Effect of eukaryotic initiation factor 4F on AUG selection in a bicistronic mRNA

Non-Canonical Translation Initiation Mechanisms Employed by Eukaryotic Viral mRNAs

Viruses exploit the translation machinery of an infected cell to synthesize their proteins. Therefore, viral mRNAs have to compete for ribosomes and translation factors with cellular mRNAs. To succeed, eukaryotic viruses adopt multiple strategies. One is to circumvent the need for m7G-cap through alternative instruments for ribosome recruitment. These include internal ribosome entry sites (IRESs), which make translation independent of the free 5' end, or cap-independent translational enhancers (CITEs), which promote initiation at the uncapped 5' end, even if located in 3' untranslated regions (3' UTRs). Even if a virus uses the canonical cap-dependent ribosome recruitment, it can still perturb conventional ribosomal scanning and start codon selection. The pressure for genome compression often gives rise to internal and overlapping open reading frames. Their translation is initiated through specific mechanisms, such as leaky scanning, 43S sliding, shunting, or coupled termination-reinitiation. Deviations from the canonical initiation reduce the dependence of viral mRNAs on translation initiation factors, thereby providing resistance to antiviral mechanisms and cellular stress responses. Moreover, viruses can gain advantage in a competition for the translational machinery by inactivating individual translational factors and/or replacing them with viral counterparts. Certain viruses even create specialized intracellular "translation factories", which spatially isolate the sites of their protein synthesis from cellular antiviral systems, and increase availability of translational components. However, these virus-specific mechanisms may become the Achilles' heel of a viral life cycle. Thus, better understanding of the unconventional mechanisms of viral mRNA translation initiation provides valuable insight for developing new approaches to antiviral therapy.

Influence of translation factor activities on start site selection in six different mRNAs

Current literature using biochemical assays, structural analyses and genetic manipulations has reported that the key factors associated with the faithful matching of the initiator met-tRNA to the start codon AUG are eIF1, eIF1A and eIF5. However, these findings were in each case based upon the utilization of a single mRNA, perhaps with variations. In an effort to evaluate this general finding, we tested six different mRNAs. Our results confirm that these three proteins are important for start site selection. However, two additional findings would not have been predicted. The first is that eIF1 plays a major role in selecting against start codons that are in close proximity to the 5′ end of the mRNA (i.e., less than 21 nucleotides). Second, the addition of eIF5B had nearly the same affect as the addition of eIF5. This is unexpected given the different roles that eIF5 and eIF5B have been proposed to play in the 80S initiation pathway. Finally, although many of the mRNAs appear to respond qualitatively in a similar manner, the quantitative differences noted suggest that there is still some mRNA specific character to our findings. This character may be the length of the 5′ UTR, involvement of an IRES element, secondary structure either 5′ or 3′ of the start codon or specific sequence/structure elements that interact with RNA binding proteins or the ribosome.

Translational control of C/EBPα and C/EBPβ isoform expression

Transcription factors derived from CCAAT/enhancer binding protein (C/EBP)α and C/EBPβ genes control differentiation and proliferation in a number of cell types. Various C/EBP isoforms arise from unique C/EBPβ and C/EBPα mRNAs by differential initiation of translation. These isoforms retain different parts of the amino terminus and therefore display different functions in gene regulation and proliferation control. We show that PKR and mTOR signaling pathways control the ratio of C/EBP isoform expression through the eukaryotic translation initiation factors eIF-2α and eIF-4E, respectively. An evolutionary conserved upstream open reading frame in C/EBPα and C/EBPβ mRNAs is a prerequisite for regulated initiation from the different translation initiation sites and integrates translation factor activity. Deregulated translational control leading to aberrant C/EBPα and C/EBPβ isoform expression or ectopic expression of truncated isoforms disrupts terminal differentiation and induces a transformed phenotype in 3T3-L1 cells. Our results demonstrate that the translational controlled ratio of C/EBPα and C/EBPβ isoform expression determines cell fate.

Translation in plants--rules and exceptions

Translation processes in plants are very similar to those in other eukaryotic organisms and can in general be explained with the scanning model. Particularly among plant viruses, unconventional mRNAs are frequent, which use modulated translation processes for their expression: leaky scanning, translational stop codon readthrough or frameshifting, and transactivation by virus-encoded proteins are used to translate polycistronic mRNAs; leader and trailer sequences confer (cap-independent) efficient ribosome binding, usually in an end-dependent mechanism, but true internal ribosome entry may occur as well; in a ribosome shunt, sequences within an RNA can be bypassed by scanning ribosomes. Translation in plant cells is regulated under conditions of stress and during development, but the underlying molecular mechanisms have not yet been determined. Only a small number of plant mRNAs, whose structure suggests that they might require some unusual translation mechanisms, have been described.

Rat phospholipid-hydroperoxide glutathione peroxidase. cDNA cloning and identification of multiple transcription and translation start sites

Phospholipid-hydroperoxide glutathione peroxidase (PhGPx) is a selenoenzyme that reduces hydroperoxides of phospholipid, cholesterol, and cholesteryl ester. Previous studies suggested that both the mitochondrial and nonmitochondrial forms of PhGPx are approximately 170 amino acids long. In this study, we isolated a full-length cDNA clone encoding rat testis PhGPx. Based on sequence analysis, the cDNA encodes a protein of 197 amino acids, with translation initiating at AUG61. The additional 27 amino acids at the N terminus contain the features of a mitochondrial targeting sequence. In vitro translation of the full-length PhGPx mRNA initiated predominantly at AUG61. However, translation initiated at AUG141 when AUG61 was deleted. An RNase protection assay was used to map the 5'-ends of PhGPx mRNAs in rat tissues. We identified two major windows of transcription initiation that are tissue-specific. Rat testis predominantly expresses larger transcripts that encode the 197-amino acid protein containing the potential mitochondrial targeting signal. The predominant smaller transcripts in somatic tissues lack AUG61 and encode a 170-amino acid protein, which may represent the nonmitochondrial forms of PhGPx. Our results suggest that the use of alternative transcription and translation start sites determines the subcellular localization of PhGPx in different tissues.

