Internal ribosomal entry sites (IRES) are cis-acting RNA sequences that permit translation initiation within the mRNA rather than at the 5'-cap end. IRES attract the eukaryotic ribosomal translation initiation complex and thus promote translation initiation independently of the presence of the commonly utilized 5'-terminal 7mG cap structure. IRES are located in the 5'UTR of RNA viruses and allow translation of the RNAs in a cap-independent manner. Some eukaryotic cellular mRNAs also have IRES.
Reporter genes are nucleic acid sequences encoding easily assayed proteins, and cistrons are segments of DNA involved in producing a polypeptide chain; these include regions preceding and following the coding DNA as well as introns between the exons. When an IRES segment is located between two reporter genes in eukaryotic mRNA molecules (bi-cistronic mRNA), the site enables translation of the downstream protein coding region independently of the 5'-cap structure. This enables the cell to produce both the 5'-cap and downstream IRES proteins. The first reporter protein (first cistron) is synthesized by the cap-dependent initiation method. Translation initiation of the second protein is directed by the IRES segment located in the intercistronic spacer region (between the two reporter protein coding regions).
Internal ribosomal entry sites are distinguished from the ribosomal shunt initiation pathway, which is an alternate viral mechanism of translation initiation in which ribosomes bind to the mRNA in a normal cap-dependent mode, then jump upstream (5'–––→3') of the initiator AUG codon.
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Showing posts with label ribosomes. Show all posts
Showing posts with label ribosomes. Show all posts
12/16/2007
12/07/2007
ribosomes
Ribosomes are large intracellular aggregates attached to the rough endoplasmic reticulum. They comprise several RNAs and scores of proteins, and function as ribozymes.
Ribosome biogenesis in eukaryotes mainly occurs in the nucleolus, a specialized nuclear compartment. The synthesis of ribosomal RNAs (rRNAs) is not achieved by simple transcription of the individual rRNA species, rather it requires a complex series of post-transcriptional processing steps.
NMD3 is a gene encoding production of Nmd3p, which is an adapter protein required for a late step in the production of the large (60S) ribosomal subunit. Nmd3p is conserved from Archaebacteria to humans, and the human Nmd3p ortholog can replace Nmd3p in yeast. Nmd3p-like proteins in Archaebacteria suggests for Nmd3p an additional function in subunit biogenesis or translation that predates the evolution of the nucleus. Nmd3p contains sequences for nuclear import and export and shuttles. Nmd3p is an adapter protein that binds to nascent 60S subunits in the nucleus, providing the export signal for transport of these subunits from the nucleus. Crm1p/Xpo1p is the the receptor for Nmd3p and 60S export, and is the receptor for leucine-rich nuclear export signals known for its role mediating the export of HIV rev protein in human cells. [s] [] diagram Nmd3p 60s transport []
The prokaryotic ribosome consists of two stable subunits, the 50 S and 30 S subunits, which can be isolated from bacteria in relatively large quantities .
Largest Computational Biology Simulation Mimics Life's Most Essential Nanomachine: "The ribosome is so fundamental to life that many portions of this molecular machine are identical in every organism ever genetically sequenced. In developing the project, the team identified a corridor inside the ribosome that the transfer RNA must pass through for the decoding to occur, and it appears to be constructed almost entirely of universal bases, implying that it is evolutionarily ancient."
Voxel simulation image : Image 2 : Image 3 : Image 4 : Image 5 : High res movie : Low res movie : See an image of the Q machine at http://www.lanl.gov/asci/.
Ribosome biogenesis in eukaryotes mainly occurs in the nucleolus, a specialized nuclear compartment. The synthesis of ribosomal RNAs (rRNAs) is not achieved by simple transcription of the individual rRNA species, rather it requires a complex series of post-transcriptional processing steps.
NMD3 is a gene encoding production of Nmd3p, which is an adapter protein required for a late step in the production of the large (60S) ribosomal subunit. Nmd3p is conserved from Archaebacteria to humans, and the human Nmd3p ortholog can replace Nmd3p in yeast. Nmd3p-like proteins in Archaebacteria suggests for Nmd3p an additional function in subunit biogenesis or translation that predates the evolution of the nucleus. Nmd3p contains sequences for nuclear import and export and shuttles. Nmd3p is an adapter protein that binds to nascent 60S subunits in the nucleus, providing the export signal for transport of these subunits from the nucleus. Crm1p/Xpo1p is the the receptor for Nmd3p and 60S export, and is the receptor for leucine-rich nuclear export signals known for its role mediating the export of HIV rev protein in human cells. [s] [] diagram Nmd3p 60s transport []
The prokaryotic ribosome consists of two stable subunits, the 50 S and 30 S subunits, which can be isolated from bacteria in relatively large quantities .
Largest Computational Biology Simulation Mimics Life's Most Essential Nanomachine: "The ribosome is so fundamental to life that many portions of this molecular machine are identical in every organism ever genetically sequenced. In developing the project, the team identified a corridor inside the ribosome that the transfer RNA must pass through for the decoding to occur, and it appears to be constructed almost entirely of universal bases, implying that it is evolutionarily ancient."
Voxel simulation image : Image 2 : Image 3 : Image 4 : Image 5 : High res movie : Low res movie : See an image of the Q machine at http://www.lanl.gov/asci/.
