If You Want to Know if Your Message Has an Ires
Review
. Sep-Oct 2012;3(5):697-705.
doi: 10.1002/wrna.1129. Epub 2012 Jun 25.
So you want to know if your message has an IRES?
Affiliations
- PMID: 22733589
- PMCID: PMC3419317
- DOI: ten.1002/wrna.1129
Free PMC article
Review
And so you want to know if your bulletin has an IRES?
Wiley Interdiscip Rev RNA. Sep-Oct 2012 .
Free PMC article
Abstract
Transcriptional regulation of cistron expression has been widely studied. More recently, in that location has been increasing appreciation of the office that translational regulation plays in gene expression, resulting in a number of new fields engaging in translational studies. Regulation of protein synthesis is critical for jail cell growth, evolution, and survival, and is primarily controlled at the initiation stride. Eukaryotic cells utilize multiple mechanisms to initiate translation, depending on cell stress, growth conditions, viral infection, or the sequences present in the mRNA. While the vast majority of mRNAs are translated in a cap-dependent way, an important subset of mRNAs uses an alternative mechanism, whereby ribosomes are recruited internally to the bulletin to initiate cap-contained translation. Some of these mRNAs incorporate an internal ribosome entry site (IRES) located in the 5' untranslated region (UTR). However, establishing that an RNA element is a functional IRES requires a number of carefully executed experiments with specific controls. This review volition clearly explain the required experiments, and the pros and cons of various assays, used to determine whether (or non) an RNA element functions as an IRES to promote initiation of translation. We hope that demystifying the accepted methods for assaying IRES activity volition open the study of this important mechanism to the broader community.
Copyright © 2012 John Wiley & Sons, Ltd.
Figures
Mechanisms of cap-dependent translation. (a) The standard cap-dependent machinery involves recognition of the v′cap (black) past a complex of eukaryotic initiation factors (eIF) eIF4E, eIF4G, and eIF4A that recruit the 43S pre-initiation circuitous (40S subunit, eIF1, eIF5, eIF1A, eIF2•met-tRNAi•GTP, and eIF3. This generates a 48S circuitous and the 40S ribosomal subunit scans down to the kickoff AUG. The 60S subunit then joins and protein synthesis begins (not shown). (b) In leaky scanning, the 40S subunit tin featherbed AUGs that are in a poor sequence context to generate proteins (bluish) that have alternate N-terminal ends. Here, the darker the AUG, the amend the sequence context, and thus the more likely that information technology will be used to initiate protein synthesis. (c) Ribosome shunting requires recruitment of the ribosome through a cap-dependent mechanism as in (a), then, post-obit scanning or translation of a short ORF the ribosome is shunted (no scanning) downstream to initiate protein synthesis. (d) The presence of uORFs can foreclose translation of the major coding region unless levels of the eIF2•met-tRNAi•GTP are low. The ribosome is brought to the 5′end equally in (a), then post-obit translation of the outset uORF the 40S subunit remains associated with the mRNA and continues to browse down the mRNA until it acquires another eIF2•met-tRNAi•GTP, enabling information technology to initiate translation at the next ORF.
Mechanisms of cap-contained translation. (a) IRESs recruit the 40S subunit internally to the mRNA using anywhere from all to none of the translation initiation factors (divers in Fig. 1a). (b) A CITE located in the three′UTR binds to the cap-binding complex (eIF4E, eIF4G, and eIF4A) and through circularization of the mRNA recruits a 43S pre-initiation complex to v′UTR of the mRNA.
Reporter assays used to assess IRES activity. (a) In the bicistronic reporter assay the IRES is placed between 2 cistrons. Expression of the first cistron (red) is cap-dependent whereas expression of the second cistron (green) is dependent upon a functional IRES. The ribosomal subunits are shown (brown). (b) Insertion of an IRES into a circular RNA ensures that expression of the reporter is due to internal initiation since there is no free 5′finish. The removal of all the stop codons in the circular RNA allows for the generation of a single continuous ORF, such that multiple rounds of translation around the circle demonstrates that the circle is intact. Insertion of a protease cleavage site (orange) allows for the plummet of the polyprotein into a single poly peptide product consequent with the ribosome transversing around the circle once. (c) Insertion of a stable hairpin in the 5′UTR can be used to cake cap-dependent translation. Therefore translation of the cistron would be dependent upon a functional IRES upstream of the reporter ORF.
Controls for the bicistronic reporter assay must be performed to rule out all possible reasons for 2d cistron expression aside from IRES activity. (a) The sequences inserted within the bicistronic reporter may contain weak or cryptic promoter activity that would result in the expression of a monocistronic mRNA that can be translated by the cap-dependent mechanism. (b) Readthrough of the stop codon in the outset cistron would generate a chimeric reporter protein that would exist translated using a cap-dependent machinery. (c) The inserted sequences in the bicistronic reporter may contain a cryptic splice site that would generate either a chimeric protein (as in (b)) or a monocistronic RNA that encodes the second gene. These spliced products would consequence in cap-dependent expression of the second cistron.
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Source: https://pubmed.ncbi.nlm.nih.gov/22733589/
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