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translation initiation factors...
what subunit & what site of the ribosome does the mRNA & initiator tRNA bind?
- small subunit binds mRNA & tRNA
- P site
what is a Shine-Dalgarno sequence?
*the closer it matches the consensus seq. the more efficiently the mRNA will be translated
- ribosome binding site (RBS) on the mRNA
- 5' AGGAGGU
non-overlapping vs. overlapping genes...
- there is a shift of reading frame in overlapping genes (backwards)
- non-overlapping has separate SD seq & protein-coding regions
- overlapping genes can share SD & protein-coding regions overlap e.g. stop/start codon
how does a Shine-Dalgarno seq. work?
SD seq. base pairs w the 3' end of the 16S rRNA subunit of the small ribosomal subunit before the start codon
why are there 2 tRNAs for Met & only 1 codon?
- initiator = Met-tRNA^fMet = charged by methionyl-tRNA synthetase, then formyltransferase adds formyl group (H-O=C-) to amide N, so it can only be positioned at the start
- Met-tRNA^mMet = internal positions => Met
Prokaryote initiation factors (3)...
function & binding
- IF1 - blocks binding of initiator tRNA to A site
- IF2 - GTPase that loads initiator tRNA to P site
- IF3 - prevents premature binding btwn large & small ribosomal subunits
The 30S binds IF1 & IF3, then mRNA
what causes IF release?
- conformational change triggers IF3 release, large subunit binds, GTP hydrolysis cause IF2 release & IF1 releases
- ...70S Initiation complex
how does translation initiation differ in eukaryotes vs prokaryotes?
- - NO SD site, so eIF4e bind 5' end cap
- - NO formylation of Met, but still 2 diff tRNAs for initiaton vs internal AUG
Eukaryotic initiation factors (6)...
function & binding
- eIF1A (IF1) - blocks A site
- eIF1 - blocks E site
- eIF3 (IF3) - prevents premature large & small subunit binding
- eIF2 (IF2) - GTPase brings Met-tRNA^(i)Met to small subunit
- eIF4F complex - mRNA assembly w 43S preinitiation complex
- eIF5B - GTPase causes removal of factors allowing 40S + 60S = 80S ribosome