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Results for “Translational repression”

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PubMed2000

Regulated translation initiation controls stress-induced gene expression in mammalian cells.

Protein kinases that phosphorylate the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) are activated in stressed cells and negatively regulate protein synthesis. Phenotypic analysis of targeted mutations in murine cells reveals a novel role for eIF2alpha kinases in regulating gene expression in the unfolded protein response (UPR) and in amino acid starved cells. When activated by their cognate upstream stress signals, the mammalian eIF2 kinases PERK and GCN2 repress translation of most mRNAs but selectively increase translation of Activating Transcription Factor 4 (ATF4), resulting in the induction of the downstream gene CHOP (GADD153). This is the first example of a mammalian signaling pathway homologous to the well studied yeast general control response in which eIF2alpha phosphorylation activates genes involved in amino acid biosynthesis. Mammalian cells thus utilize an ancient pathway to regulate gene expression in response to diverse stress signals.

Activating Transcription Factor 4↗

PubMed1995

Translational control in spermatogenesis.

Translational silencing phenomena during spermatogenesis in the two model systems Drosophila and mouse are reviewed. Cis-acting sequences were identified in both species that are necessary for translational repression. While in Drosophila these elements so far have only been found in the 5' untranslated region (5' UTR), in the mammals such regions were identified both in the 5' as well as in the 3' UTR. In all cases, RNA-binding proteins interact with these regions, yet their specific role in the observed negative regulation of translation has to be established.

Animals↗

PubMed1997

Organisation of regulatory elements in two closely spaced Drosophila genes with common expression characteristics.

Sperm tail proteins that are components of a specific structure formed late during spermatid elongation have been found to be encoded by the Mst(3)CGP gene family. These genes have been demonstrated to be regulated both at the transcriptional as well as at the translational level. We report here on the dissection of the regulatory regions for two members of the gene family, Mst84Da and Mst84Db. While high level transcription and negative translational control of Mst84Da is mediated by a short gene segment of 205 nt (-152/+53), Mst84Db expression is controlled by a number of distinct regulatory elements with different effects that all reside within the gene itself. We identify a transcriptional control element between +154 and +216, a translational repression element around +216 to +275 and an RNA stability element within the 3'UTR. Irrespective of the final common expression characteristics, correct regulation for any individual member of the gene family seems to be achieved by very different means. This confirms earlier observations that did not detect any other sequence elements in common apart from the TCE (translational control element).

Animals↗

PubMed2003

Drosophila sex-lethal inhibits the stable association of the 40S ribosomal subunit with msl-2 mRNA.

The inhibition of male-specific lethal-2 (msl-2) mRNA translation in female flies is essential for X chromosome dosage compensation in Drosophila melanogaster. Translational repression of msl-2 requires sex-lethal (SXL) binding to uridine-rich sequences in both the 5' and 3' untranslated regions (UTRs) of the message. We delineate the msl-2 mRNA sequence elements that are important for regulation by SXL and identify functionally critical sequences adjacent to regulatory SXL binding sites. We demonstrate that SXL inhibits translation initiation and prevents the stable association of the 40S ribosomal subunit with the mRNA in a manner that does not require the presence of a cap structure at the 5' end of the mRNA. These results elucidate a critical regulatory step for dosage compensation in Drosophila melanogaster.

3' Untranslated Regions↗

PubMed1995

Autoregulation of poly(A)-binding protein synthesis in vitro.

