Lecture 20

Lecture Slides

• correct identification of the initiator codon during initiation determines the reading frame

Elongation follows initiation

• cycles of 3 steps, repeated until a stop codon is reached:

  1. binding of a charged tRNA

  2. peptide bond formation

  3. translocation

1. binding of a charged tRNA

   • completed initiation complex has fMet in P site

   • if the newly-entered tRNA into A site has an anti-codon that pairs with the codon, it is allowed to stay

   • elongation factor EF-Tu-GTP is attached to the charged tRNA

   • hides the amino acid connected to the tRNA

   • if the codon match is correct, the elongation factor dissociates

   • GTP → GDP + Pi

   • amino acid can now connect

   • correct pairing of codon and anticodon stimulates GTP hydrolysis and release of EF-Tu

2. peptide bond formation

   • the amino acid in the P-site has their C-terminus attached to the tRNA

   • in the ribosome active site, the ribosome breaks that bond and forms a peptide bond between the C-terminus of the P site amino acid and N-terminus of the amino acid in the A site

3. translocation

   • ribosome slides 3 bases down

   • tRNAs never more from mRNA

   • requires another EF (EF-G) and hydrolysis of another GTP

   • uncharged tRNA is displaced; A site is open for next charged tRNA

termination follows elongation

• elongation continues until one of three stop codons reaches A site

• no new tRNA is involved in termination (no tRNA^stop)

• release factors (proteins whose 3D shape resembles charged tRNA) recognize stop codons, and high energy bond between protein and tRNA is broken

• usually lots of protein made from mRNA

  • once the ribosome has moved away from the initiator codon, the initiator codon is available for a new initiation complex

  • the 5’ end is now available for another ribosome

  • “assembly line” style production of protein on mRNA called a polysome (or polyribosome)

• simultaneous transcription and translation possible in prokaryotic cells

  • called “co-transcriptional translation”