MBB331_Week11-Fall-2011

Coli ribosomes dissociate into subunits at the end of

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Unformatted text preview: Prokaryotic initiation codon is: – Usually AUG – Can be GUG – Rarely UUG • Initiating aminoacyl-tRNA is N-formyl-methionyltRNA • N-formyl-methionine (fMet) is the first amino acid incorporated into a polypeptide • This amino acid is frequently removed from the protein during maturation 29 Dissociation of Ribosomes • E. coli ribosomes dissociate into subunits at the end of each round of translation • IF1 actively promotes this dissociation • IF3 binds to free 30S subunit and prevents reassociation with 50S subunit to form a whole ribosome 17-30 • The 30S initiation complex is formed from a free 30S ribosomal subunit plus mRNA and fMet-tRNA • Binding between the 30S prokaryotic ribosomal subunit and the initiation site of a message depends on base pairing (mediated by IF3) between: – Short RNA sequence • Shine-Dalgarno sequence (AGGAGGU) • Upstream of initiation codon – Complementary sequence • 3’-end of 16S RNA of the 30S subunit 31 Bacterial Translation Initiation 1. IF1 influences dissociation of 70S ribosome to 50S and 30S 2. Binding IF3 to 30S, prevents subunit reassociation 3. IF1, IF2, GTP bind alongside IF3 4. Binding mRNA to fMet-tRNA forming 30S initiation complex a. Can bind in either order b. IF2 sponsors fMet-tRNA c. IF3 sponsors mRNA 5. Binding of 50S with loss of IF1 and IF3 6. IF2 dissociation and GTP hydrolysis 17-32 (ii) TRANSLATIONAL CONTROL: translational repressors translational -coordinate regulation of groups of structural genes in operons useful when those genes encode protreins that are required in equimolar ratios (e.g. members of a linear pathway) -an additional layer of regulation, which can allow differences in extent to which individual genes are expressed can occur with regulation at translation Lewin Genes VII 2000 33 Oxford University Press (ii) TRANSLATIONAL CONTROL: translational repressors translational -repressor function provided by protein that binds mRNA, preventing ribosomes from recognizing initiation region -table below shows some proteins that bind to initiation regions of mRNAs and may act as translational repressors -note that some of these proteins regulate translation of own mRNA Lewin Genes VII 2000 34 Oxford University Press TRANSLATIONAL CONTROL: r-proteins • • Autogenous Autogenous control: permits differential expression from polycistronic mRNA mRNA eg. ribosomal ribosomal protein operons: encode proteins for mixture of functions other than just ribosomes (other players in gene expression) expression) A 35 Lewin Genes VII 2000 Oxford University Press TRANSLATIONAL CONTROL: r-proteins • • • • Whe...
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