Lecture 17

Question 1

Describe some differences between translation in eukaryotes and prokaryotes

Answer 1

1. Methionine is first amino acid in both, but not formulated in eukaryotes
2. No shine dalgarno sequence on mRNA - 7-methylG cap assists in binding of ribosome and ribosome moves to first AUG
3. Start codons in mammals - RXXAUGG (Kozak sequence)

Question 2

Eukaryotic mRNA

Answer 2

7-methylguanosine cap

PolyA tail

Both stabilize mRNA and prevent degradation

Not all mRNA acts as template - 5’ and 3’ untranslated regions

5’UTR (<100 bases), 3’UTR (1000 bp)
Methylated cap plays pivotal role

Question 3

Eukaryotic initiation factors (eIFs)

Answer 3

12 eIFs identified

eIFs number 1-6, but each number is a group of different types and further named with a letter e.g. eIF4A, eIF4B

Question 4

Initiation

Answer 4

1. mRNA bound by eIF4 family, where eIF4E recognises cap, eIF4G acts as scaffold between cap and polyA tail - forming closed loop complex
2. Small subunit bound by initiator tRNA, initiation factors recruited via eIF4G/eIF3 interaction
3. Pre-initiation complex scans for start codon - eIF4A/4B complex has ATP-dependent helicase activity - draws mRNA through till start codon located
4. activation of GTPase activity of eIF2 cause conformational shift after start codon identified
5. Large subunit binds, majority eIFs released

Question 5

Elongation in eukaryotes

Answer 5

eEF1A equivalent to EFTu - binds aminoacyl tRNA and GTP

eEF1B equivalent to EFTs and exchanges GTP for GDP

eEF2 equivalent to EFG - involved in ribosome translocation