The poly(A)-binding protein (PABP), in a complex with the 3'poly(A) tail of eukaryotic mRNAs, plays important roles in the control of translation and message stability. All known examples of PABP mRNAs contain an extensive A-rich sequence in their 5' untranslated regions. Studies in mammalian cells undergoing growth stimulation or terminal differentiation indicate that PABP expression is regulated at the translational level. Here we examine the hypothesis that synthesis of the PABP is autogenously controlled. We show that the endogenous inactive PABP mRNA in rabbit reticulocytes can be specifically stimulated by addition of low concentrations of poly(A) and that this stimulation is also observed with in vitro transcribed human PABP mRNA. By deleting the A-rich region from the leader of human PABP mRNA and adding it upstream of the initiator AUG in a reporter mRNA we show that the adenylate tract is sufficient and necessary for mRNA repression and poly(A)-mediated activation in the reticulocyte cell-free system. UV cross-linking experiments demonstrate that the leader adenylate tract binds PABP. Furthermore, addition of recombinant GST-PABP to the cell-free system represses translation of mRNAs containing the A-rich sequence in their 5'UTR, but has no effect on control mRNA. We thus conclude that in vitro PABP binding to the A-rich sequence in the 5' UTR of PABP mRNA represses its own synthesis.

Adenosine Monophosphate↗

PubMed2006

The 3' untranslated region of a soybean cytosolic glutamine synthetase (GS1) affects transcript stability and protein accumulation in transgenic alfalfa.

Higher plants assimilate nitrogen in the form of ammonia through the concerted activity of glutamine synthetase (GS) and glutamate synthase (GOGAT). The GS enzyme is either located in the cytoplasm (GS1) or in the chloroplast (GS2). Glutamine synthetase 1 is regulated in different plants at the transcriptional level and there are some reports of regulation at the level of protein stability. Here we present data that clearly establish that GS1 in plants is also regulated at the level of transcript turnover and at the translational level. Using a Glycine max (soybean) GS1 transgene, with and without its 3' untranslated region (UTR), driven by the constitutive CaMV 35S promoter in Medicago sativa (alfalfa) and Nicotiana tabacum (tobacco), we show that the 3' UTR plays a major role in both transcript turnover and translation repression in both the leaves and the nodules. Our data suggest that the 3' UTR mediated turnover of the transcript is regulated by a nitrogen metabolite or carbon/nitrogen ratios. We also show that the 3' UTR of the gene for the soybean GS1 confers post-transcriptional regulation on a reporter gene. Our dissection of post-transcriptional and translational levels of regulation of GS in plants shows that the situation in plants strongly resembles that in other organisms where GS is regulated at almost all levels. Multistep regulation of GS shows the high priority given by organisms to regulating and ensuring optimal control of nitrogen substrates and preventing overproduction of glutamine and drainage of the glutamate pool.

3' Untranslated Regions↗

PubMed1995

mRNA localisation during development.

