Coordinate translational regulation in the syntheses of elongation factor 1 alpha and ribosomal proteins in Xenopus laevis

Ribosomal stress activates eEF2K-eEF2 pathway causing translation elongation inhibition and recruitment of terminal oligopyrimidine (TOP) mRNAs on polysomes

The synthesis of adequate amounts of ribosomes is an essential task for the cell. It is therefore not surprising that regulatory circuits exist to organize the synthesis of ribosomal components. It has been shown that defect in ribosome biogenesis (ribosomal stress) induces apoptosis or cell cycle arrest through activation of the tumor suppressor p53. This mechanism is thought to be implicated in the pathophysiology of a group of genetic diseases such as Diamond Blackfan Anemia which are called ribosomopathies. We have identified an additional response to ribosomal stress that includes the activation of eukaryotic translation elongation factor 2 kinase with a consequent inhibition of translation elongation. This leads to a translational reprogramming in the cell that involves the structurally defined group of messengers called terminal oligopyrimidine (TOP) mRNAs which encode ribosomal proteins and translation factors. In fact, while general protein synthesis is decreased by the impairment of elongation, TOP mRNAs are recruited on polysomes causing a relative increase in the synthesis of TOP mRNA-encoded proteins compared to other proteins. Therefore, in response to ribosomal stress, there is a change in the translation pattern of the cell which may help restore a sufficient level of ribosomes.

The race to decipher the top secrets of TOP mRNAs

Cells encountering hostile growth conditions, like those residing in the middle of a newly developing solid tumor, conserve resources and energy by downregulating protein synthesis. One mechanism in this response is the translational repression of multiple mRNAs that encode components of the translational apparatus. This coordinated translational control is carried through a common cis-regulatory element, the 5' Terminal OligoPyrimidine motif (5'TOP), after which these mRNAs are referred to as TOP mRNAs. Subsequent to the initial structural and functional characterization of members of this family, the research of TOP mRNAs has progressed in three major directions: a) delineating the landscape of the family; b) establishing the pathways that transduce stress cues into selective translational repression; and c) attempting to decipher the most proximal trans-acting factor(s) and defining its mode of action--a repressor or activator. The present chapter critically reviews the development in these three avenues of research with a special emphasis on the two "top secrets" of the TOP mRNA family: the scope of its members and the identity of the proximal cellular regulator(s). This article is part of a Special Issue entitled: Translation and Cancer.

Enhanced translation of mRNAs encoding proteins involved in mRNA translation during recovery from heat shock

The mRNAs encoding poly (A) binding protein (PABP1), eukaryotic elongation factor 1A (eEF1A) and ribosomal protein S6 (RPS6) belong to the family of terminal oligo pyrimidine tract (TOP) containing mRNAs. Translation of the TOP mRNAs is regulated by growth signals and usually codes for proteins involved in mRNA translation. Previous studies from our laboratory showed that translation of PABP1 mRNA was preferentially enhanced during recovery of HeLa cells from heat shock. Presence of the 5′ TOP cis element was required for the observed increase of PABP1 mRNA translation. In the studies reported here we showed that translation of two additional TOP mRNAs such as, eEF1A and RPS6 was similarly enhanced during recovery. In addition, we showed by in vivo cross-linking experiments that the cellular nucleic acid binding protein ZNF9 binds to all three TOP mRNAs examined in these studies as well as to the β-actin mRNA that lacks a TOP cis element. Binding of ZNF9 to mRNAs was observed in both heat-shocked and non heat- shocked cells. However, depletion of ZNF9 by siRNA prevented the preferred stimulation of PABP1, eEF1A and RPS6 expression during recovery from heat shock. There was no detectable effect of ZNF9 depletion on the basal level of expression of either β-actin or PABP1, eEF1A and RPS6 in HeLa cells following recovery from heat shock.

