursor protein , and fragile X mental retardation protein based on experimental evidence. The anabolic effect of the secreted alpha form of the amyloid-beta precursor protein may contribute to the state of brain overgrowth implicated in autism and FXS. Our previous report demonstrated that higher plasma sAPPalpha levels associate with more severe symptoms of (...) autism, including aggression. This molecular effect could contribute to intellectual disability due to repression of cell-cell adhesion, promotion of dense, long, thin dendritic spines, and the potential for disorganized brain structure as a result of disrupted neurogenesis and migration. At the molecular level, APP and FMRP are linked via the metabotropic glutamate receptor 5 . Specifically, mGluR5 activation releases FMRP repression of APP mRNA translation and stimulates sAPP secretion. The relatively lower sAPPalpha level in AD may contribute to AD symptoms that significantly contrast with those of FXS and autism. Low sAPPalpha and production of insoluble Abeta would favor a degenerative process, with the brain atrophy seen in AD. Treatment with mGluR antagonists may help repress APP mRNA translation and reduce secretion of sAPP in FXS and perhaps autism. (shrink)

The expression of protein‐encoding genes is a complex process culminating in the production of mature mRNA and its translation by the ribosomes. The production of a mature mRNA involves an intricate series of processing steps. The majority of eukaryotic protein‐encoding genes contain intron sequences that disrupt the protein‐encoding frame, and hence have to be removed from immature mRNA prior to translation into protein. The mechanism involved in the selection of correct splice sites is incompletely understood. (...) A considerable body of evidence suggests that the splicing machinery has suboptimal efficiency and fidelity leading to substantial processing inaccuracy. Here we discuss a recently published article1 that extends observations that cells rely on nonsense‐mediated mRNA decay (NMD) to compensate for such suboptimal processing accuracy. Intriguingly these authors provide evidence for a strong selective pressure in favour of premature termination of mRNA translation in the event of intron retention. The analysis presented implies a positive role of NMD in transcript diversification through alternative splicing and suggest that this ancient surveillance mechanism may have co‐evolved with intron acquisition born from the need for quality control of splicing patterns. BioEssays 30:926–928, 2008. © 2008 Wiley Periodicals, Inc. (shrink)

The integrated stress response (ISR) is a key determinant of tumorigenesis in response to oncogenic forms of stress like genotoxic, proteotoxic and metabolic stress. ISR relies on the phosphorylation of the translation initiation factor eIF2 to promote the translational and transcriptional reprogramming of gene expression in stressed cells. While ISR promotes tumor survival under stress, its hyperactivation above a level of tolerance can also cause tumor death. The tumorigenic function of ISR has been recently demonstrated for lung adenocarcinomas (LUAD) (...) with KRAS mutations. ISR mediates the translational repression of the dual‐specificity phosphatase DUSP6 to stimulate ERK activity and LUAD growth. The significance of this finding is highlighted by the strong anti‐tumor responses of ISR inhibitors in pre‐clinical LUAD models. Elucidation of the mechanisms of ISR action in LUAD progression via cell‐autonomous and immune regulatory mechanisms will provide a better understanding of its tumorigenic role to fully exploit its therapeutic potential in the treatment of a deadly form of cancer. (shrink)

The noncoding RNA 7SK is a critical regulator of transcription by adjusting the activity of the kinase complex P‐TEFb. Release of P‐TEFb from 7SK stimulates transcription at many genes by promoting productive elongation. Conversely, P‐TEFb sequestration by 7SK inhibits transcription. Recent studies have shown that 7SK functions are particularly important for neuron development and maintenance and it can thus be hypothesized that 7SK is at the center of many signaling pathways contributing to neuron function. 7SK activates neuronal gene expression (...) programs that are key for terminal differentiation of neurons. Proteomics studies revealed a complex protein interactome of 7SK that includes several RNA‐binding proteins. Some of these novel 7SK subcomplexes exert non‐canonical cytosolic functions in neurons by regulating axonal mRNA transport and fine‐tuning spliceosome production in response to transcription alterations. Thus, a picture emerges according to which 7SK acts as a multi‐functional RNA scaffold that is integral for neuron homeostasis. (shrink)

It is of fundamental importance to understand how the individual processes of gene expression, transcription, and translation, as well as mRNA and protein stability, act in concert to produce dynamic cellular proteomes. We use the concept of response times to illustrate the relation between degradation processes and responsiveness of the proteome to system changes and to provide supporting experimental evidence: proteins with short response times tend to be more strongly up‐regulated after 1 hour of TNFα stimulation than proteins (...) with longer response times. Furthermore, based on process‐dependent response times, we demonstrate that synthesis and degradation act in concert to enable rapid responses. Finally, by building on a previously published data set quantifying the mammalian gene expression cascade, we speculate on how combinations of stable and unstable mRNAs and proteins may be wired to transcriptional or translational regulation to support gene function. (shrink)

