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Unformatted text preview: zation before translation 3´ - ensures protein translation at right place when required required • In Drosophila, specific 3´ UTRs localize mRNAs to In Drosophila specific specific regions of precellular embryo or cell during embryonic development embryonic 57 -RNA-binding proteins associated with sequences in 3´ UTR link mRNAs to motor proteins that track along cytoskeleton and carry mRNA to final destination Molecular Biology of the Cell, 3rd edition 58 -e.g. localization of Drosophila runt mRNA requires microtubules 59 3. Control at level of mRNA translation 3. Control at level of mRNA translation REVIEW OF TRANSLATION INITIATION IN EUKARYOTES -Begins with methionine -Initiating tRNA not same as tRNA for interior -No Shine-Dalgarno -mRNAs have caps at 5’end 60 Scanning Model of Eukaryotic Initiation -Eukaryotic 40S ribosomal subunits locate start codon by binding to 5’-cap and scanning downstream to find the 1st AUG in a favorable context -Kozak’s Rules are a set of requirements -Best context uses A of ACCAUGG as +1: Purine in -3 position G in +4 position -5-10% cases ribosomal subunits bypass 1st AUG scanning for more favorable one 61 62 Effects of mRNA Secondary Structure -Secondary structure near the 5’-end of an mRNA can have either positive or negative effects -Hairpin just past an AUG can force a pause by ribosomal subunit and stimulate translation -Very stable stem loop between cap and initiation site can block scanning and inhibit translation 63 Translation Initiation in Eukaryotes -Bacterial translation initiation requires initiation factors as does eukaryotic initiation of translation -Eukaryotic system is more complex than bacterial Scanning process Factors to recognize the 5’-end cap Eukaryotic initiation factors and general functions: -eIF2 binds Met-tRNA to ribosomes -eIF2B activates eIF2 replacing its GDP with GTP -eIF1 and eIF1A aid in scanning to initiation codon -eIF3 binds to 40S ribosomal subunit, inhibits reassociation with 60S subunit 64 Translation Initiation in Eukaryotes -eIF4F is a multi subunit (A, G, E) cap-binding protein allowing 40S subunit to bind 5’-end of mRNA -eIF5 encourages association between 60S ribosome subunit and 48S complex -eIF6 binds to 60S subunit, blocks reassociation with 40S subunit 65 Function of eIF4G -eIF4G is an adaptor protein capable of binding to other proteins including: -eIF4E, cap-binding protein -eIF3, 40S ribosomal subunit-binding protein -Pab1p, a poly[A]-binding protein -Interacting with these proteins lets eIF4G recruit 40S ribosomal subunits to mRNA and stimulate translation 66 Function of eIF4G -from Lodish et al. Molecular Cell Biology, 4th edition 67...
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