Translation Flashcards
Prokaryotic Ribosome
70S (made up of 50S and 30S subunits)
50S subunit made up of 23S rRNA and 5S rRNA and proteins
30S subunit made up of 16S and proteins
EPA sites
Polysomes
in prokaryotes, several ribosomes can translate one mRNA simultaneously, even at the same time of transcription occurrence
Translation Initiation
mRNA and small subunit combine with initiator tRNA, which binds in the P site and base pairs with AUG (start codon)
large subunit binds and translation can begin
Initatiator tRNA
different from other methionyl-tRNAs
prokaryotes: formylated tRNA fMET
eukaryotes: special tRNA iMet
Shine Dalgarno Sequence
in prokaryotes
sequence preceding the start codon that tells the ribosome where to bind
binds to 16S rRNA of 30S subunit, near the E site
positions the mRNA correctly on the bacterial ribosome
Kozak Sequence
in eukaryotes
sequence enclosing the start codon in mRNA
that, with the 5’ cap, allows eukaryotic ribosomes to recognize and bind mRNA
highly conserved among organisms, the more conserved the higher the translation efficiency
Translation Initiation in Prokaryotes
IF3 binds to the 30S subunit in E site
IF1 binds to 30S subunit in A site
mRNA binds to 30S subunit, with Shine-Dalgarno sequence bound to 16S rRNA and AUG start codon positioned in the P site
fMet-tRNA binds to IF2, and then base pairs with the start codon
Initiation Factor 3 (IF3)
prevents association with 50S subunit and blocks the E site
Initiation Factor 1 (IF1)
blocks the A site of the 30S subunit
Initiation Factor 2 (IF2)
GTPase, binds to initiator tRNA with GTP attached
then helps position the initiation tRNA in the P site
hydrolyzes GTP if the correct base pair is achieved and 50S subunit attaches to begin initiation and have all three initiation factors dissociate (create the initiation complex)
Similarities in Mechanism of Translation Initiation Between Prokaryotes and Eukaryotes
initiation factors keep the ribosomal subunits separate, one helps the first tRNA to bind, which binds to start codon AUG
larger subunit binds when initiation factors unattach
GTP is hydrolyzed in both to cause initiation factors to unattach
Differences in Mechanism of Translation Initiation Between Prokaryotes and Eukaryotes
in EK, 5’ cap is bound to an IF, guides the mRNA to the small ribosomal subunit
Shine-Dalgarno sequence (PK) vs Kozak Sequence (EK)
eukaryotic translation can become circular to facilitate gene expression, polycistronic translation in prokaryotes
EK can have cap dependent or independent translation
Elongation
successive cycles of aminoacyl-tRNA binding and peptide bond formation occur until the ribosome reaches a stop codon
Elongation Overview
each tRNA must be activated (aminoacylated) by attaching the correct amino acid; uses ATP, done by aminoacyl transferase
correct aminoacyl tRNA binds in A site
peptide bond forms between aa on tRNA in P site and tRNA in A site (no ATP needed)
translocation: the tRNA in P moves to E site, and tRNA in A moves to P site
Aminoacyl tRNA Binding in A site in Elongation
binds to A site
facilitated by EF-Tu, which is a GTP-ase bound to the tRNA
only releases if they’re paired correctly
EF-Tu
GTPase that facilitates the aminoacyl tRNA binding into the A site
only releases the tRNA if the base pairs are correctly matched
Peptide Bond Formation in Elongation
the proximity and positioning in large subunit catalyzes peptide bond formation
high energy bond from aminoacylation means no ATP required
N-terminus of A site attacks C-terminus of P site
Hybrid State in Elongation
momentary, when amino acid chain is attached to the tRNA in the A site
Translocation in Elongation
EF-G (a GTPase) binds to aminocyl tRNA in A site
GTP hydrolysis causes a conformational change, leading to ribosome translocation
causes tRNA in A to move to P, and that in P to move to E and exit
Colinearity of Proteins and mRNA
5’-3’ polarity of mRNA corresponds to N-C polarity of protein
Translation Termination
translation stops when a stop codon is encountered
mRNA and protein dissociate and ribosomal subunits are recycled
Termination: Release of the Polypeptide
RF1 or RF2 recognize the stop codons and bind in the a site (instead of a tRNA)
peptide chain is transferred to the H2O instead of an amino acid, causing hydrolysis and release of aa chain
RF3 attaches to RF1/2, causing their release, then hydrolyzes GTP to release self
Class I Release Factors
RF1 and RF2
recognize stop codons
shaped like tRNA
Class II Release Factors
RF3
attaches to RF1/2 to dissociate it, then hydrolyzes GTP to dissociate itself