A late adenovirus factor induces eIF-4E dephosphorylation and inhibition of cell protein synthesis

Adenovirus prevents host cell protein synthesis during its late phase of replication in large part by causing the underphosphorylation of translation initiation factor eIF-4E, a component of initiation factor eIF-4F (cap-binding protein complex). Late adenovirus mRNAs are preferentially translated because they possess a reduced requirement for eIF-4F. This study continues the characterization of the mechanism by which adenovirus inhibits cellular protein synthesis. First it is shown that adenovirus blocks the addition of phosphate to eIF-4E rather than enhancing its removal, establishing that the virus impairs a signalling pathway or protein kinase activity involved in eIF-4E phosphorylation. It is then shown that shutoff of cell protein synthesis and translation of late viral mRNAs are uncoupled, in that shutoff actually occurs a short time (1 to 3 h) after late adenovirus mRNAs are already undergoing translation. Finally, by using a variety of genetic mutants stalled at different stages in the viral life cycle, it was found that dephosphorylation of eIF-4E and inhibition of cell translation are not caused by early adenovirus gene products acting at late times or by events related to viral DNA replication. Instead, it is shown that inhibition of eIF-4E phosphorylation and cell translation are mediated upon activation of the viral major late transcription unit. These and other results presented indicate that the adenovirus signal which induces eIF-4E dephosphorylation and shutoff of cell protein synthesis is linked either to an activity of one or more late viral polypeptides, to double-stranded RNA produced by opposition of the early and late viral transcription units, or to both.

Initiation codons within 5'-leaders of mRNAs as regulators of translation

Why should a preponderance of proto-oncogenes, growth factor genes and growth factor receptor genes contain translation initiation codons and associated open reading frames in their 5'-leaders? An increasing number of open reading frames are being shown to function as cis-acting regulatory signals able to moderate expression of the downstream reading frame. These regulatory elements could play a fundamental role in the regulation of proliferation of vertebrate cells.

Effects of mouse hepatitis virus infection on host cell metabolism

A time dependent decrease in cell surface expression of major histocompatibility complex (MHC) class 1 proteins was found during JHMV infection of the mouse macrophage J774.1 cells line by radioimmunoassay. MHC class I, actin and CSF-1 receptor mRNA levels were also found to decrease during infection. Surprisingly, not all host cell mRNA were similarly affected, suggesting that the apparent MHV-induced translational shut off of host cell protein synthesis during infection was specific for only some host cell mRNAs. Interestingly, two mRNAs found to be refractory to JHMV infection encode monokines, suggesting a role in pathogenesis. To understand the mechanism(s) of this preferential mRNA stability and the apparent shut off of host cell mRNA, translation lysates were prepared from infected and uninfected cells. Translation of host mRNAs in these extracts showed no apparent loss of translational ability in the infected cells vs. the uninfected cells; however, viral mRNAs were preferentially translated in the lysates from the infected cells. Chimeric mRNAs containing the MHV leader upstream of a globin reporter gene showed that preferential translation was a property of the MHV leader RNA. Deletional analysis showed that the sequences responsible for this cis translational augmentation are in a 12 nucleotide (nt) tract at the 3' end of the leader. The previously reported interaction of the nucleocapsid protein with these nts suggest that it may play a role in translational augmentation of MHV mRNAs.

Characterization and expression of C/EPB-like genes in the liver of Rana catesbeiana tadpoles during spontaneous and thyroid hormone-induced metamorphosis

Tissue-specific changes in gene expression occur in the liver of Rana catesbeiana tadpoles undergoing metamorphosis. Many of these changes can be induced precociously by administration of thyroid hormone (TH) to a tadpole or to cultured tadpole liver. While the precise molecular means by which TH exerts a tissue-specific response is unknown, recent studies suggest that the expression of genes which are liver-specific and characteristic of the adult liver phenotype may rely on TH-induction of tissue-specific transcription factors, as well as the thyroid hormone receptor proteins. Guided by this notion, we screened our Rana catesbeiana liver cDNA library and isolated clones, RcC/EBP-1 and -2, encoding Rana homologues of a mammalian transcription factor, C/EBP (CCAAT/enhancer core binding protein), implicated in the expression of liver-specific genes and terminal differentiation of hepatocytes. Gel mobility shift assays demonstrate that the proteins synthesized from these cDNAs bind specifically to the consensus binding site for C/EBP-related proteins. Characterization of the amino acid sequence in the bZIP DNA-binding domains of these proteins suggests that RcC/EBP-1 and -2 encode Rana homologues of C/EBP alpha and delta, respectively. Hybridization analyses demonstrate that the amount of RcC/EBP-2 mRNAs in tadpole liver remains constant throughout metamorphosis, whereas RcC/EBP-1 mRNAs are up-regulated during both spontaneous and TH-induced metamorphosis. The TH-induced up-regulation of RcC/EBP-1 mRNAs precedes the up-regulation of liver-specific urea cycle enzyme mRNAs by 6 to 12 hours. These results, coupled with in situ hybridization studies, suggest that RcC/EBP-1 mRNAs encode a transcription factor which may play an early role(s) in the terminal differentiation and/or reprogramming of gene expression in this tadpole's liver cells during both spontaneous and TH-induced metamorphosis.