Conclusion

Although the presence of ZNF9 was required for the translational stimulation of PABP1, eEF1A and RPS6 mRNAs, the mechanistic details of this process are still unclear. Since ZNF9 was shown to bind both TOP and non-TOP mRNAs, it is uncertain whether ZNF9 exerts its stimulatory effect on TOP mRNA translation following recovery from heat shock through the TOP cis-element. Perhaps additional factors or post-translational modification(s) of ZNF9 following heat shock are necessary for the preferred increase of TOP mRNA translation.

hnRNP H1 and intronic G runs in the splicing control of the human rpL3 gene

By generating mRNA containing a premature termination codon (PTC), alternative splicing (AS) can quantitatively regulate the expression of genes that are degraded by nonsense-mediated mRNA decay (NMD). We previously demonstrated that AS-induced retention of part of intron 3 of rpL3 pre-mRNA produces an mRNA isoform that contains a PTC and is targeted for decay by NMD. We also demonstrated that overexpression of rpL3 downregulates canonical splicing and upregulates the alternative splicing of its pre-mRNA. We are currently investigating the molecular mechanism underlying rpL3 autoregulation. Here we report that the heterogeneous nuclear ribonucleoprotein (hnRNP) H1 is a transacting factor able to interact in vitro and in vivo with rpL3 and with intron 3 of the rpL3 gene. We investigated the role played by hnRNP H1 in the regulation of splicing of rpL3 pre-mRNA by manipulating its expression level. Depletion of hnRNP H1 reduced the level of the PTC-containing mRNA isoform, whereas its overexpression favored the selection of the cryptic 3' splice site of intron 3. We also identified and characterized the cis-acting regulatory elements involved in hnRNP H1-mediated regulation of splicing. RNA electromobility shift assay demonstrated that hnRNP H1 specifically recognizes and binds directly to the intron 3 region that contains seven copies of G-rich elements. Site-directed mutagenesis analysis and in vivo studies showed that the G3 and G6 elements are required for hnRNP H1-mediated regulation of rpL3 pre-mRNA splicing. We propose a working model in which rpL3 recruits hnRNP H1 and, through cooperation with other splicing factors, promotes selection of the alternative splice site.

All translation elongation factors and the e, f, and h subunits of translation initiation factor 3 are encoded by 5'-terminal oligopyrimidine (TOP) mRNAs

Terminal oligopyrimidine (TOP) mRNAs (encoded by the TOP genes) are identified by a sequence of 6-12 pyrimidines at the 5' end and by a growth-associated translational regulation. All vertebrate genes for the 80 ribosomal proteins and some other genes involved, directly or indirectly, in translation, are TOP genes. Among the numerous translation factors, only eEF1A and eEF2 are known to be encoded by TOP genes, most of the others having not been analyzed. Here, we report a systematic analysis of the human genes for translation factors. Our results show that: (1) all five elongation factors are encoded by TOP genes; and (2) among the initiation and termination factors analyzed, only eIF3e, eIF3f, and eIF3h exhibit the characteristics of TOP genes. Interestingly, these three polypeptides have been recently shown to constitute a specific subgroup among eIF3 subunits. In fact, eIF3e, eIF3f, and eIF3h are the part of the functional core of eIF3 that is not conserved in Saccharomyces cerevisiae. It has been hypothesized that they are regulatory subunits, and the fact that they are encoded by TOP genes may be relevant for their function.

Gene expression profiling of chicken primordial germ cell ESTs

BACKGROUND

Germ cells are the only cell type that can penetrate from one generation to next generation. At the early embryonic developmental stages, germ cells originally stem from primordial germ cells, and finally differentiate into functional gametes, sperm in male or oocyte in female, after sexual maturity. This study was conducted to investigate a large-scale expressed sequence tag (EST) analysis in chicken PGCs and compare the expression of the PGC ESTs with that of embryonic gonad.

RESULTS

We constructed 10,851 ESTs from a chicken cDNA library of a collection of highly separated embryonic PGCs. After chimeric and problematic sequences were filtered out using the chicken genomic sequences, there were 5,093 resulting unique sequences consisting of 156 contigs and 4,937 singlets. Pearson chi-square tests of gene ontology terms in the 2nd level between PGC and embryonic gonad set showed no significance. However, digital gene expression profiling using the Audic's test showed that there were 2 genes expressed significantly with higher number of transcripts in PGCs compared with the embryonic gonads set. On the other hand, 17 genes in embryonic gonads were up-regulated higher than those in the PGC set.

