Objectives V and VI

What components are necessary for bacterial translation initiation?

30S subunit, mRNA, initiator tRNA (fMet-tRNAᶠᴹᵉᵗ), initiation factors IF-1, IF-2, IF-3, and GTP.

What is the Shine-Dalgarno sequence?

A purine-rich sequence on mRNA that aligns the ribosome with the start codon by base pairing with 16S rRNA.

Describe the process of initiation in bacteria.

IF-3 binds 30S subunit to prevent premature 50S association. IF-1 helps tRNA positioning. IF-2 brings fMet-tRNAᶠᴹᵉᵗ to the P site. After codon–anticodon pairing, 50S joins and GTP is hydrolyzed.

What are the three steps of elongation in bacteria?

(1) Aminoacyl-tRNA entry to A site,

(2) Peptide bond formation,

(3) Translocation of the ribosome.

What elongation factors are required and their functions?

EF-Tu: delivers aminoacyl-tRNA to A site;
EF-Ts: recycles EF-Tu;
EF-G: catalyzes translocation.

What is peptidyl transferase?

A ribozyme activity of the 23S rRNA in the 50S subunit.

What is the function of peptidyl transferase?

Forms peptide bonds between amino acids in the A and P sites.

What protein factors are required for termination in bacteria?

Release factors RF-1 (UAA, UAG), RF-2 (UAA, UGA), and RF-3 (GTPase for RF recycling).

Describe the bacterial termination process.

Stop codon in A site → RF binds → peptidyl-tRNA bond hydrolyzed → peptide released → ribosome dissociates.

What components are needed for eukaryotic initiation?

40S subunit, Met-tRNAᵢᴹᵉᵗ, mRNA (with 5′ cap), eIFs (eukaryotic initiation factors), GTP, and 60S subunit.

Describe the process of eukaryotic initiation.

eIFs guide 40S subunit with Met-tRNAᵢ to 5′ cap → scans for AUG → GTP hydrolysis → 60S joins → functional 80S ribosome forms.

How does eukaryotic initiation differ from bacterial?

Eukaryotes use more initiation factors, require a 5′ cap and poly-A tail, and scan for the AUG (no Shine-Dalgarno).

What are the three repeating steps of elongation in eukaryotes?

(1) Entry of aminoacyl-tRNA,

(2) Peptide bond formation,

(3) Ribosome translocation.

What elongation factors are used in eukaryotes and their functions?

eEF1α: delivers aminoacyl-tRNA;
eEF1βγ: regenerates eEF1α;
eEF2: drives translocation.

What is peptidyl transferase in eukaryotes?

A ribozyme activity in the 28S rRNA of the large (60S) subunit.

Function of peptidyl transferase in eukaryotes?

atalyzes peptide bond formation between amino acids.

How does eukaryotic elongation differ from bacterial?

Uses different elongation factors (eEFs), but core mechanism is conserved.

What protein factors are required for eukaryotic termination?

eRF1 (recognizes all stop codons), eRF3 (GTPase that helps eRF1 function).

Describe the eukaryotic termination process.

eRF1 binds to stop codon in A site → peptidyl-tRNA bond is hydrolyzed → peptide released → ribosome dissociates with help from eRF3.

Compare bacterial and eukaryotic termination.

Bacteria use multiple RFs for different stop codons; eukaryotes use a single eRF1 for all three.