Chapter 17: Initiation of Translation

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Chapter 17 Initiation of Translation

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• Chapter 17 Translation
• Prokaryoticinitiation
• Eukaryotic initiation

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Initiation of Translation in Prokaryotes

• T-RNA charging
• Aminoacyl-tRNA synthetase
• Activate amino acid using ATP
• Transfer the amino acid to the tRNA
• 20 synthetases, one for each a.a.

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Dissociation of Ribosomes
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Initiation Factors

• IF-1
• Promotes dissociation
• IF-2
• Binds 30S along with others, stabilizes their
  binding
• IF-3
• Binds 30S subunit and prevents reassociation with
  50S subunit

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N-Formyl Methionine
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The Start Codon, and Binding of the mRNA to the
Ribosome

• How do you recognize he start codon? (AUG or GUG)
• Secondary structure or binding sequence?
• Shine-Delgarno Sequence
• 16S rRNA
• GGAGG in mRNA (CCUCC in rRNA)
• Binding mediated by IF3 with help from IF1 and IF2

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Binding of fMet-tRNA

• IF2 mediates binding along with 1 GTP
• GTP is not hydrolyzed until 50S subunit joins

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Formation of the 70S Initiation Complex

• 50S subunit joins
• Together with IF2 forms a GTPase which hydrolyzes
  GTP to GDP
• This energy is used to dissociate IF2

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• IF1 binds to dissociate 70S ribosome
• IF3 binds 30S
• IF1, IF2 and GTP bind 30S
• MRNA stabilized by IF2 and fMet-tRNA stabilized
  by IF3 bind 30 S
• 50S binds and IF1 and IF3 leave
• IF2 leaves with hydrolysis of GTP to GDP

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Eukaryotic Translation Initiation

• Starts with met, not fmet
• Initiator tRNA
• No Shine-Delgarno sequence, instead use cap
  binding protein
• Monocistronic messages,
• Kozak's rule ACCAUGG
• Hairpin loops12-15 nt downstream can promote
  initiation in bad neighborhoods
• Initiator tRNA recognizes start codon

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Scanning Model
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Initiation Factors

• eIF2
• Binds the initiator tRNA
• Requires GTP
• Exchange factor (eIF2Bor GEF removes GDP after
  dissociation)
• EIF5B helps catalyze rxn of GTP to GDP
• eIF3-Bind 40S to prevent reassociation with 60S
• eIF4F- cap binding protein which binds mRNA to
  40S
• eIF1 and eIF1A- scan to start codon
• eIF5 stimulates association with 60S
• eIF6- binds 60S and prevents reassociation

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eIF4F

• 3 polypeptides
• Form cap-binding protein
• eIF4E
• Actual cap binding
• eIF4A
• DEAD family
• RNA helicase
• Requires eIF4B which has RNA binding domain

• EIF4G
• Adaptor
• Helps bind poly A,-Pab1p
• eIF4E and eIF3

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Summary of Eukaryotic Translation Initiation
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Prokaryotic Initiation Control

• mRNA's short-lived
• Secondary structure

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Eukaryotic Translational Control

• mRNA's long-lived
• Phosphorylation of initiation factor

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MRNA Binding Proteins

• Repressor protein binds
• Inducer protein removes repressor
• Ferritin mRNA aconitase apoprotein binds to stem
  loop iron response element (IRE) near 5' UTR,
  Iron removes represson