CONCLUSION

Our results in this study contribute to knowledge of mining novel transcripts and genes involved in germline cell proliferation and differentiation at the early embryonic stages.

Endotoxin induces differential regulation of mTOR-dependent signaling in skeletal muscle and liver of neonatal pigs

In the present study, differential responses of regulatory proteins involved in translation initiation in skeletal muscle and liver during sepsis were studied in neonatal pigs treated with lipopolysaccharide (LPS). LPS did not alter eukaryotic initiation factor (eIF) 2B activity in either tissue. In contrast, binding of eIF4G to eIF4E to form the active mRNA-binding complex was repressed in muscle and enhanced in liver. Phosphorylation of eIF4E-binding protein, 4E-BP1, and ribosomal protein S6 kinase, S6K1, was reduced in muscle during sepsis but increased in liver. Finally, changes in 4E-BP1 and S6K1 phosphorylation were associated with altered phosphorylation of the protein kinase mammalian target of rapamycin (mTOR). Overall, the results suggest that translation initiation in both skeletal muscle and liver is altered during neonatal sepsis by modulation of the mRNA-binding step through changes in mTOR activation. Moreover, the LPS-induced changes in factors that regulate translation initiation are more profound than previously reported changes in global rates of protein synthesis in the neonate. This finding suggests that the initiator methionyl-tRNA-rather than the mRNA-binding step in translation initiation may play a more critical role in maintaining protein synthesis rates in the neonate during sepsis.

The 5' terminal oligopyrimidine tract of human elongation factor 1A-1 gene functions as a transcriptional initiator and produces a variable number of Us at the transcriptional level

The human elongation factor 1A-1 (eEF1A-1) gene is a member of the 5' terminal oligopyrimidine tract (5' TOP) gene family, and the number of thymidines (Ts) at the 5' TOP of cDNAs corresponding to this gene is known to show variation. Here we determined the 5'-end sequences of 125 eEF1A-1 clones and the complete sequences of 19 eEF1A-1 clones from an oligo-capped cDNA library and showed that variation in the number of Ts is generated by an in vivo process, not by an in vitro artifact during the construction of the cDNA library. Moreover, using green fluorescent protein transgenic mice, we demonstrated that the variation in T number is probably generated during or after transcription. We also introduced various mutations in the mRNA start site of this gene, particularly in the T stretch at the 5' TOP, and examined the effects on the promoter activity. The results showed that at least three Ts must exist at the 5' TOP for the high transcriptional activity of the eEF1A-1 gene promoter. Many other housekeeping genes, including ribosomal protein genes, are also members of the 5' TOP gene family, and the 5' TOP sequence may be an important core-promoter element of these genes.

Regulation of elongation factor-1alpha expression by growth factors and anti-receptor blocking antibodies

The epidermal growth factor (EGF) family and its receptors regulate normal and cancerous epithelial cell proliferation, a process that could be suppressed by anti-receptor blocking antibodies. Polypeptide elongation factor-1alpha (EF-1alpha) is a multifunctional protein whose levels are positively correlated with the proliferative state of cells. To identify genes, whose expression may be modulated by anti-receptor blocking antibodies, we performed a differential display screening and isolated differentially expressed cDNAs. Isolates from one clone were 100% identical to human EF-1alpha. Both EGF and heregulin-beta1 (HRG) induced EF-1alpha promoter activity and mRNA and protein expression. Growth factor-mediated EF-1alpha expression was effectively blocked by pretreatment with humanized anti-EGF receptor antibody C225 or anti-human epidermal growth factor receptor-2 (HER2) antibody herceptin. Mutants and pharmacological inhibitors of p38(MAPK) and MEK, but not phosphatidylinositol 3-kinase, suppressed both constitutive and HRG-induced stimulation of EF-1alpha promoter activity in MCF-7 cells. Deletion analysis of the promoter suggested the requirement of the -393 to -204 region for growth factor-mediated transcription of EF-1alpha. Fine mapping and point mutation studies revealed a role of the SP1 site in the observed HRG-mediated regulation of the EF-1alpha promoter. In addition, we also provide new evidence to suggest that HRG stimulation of the EF-1alpha promoter involves increased physical interactions with acetylated histone H3 and histone H4. These results suggest that regulation of EF-1alpha expression by extracellular signals that function through human EGF receptor family members that are widely deregulated in human cancers and that growth factor regulation of EF-1alpha expression involve histone acetylation.