Although there are many differences, mRNA localisations in the Xenopus oocyte show some tantalizing similarities to those occurring in Drosophila development. As in Drosophila, transcripts localise to opposite poles of the oocyte, this localisation is hierarchical and occurs in a multistep process in which localisation is followed by anchoring at the cortex. This distinction between initial transport and long-term maintenance reflects the dynamic nature of the cytoskeleton: the microtubule tracks form and reform according to the needs of the cell so that stable localisation must be mediated by a more constant structure--the cortex. A possible exception is the localisation of gurken mRNA where it is unknown whether there are separate mechanisms for transport to and maintenance at the oocyte nucleus. However, gurken is responsible for the transmission of a transitory signal; once this has been received, and the fate of the recipient follicle cells determined, there is no further need for localisation. It is possible that the time scale over which the localisation machinery is stable is sufficient for transmission of this signal without the need for a separate maintenance phase. The existence of a nanos homologue, Xcat-2 (Mosquera et al., 1993), associated with the Xenopus germ plasm is particularly interesting because of the morphological and functional similarities between Drosophila polar granules, Caenorhabditis P-granules, and Xenopus germ plasm. These electron-dense protein-RNA complexes are maternally supplied and in each case segregate with the germ line. These granules may represent a fundamental conserved pathway to germ-cell specification and it is now at least a possibility that they are also involved in establishing the embryonic axis through translational repression. In the case of Drosophila, this occurs through localised nanos acting (via Pumilio) on nanos response elements in hunchback mRNA. No such regulatory pair has yet been demonstrated in C. elegans or X. laevis, but each contains a candidate for one half of the interaction: glp-1 could be a target for an unidentified nanos-like protein; Xcat-2 may control translation of an unknown NRE-containing mRNA. Another common feature of mRNA localisation is that in every case where the targeting signal has been determined, it has been mapped to a region of the 3' UTR capable of forming an extensive secondary structure (e.g., David and Ish-Horowicz, 1991; Dalby and Glover, 1992; Gavis and Lehmann, 1992; Kim-Ha et al., 1993; Kislauskis et al., 1993, 1994; Lantz and Schedl, 1994). In several cases, translational control and transcript stability signals have also been mapped to these regions (Jackson and Standart, 1990; Standart et al., 1990; Standart and Hunt, 1990; Davis and Ish-Horowicz, 1991; Wharton and Struhl, 1991; Dalby and Glover, 1993; Evans et al., 1994; Kim-Ha et al., 1995). The large secondary structures may provide a means for stably exposing sequence-specific regions of RNA to proteins. Due to the ease with which RNA forms base pairs, it is likely that rather than remaining single-stranded, RNA within the cell forms some sort of secondary structure. The geometry of purely double-stranded RNA does not permit sequence specific interactions between proteins and the bases because the major groove is inaccessible to amino acid side chains (Weeks and Crothers, 1993). However, the distortions to the dsRNA helix provided by bulges, pseudoknots, and the single-strand loop regions in stem-loop structures do present sequence information that can be "read" by proteins. The extensive 3'UTRs may produce a stable secondary structure which ensures that regulatory elements remain exposed in such regions rather than hidden in double-stranded stems. (ABSTRACT TRUNCATED)

Animals↗

PubMed2005

Alternative splicing facilitates internal ribosome entry on the ornithine decarboxylase mRNA.

Ornithine decarboxylase (ODC) is the ratelimiting enzyme in the biosynthesis of polyamines, which are required for optimal cell growth and proliferation. ODC is overexpressed in many tumors and, conversely, its overexpression induces transformation. We have previously reported that ODC mRNA alternative splicing relieves the translation repression normally imposed by a long and structured 5' untranslated region (UTR), and that the ODC 5' UTR contains an internal ribosome entry site (IRES). Here we show that ODC IRES activity is enhanced following inclusion of alternative sequences generated by splicing at cryptic acceptor sites. Furthermore, the alternative ODC IRES is more sensitive to cell-cycledependent changes in the rate of translation. These findings uncover a new biological property of differentially spliced transcripts. This is the first example of alternative splicing that modulates mRNA translation through the cell cycle in a cap-independent manner.

5' Untranslated Regions↗

PubMed2004

Evidence for cyclin D3 as a novel target of rapamycin in human T lymphocytes.

The immunosuppressant rapamycin has been shown to inhibit G(1)/S transition of the cell cycle. This inhibition is thought to be mediated by maintenance of the threshold levels of cyclin-dependent kinase (CDK) inhibitor p27(Kip1) (p27) and inhibition of p70 s6 kinase (p70(s6k)). However, recent evidence suggests that cells still remain sensitive to rapamycin in the absence of functional p27 or p70(s6k). Here, we show that rapamycin represses cyclin D3 levels in activated human T lymphocytes with no inhibitory effects on cyclin D2. Furthermore, rapamycin elicits similar cyclin D3 modulatory effects in B lymphocytes. The overall effect of rapamycin on cyclin D3 leads to impaired formation of active complexes with Cdk4 or Cdk6 and subsequent inhibition of cyclin D3/CDK kinase activity. Decrease in cyclin D3 protein levels is due to translational repression and not due to attenuated transcription of the cyclin D3 gene. Importantly, stable overexpression of cyclin D3 (2-2.5 fold) in Jurkat T cell transfectants renders them resistant to lower doses (1-10 ng/ml) of rapamycin. These results point to a critical role of cyclin D3 in rapamycin-mediated immunosuppressive effects in T cells and cell cycle regulation in lymphocytes in general.

