Spring 09 lecture 31

Spring 09 lecture 31 - Translation Initiation Protein...

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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 tRNAMet- for delivering methionine at internal sites of growing polypeptide chain tRNAiMet- 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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In prokaryotes ( E. coli ) there is a conserved sequence in mRNA 8-13 nt upstream from the first codon to be translated Discovered by Shine and Dalgarno - Shine-Dalgarno sequence – = ribosome 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 Very important:
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In eukaryotes - Kozak proposed scanning hypothesis 40S ribosomal subunit ( already having initiator tRNA bound ) 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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Keep in mind: polycistronic mRNAs have multiple internal Shine-Dalgarno sequences resulting in synthesis of different proteins simultaneously 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 IF3 is not needed any more- released This is 30S initiation complex 50S subunit binds This displaces IF1 and IF2, and GTP is hydrolyzed –energy consuming step Bacterial initiation of translation
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Eukaryote initiation of translation Cont. Free 40S subunit complexes with eIF3 (large protein) and eIF4C and eIF1A (keep it apart from 60S subunit) ternary complex forms: 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 structures using the energy from ATP mRNA will bind to 43S complex through the 5’ methyl cap
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mRNA is scanned by 43S complex to find right AUG (anticodon from initiator tRNA has to base pair with the first Met codon) – role of the Kozak sequence
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This note was uploaded on 08/03/2009 for the course BIOL BIOL308 taught by Professor Miskovic during the Spring '09 term at Waterloo.

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Spring 09 lecture 31 - Translation Initiation Protein...

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