In molecular biology, translation initiation factor IF-3 (gene infC) is one of the three factors required for the initiation of protein biosynthesis in bacteria. IF-3 is thought to function as a fidelity factor during the assembly of the ternary initiation complex which consists of the 30S ribosomal subunit, the initiator tRNA and the messenger RNA. IF-3 is a basic protein that binds to the 30S ribosomal subunit.[1] The chloroplast homolog enhances the poly(A,U,G)-dependent binding of the initiator tRNA to its ribosomal 30s subunits.[2] IF1–IF3 may also perform ribosome recycling.[3]
IF3 is not universally found in all bacterial species. However, in E. coli, it is required for the 30S subunit to bind to the initiation site in mRNA. In addition, it has several other jobs including the stabilization of free 30S subunits, enables 30S subunits to bind to mRNA and checks for accuracy against the first aminoacyl-tRNA. It also allows for rapid codon-anticodon pairing for the initiator tRNA to bind quickly. IF3 is required by the small subunit to form initiation complexes, but has to be released to allow the 50S subunit to bind.
IF3 is made up of two domains connected by a flexible linker. Together they allow IF3 to carry out its function.[4]
Human mitochondria use a nuclear-encoded homolog MTIF3 for translation initiation.[5] Some bacteria, chloroplasts, and mitochondria have multiple copies of IF3.[6]
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Initiation of translation in prokaryotes: The small ribosomal subunit is separated from the large subunit with the help of two initiation factors: IF1 and IF3. This complex then binds a to purine-rich region -- the Shine-Dalgarno sequence -- upstream of the AUG start codon on the mRNA. The Shine-Dalgarno sequence is base-paired to a complementary sequence on the 16S rRNA - a component of the small subunit. This alignment ensures that the start codon is in the right position within the ribosome. Another initiation factor - IF2 - brings in the initiator tRNA charged with the initiator amino acid N-formyl-methionine. The large ribosomal subunit joins the complex and all initiation factors are released. The ribosome has three sites: the A-site is the entry site for new tRNA charged with amino-acid or aminoacyl-tRNA; the P-site is occupied by peptidyl-tRNA - the tRNA that carries the growing polypeptide chain; the E-site is the exit site for the tRNA after it's done delivering the amino acid. The initiator tRNA is positioned in the P-site. Elongation: A new tRNA carrying an amino acid enters the A-site of the ribosome. On the ribosome, the anticodon of the incoming tRNA is matched against the mRNA codon positioned in the A-site. During this proof-reading, tRNA with incorrect anticodons are rejected and replaced by new tRNA that are again checked. When the right aminoacyl-tRNA enters the A-site, a peptide bond is made between the two now-adjacent amino-acids. As the peptide bond is formed, the tRNA in the P-site releases the amino-acids onto the tRNA in the A-site and becomes empty. At the same time, the ribosome moves one triplet forward on the mRNA. As a result, the empty tRNA is now in the E-site and the peptidyl tRNA is in the P-site. The A-site is now unoccupied and is ready to accept a new tRNA. The cycle is repeated for each codon on the mRNA. Termination: Termination happens when one of the three stop codons is positioned in the A-site. No tRNA can fit in the A-site at that point as there are no tRNA that match the sequence. Instead, these codons are recognized by a protein, a release factor. Binding of the release factor catalyzes the cleavage of the bond between the polypeptide and the tRNA. The polypeptide is released from the ribosome. The ribosome is disassociated into subunits and is ready for a new round of translation.
References
- ^ Liveris D, Schwartz JJ, Geertman R, Schwartz I (September 1993). "Molecular cloning and sequencing of infC, the gene encoding translation initiation factor IF3, from four enterobacterial species". FEMS Microbiol. Lett. 112 (2): 211–6. doi:10.1111/j.1574-6968.1993.tb06450.x. PMID 8405963.
- ^ Lin Q, Ma L, Burkhart W, Spremulli LL (April 1994). "Isolation and characterization of cDNA clones for chloroplast translational initiation factor-3 from Euglena gracilis". J. Biol. Chem. 269 (13): 9436–44. doi:10.1016/S0021-9258(17)36900-4. PMID 8144528.
- ^ Pavlov, MY; Antoun, A; Lovmar, M; Ehrenberg, M (18 June 2008). "Complementary roles of initiation factor 1 and ribosome recycling factor in 70S ribosome splitting". The EMBO Journal. 27 (12): 1706–17. doi:10.1038/emboj.2008.99. PMC 2435134. PMID 18497739.
- ^ Hussain, T; Llácer, JL; Wimberly, BT; Kieft, JS; Ramakrishnan, V (22 September 2016). "Large-Scale Movements of IF3 and tRNA during Bacterial Translation Initiation". Cell. 167 (1): 133–144.e13. doi:10.1016/j.cell.2016.08.074. PMC 5037330. PMID 27662086.
- ^ D'Souza, AR; Minczuk, M (20 July 2018). "Mitochondrial transcription and translation: overview". Essays in Biochemistry. 62 (3): 309–320. doi:10.1042/EBC20170102. PMC 6056719. PMID 30030363.
- ^ Gutu, A.; Nesbit, A. D.; Alverson, A. J.; Palmer, J. D.; Kehoe, D. M. (18 September 2013). "Unique role for translation initiation factor 3 in the light color regulation of photosynthetic gene expression". Proceedings of the National Academy of Sciences. 110 (40): 16253–16258. Bibcode:2013PNAS..11016253G. doi:10.1073/pnas.1306332110. PMC 3791720. PMID 24048028.
This page was last edited on 22 September 2021, at 18:41