Anti-Inflammatory Agents, Non-Steroidal↗

PubMed1992

Synthesis and modification of D7 protein during Xenopus oocyte maturation.

The Xenopus maternal mRNA D7 is translationally repressed during oogenesis, only becoming recruited into polysomes during oocyte maturation, with D7 protein being detectable for the first time prior to germinal vesicle breakdown (GVBD). The synthesis of D7 protein was found to be induced by a variety of maturation-promoting agents including cyclin, c-mos and crude preparations of MPF. D7 protein induced by all these agents is post-translationally modified and exists as a number of variants of differing molecular weight. In contrast to endogenous D7 mRNA, D7 RNA injected into the stage VI oocyte is efficiently translated, resulting in the accumulation of predominantly unmodified D7 polypeptides, which become increasingly modified during oocyte maturation to produce a pattern of polypeptides similar to those derived from endogenous D7 mRNA. Thus, the system that results in the post-translational modification of the D7 protein is itself activated during oocyte maturation. The nature of the protein modification is not known but does not appear to be phosphorylation. The translation of exogenous D7 RNA in the stage VI oocyte does not lead to translational derepression of endogenous D7 mRNA.

Animals↗

PubMed2003

Interaction of the RNA chaperone Hfq with mRNAs: direct and indirect roles of Hfq in iron metabolism of Escherichia coli.

The Escherichia coli Sm-like host factor I (Hfq) is thought to play direct and indirect roles in post-transcriptional regulation by targeting small regulatory RNAs and mRNAs. In this study, we have used proteomics to identify new mRNA targets of Hfq. We have identified 11 candidate proteins, synthesis of which was differentially affected in a hfq- background. The effect of Hfq on some of the corresponding mRNAs including fur, gapA, metF, ppiB and sodB mRNA was assessed, using different in vitro and in vivo methods. This allowed us to distinguish between direct and indirect effects of Hfq in modulating the translational activities of these mRNAs. From the collection of mRNAs tested, only fur and sodB mRNA, encoding the master regulator of iron metabolism and the iron superoxide dismutase, respectively, were found to be regulated by Hfq. Fur is known to be a negative regulator of transcription of the small RNA RyhB. Mutations in the sodB leader and compensating mutations in RyhB revealed that RyhB in turn represses translation of sodB mRNA, explaining the previously reported positive control of sodB by Fur. These data assign a role to Hfq in regulation of iron uptake and in switching off of iron scavenger genes.

5,10-Methylenetetrahydrofolate Reductase (FADH2)↗

PubMed1993

Interaction of proteins with the mRNA for ribosomal protein L1 in Xenopus: structural characterization of in vivo complexes and identification of proteins that bind in vitro to its 5'UTR.

Xenopus r-protein mRNAs are known to be coordinately regulated at the translational level. To find out if RNA/protein interactions are involved in this control mechanism, we have characterized the particles containing the translationally repressed rp-mRNA and we have investigated the proteins that specifically bind to this type of mRNA. By sedimentation analysis and isopycnic centrifugation we have found that the repressed rp-mRNAs are assembled in slow sedimenting complexes where the RNA is prevalent over the protein mass (2.3 to 1). This composition is maintained also after in vitro reconstitution of the particle. We carried out also a detailed analysis of in vitro RNA/protein complex formation by focusing our attention on the 5'UTR, very similar in different rp-mRNAs and important in the translational regulation. We describe specific interactions of L1 mRNA with four proteins. The binding site of two of them, 57 kD and 47 kD, is in the typical pyrimidine sequence at the 5' end and is position dependent. Proteins of the same size interact also with the analogous region of r-protein S1 and L14 mRNA, not with unrelated RNAs. Binding of two other proteins, 31 kD and 24 kD, in the downstream region of the 5'UTR was also observed. The most evident 57 kD protein has been partially purified. Although the binding of these proteins to the r-protein mRNA 5'UTR is specific, their involvement in the translation regulation remains to be proved.

