6.4
Translation in Prokaryotes
Prokaryote translation is a complex, highly coordinated process that converts genetic information from mRNA into functional proteins. It involves thre…
Initiation, elongation, and termination are translation steps shared by bacteria and archaea. The initiation begins when the 30S and 50S ribosomal subunits bind to the mRNA.
In bacteria, the 30S subunit recognizes the Shine-Dalgarno sequence on the mRNA. Then, the initiator fMet-tRNA loaded with N-formylmethionine, initiation factors, and GTP binds to the mRNA
The 50S subunit then binds to the initiation complex, forming the active 70S ribosome with three key sites.
While the fMet-RNA is held at the P site, an aminoacyl-tRNA enters the A site, aligning its anticodon with the second codon.
The enzyme peptidyl transferase in the 50S subunit transfers N-formylmethionine from fMet-tRNA in the P site to the amino acid on the tRNA in the A site.
The elongation factors and GTP keep translocating the ribosome, exposing a new codon in the A site and pushing the empty tRNA via the E site.
Translation terminates when the ribosome encounters a stop codon. Release factors cleave the polypeptide from the last tRNA.
View the full transcript and gain access to JoVE Core videos
Q1: How do bacterial ribosomes recognize where to start translation?
In bacteria, the 30S ribosomal subunit recognizes the Shine-Dalgarno sequence, a conserved region upstream of the start codon on the mRNA. This sequence aligns the ribosome so the start codon is positioned correctly in the P site. The initiator fMet-tRNA then binds to the start codon with help from initiation factors and GTP, ensuring accurate translation initiation.
Q2: What role does peptidyl transferase play during protein synthesis?
Peptidyl transferase is an enzymatic activity of the 50S ribosomal subunit that catalyzes peptide bond formation between amino acids. It transfers the growing polypeptide chain from the tRNA in the P site to the incoming amino acid on the tRNA in the A site, extending the protein chain one residue at a time during elongation.
Q3: How does the ribosome move along the mRNA during translation?
Elongation factors and GTP facilitate ribosomal translocation, moving the tRNA from the A site to the P site and shifting the empty tRNA to the E site for exit. This movement exposes a new codon in the A site, allowing the next aminoacyl-tRNA to enter and continue the elongation cycle efficiently.
Q4: What happens when the ribosome encounters a stop codon?
Translation terminates when the ribosome encounters a stop codon (UAA, UAG, or UGA). Since no tRNA recognizes these codons, release factors bind to the ribosome and catalyze cleavage of the polypeptide from the tRNA in the P site. The ribosomal subunits, mRNA, and tRNA then dissociate, completing translation.
Q5: What is the role of the 70S ribosome in bacterial translation?
The 70S ribosome forms when the 50S subunit joins the initiation complex containing the 30S subunit, mRNA, and fMet-tRNA. This active ribosome contains three key sites: the P site holding the initiator tRNA, the A site for incoming aminoacyl-tRNAs, and the E site for empty tRNA exit during elongation.
Q6: Why is N-formylmethionine special in bacterial translation initiation?
N-formylmethionine (fMet) is the first amino acid incorporated into bacterial proteins because the initiator tRNA is specifically charged with fMet rather than regular methionine. This formylated amino acid signals the start of translation and is carried by the initiator fMet-tRNA, which uniquely recognizes the start codon during initiation.
Q7: How does translation connect to broader gene expression in bacteria?
Translation is the final step in converting genetic information into functional proteins, following transcription. Understanding translation mechanisms is essential for comprehending coordination of gene expression processes in bacteria, where transcription, translation, and protein maturation work together to regulate cellular function and response to environmental changes.