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Bacterial Physiology (Micr430) Lecture 8 Macromolecular Synthesis and Processing: Proteins (Text Chapter: 10)
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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
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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
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Charging of tRNA Fig. 10.32
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Composition of ribosome
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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
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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
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Initiation Complex Formation Fig. 10.33
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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
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Elongation Fig. 10.35
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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
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Termination Fig. 10.35
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