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Chapter 9-4.docx

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BIOL 2000
Joseph Rasmussen

Chapter 9-4 Key sites of interaction in the ribosome   A site- decoding center o Aminoacyl o Binding site for aminoacylated  P site- o Peptidyl o tRNA has a peptide attached at this site  E site o Exit site o After tRNA has delivered amino acid it comes here  Peptidyl transferase center o Business end of ribosome where enzymatic activity occurs o Where peptide bond formation occurs o Between new amino acid and carboxyl terminus end of the polypeptide chain  tRNA movement o A site to P site to E site then leave ribosome Inititation of translation  1) Binding of the mRNA to the small subunit (SSU)  2) Recognition of a start codon o Usually AUG o Utilizes a specialized initiator tRNA  Prokaryotes tRNA fet  F met= formyl methionine  Methionine is a standard amino acid with formyl group  Eukaryotes tRNA iMet  No formyl group on methionine  I indicates it is an initiator tRNA  3) Requires protein translation factors called initiation factors o Not ribosomal proteins o Used exclusively for initiation Question: How is correct AUG codon chosen?  In prokaryotes, a sequence upstream of an AUG start codon defines the start site by binding to the small subunit of the ribosome Shine-Dalgarno Sequence  Base pairing between Shine-Dalgarno sequence in mRNA and anti-Shine- Dalgarno sequence at 3’ end of small subunit rRNA (16S rRNA)  Small subunit binds to this position  The 16S rRNA base pairs to this sequence  Anti-Shine-Dalgarno is the sequence on the small subunit Translation initiatio
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