Synthesis of the translational apparatus is regulated at the translational level

The synthesis of many mammalian proteins associated with the translational apparatus is selectively regulated by mitogenic and nutritional stimuli, at the translational level. The apparent advantages of the regulation of gene expression at the translational level are the speed and the readily reversible nature of the response to altering physiological conditions. These two features enable cells to rapidly repress the biosynthesis of the translational machinery upon shortage of amino acids or growth arrest, thus rapidly blocking unnecessary energy wastage. Likewise, when amino acids are replenished or mitogenic stimulation is applied, then cells can rapidly respond in resuming the costly biosynthesis of the translational apparatus. A structural hallmark, common to mRNAs encoding many components of the translational machinery, is the presence of a 5' terminal oligopyrimidine tract (5'TOP), referred to as TOP mRNAs. This structural motif comprises the core of the translational cis-regulatory element of these mRNAs. The present review focuses on the mechanism underlying the translational control of TOP mRNAs upon growth and nutritional stimuli. A special emphasis is put on the pivotal role played by ribosomal protein S6 kinase (S6K) in this mode of regulation, and the upstream regulatory pathways, which might be engaged in transducing external signals into activation of S6K. Finally, the possible involvement of pyrimidine-binding proteins in the translational control of TOP mRNAs is discussed.

Identification of mRNA-binding proteins during development: characterization of Bufo arenarum cellular nucleic acid binding protein

Ultraviolet irradiation was used to covalently cross-link poly(A)+RNA and associated proteins in eggs and embryos of the toad Bufo arenarum. Four major proteins with apparent sizes of 60, 57, 45 and 30-24 kDa were identified. It was observed that the same mRNA-binding proteins were isolated from eggs to gastrula and neural stages of development. The 30 kDa polypeptide, p30, appeared as the main ultraviolet (UV) cross-linked protein in the developmental stages analyzed. By means of polyclonal antibodies, it was determined that this polypeptide has a cytoplasmic localization and it was detected in liver, eggs and embryos. The presence of p30 was also analyzed by western blot during oogenesis and development. The 30 kDa polypeptide was present in all stages analyzed but it could not be detected in stages I-II of oogenesis. At the neural stage, the relative amount of p30 began to decrease, reaching its lowest levels after stages 26-30 (tail-bud in Bufo arenarum). On the basis of purification, immunoprecipitation and western blot assays the 30 kDa protein was identified as the Bufo arenarum cellular nucleic acid binding protein.

Involvement of the Xenopus laevis Ro60 autoantigen in the alternative interaction of La and CNBP proteins with the 5'UTR of L4 ribosomal protein mRNA

In vertebrates the synthesis of ribosomal proteins is co-ordinately regulated at the translational level. The 5'-untranslated region (5'UTR) of this class of mRNAs contains conserved regions that are necessary and sufficient for translational regulation. Recently, we found that two proteins, the Xenopus laevis La autoantigen and the cellular nucleic acid binding protein (CNBP), are able to bind in vitro a pyrimidine tract at the 5' end and a downstream region, respectively. These regions are considered the common cis-acting elements of translational regulation. It was previously observed that the binding of both these putative trans-acting factors to their RNA sequences is assisted by a protease-sensitive factor(s) that dissociates from the complex after its formation. Here we provide evidence that the requirement for an ancillary factor assisting La binding to the pyrimidine tract of ribosomal protein mRNAs is typical of this RNA, and secondly that it may involve an RNA recognition motif of the La protein not clearly characterized previously. We also show that the Ro60 autoantigen is involved in the common factor activity necessary for the binding of La and CNBP proteins to their respective sequences. In addition, our findings suggest that an RNA also participates in this process. We show that CNBP can multimerise and that it binds to the 5'UTR as a dimer. Both La and CNBP compete for the interaction with the factor, and their binding to the 5'UTR is mutually exclusive. Our results from the binding analysis of mutations in the 5'UTR, which are known to disrupt the translational control in vivo, suggest a model in which the protein interactions and the 5'UTR RNA structure may co-operate in regulating the translational fate of ribosomal protein mRNAs.

