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Bacterial Physiology (Micr430)

Bacterial Physiology (Micr430). Lecture 8 Macromolecular Synthesis and Processing: Proteins (Text Chapter: 10). PROTEIN SYNTHESIS. Also termed translation A triplet code specifies for an amino acid There may be more than one code for a particular amino acid (degeneracy)

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Bacterial Physiology (Micr430)

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  1. Bacterial Physiology (Micr430) Lecture 8 Macromolecular Synthesis and Processing: Proteins (Text Chapter: 10)

  2. PROTEIN SYNTHESIS • Also termed translation • A triplet code specifies for an amino acid • There may be more than one code for a particular amino acid (degeneracy) • Among 64 combinations of four bases, three (UAG, UAA and UGA) do not encode any amino acid - nonsense codons

  3. Transfer RNA • tRNA possesses anticodon that complements a specific codon on mRNA • It has a specific amino acid attachment site for binding a particular amino acid • It has a ribosome recognition site • It consists of about 80 nucleotides, many of which are of unusual nucleotides

  4. Charging of tRNA Fig. 10.32

  5. Composition of ribosome

  6. Initiation of Protein Synthesis • In the absence of mRNA, 30S and 50S subunits remain separated, ensured by the activity of initiation factors 1 and 3 (IF-1 and IF-3) associated with 30S. • If mRNA is present, IF-3 binds to mRNA and thus brings mRNA and 30S together. • The Shine-Dalgarno sequence on mRNA will pair with a region located at the 3’ end of 16S rRNA

  7. Initiation of Protein Synthesis • Next, cytosolic initiator factor 2 (IF-2) complexes with GTP and initiator tRNA (fMet-tRNA) • IF-2 directs binding of fMet-tRNA to P site within 30S subunit, permitting the association of 30S and 50S subunits • IF-3 is then removed from the complex

  8. Initiation Complex Formation Fig. 10.33

  9. Elongation of Protein Synthesis • Elongation factor Tu shuttles proper aa-tRNA into the A site, forming anticodon:codon match • The peptide bond is formed between the incoming aa and the C terminal of the elongated polypeptide, catalyzed by a ribozyme located on 50S subunit • Growing peptide bound to tRNA on A site is moved to the P site with help by Elongation factor G and GTP

  10. Elongation Fig. 10.35

  11. Termination • When translocation brings one of the termination codons into the A site, the ribosome will bind one of the two peptide release factors (RF-1 and RF-2) • The release factor will activate peptidyl transferase, hydrolyzing the bond joining polypeptide to the tRNA at P site. • Ribosome disassembles and recycle to another round of protein synthesis

  12. Termination Fig. 10.35

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