In a recent issue of Nature Communications Ukleja and co‐workers reported a cryo‐EM 3D reconstruction of the Ccr4‐Not complex from Schizosaccharomyces pombe with an immunolocalization of the different subunits. The newly gained architectural knowledge provides cues to apprehend the functional diversity of this major eukaryotic regulator. Indeed, in the cytoplasm alone, Ccr4‐Not regulates translational repression, decapping and deadenylation, and the Not module additionally plays a positive role in translation. The spatial distribution of the subunits within the structure is compatible (...) with a model proposing that the Ccr4‐Not complex interacts with the 5′ and 3′ ends of target mRNAs, allowing different functional modules of the complex to act at different stages of the translation process, possibly within a circular constellation of the mRNA. This work opens new avenues, and reveals important gaps in our understanding regarding structure and mode of function of the Ccr4‐Not complex that need to be addressed in the future. (shrink)

Numerous accessory factors modulate RNA polymerase response to regulatory signals and cellular cues and establish communications with co‐transcriptional RNA processing. Transcription regulators are astonishingly diverse, with similar mechanisms arising via convergent evolution. NusG/Spt5 elongation factors comprise the only universally conserved and ancient family of regulators. They bind to the conserved clamp helices domain of RNA polymerase, which also interacts with non‐homologous initiation factors in all domains of life, and reach across the DNA channel to form processivity clamps that enable (...) uninterrupted RNA chain synthesis. In addition to this ubiquitous function, NusG homologs exert diverse, and sometimes opposite, effects on gene expression by competing with each other and other regulators for binding to the clamp helices and by recruiting auxiliary factors that facilitate termination, antitermination, splicing, translation, etc. This surprisingly diverse range of activities and the underlying unprecedented structural changes make studies of these “transformer” proteins both challenging and rewarding. (shrink)

NIK1 is a receptor‐like kinase involved in plant antiviral immunity. Although NIK1 is structurally similar to the plant immune factor BAK1, which is a key regulator in plant immunity to bacterial pathogens, the NIK1‐mediated defenses do not resemble BAK1 signaling cascades. The underlying mechanism for NIK1 antiviral immunity has recently been uncovered. NIK1 activation mediates the translocation of RPL10 to the nucleus, where it interacts with LIMYB to fully down‐regulate translational machinery genes, resulting in translation inhibition of host and (...) viral mRNAs and enhanced tolerance to begomovirus. Therefore, the NIK1 antiviral immunity response culminates in global translation suppression, which represents a new paradigm for plant antiviral defenses. Interestingly, transcriptomic analyses in nik1 mutant suggest that NIK1 may suppress antibacterial immune responses, indicating a possible opposite effect of NIK1 in bacterial and viral infections. (shrink)

Many viruses evolved mechanisms for capping the 5′‐ends of their plus‐strand RNAs as a means of hijacking the eukaryotic messenger RNA (mRNA) splicing/translation machinery. Although capping is critical for replication, the RNAs of these viruses have other essential functions including their requirement to be packaged as either genomes or pre‐genomes into progeny viruses. Recent studies indicate that human immunodeficiency virus type‐1 (HIV‐1) RNAs are segregated between splicing/translation and packaging functions by a mechanism that involves structural sequestration of (...) the 5′‐cap. Here, we examined studies reported for other viruses and retrotransposons that require both selective packaging of their RNAs and 5′‐RNA capping for host‐mediated translation. Our findings suggest that viruses and retrotransposons have evolved multiple mechanisms to control 5′‐cap accessibility, consistent with the hypothesis that removal or sequestration of the 5′ cap enables packageable RNAs to avoid capture by the cellular RNA processing and translation machinery. (shrink)

The primary transcripts, pre‐mRNAs, of almost all protein‐coding genes in higher eukaryotes contain multiple non‐coding intervening sequences, introns, which must be precisely removed to yield translatable mRNAs. The process of intron excision, splicing, takes place in a massive ribonucleoprotein complex known as the spliceosome. Extensive studies, both genetic and biochemical, in a variety of systems have revealed that essential components of the spliceosome include five small RNAs–U1, U2, U4, U5 and U6, each of which functions as a RNA, protein complex (...) called an snRNP (small nuclear ribonucleoprotein). In addition to snRNPs, splicing requires many non‐snRNP protein factors, the exact nature and number of which has been unclear. Technical advances, including new affinity purification methods and improved mass spectrometry techniques, coupled with the completion of many genome sequences, have now permitted a number of proteomic analyses of purified spliceosomes. These studies, recently reviewed by Jurica and Moore,1 reveal that the spliceosome is composed of as many as 300 distinct proteins and five RNAs, making it among the most complex macromolecular machines known. BioEssays 25:1147–1149, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

Nonsense‐mediated RNA decay (NMD) represents an established quality control checkpoint for gene expression that protects cells from consequences of gene mutations and errors during RNA biogenesis that lead to premature termination during translation. Characterization of NMD‐sensitive transcriptomes has revealed, however, that NMD targets not only aberrant transcripts but also a broad array of mRNA isoforms expressed from many endogenous genes. NMD is thus emerging as a master regulator that drives both fine and coarse adjustments in steady‐state RNA levels (...) in the cell. Importantly, while NMD activity is subject to autoregulation as a means to maintain homeostasis, modulation of the pathway by external cues provides a means to reprogram gene expression and drive important biological processes. Finally, the unanticipated observation that transcripts predicted to lack protein‐coding capacity are also sensitive to this translation‐dependent surveillance mechanism implicates NMD in regulating RNA function in new and diverse ways. (shrink)

