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Powerpoint file - University of Evansville Faculty Web sites

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									   Chapter 14


    Translation




18 and 20 October, 2004
                            Overview
•   Translation uses the nucleotide sequence of mRNA to specify protein sequence.
•   Each ORF specifies a polypeptide.
•   Ribosome components and / or tRNAs recognize structures and sequences near the
    5’ end of the transcript to identify the correct start codon.
•   tRNAs are highly modified short RNAs that are the adaptors between codons and
    amino acids.
•   Amino acyl tRNA synthetases recognize structural features of tRNAs and charge
    only the correct tRNA with the correct amino acid.
•   The large and small ribosomal subunits are extremely complex ribonucleoprotein
    structures that dissociate and reassociate in each round of translation.
•   Peptide synthesis is catalyzed by a ribozyme, and proceeds in the N-to-C terminal
    direction.
•   The ribosome uses three tRNA binding sites: A, P, and E.
•   tRNAs are delivered to the ribosome by EF-Tu.
•   EF-G GTP hydrolysis along with peptide bond formation drive ribosomal
    translocation.
•   Translation termination involves release factors and GTP hydrolysis.
•   Translation-dependent RNA stability assures the degradation of damaged messages.
Three possible open reading frames.
Shine-Dalgarno and Kozak Sequences
Kozak: Identification of Consensus
Kozak: Correct context makes a better barrier
        to downstream initiation.
tRNA Structures
Two-step charging of tRNA
Two-step charging of tRNA
tRNA Structural Elements Recognized by
    Aminoacyl-tRNA Synthetase
Synthetase-tRNA cocrystal
The Problem Solved by Editing Pockets
The ribosome cannot distinguish incorrectly
             charged tRNAs
There are twenty-one amino acids.
Prokaryotic transcription and translation are
                  linked.
Composition
of Ribosomes
Translation Overview
The Peptidyl Transferase Reaction
The Ribosome
Ribosome -
   tRNA
interactions
   tRNA
Interactions
 Within the
 Ribosome
Ribosome Channels
Initiation in Prokaryotes
Initiation in Prokaryotes
Initiation in
Prokaryotes
Initiation in
Eukaryotes
 Start Codon
Identification
Interactions between PABP and eIF4F
       circularize the transcript.
uORFs
IRES
  Aminoacyl-
 tRNAs bind to
the ribosome in
a complex with
 EF-Tu. Ef-Tu
release requires
  correct base
    pairing.
The ribosome also uses minor-groove interactions
between the 16S rRNA and the codon-anticodon to
            drive correct base pairing
Accommodation
 (rotation) of the
tRNA strains the
codon-anticodon
    interaction
      causing
    incorrectly
paired tRNAs to
    dissociate.
 Peptidyl
Transferase
Ribozyme
Peptide bond formation and EF-G GTP hydrolysis
               drive translocation.
EF-G is a structural homolog of EF-Tu-tRNA
GTP hydrolysis drives conformational change.
Peptide anticodons allow release factors to
        recognize the stop codon.
 GGQ on
 the RF-I
stimulates
 peptidyl
transfer to
  water.
RRF and EF-
 G stimulate
dissociation
    of the
 terminated
  ribosome.
tmRNA and SsrA rescue stalled complexes
Normal translation displaces exon-junction
               complexes.
Nonsense-mediated decay is caused by
undisplaced exon-junction complexes.
In eukaryotes, abnormal termination causes
          message degradation.
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