Termination of Protein Synthesis
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Questions and Answers

Which of the following stop codons are recognized by RF-1 in termination of translation?

  • UGA and UAG
  • UAA and UGA
  • UAA and UAG (correct)
  • UGA and UAA

    • Which release factor recognizes all three stop codons (UGA, UAA, UAG)?

  • eRF-1 (correct)
  • RF-1
  • RF-2
  • RF-3

    • In coupled transcription and translation in bacteria, what is the direction of translation?

  • 5' to 3' (correct)
  • 3' to 5'
  • 5' to 5'
  • 3' to 3'

    • Release factors act at which site of the ribosome during termination?

    <p>A site</p> Signup and view all the answers

    Which of the following release factors is responsible for recognizing UGA and UAA stop codons?

    <p>RF-2</p> Signup and view all the answers

    Which of the following is NOT a distinct step in translation?

    <p>Replication</p> Signup and view all the answers

    How many nucleotides does a typical tRNA molecule contain?

    <p>73 to 93</p> Signup and view all the answers

    Which molecule is NOT involved in the translation process?

    <p>DNA</p> Signup and view all the answers

    What role do release factors (RFs) play during translation?

    <p>Terminate polypeptide synthesis</p> Signup and view all the answers

    Which of the following is true about aminoacyl-tRNA synthetases?

    <p>They link amino acids to tRNA</p> Signup and view all the answers

    Which step involves matching each amino acid with the correct tRNA?

    <p>Activation</p> Signup and view all the answers

    Where does the initiation complex form in eukaryotic translation?

    <p>At the 5' terminus of the mRNA</p> Signup and view all the answers

    What describes the optimal sequence for efficient translation initiation in eukaryotes?

    <p>Kozak's Rules</p> Signup and view all the answers

    What is required to form the initiation complex in prokaryotes?

    <p>50S subunit of the ribosome</p> Signup and view all the answers

    Which initiation factor binds to the 40S ribosomal subunit?

    <p>eIF1, 1A, 3 &amp; 5</p> Signup and view all the answers

    Which component binds the first amino acid in translation initiation in eukaryotes?

    <p>eIF2-GTP</p> Signup and view all the answers

    What guides the mRNA to the 30S ribosomal subunit in prokaryotic translation initiation?

    <p>Shine-Dalgarno sequence</p> Signup and view all the answers

    Which initiation factor mediates the binding of formylmethionine tRNA to the peptidyl site (P site)?

    <p>IF-2-GTP</p> Signup and view all the answers

    What role does the eIF4F complex play during translation initiation?

    <p>It binds to the 5' CAP of the mRNA</p> Signup and view all the answers

    How is the initiation complex completed in prokaryotic translation?

    <p>Attachment of large 50S subunit</p> Signup and view all the answers

    Which antibiotic targets the elongation phase of bacterial protein synthesis?

    <p>Streptomycin</p> Signup and view all the answers

    Which site on the ribosome does the initiator tRNA occupy during elongation?

    <p>P site</p> Signup and view all the answers

    Which of the following antibiotics inhibit peptide-bond formation?

    <p>Linezolid</p> Signup and view all the answers

    What is the function of the PolyA Binding Protein (PABP) during translation initiation?

    <p>It checks the ends of the mRNA</p> Signup and view all the answers

    What happens to the A site during translocation in prokaryotic translation?

    <p>It becomes empty</p> Signup and view all the answers

    What sequence is generally located 8 bp upstream of the AUG start codon?

    <p>Shine-Dalgarno sequence</p> Signup and view all the answers

    Which of the following is NOT an initiation factor involved in prokaryotic translation initiation?

    <p>EF-Tu</p> Signup and view all the answers

    Study Notes

    Termination

    • STOP codons: UGA, UAA, and UAG
    • No tRNAs for these codons
    • Recognized by release factors (RF)
    • RF-1 for UAA and UAG
    • RF-2 for UGA and UAA
    • eRF-1 recognizes all 3 codons
    • RF acts at A site and needs an occupied P site
    • RF causes the chain to be released from the P site tRNA
    • RF then escapes from the complex

    Coupled Transcription and Translation in Bacteria

    • Transcription occurs from 3' to 5' end
    • mRNA is translated by a ribosome from 5' to 3' end
    • Ribosome is composed of two tRNAs, each with a +NH3 attached

    Translation - The Process

    The Macromolecules of Translation

    • Polypeptides (>50) and RNA molecules (3-5) make up the ribosome
    • Amino acid activating enzymes (20)
    • tRNA molecules (32-60)
    • Soluble proteins involved in polypeptide chain initiation, elongation, and termination
    • ATP and Mg2+