Primary sequence of the human, lysine-rich, ribosomal protein RPL38 and detection of an unusual RPL38 processed pseudogene in the promoter region of the type-1 angiotensin II receptor gene

We have isolated a cDNA clone encoding the human ribosomal protein L38 (HSRPL38). The longest ORF of the cDNA predicts a lysine-rich small polypeptide identical to the rat RPL38 protein (100% identity), and sharing a 84% of identity to the tomato RPL38 protein sequence. Northern blot analysis of a number of epithelial cell lines showed that the HSRPL38 is encoded by a mRNA ubiquitously expressed. Southern blot analysis of human genomic DNA suggested that the RPL38 does not constitute a multigene family but it is encoded by a reduced set of active genes, among which we have also found a RPL38 processed pseudogene located in the promoter region of the human type-1 angiotensin II receptor gene. This RPL38 pseudogene is very unusual among processed pseudogenes in that the poly A tail and the entire 5'-UTR of the original RPL38 mRNA were deleted during the retrotransposition process.

Cellular nucleic acid binding protein binds a conserved region of the 5' UTR of Xenopus laevis ribosomal protein mRNAs

Vertebrate ribosomal protein mRNAs share structural features in the 5' untranslated region implicated in the control of their translation. A pyrimidine tract, at the 5' end, is considered the common cis-acting element, but the control requires also the integrity of the conserved downstream region. These sequences interact in vitro with proteins, which may represent the trans-acting factors for a common regulation. The protein that binds the pyrimidine tract has been identified as La and its binding in vitro depends on interaction with a protein factor. In the present study, by purification, microsequencing and immunoprecipitation analysis we have identified the protein that interacts with the region downstream of the pyrimidine tract as the Xenopus laevis cellular nucleic acid binding protein (CNBP). The interaction of this protein with the conserved region of various ribosomal protein (rp)-mRNAs suggests a class-specific recognition. The binding of CNBP to the target region requires the assistance of a protease-sensitive factor, that dissociates after complex formation. Some evidence suggests that this may be the same factor that assists the binding of La to the 5' untranslated region (UTR) of the rp-mRNAs. Considering that CNBP and La come in contact with two typical regions of the 5' UTR, essential for regulation, their interaction with the assisting factor may exert a modulating activity on the translational control of ribosomal protein mRNAs.

Structure and transcription of the gene for translation elongation factor 1 subunit alpha of zebrafish (Danio rerio)

The zebrafish gene for translation elongation factor 1 alpha (EF1 alpha) was isolated from a phage Lambda genomic library and sequence and structure determined. One gene copy of EF1 alpha per haploid set of chromosomes was found and no processed pseudogenes. A highly active promoter region was localized to a 277 bp PstI/PvuII fragment beginning 240 bp upstream from the tsp, but no transcription enhancing, or silencing activity was observed within 1 kbp upstream, or downstream from the promoter. Expression of EF1 alpha appears to be developmentally regulated.

