Due to the large size andĭifference in expression levels between genes before and after IRES, however, a Ribosomal entry site (IRES) has been widely used. Among various strategiesĮmployed to construct bicistronic or multicistronic vectors, an internal Multicistronic expression vectors have been used. Similarly, the fact that IRES activity can be modulated in response to mitotic stimuli, hypoxia and other stresses would add further control over expression of a given transgene.When expression of more than one gene is required in cells, bicistronic or On the other hand, the tissular tropism displayed by some cellular IRESes could be useful to target specific organs ( 9, 10). One advantage of viral IRESes is their capability to sustain protein synthesis in a broad range of cellular types and tissues, albeit with different efficiencies. A direct application is the synthesis of several proteins of interest from one multicistronic mRNA. It is generally admitted that the primary RNA sequence determines two main requirements of IRES function that can act either together or independently: (i) the RNA structure, (ii) the binding sites for trans-acting factors that will contribute to the recruitment of the ribosome ( 2, 3).Īs the understanding of the internal ribosome entry mechanism goes forward, IRESes begin to be used as biotechnological tools, more particularly for gene therapy. For instance, an IRES can be a short 9 nt-long sequence ( 6) and can also be a large highly structured 600 nt-long sequence ( 7, 8). The mechanism of internal entry of ribosomes is not fully elucidated because IRES sequences are very diverse.
More importantly, evidence that internal entry of ribosomes is controlled during the cell cycle or apoptosis demonstrates the importance of this new mechanism of translation initiation and suggests that internal entry of ribosomes could be a key player of cellular proliferation and/or differentiation ( 4, 5). The important feature is that cellular IRESes are found in crucial messenger RNAs encoding key regulatory proteins (transcription factors, growth factors and kinases). There is also a continuously growing list of IRES-containing cellular mRNAs. Indeed, IRESes not only exist in picornaviruses but are also present in flaviviruses, plant viruses, retroviruses and even DNA viruses. However, the process of internal entry of ribosomes appears to be far more extended than previously described. While the role of the 5′ cap structure and the 3′ poly(A) tail in translation initiation were directly envisaged to be of general importance, the role of an Internal Ribosome Entry Sites (IRES) sequence was initially thought to be restricted to the picornavirus group. These three features can act either together or independently, depending on the mRNA or the cell status, to recruit the small subunit of the ribosome. The first step in eukaryotic translation initiation, which consists in the recruitment of the 40S small ribosomal subunit to the eukaryotic messenger RNA (mRNA), requires several features of RNA polymerase II-transcribed mRNAs: (i) the 7methyl guanosine capped structure at the 5′ end, (ii) the poly(A) tail at the 3′ end and (iii) the internal ribosome entry site located upstream of the translation initiation codon ( 1– 3). Several subsets of data are classified according to the viral taxon (for viral IRESes), to the gene product function (for cellular IRESes), to the possible cellular regulation or to the trans-acting factor that mediates IRES function. The IRES database is a comprehensive Sequences are presented in FASTA form and hotlinked to NCBI GenBank files. Novel IRES sequences continue to be added to public databases every year and the list of unknown IRESes is certainly still very large. These sequences are very diverse and are present in a growing list of mRNAs. Internal Ribosome Entry Sites (IRES) are cis-acting RNA sequences able to mediate internal entry of the 40S ribosomal subunit on some eukaryotic and viral messenger RNAs upstream of a translation initiation codon.
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