Chapter 40 - The Mechanism of Protein Synthesis

What are the stages of translation?

Initiation, elongation, and termination.

What cis-elements and trans-factors are involved in initiation?

Shine-Dalgarno sequence (cis), 30S ribosomal subunit, IF1, IF2, IF3 (trans).

What cis-elements and trans-factors are involved in elongation?

mRNA codons (cis); EF-Tu, EF-G, tRNAs (trans).

What cis-elements and trans-factors are involved in termination?

Stop codons (cis); Release factors RF1/RF2 and RF3 (trans).

How is the appropriate AUG start site found in prokaryotes?

The Shine-Dalgarno sequence base pairs with the 16S rRNA to position the ribosome at the correct AUG.

What elongation factors are involved in elongation and what are their roles?

• EF-Tu brings aminoacyl-tRNA to the A site

• EF-G promotes translocation.

Which ribosomal subunit initially binds mRNA?

The 30S subunit.

Where is the peptidyl transferase center (PTC) located?

In the large 50S ribosomal subunit.

What is the Shine-Dalgarno sequence?

A purine-rich sequence upstream of the start codon that aligns the mRNA on the ribosome.

What does IF2 do in prokaryotic initiation?

IF2 delivers fMet-tRNA to the P site using GTP.

Which elongation factors are GTPases?

• EF-Tu

• EF-G

What are the A, P, and E sites in a ribosome?

• A site = Aminoacyl site

• P site = Peptidyl site

• E site = Exit site.

What is the initiating tRNA in prokaryotes?

tRNA fMet

What is distinctive about fMet-tRNA^fMet?

It carries a formylated methionine and directly binds the P site.

What is the peptidyl transferase center (PTC) and what does it catalyze?

It is the site of peptide bond formation; the amino group of the A site tRNA attacks the ester bond of the P site tRNA.

What is translocation and why is it necessary?

It moves the ribosome along the mRNA by one codon to free the A site for the next aminoacyl-tRNA.

What are key differences between prokaryotic and eukaryotic translation?

• Eukaryotes use a 5’ cap and Kozak sequence for initiation

• more initiation factors

• translation occurs in the cytoplasm separate from transcription.

What is the role of the SRP in the secretory pathway?

SRP binds to the signal sequence and halts translation, directing the ribosome to the ER membrane.

How is ferritin translation regulated by iron?

• In low iron, IRP binds the IRE in the 5' UTR and blocks translation

• In high iron, IRP does not bind, leading to degradation