Coordinate regulation of ribosomal component synthesis in Acanthamoeba castellanii: 5S RNA transcription is down regulated during encystment by alteration of TFIIIA activity

Transcription of large rRNA precursor and 5S RNA were examined during encystment of Acanthamoeba castellanii. Both transcription units are down regulated almost coordinately during this process, though 5S RNA transcription is not as completely shut down as rRNA transcription. The protein components necessary for transcription of 5S RNA and tRNA were determined, and fractions containing transcription factors comparable to TFIIIA, TFIIIB, and TFIIIC, as well as RNA polymerase III and a 3'-end processing activity, were identified. Regulation of 5S RNA transcription could be recapitulated in vitro, and the activities of the required components were compared. In contrast to regulation of precursor rRNA, there is no apparent change during encystment in the activity of the polymerase dedicated to 5S RNA expression. Similarly, the transcriptional and promoter-binding activities of TFIIIC are not altered in parallel with 5S RNA regulation. TFIIIB transcriptional activity is unaltered in encysting cells. In contrast, both the transcriptional and DNA-binding activities of TFIIIA are strongly reduced in nuclear extracts from transcriptionally inactive cells. These results were analyzed in terms of mechanisms for coordinate regulation of rRNA and 5S RNA expression.

Xenopus laevis ribosomal protein L22: full-length cDNA sequence and expression analysis

A cDNA clone was isolated from a Xenopus laevis embryo library and sequenced. Primer extension experiments indicated the full-length nature of the insert and the encoded product was identified on a two dimensional gel as ribosomal protein (r-protein) L22. The 510-bp L22 cDNA sequence presents short untranslated regions and a 5'-end polypyrimidine tract found in all other vertebrate r-protein mRNA (rp mRNA) so far analyzed. Both the nucleotide (nt) and the deduced amino acid (aa) sequences have been compared with the homologous sequences from other species. The L22 nt sequence is about 70% similar to the mammalian L27a rp mRNA and about 60% homologous to the Drosophila, Tetrahymena and yeast corresponding mRNAs. The 148-aa sequence presents a higher conservation, being 90% similar to the mammalian sequence and more than 70% to the other species. Expression analysis showed that, both during X. laevis embryogenesis and in X. laevis cultured cells during growth-rate changes, L22 synthesis is translationally regulated. Therefore X. laevis L22 mRNA is a new example of the correlation between the polypyrimidine terminal tract and the translational regulation observed in other rp mRNAs.

mRNA encoding the translation initiation factor eIF-4E is expressed early in Xenopus embryogenesis

The translation initiation factor eIF-4E plays an important role in regulating the overall rate of translation in eukaryotic cells. To investigate the expression of eIF-4E itself, we characterized the eIF-4E mRNA expressed in Xenopus embryos. 5'-RACE was performed to determine the 5'-end of the mRNA and the result predicts isoforms differing at the amino-terminal end. Expression of the eIF-4E mRNA in Xenopus oocytes and embryos was examined by RT-PCR. Xenopus eIF-4E mRNA is produced during oogenesis and persists during the early stages of embryogenesis as a maternal mRNA.

Vertebrate mRNAs with a 5'-terminal pyrimidine tract are candidates for translational repression in quiescent cells: characterization of the translational cis-regulatory element

The translation of mammalian ribosomal protein (rp) mRNAs is selectively repressed in nongrowing cells. This response is mediated through a regulatory element residing in the 5' untranslated region of these mRNAs and includes a 5' terminal oligopyrimidine tract (5' TOP). To further characterize the translational cis-regulatory element, we monitored the translational behavior of various endogenous and heterologous mRNAs or hybrid transcripts derived from transfected chimeric genes. The translational efficiency of these mRNAs was assessed in cells that either were growing normally or were growth arrested under various physiological conditions. Our experiments have yielded the following results: (i) the translation of mammalian rp mRNAs is properly regulated in amphibian cells, and likewise, amphibian rp mRNA is regulated in mammalian cells, indicating that all of the elements required for translation control of rp mRNAs are conserved among vertebrate classes; (ii) selective translational control is not confined to rp mRNAs, as mRNAs encoding the naturally occurring ubiquitin-rp fusion protein and elongation factor 1 alpha, which contain a 5' TOP, also conform this mode of regulation; (iii) rat rpP2 mRNA contains only five pyrimidines in its 5' TOP, yet this mRNA is translationally controlled in the same fashion as other rp mRNAs with a 5' TOP of eight or more pyrimidines; (iv) full manifestation of this mode of regulation seems to require both the 5' TOP and sequences immediately downstream; and (v) an intact translational regulatory element from rpL32 mRNA fails to exert its regulatory properties even when preceded by a single A residue.

