32 - Translation; Initiation

32 - Translation; Initiation - Translation Translation...

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Translation Learning Objectives • Compare the mechanism for finding the correct reading frame of an mRNA in prokaryotes versus in eukaryotes • Identify the energy requirements for protein synthesis Translation initiation
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Mechanism of protein synthesis can be divided into three stages: Initiation –the assembly of a complete ribosome on a mRNA molecule at a correct point Elongation –repeated cycles of amino acid addition Termination –the release of the new protein chain Lots of factors involved: prokaryotic initiation and elongation factors eukaryotic initiation and elongation factors Differences in details
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Translation Initiation Protein synthesis begins with initiator tRNA correctly positioned at start codon (AUG = methionine) Eukaryotes and prokaryotes have two types of methionine tRNAs charged with the same enzyme (amino-acyl) methyonil tRNA synthetase – tRNA Met - for delivering methionine at internal sites of growing polypeptide chain – tRNA i Met - for initiation (bacteria have a modification - formyl- methionine ) Initiator tRNA allows more flexibility in base pairing -AUG and GUG have to be recognized in bacteria
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Prokaryotic initiation of translation
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In prokaryotes - conserved sequence in 5‟ UTR of a mRNA, 8-13 nt upstream from the first codon to be translated Ribosomal binding site (RBS) 5’-AGGAGGU-3’ •It base pairs with the complementary sequence at 3‟ end of 16S rRNA in the small 30S subunit •Positions the ribosome correctly with respect to the initiation codon Keep in mind: polycistronic mRNAs have multiple internal RBSs resulting in synthesis of different proteins simultaneously sites of ribosome „re-cycling‟
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IF3 binds to free 30S subunit (prevention of binding of 50S subunit) IF1 binds (prevents potential binding of tRNA to A site) IF2 (GTPase) complexes with GTP and binds mRNA binds to 30S subunit through interaction of Shine-Dalgarno sequence with 16S rRNA Initiator tRNA binds (anticodon-codon base pairing) to P site This is 30S initiation complex 50S subunit binds This displaces IF1 and IF3; GTP is hydrolyzed – energy consuming step – IF2 is released This is 70S initiation complex Steps:
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Eukaryotic initiation of translation
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In eukaryotes - 40S ribosomal subunit first binds initiator tRNA; then 40S subunit-initiator tRNA complex binds mRNA and scans along mRNA until it reaches an appropriate AUG and positions initiator tRNA there •This first AUG has to be in the correct sequence context Optimal sequence context – Kozak consensus sequence: 5’- CC RCC AUG G-3’ R = purine (A or G)
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•Free 40S subunit complexes with eIF3 (large protein) and eIF1A keep it apart from 60S subunit •ternary complex forms separately: Initiator tRNA, eIF2 and GTP •It binds to 40S- this is 43S preinitiation complex •On the other side: Different eIF4 factors are involved in recognition of 5‟methyl cap; they keep mRNA free of any secondary
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This note was uploaded on 12/23/2011 for the course BIOL 308 taught by Professor Miskovic during the Fall '09 term at Waterloo.

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32 - Translation; Initiation - Translation Translation...

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