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Translation of picornavirus RNAs is mediated by internal ribosomal entry site (IRES) elements and requires both standard eukaryotic translation initiation factors (eIFs) and IRES-specific cellular trans-acting factors (ITAFs).After infection of a susceptible cell, translation of the picornavirus plus-strand RNA genome is controlled by the internal ribosome entry site (IRES). This cis-regulatory RNA element of about 450 nucleotides folds into complex and highly conserved secondary structures and facilitates translation by direct binding of ribosomes to an internal site of the viral RNA. IRES elements have been found in various viral RNAs and also in cellular mRNAs (for reviews, see references 1 and 16).Internal initiation of picornavirus translation seems to require all canonical eukaryotic initiation factors (eIFs) also involved in cellular cap-dependent translation (28, 33), except for the actual cap-binding protein eIF4E. For the majority of eukaryotic mRNAs that are translated by a ribosome-scanning mechanism (13,18,38), the initiation of translation is the most important point of regulation in the overall process of protein synthesis (29). Modulation of the activity of these eIFs alters the general rate of protein synthesis, and the signal transduction routes leading to the eIFs are becoming clear now (36), supporting the idea that translational control substantially contributes to the regulation of gene expression in eukaryotes.For picornaviruses, the initiation of translation also appears to be a major point of control. In vitro studies revealed that additional noncanonical translation initiation factors are involved in picornavirus IRES-mediated translation (1, 41). Accordingly, there is significant genetic evidence that picornavirus IRES elements contain determinants of cell specificity.Analysis of poliovirus IRES mutants showed that translation defects could be cell type specific, since a decreased translation capacity of mutant templates was evident in cell extracts of neuronal origin, but not in HeLa cells (26). Cell-specific determinants were recently demonstrated to exist in the poliovirus 5Ј-untranslated region (UTR) by experiments using viruses with chimeric genomes. When the poliovirus IRES was replaced with that of human rhinovirus (HRV), neuropathogenicity in a mouse model was abrogated (15). The differential translation of wild-type and attenuated Sabin vaccine strains of poliovirus in different cell types indicates that cellular factors influencing translational activity may be differentially expressed, consistent with the idea that factors distinct from the standard initiation factors determine picornavirus translation efficiency (for review, see reference 1). The general idea that IRES elements may allow fine-tuning of gene expression was also supported for cellular IRES elements. The ornithine decarboxylase and p58 PISTLRE IRES elements specifically function during the G 2 /M period (8, 37), the Ultrabithorax and Antennapedia IRES activities exhibit a high degree of developmental regulatio...
Translation of picornavirus RNAs is mediated by internal ribosomal entry site (IRES) elements and requires both standard eukaryotic translation initiation factors (eIFs) and IRES-specific cellular trans-acting factors (ITAFs).After infection of a susceptible cell, translation of the picornavirus plus-strand RNA genome is controlled by the internal ribosome entry site (IRES). This cis-regulatory RNA element of about 450 nucleotides folds into complex and highly conserved secondary structures and facilitates translation by direct binding of ribosomes to an internal site of the viral RNA. IRES elements have been found in various viral RNAs and also in cellular mRNAs (for reviews, see references 1 and 16).Internal initiation of picornavirus translation seems to require all canonical eukaryotic initiation factors (eIFs) also involved in cellular cap-dependent translation (28, 33), except for the actual cap-binding protein eIF4E. For the majority of eukaryotic mRNAs that are translated by a ribosome-scanning mechanism (13,18,38), the initiation of translation is the most important point of regulation in the overall process of protein synthesis (29). Modulation of the activity of these eIFs alters the general rate of protein synthesis, and the signal transduction routes leading to the eIFs are becoming clear now (36), supporting the idea that translational control substantially contributes to the regulation of gene expression in eukaryotes.For picornaviruses, the initiation of translation also appears to be a major point of control. In vitro studies revealed that additional noncanonical translation initiation factors are involved in picornavirus IRES-mediated translation (1, 41). Accordingly, there is significant genetic evidence that picornavirus IRES elements contain determinants of cell specificity.Analysis of poliovirus IRES mutants showed that translation defects could be cell type specific, since a decreased translation capacity of mutant templates was evident in cell extracts of neuronal origin, but not in HeLa cells (26). Cell-specific determinants were recently demonstrated to exist in the poliovirus 5Ј-untranslated region (UTR) by experiments using viruses with chimeric genomes. When the poliovirus IRES was replaced with that of human rhinovirus (HRV), neuropathogenicity in a mouse model was abrogated (15). The differential translation of wild-type and attenuated Sabin vaccine strains of poliovirus in different cell types indicates that cellular factors influencing translational activity may be differentially expressed, consistent with the idea that factors distinct from the standard initiation factors determine picornavirus translation efficiency (for review, see reference 1). The general idea that IRES elements may allow fine-tuning of gene expression was also supported for cellular IRES elements. The ornithine decarboxylase and p58 PISTLRE IRES elements specifically function during the G 2 /M period (8, 37), the Ultrabithorax and Antennapedia IRES activities exhibit a high degree of developmental regulatio...
The kinetics of pre-mRNA processing in living cells is poorly known, preventing a detailed analysis of the regulation of these reactions. Using tetracycline-regulated promoters we performed, during a transcriptional induction, a complete analysis of the maturation of two cellular mRNAs, those for LT-␣ and -globin. In both cases, splicing was appropriately described by first-order reactions with corresponding half-lives ranging between 0.4 and 7.5 min, depending on the intron. Transport also behaved as a first-order reaction during the early phase of -globin expression, with a nuclear dwelling time of 4 min. At a later time, analysis was prevented by the progressive accumulation within the nucleus of mature mRNA not directly involved in export. Our results further establish for these genes that (i) splicing components are never limiting, even when expression is induced in naive cells, (ii) there is no significant RNA degradation during splicing and transport, and (iii) precursor-to-product ratios at steady state can be used for the determination of splicing rates. Finally, the comparison between the kinetics of splicing during transcriptional induction and during transcriptional shutoff reveals a novel coupling between transcription and splicing.In eukaryotic cells pre-mRNA maturation is a complex process that can entail the removal of multiple introns before a mature transcript can be exported to the cytoplasm. Moreover, the fidelity of processing and the control of alternative pathways are crucial for the correct execution of a genetic program. While our knowledge of the biochemical processes behind splicing and transport has greatly increased over the last years, we still know very little about their kinetics in vivo. This lack is all the more serious since the current view of nuclear RNA processing is that of a large number of regulators competing for the same substrates. This stresses the importance of the local concentration of regulators but also of the kinetics of the corresponding reactions. In addition, it is usually assumed that the RNA species generated during pre-mRNA processing are very unstable (7,21). If this is true, the level of expression should be critically dependent on the kinetics of processing.Our lack of knowledge of the kinetics of splicing and transport reflects the paucity of adequate experimental approaches. In vivo labeling studies have been used to investigate mRNA metabolism in mammalian cells. While this approach should provide access to the processing rates of pre-mRNA, two factors seriously limit its usefulness. (i) Studies on the fate of a specific RNA are hampered by the lack of sensitivity. (ii) The equilibration of the labeled compound with the intracellular nucleotide pool requires several minutes, which is the time scale anticipated for splicing and transport reactions. Thus the labeling is too slow for a stop flow type of kinetic analysis and too rapid for an approach to equilibrium analysis. Consequently, only some general information on pre-mRNA processing can be extr...
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