2.4 Expression of Genetic Information Flashcards
tRNA Wobble
In some cases the 5’ nucleotide end of the tRNA is capable of pairing with more than one nucleotide at the 3’ end of the mRNA codon. Consequently, more than one codon can use the same tRNA
Specificity of Amino Acid Incorporation
Chemical modification of cysteine to alanine
Anticodon part is bottom
that means that as long as the anticodon is right, literaly any a.a could bind and it would not affect the specificity for the mRNA at all.
Translation Initiation
- Ribosome attaches at initiation codon (AUG)
- Establishes reading frame
- Step 1: Small ribosomal subunit associates with initiation codon
- Aligned by Shine-Dalgarno Sequence
- GGAGGA
Prokaryotic Translation Initiation
•Step 1: Bringing the small subunit to the initiation codon
•IF2 is a GTP-binding protein required for attachment of the first aminoacyl-tRNA.
•IF3 may prevent the large (50S) subunit from joining prematrely to the 30S subunit, and also facilitate entry of the initial aa-tRNA.
•IF1 facilitates attachment of the 30S subunit to the mRNA and prevent the aa-tRNA from entering the wrong site on the ribosome
•Step 2: Bringing the first aa-tRNA into the ribosome
Initiator met-tRNA is different from met-tRNA used to elongate polypeptide
Enters P site and binds to AUG codon and IF2 initiation factor
IF1 and IF3 are released
The initiator enters at P site, all tRNAs after enter at A site
Step 3: Assembling the complete initiation complex
Large subunit joins the complex and the GTP bound to IF2 is hydrolyzed..
Translation Initiation in Eukaryotes
- At least 12 initiation factors
- Several factors bind to the 40S subunit which prepares it to bind to the mRNA
- Initiator tRNA-met also binds to 40S subunit before it binds to mRNA
- Initiator tRNA-met enters the P site in association with eIF2-GTP
- Entire complex (43S complex) then binds to mRNA
- Initiation factors already bound to mRNA help 43S complex bind.
- eIF4E binds to 5’ cap of the mRNA
- eIF4A moves along the 5’ end removing double stranded regions
- eIF4G serves as a linker between the 5’ capped end and the 3’ poly A tail.
- 43S complex scans along mRNA for initiation codon
- eIF2-GTP is hydrolyzed, eIF2-GDP is released and the large (60S) subunit joins the complex
union is mediated bye elF3 on 43S complex and eIF4G on mRNA comple
Bacterial Ribosome Model
- Interface of two subunits contains almost exclusively RNA
- Active site also consists of RNA (Deep cleft)
- mRNA in narrow channel that winds around the neck of the small subunit
- Tunnel runs completely through the core of the large subunit beginning at the active site
- Proteins of ribosomal subunits have multiple RNA binding sites
- A=aminoacyl
- P=peptidyl
- E=Exit
Translation Elongation (Prokaryotic)
- Step 1: Aminoacyl-tRNA selection
- Second aminoacyl-tRNA enters vacant A site. First binds to protein elongation factor bound to GTP (Ef-Tu)
- Step 2: Peptide Bond Formation
- Amino group of aa-tRNA in A site reacts with carbonyl group of amino acid in P site displacing the P-site tRNA
- tRNA bound to second codon in the A site has a dipeptide and the tRNA in the P site is deacylated
- Reaction catalyzed by peptidyl transferase (Large subunit RNA)
- Step 3: Translocation
- Ribosome moves 3 nucleotides along mRNA and tRNA-peptide moves from A site to the P site
- Deacylated tRNA moves from P site to the E site
- Driven by conformational changes in elongation factor EF-G following hydrolysis of bound GTP
- Step 4: Release of deacylated tRNA
Translation Termination
•Termination codons UAA, UAG, UGA •Requires release factors •Prokaryotes •RF1 recognizes UAA and UAG •RF2 recognizes UAA and UGA •RF3-not codon specific •Eukaryotes •eRF1 and eRF3 combine and recognize all termination codons
Polyribosomes
- More than one ribosome associated with a transcript
* Increases rate of protein synthesis