Aspects of regulation of ribosomal protein synthesis in Xenopus laevis. Review

The work carried out in the authors' laboratories on the structure and expression of ribosomal protein genes in Xenopus is reviewed, with some comparisons with other systems. These genes form a class that shares several structural features, especially in the region surrounding the 5' ends. These similar structures appear to be involved in coregulated expression that is attained at various regulatory levels: transcriptional, transcript processing and stability, and translational. Particular attention is paid here to the one operating at the translational level, which has been studied during Xenopus oogenesis and embryogenesis, and also during nutritional changes of Xenopus cultured cells. This regulation, which responds to the cellular need for new ribosomes, operates by changing the fraction of rp-mRNA engaged on polysomes, leaving each translated rp-mRNA molecule always fully loaded with ribosomes. Responsible for this translational behaviour is the typical 5'UTR, which characterizes all rp-mRNAs analyzed up to now, and that can bind in vitro some proteins, putative trans-acting factors for this translational regulation.

Vertebrate mRNAs with a 5'-terminal pyrimidine tract are candidates for translational repression in quiescent cells: characterization of the translational cis-regulatory element

The translation of mammalian ribosomal protein (rp) mRNAs is selectively repressed in nongrowing cells. This response is mediated through a regulatory element residing in the 5' untranslated region of these mRNAs and includes a 5' terminal oligopyrimidine tract (5' TOP). To further characterize the translational cis-regulatory element, we monitored the translational behavior of various endogenous and heterologous mRNAs or hybrid transcripts derived from transfected chimeric genes. The translational efficiency of these mRNAs was assessed in cells that either were growing normally or were growth arrested under various physiological conditions. Our experiments have yielded the following results: (i) the translation of mammalian rp mRNAs is properly regulated in amphibian cells, and likewise, amphibian rp mRNA is regulated in mammalian cells, indicating that all of the elements required for translation control of rp mRNAs are conserved among vertebrate classes; (ii) selective translational control is not confined to rp mRNAs, as mRNAs encoding the naturally occurring ubiquitin-rp fusion protein and elongation factor 1 alpha, which contain a 5' TOP, also conform this mode of regulation; (iii) rat rpP2 mRNA contains only five pyrimidines in its 5' TOP, yet this mRNA is translationally controlled in the same fashion as other rp mRNAs with a 5' TOP of eight or more pyrimidines; (iv) full manifestation of this mode of regulation seems to require both the 5' TOP and sequences immediately downstream; and (v) an intact translational regulatory element from rpL32 mRNA fails to exert its regulatory properties even when preceded by a single A residue.

Regulation of translation by specific protein/mRNA interactions

This review will focus on cases of specific translational control by protein/RNA interactions in the 5'- or 3'-UTR of eukaryote mRNA where either the cis-acting RNA determinant or the trans-acting protein (or preferably both) have been identified with fair certainty. Examples of messages that are regulated by 5' motifs, which are proposed to occlude ribosome binding when bound by their specific factors, include ferritin and ribosomal protein mRNAs and the autoregulated thymidylate synthase and poly(A)-binding mRNAs. However, it has become increasingly evident recently that 3' UTR determinants and their specific binding proteins also regulate translation efficiency either directly, or indirectly via an influence on the polyadenylation status of the mRNA. It is still unclear how events at the 3' end of mRNA influence ribosome binding. Most, if not all, of the mRNAs known to be regulated by 3' UTR motifs are subject to regulation during early development or during differentiation such as several spermatocyte and oocyte mRNAs and erythroid lipoxygenase mRNA. To date, in all cases where translation is controlled directly by specific protein/mRNA interactions, the protein seems to act as a negative regulator, a translational repressor, whose binding to the specific site on the mRNA results in inhibition of initiation. The only cases of translational activation known so far concern internal initiation of translation of picornaviral RNAs, but this topic is beyond the scope of this review.