Camerini 3

Camerini 3 - Lecture 3: tRNA, mRNA and Ribosomes Today, we...

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1 Lecture 3: tRNA, mRNA and Ribosomes Today, we will discuss the following: The structure of tRNA The “adaptor hypothesis” and the “second genetic code” tRNA synthetases and tRNA synthesis reactions; double sieve model The “wobble” hypothesis mRNA structure; signals that regulate protein synthesis Ribosomes and protein synthesis; polysomes Composition of ribosomes Ribosome structure
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2 tRNA functional sites A-form double helix
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3 Tertiary Structure of tRNA Acceptor (AA added here) D-loop folds over to contact the TyC loop Anticodon
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4 “Adaptor hypothesis” Raney Nickel experiment Crick (1958): soluble RNA (sRNA) acts as adaptor for incorporation of AAs into proteins. Günther Von Ehrenstein (1961): chemically modified an AA attached to tRNA; (Cys to Ala, Raney Nickel expt.) Result: Ala inserted at site of Cys! Demonstrates that specificity is dictated by the “adaptor” not by the protein synthesis machinery (e.g. ribosome)
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5 tRNA Synthetases Class I tRNA synthetase 3D structure, glutaminyl-tRNA synthetase (blue; tRNA in 20 different synthetases, one for each AA Two classes: type I and type II Type I: transfer AA to 2’ OH of Adenine at end of acceptor stem (CCA); requires trans-esterification to transfer AA from 2’ to 3’ OH of tRNA. Bind minor-groove of acceptor stem. , both 2 domain structures Type II: transfer AA to 3’ OH of Adenine. Bind to major groove of acceptor stem. Both classes have 2 domains: -catalytic domain (interacts with 3’ CCA) -variable domain: binds to tRNA recognition domain tRNAs also have 2 functional domains: -anticodon -CCA acceptor (3’)
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6 tRNA synthesis reactions Two types of tRNA synthetases: Class I: usually monomeric; AA added to 2’ OH of A Class II: oligomeric; AA added to 3’ OH Two phases of reaction catalyzed by tRNA synthetase: Type II Type I
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7 Aminoacyl-tRNA synthetases: the “second genetic code” Note that Ile and Val have similar size and charge. The specificity of first reaction not absolute; Ile-tRNA synthetase will catalyze transfer of adenine (from ATP) to valine. Prior to 2nd step (transfer of valyl-adenylate to acceptor of Ile-tRNA), Valine will trigger deacylase site of Ile- tRNA synthetase , which hydrolyzes the aminoacyl- adenylate. As described earlier, ribosomes don’t provide the specificity, the aminoacyl-tRNA synthetases do. How?
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8 Specificity of tRNA charging: the “double sieve” model Two ways for tRNA synthetase to “know” if the wrong AA is bound (prior to charging): 1. If AA is too large, it cannot enter. 2.
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This note was uploaded on 03/14/2009 for the course BIOL 97 taught by Professor Edinger during the Fall '08 term at UC Irvine.

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Camerini 3 - Lecture 3: tRNA, mRNA and Ribosomes Today, we...

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