    Distinct Steps in Translation

    • Initiation
    • Elongation
    • Termination

    tRNA

    • 73 to 93 nucleotides long

    Stage 1: Activation of the tRNA

    • Catalyzed by aminoacyl-tRNA synthetases
    • Each enzyme links a specific amino acid to the matching tRNA
    • 20 different aminoacyl-tRNA synthetases (1 for each amino acid)
    • Some cells contain less than 20 synthetases, in which case one amino acid is converted to another after charging the tRNA
    • Matching each amino acid with the correct tRNA is the "second genetic code"
    • Codon specificity resides in the tRNA, not the attached amino acid

    Stage 2: Initiation in Prokaryotes

    Ingredients

    • 30S subunit of the ribosome
    • Initiator tRNA (tRNAMet)
    • mRNA
    • Initiation Factors IF-1, IF-2, and IF-3
    • One molecule of GTP
    • 50S subunit of the ribosome

    Formation of the Initiation Complex

    • mRNA is guided to the 30S ribosomal subunit by the Shine-Dalgarno sequence
    • Shine-Dalgarno sequence is a ribosomal binding site in the mRNA
    • Generally located 8 bp upstream of the AUG start codon

    tRNA Binding Sites on the Ribosome

    • E or exit tRNA
    • P or peptidyl binding site
    • A or aminoacyl binding site

    Formation of the Initiation Complex

    • IF-1, IF-2, and IF-3 are initiation factors
    • P site - peptidyl site
    • A site - aminoacyl site
    • E site - exit site

    Translocation

    • Elongation needs another elongation factor for translocation
    • All amino acids move along one space, resulting in an empty A site
    • When the A site is filled, the E site is emptied
    • The 3rd EF recycles the first elongation factor
    • This continues until a STOP codon is reached

    Elongation

    • All ribosomes have 3 sites: A, P, and E
    • Initiator tRNA is in P site
    • Next amino acid is escorted to A site
    • The small subunit and base pairing "check" amino acid is correct
    • Correct amino acid causes conformational change and hydrolysis of bound GTP
    • EF's released and peptide bond formed

    Translation Initiation in Eukaryotes

    • The amino group of the methionine on the initiator tRNA is not formylated (i.e., Met, not fMet)
    • The initiation complex forms at the 5' terminus of the mRNA
    • No zone of Shine-Dalgarno--AUG start sequences
    • The initiation complex scans the mRNA for an AUG initiation codon
    • Translation usually begins at the 1st AUG found
    • Kozak's Rules describe the optimal sequence for efficient translation initiation in eukaryotes

    Stage 2: Initiation in Eukaryotes

    • Prepare the Ribosome
    • 40S subunit binds initiation factors (eIF1, 1A, 3, and 5)
    • Prepare the 1st amino acid
    • tRNA-Met binds to eIF2-GTP
    • Prepare the RNA
    • eIF4 family + PolyA Binding Protein (PABP) check ends of mRNA
    • All merge, ATP -> ADP, all factors released
    • 60S subunit binds

    Antibiotics Target Ribosome Sites During Bacterial Protein Synthesis

    • Initiation: Edeine (Ede), kasugamycin (Ksg), pactamycin (Pct), thermorubin (Thb) (30S subunit)
    • Initiation: orthosomycins avilamycin (Avn), evernimicin (Evn), thiostrepton (Ths) (50S subunit)
    • Elongation (by EF-Tu): inhibited by streptomycin (Stp), tetracyclines (Tet), glycylcyclines (tigecycline (Tig))
    • Peptide-bond formation: blasticidin S (Bls), chloramphenicol (Cam), lincosamides (clindamycin (Cln)), oxazolidinones (linezolid (Lnz)), pleuromutilins (Plu), puromycin (Pmn), streptogramin A (S) and sparsomycin (Spr)
    • tRNAs translocation (by EF-G): tuberactinomycins capreomycin (Cap), viomycin (Vio), hygromycin B (HygB), neomycin (Neo), paromomycin (Par), fusidic acid (Fus), spectinomycin (Spt), Ths
    • Chain elongation: macrolides (erythromycin (Ery)), streptogramin B (S), ketolides (telithromycin (Tel))
    • Termination: peptidyl-transferase inhibitors Bls, Cam, Pmn, Spr
    • Recycling: translocation inhibitors, especially Fus

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    Description

    This quiz covers the termination stage of protein synthesis, including STOP codons, release factors, and the process of chain release. It also touches on coupled transcription and translation in bacteria.

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