Translation Elongation in Prokaryotes

Question 1

How does prokaryotic translation elongation occur?

Answer 1

Protein synthesized one amino acid at a time in the N –> C direction

Question 2

What is required for prokaryotic translation elongation?

Answer 2

GTP and non-ribosomal protein elongation factors (EF)

Question 3

List the three non-ribosomal protein elongation factors used in prokaryotic translation elongation.

Answer 3

EF-Tu, EF-Ts, EF-G

Question 4

What does the 30S subunit do in elongation? The 50S subunit?

Answer 4

30S aligns two tRNA in P- and A-sites, thereby aligning amino acids.
50S subunit catalyzes transpeptidation.

Question 5

Outline the 4 step elongation process.

Answer 5

1. Aminoacyl-tRNA enters A-site (substrate binding)
2. Peptide bond formation
   3/4. Ribosome moves to next codon

Question 6

How does substrate binding occur?

Answer 6

EF-Tu-GTP binds to aminoacyl tRNA and carries it to the ribosome’s A site

Question 7

What happens when the 30S subunit detects correct codon-anticodon match?

Answer 7

Conformational change which signals EF-Tu-GTP to hydrolzye GTP. GTP hydrolysis causes Ef-Tu- to release aminoacyl tRNA in A-site

Question 8

What happens if the 20S subunit detects the incorrect codon-anticodon match?

Answer 8

Entire EF-Tu-GTP-aminoacyl-tRNA complex is released so another complex can bind to the A-site.

Question 9

Outline the structure of EF-Tu.

Answer 9

Domain 1, 2, 3, switch helic

Question 10

What does the switch helix of EF-Tu complex do?

Answer 10

Detects the state of nucleotide and controls protein conformation

Question 11

What is the closed structure of EF-Tu?

Answer 11

GTP is bound, has a functional tRNA binding surface

Question 12

What is the open structure of EF-Tu?

Answer 12

GTP hydrolysis, tRNA binding surface disabled

Question 13

Is a new EF-Tu made for each new GTP + aminoacyl-tRNA binding?

Answer 13

No, the EF-Tu is recycled.

Question 14

How does transpeptidation occur?

Answer 14

Disjoins fMet from P-site and joins it to the amino acid bound to the A-site tRNA. Growing polypeptide transferred from P-site tRNA to A-site tRNA.

Question 15

What catalyzes transpeptidation?

Answer 15

23S rRNA of the 50S subunit

Question 16

What is ratcheting?

Answer 16

30S subunit turns 7-8 degrees clockwise following transpeptidation.

Question 17

What does ratcheting result in?

Answer 17

tRNA moving one site to the left in 50S region but tRNA anticodons only move a half-site.

Question 18

What is required to complete translocation after the ratchet?

Answer 18

EF-G-GTP

Question 19

Outline the power stroke.

Answer 19

1. EF-G-GTP inserts itself into the A-site and pushes the tRNA anticodon stems to new sites
2. GTP is hydrolyzed to power this process
3. EF-G-GDP dissociates after the power stroke to allow EF-Tu-GTP to carry new aminoacyl tRNA into the free A-site

Question 20

Where does the growing polypeptide chain exit the ribosome?

Answer 20

Through the 50S subunit