An internal ribosome entry site, abbreviated IRES, is an
protein synthesis. Initiation of eukaryotic translation nearly always occurs at and is dependent on the 5' cap of mRNA molecules, where the translation initiation complex forms and ribosomes
engage the mRNA. IRES elements, however allow ribosomes to engage the mRNA and begin translation independently of the 5' cap.
History
IRES sequences were first discovered in 1988 in the
primary
or secondary structure that are common to all IRES segments have not been reported to date.
Use of IRES sequences in molecular biology soon became common as a tool for expressing multiple genes from a single transcriptional unit in a genetic vector. In such vectors, translation of the first cistron is initiated at the 5' cap, and translation of any downstream cistron is enabled by an IRES element appended at its 5' end.[3]
stress survival, and other processes critical to survival. As of September 2009, there are 60 animal and 8 plant viruses reported to contain IRES elements and 115 mRNA sequences containing them as well.[6]
Activation
IRESs are often used by viruses as a means to ensure that viral translation is active when host translation is inhibited. These mechanisms of host translation inhibition are varied, and can be initiated by both virus and host, depending on the type of virus. However, in the case of most picornaviruses, such as
eukaryotic initiation factors (eIFs) of the eIF4F complex is necessary for 40S ribosomal subunit recruitment to the 5' end of mRNAs, which is further thought to occur with mRNA 5'cap to 3' poly(A) tail loop formation. The virus may even use partially-cleaved eIF4G
to aid in initiation of IRES-mediated translation.
Cells may also use IRESs to increase translation of certain proteins during mitosis and programmed cell death. In mitosis, the cell dephosphorylates eIF4E so that it has little affinity for the 5'cap. As a result, the 40S ribosomal subunit, and the translational machinery is diverted to IRES within the mRNA. Many proteins involved in mitosis are encoded by IRES mRNA. In programmed cell death, cleavage of eIF-4G, such as performed by viruses, decreases translation. Lack of essential proteins contributes to the death of the cell, as does translation of IRES mRNA sequences coding proteins involved in controlling cell death.[7]
Mechanism
To date, the mechanism of viral IRES function is better characterized than the mechanism of cellular IRES function,
eIF3, eIF5, and eIF5B, but do not require the factors eIF1, eIF1A, and the eIF4F complex. In contrast, picornavirus IRESs do not bind the 40S subunit directly, but are recruited instead through the eIF4G-binding site.[9]
Many viral IRES (and cellular IRES) require additional proteins to mediate their function, known as IRES trans-acting factors (ITAFs). The role of ITAFs in IRES function is still under investigation.
Validity of Tests for IRES function
Testing of sequences for potential IRES function has generally relied on the use of
promoter function. Unexpected splicing activity within several reported IRES elements have also been shown to be responsible for the apparent IRES function observed in bicistronic reporter tests.[10] A promoter or splice acceptor within a test sequence can result in the production of monocistronic mRNA from which the downstream cistron is translated by conventional cap-dependent, rather than IRES-mediated, initiation. A later study that documented a variety of unexpected aberrant mRNA species arising from reporter plasmids revealed that splice acceptor sites can mimic both IRES and promoter elements in tests employing such plasmids, further highlighting the need for caution in the interpretation of reporter assay results in the absence of careful RNA analysis.[11]
Applications
IRES sequences are often used in molecular biology to co-express multiple genes under the control of the same promoter, thereby mimicking a polycistronic mRNA. Within the past decades, IRES sequences have been used to develop hundreds of genetically modified rodent animal models.[12] The advantage of this technique is that molecular handling is improved. The problem about IRES is that the expression for each subsequent gene is decreased.[13]
Another viral element to establish polycistronic mRNA in eukaryotes are
2A-peptides. Here, the potential decrease in gene expression and the degree of incomplete separation of proteins is context dependent.[14]
Malys N, McCarthy JE (March 2011). "Translation initiation: variations in the mechanism can be anticipated". Cellular and Molecular Life Sciences. 68 (6): 991–1003.
