Translation Termination

Question 1

What are the steps of translation termination?

Answer 1

1. Termination reaction: the new peptide is released from the last tRNA
2. Post termination reaction: dissociation of the ribosome

Question 2

What are the termination codons?

Answer 2

1. UAG
2. UAA
3. UGA
   • No tRNA recognize these (no anticodon)

Question 3

What recognize termination codons?

Answer 3

• No tRNA recognize these (no anticodon)
• They are recognized by released factors (protein complex)
1. Class 1 release factors
2. Class 2 release factors

Question 4

What are Class 1 release factors in E. coli and what do they recognize?

Answer 4

1. RF1: recognizes UAA and UAG

2. RF2: UAA and UGA

Question 5

What are the roles/activity of the class 1 release factors?

Answer 5

• RFs bind to the intact (empty) A site and docks into the A site
o The docking activates hydrolytic activity in LSU
o The hydrolysis cleaves the bond between the last AA and the 3’ end of tRNA, releasing the peptide

Question 6

What are the Class 2 release factors found in E. coli? What do they recognize?

Answer 6

• In E. Coli this is RF3

* Not codon specific

Question 7

What are the roles and activity of class 2 release factors?

Answer 7

• Role: release class 1 release factor from the A site
• GTP binding: similar cycle as EF-Tu and EF-G
o RF3 exists in active state RF3(GTP) and inactive state RF3(GDP)
o Prior to binding, RF3 needs to be activated
o It binds to LSU binding site (same site as EF-Tu and EF-G, adjacent to A site)
 It binds the class 1 release factor
 The class 1 factor is released, also releasing RF3 (since it is bound to it)

Question 8

What is the goal of the post-termination reaction?

Answer 8

• Following the termination reaction, the intact ribosome is detached from the deacetylated tRNA

Question 9

What factors are involved in post-termination?

Answer 9

RRF, EF-G, IF-3

Question 10

What are the roles/activity of RRF?

Answer 10

o RRF: ribosome recycling factor

 Binds the empty A site (adjacent to P, which contains the tRNA)

Question 11

What are the roles/activity of EF-G in post-termination?

Answer 11

o EF-G: translocation factor
 Binds same site (LSU, adjacent to A)
 Ribosome moves forward (the RRF is now in the P site)
 Hydrolysis: facilitates SU dissociation

Question 12

What are the roles/activity of IF-3 in post-termination?

Answer 12

o IF-3

 Binds SSU and maintains dissociation states (like in initiation)