lecture 22- translation pt 2 Flashcards
the ribosome provides environment for interaction of ___ and ___, and catalyzes ___
tRNA and mRNAs
peptide bond formation
ribosomes, RNA-protein complex:
ribosomal proteins (r-proteins) and ribosomal RNAs (rRNAs)
prokaryotic ribosome: 70S (50 & 30)
eukaryotic ribosome: 80S (60 & 40)
small subunit of ribosome contains the ___
decoding center
– where aminoacylated-tRNAs “read” the genetic code by base pairing with each triplet codon in mRNA
large subunit of ribosome contains the ___
peptidyl transferase center (active site)
peptidyl transferase: catalyzes peptide bond formation between adjacent AA’s in a growing polypeptide chain
the ribosome is a ___
describe ribosomal RNA
ribozyme
ribosomal RNA has a fundamental importance in catalyzing protein synthesis
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what is responsible for catalysis of peptide bond formation?
ribosomal RNA, not protein
describe ribosomal RNA in ribosomal active site
- deproteinized ribosome retains peptidyl transferase activity
there is no protein within the peptidyl transferase active site within the crystal structure of the 50s ribosomal subunit
3 stages of translation
1- initiation- involves assembly of active ribosome with mRNA and the first aminoacyl-tRNA before first peptide bond formation
2- elongation- all rxn’s from 1st to last peptide bond formation
3- termination- ribosome reaches a stop codon, release of completed peptide and dissociation of ribosome from mRNA
ribosome has 3 tRNA binding sites
1- (A): aminoacyl
2- (P) peptidyl
3- (E) exit- exit site occupied by the tRNA molecule released after the growing polypeptide chain is transferred to the aminoacyl-tRNA
initiation of translation in prokaryotes
the Shine-Dalgarno sequence positions the mRNA on the bacterial ribosome by binding to the 16S rRNA
- Shine-Dalgarno sequence also called ribosome binding site (RBS)
- consensus sequence (initiation signal) of 5-9 purine residues, situated 8 to 13 nucleotides on the 5’ side of the start codon
- base pairing with a complementary pyrimidine-rich sequence near the 3’ end of the 16s rRNA of 30s ribosomal subunit
- mRNA-rRNA interaction positions the initiating 5’-AUG sequence of mRNA in precise location on 30s subunit where required for translation initiation
a specific amino acid initiates protein synthesis
met
AUG codon represents met, two tRNAs
fMet-tRNA^(fMet) charged used only for initation
tRNA^fMet for initiation
tRNA^Met for elongation
initiation factors in bacteria (IFs)
IF-3 prevents premature association of 30S with 50S
IF-1 binds at the A site and blocks tRNA binding during initiation
IF-2 (GTPase) facilitates binding of charged initiator tRNA to partial P site
translation initiation in bacteria
mRNA, fMet-tRNA^fMet and IFs bind to 30s
IF-3 leaves when fMet-tRNA^fMet pairs with mRNA
50s joins in as IF-2 hydrolyzes GTP and IF2-GDP and IF-1 leave
70s initiation complex ready for elongation
initiation of translation in eukaryotes
similar to prokaryotic except:
- Met-tRNAi^Met (not formylated)
- more eIFs required (at least 12)
- Met-tRNAi^Met always binds to 40s before mRNA binding
- ribosomes are recruited to mRNA to 5’ cap (no RBS)
“initiation sites” in eukaryotic mRNAs
no shine dalgarno sequence
- modifications at both ends are important for initiation
- 43s recognizes 5’ cap and then “migrates along” mRNA
- leader region is “scanned” (in 5’-3’ direction)
- AUG recognized by 43 s complex
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