Animals↗

PubMed2005

Translational autocontrol of the Escherichia coli hfq RNA chaperone gene.

The conserved bacterial RNA chaperone Hfq has been shown to play an important role in post-transcriptional regulation. Here, we demonstrate that Hfq synthesis is autoregulated at the translational level. We have mapped two Hfq binding sites in the 5'-untranslated region of hfq mRNA and show that Hfq binding inhibits formation of the translation initiation complex. In vitro translation and in vivo studies further revealed that Hfq binding to both sites is required for efficient translational repression of hfq mRNA.

5' Untranslated Regions↗

PubMed2006

Regulated Pumilio-2 binding controls RINGO/Spy mRNA translation and CPEB activation.

CPEB is a sequence-specific RNA-binding protein that controls the polyadenylation-induced translation of mos and cyclin B1 mRNAs in maturing Xenopus oocytes. CPEB activity requires not only the phosphorylation of S174, but also the synthesis of a heretofore-unknown upstream effector molecule. We show that the synthesis of RINGO/Spy, an atypical activator of cyclin-dependent kinases (cdks), is necessary for CPEB-directed polyadenylation. Deletion analysis and mRNA reporter assays show that a cis element in the RINGO/Spy 3'UTR is necessary for translational repression in immature (G2-arrested) oocytes. The repression is mediated by 3'UTR Pumilio-Binding Elements (PBEs), and by its binding protein Pumilio 2 (Pum2). Pum2 also interacts with the Xenopus homolog of human Deleted for Azoospermia-like (DAZL) and the embryonic poly(A)-binding protein (ePAB). Following the induction of maturation, Pum2 dissociates not only from RINGO/Spy mRNA, but from XDAZL and ePAB as well; as a consequence, RINGO/Spy mRNA is translated. These results demonstrate that a reversible Pum2 interaction controls RINGO/Spy mRNA translation and, as a result, CPEB-mediated cytoplasmic polyadenylation.

3' Untranslated Regions↗

PubMed2005

Folded RNA from an intron of one gene might inhibit expression of a counteracting gene.

Homeostatic maintenance of mRNA levels including prompt availability of mRNAs for translation in response to changing protein demands might be partly enabled by a system of combinatorial controls involving noncoding RNA blocking agents. This article proposes a specific version of that control mechanism, namely, a double-stranded RNA folding from transcription of an intron of one gene might and leading to an agent that inhibits mRNA of a counteracting gene. Thus transcription of the first gene would automatically repress translation of the second. On the basis of a bioinformatics search, we suggest a possible example, namely, that pro-apoptosis gene PAR4 might inhibit anti-apoptosis gene XIAP. Part of an intron from PAR4 folds to form a large, stable hairpin, and reverse complement of the hairpin stem (with approximately 280 nucleotides) matches a sub-sequence of an exon in XIAP. This would be part of an efficient system to drive initiated apoptosis to its conclusion. Figuratively speaking, it replaces two control knobs with one. Since repeats, some with many thousands of copies, occur throughout the genome with complex distributions, care must be taken before asserting that the presence of any repeat in any gene is significant. Our apoptosis example involves repeats, so experimental verification is needed and planned. However, if it is found that the noncoding RNA by-product of one gene folds into a hairpin that is processed into an agent that inhibits a counteracting gene, then the same type of control unit might be found extensively among counteracting families of genes of many types.

Animals↗

PubMed1990

Regulation of rat ornithine decarboxylase mRNA translation by its 5'-untranslated region.

Ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis, is a highly inducible protein whose expression involves a complex and variable array of regulatory mechanisms. We investigated the influence of the 5'-untranslated region (5'UTR) of the rat ODC mRNA on translation of the mRNA in a cell-free system and in cultured mammalian cells. ODC mRNA containing the full-length 5'UTR was translated in reticulocyte lysates at approximately 5% of the rate of mRNA containing no ODC 5' leader sequences. The complete 5'UTR inhibited expression of a heterologous gene product, human growth hormone, to the same extent in cultured mammalian cells. Furthermore, the 5'-most 130 bases of the rat ODC 5'UTR, a conserved G/C-rich region predicted to form a stable stem-loop structure (delta G = -68 kcal/mol), repressed translation to the same extent as the entire 5'UTR, both in the lysates and in intact cells. The 3'-most 160 bases of the 5'UTR, containing a small upstream open reading frame, decreased expression by 50-65% both in vitro and in intact cells, compared with controls lacking any ODC 5'UTR sequences. Mutation of the initiation codon AUG beginning this upstream open reading frame to GCG restored expression to rates equivalent to those seen in constructions containing no ODC 5'UTR sequences. We conclude that the rat ODC mRNA 5'UTR can inhibit translation of ODC mRNA both in vitro and in vivo, and that the predicted stem-loop structure at the 5' end of the 5'UTR is both necessary and sufficient for this inhibition.

Animals↗

PubMed2001

Post-transcriptional regulation through the HO 3'-UTR by Mpt5, a yeast homolog of Pumilio and FBF.

Drosophila Pumilio (Pum) and Caenorhabditis elegans FBF bind to the 3'-untranslated region (3'-UTR) of their target mRNAs and repress translation. Pum and FBF are members of a large and evolutionarily conserved protein family, the Puf family, found in Drosophila, C.elegans, humans, and yeasts. Budding yeast, Saccharomyces cerevisiae, has five proteins with conserved Puf motifs: Mpt5/Uth4, Ygl014w, Yll013c, Jsn1, and Ypr042c. Here we report that Mpt5 negatively regulates expression of the HO gene. Loss of MPT5 increased expression of reporter genes integrated into the ho locus, whereas overexpression of MPT5 decreased expression. Repression required the 3'-UTR of HO, which contains a tetranucleotide, UUGU, also found in the binding sites of Pum and FBF. Mutation of UUGU to UACU in the HO 3'-UTR abolished Mpt5-mediated repression. Studies using a three-hybrid assay for RNA binding indicate that Mpt5 binds to the 3'-UTR of HO mRNA containing a UUGU sequence but not a UACU sequence. These observations suggest that the yeast Puf homolog, Mpt5, negatively regulates HO expression post-transcriptionally.

3' Untranslated Regions↗

PubMed2006

Mutations in the PI3K/PTEN/TSC2 pathway contribute to mammalian target of rapamycin activity and increased translation under hypoxic conditions.

Decreased oxygen causes a rapid inhibition of mRNA translation. An important regulatory mechanism of translational repression under hypoxic conditions involves inhibition of the mammalian target of rapamycin (mTOR). mTOR is a target of the phosphatase and tensin homologue detected on chromosome 10 (PTEN)/phosphatidylinositol 3-kinase/AKT/TSC2 pathway, a pathway that is frequently mutated in human cancers. Although hypoxia has been shown to inhibit mTOR activity, we show here that the hypoxia-induced inhibition of mTOR activity is attenuated in cells lacking TSC2 or PTEN, resulting in a higher translation rate even under hypoxic conditions. Comparison of mTOR inhibition by hypoxia alone or in combination with rapamycin showed that prolonged exposure to hypoxia was required to fully inhibit mTOR activity even in wild-type cells. Increased mTOR activity and protein synthesis did not translate into enhanced cell proliferation rates. However, lack of TSC2 resulted in a survival advantage when cells were exposed to hypoxia. Protection against hypoxia-induced cell death due to TSC2 deficiency is rapamycin-resistant, suggesting that TSC2 affects an apoptotic pathway. Tumors derived from TSC2 wild-type cells exhibited a growth delay compared with TSC2-deficient tumors, indicating that enhanced mTOR activity is advantageous in the initial phase of tumor growth. Therefore, failure to inhibit mTOR under oxygen-limiting conditions can be affected by upstream activating mutations and increases the survival and growth of hypoxic tumor cells.

Cell Growth Processes↗