The conserved ribosomal protein uS3 in eukaryotes has long been known as one of the essential components of the small (40S) ribosomal subunit, which is involved in the structure of the 40S mRNA entry pore, ensuring the functioning of the 40S subunit during translation initiation. Besides, uS3, being outside the ribosome, is engaged in various cellular processes related to DNA repair, NF‐kB signaling pathway and regulation of apoptosis. This review is devoted to recent data opening new horizons in (...) understanding the roles of uS3 in such processes as the assembly and maturation of 40S subunits, ensuring proper structure of 48S pre‐initiation complexes, regulation of initiation and ribosome‐based RNA quality control pathways. Besides, we summarize novel results on the participation of the protein in processes beyond translation and consider biomedical implications of previously known and recently found extra‐ribosomal functions of uS3, primarily, in oncogenesis. (shrink)

Auxin, a class of plant hormones which affects a wide array of growth and developmental processes including cell elongation and cell division, alters gene expression in a very rapid, selective, and dramatic way. The relative level of some mRNAs decreases several fold, while that of other mRNAs increases many fold. These changes are mediated, at least in some cases, by very fast (within 5–10 min) modulation by auxin of transcription as measured by run‐off transcription assays using nuclei isolated from (...) control and auxin‐treated tissues. Rapid turnover of mRNAs following auxin treatment also contributes to large changes in steady state concentration in some cases. The data are suggestive of multiple and complex mechanisms of regulation of expression of those genes which have been studied, using cloned cDNAs for direct quantitation of mRNA steady state levels and relative transcription rates. While there is no definitive evidence that auxin‐regulated gene expression mediates any of the growth responses effected by auxin, several lines of evidence are supportive of a very close relationship between these processes. The working hypothesis is that there is a causal relationship between the effects of auxin on gene expression and at least some of the physiological and growth responses to auxin. (shrink)

The fast advancing RNA‐seq technology has unveiled an unexpected diversity and expression specificity of 3′ untranslated regions (3′UTRs) of mRNAs. In particular, neural mRNAs seem to express significantly longer 3′UTRs, some of which are over 10 kb in length. The extensive elongation of 3′UTRs in neural tissues provides intriguing possibilities for cell type‐specific regulations that are governed by miRNAs, RNA‐binding proteins and ribonucleoprotein aggregates. In this article, we review recent progress in the characterization of mRNA 3′UTRs and discuss (...) their implications in the understanding of 3′UTR‐mediated gene regulation. (shrink)

All the conserved detailed results of evolution stored in DNA must be read, transcribed, and translated via an RNAmediated process. This is required for the development and growth of each individual cell. Thus, all known living organisms fundamentally depend on these RNA-mediated processes. In most cases, they are interconnected with other RNAs and their associated protein complexes and function in a strictly coordinated hierarchy of temporal and spatial steps (i.e., an RNA network). Clearly, all cellular life as we know it (...) could not function without these key agents of DNA replication, namely rRNA, tRNA, and mRNA. Thus, any definition of life that lacks RNA functions and their networks misses an essential requirement for RNA agents that inherently regulate and coordinate (communicate to) cells, tissues, organs, and organisms. The precellular evolution of RNAs occurred at the core of the emergence of cellular life and the question remained of how both precellular and cellular levels are interconnected historically and functionally. RNA-networks andRNA-communication can interconnect these levels.With the reemergence of virology in evolution, it became clear that communicating viruses and subviral infectious genetic parasites are bridging these two levels by invading, integrating, coadapting, exapting, and recombining constituent parts in host genomes for cellular requirements in gene regulation and coordination aims. Therefore, a 21st century understanding of life is of an inherently social process based on communicating RNA networks, in which viruses and cells continuously interact. (shrink)

One distinguishing feature of vertebrate oocyte meiosis is its discontinuity; oocytes are released from their prophase I arrest, usually by hormonal stimulation, only to again halt at metaphase II, where they await fertilization. The product of the c‐mos proto‐oncogene, Mos, is a key regulator of this maturation process. Mos is a serine‐threonine kinase that activates and/or stabilizes maturation‐promoting factor (MPF), the master cell cycle switch, through a pathway that involves the mitogen‐activated protein kinase (MAPK) cascade. Oocytes arrested at prophase I (...) lack detectable levels of Mos, which must be synthesized from a pool of maternal mRNAs for proper maturation. While Mos is necessary throughout maturation in Xenopus, it seems to be required only for meiosis II in the mouse. The translational activation of c‐mos mRNA at specific times during meiosis requires cytoplasmic polyadenylation. Cis‐ and trans‐acting factors for polyadenylation are, therefore, essential elements of maturation. (shrink)