Lecture17S10 - BIS101/Engebrecht Lecture17 5/7/10 Like...

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BIS101/Engebrecht Lecture17 5/7/10 Like transcription, we can think of translation in three distinct steps: initiation, elongation and termination. Initiation : requires initiation factors (IF) 1,2,3. 1) The 30S subunit of the ribosome associates with IF3 and binds mRNA via base pairing with the 16S RNA using the Shine-Dalgarno sequence of the mRNA. 2) Initiator formylmet-tRNA met is bound by IF2-GTP and delivered to the 30S-mRNA complex and moves into the P-site on the 30S subunit. (Formylmethionine can only be inserted as the initial amino acid, it can not be inserted into a growing peptide chain. However, it is cleaved by enzymes so that most proteins start with methionine). 3) IF3 is released and the large subunit binds. GTP hydrolysis on IF2 occurs and IF2 and IF1 are released. Elongation: requires elongation factors (EF) Tu, G and Ts. These are also GTP binding proteins: energy derived from hydrolysis of GTP. 1) EF-Tu (bound to GTP) mediates entry of charged tRNA into the A site by binding the tRNA. Hydrolysis of GTP to GDP helps drive the binding of the tRNA to the A site. 2) EF-Ts mediate the release of EF-Tu-GDP from the ribosome and the regeneration of EF-Tu-GTP. 3) Translocation occurs when the polypeptide chain on the peptidyl-tRNA is transferred to the aminoacyl-tRNA on the A site. This is catalyzed by peptidyltransferase, this is an RNA based enzyme in the 23S rRNA. The ribosome then translocates by moving one codon along the mRNA (5’ 3’ direction). This is mediated by EF-G and GTP hydrolysis. This releases the uncharged tRNA from the P site (to the E site) and transfers the newly formed peptidyl-tRNA from the A site to the P site. Termination: The elongation cycle continues until the codon in the A site is either UGA, UAA, or UAG. No tRNAs recognize these codons (unless what happens? ). Release factors RF1 and RF2 do recognize these codons. RF1 recognizes UAA or UAG and RF2 recognizes UAA or UGA. These release factors are assisted by a third, RF3. Upon binding, these factors induced a release of the polypeptide and free tRNA. The 70S ribosome then dissociates into the 30S and 50S subunits. The initiation, elongation and release factors are not considered components of the ribosome
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Lecture17S10 - BIS101/Engebrecht Lecture17 5/7/10 Like...

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