Control of Translation Initiation (Bacteria).
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When does a complete Ribosome form?
On mRNA, just before translation.
What are the two Bacterial Ribosomal Subunits?
30S and 50S
Which Subunit Initiates Translation?
The 30s (Small) Subunit.
What three things are required for initiation on the 30S subunit?
- Initiator aminoacyl-tRNA
- Inition Factors
What are some Bacterial Initiation Factors?
IF 1, IF 2 and IF 3
What do Bacterial Initiation Factors do?
- Promote Subunit Dissociation
- Prevent Subunit Association
- Stimulate association of Initiator Aminoacyl-tRNA and 30s subunit
What are the three most common Translational start codons in Prokaryotes?
- AUG (90%)
- GUG (8%)
- UUG (1%)
What is the first tRNA used in Bacterial Translation?
What is the fist coded placed Amino Acid in Bacterial Translation?
What is N-formyl-methionine?
- The first Amino Acid in Bacterial Translation.
- Methionine with a Carboxyl group added to the N "terminus".
What happens to the N-formyl-methionine after Translation?
Often the formyl group, or whole amino acid is removed.
The three main bacterial start codons all code for different amino acids, yet N-formyl methionyl tRNAfmet recognizes all of them. Is this an example of a wobble interaction?
- No, because the changing base is on the left side of the codon.
- In wobble interactions the variable base is on the right side.
What does the Shine-Delgarno sequence consist of a large number of?
Purines (Purine Rich Sequence).
Where is the Purine Rich Sequence Located?
Just upstream of the start codon on mRNA.
Where is the Pyrimidine rich sequence located?
Towards the 3' end of the small subunit rRNA.
How/Why do the Purine and Pyrimide rich sequences interact?
- The two sequences interact through complimentary base pairing.
- The interactions serve to correctly place the small subunit for Translation Initation.
Which sequence (Purine/Pyrimidine Rich) is known as the Shine-Delgarno Site?
The Purine Rich Sequence.
What is another name for the Pyrimidine Rich Sequence?
The Anti-SD sequence.
What are polycistronic RNA's?
mRNA's producing multiple different peptide products.
How can mRNA's control timing of Transcription?
Using a secondary structure that unfolds on Translation of prior Polypeptides.
Give an Example of an Organism that uses it's secondary mRNA structure to time Translation?
The MS2 phages replicase peptide.
What is the order of cistrons in the MS2 phage?
- 1. A Protein Protein
- 2. Coat Protein
- 3. Replicase Protein
How does e.coli regulate Translation of the (σ32) heat shock factor?
- The mRNA Ribosensor has internal base pairing (blocks start codon) that breaks at 42°c.
- The newly exposed Start Codon allows for translation.
Why is it advantageous to produce σ32 mRNA even when the temperature is below 42°c?
Because a rapid response to changing in temperature is essential.
What is another name for the σ32 Ribosensor?
An RNA Thermometer.
What is the σs factor?
A general stress factor.
When is the σs factor translated?
- When optimal growth conditions are not met.
- Temp not 37c
- Nutrient Starvation
What gene codes for the σs factor?
The rpoS gene
Why does the rpoS gene only exhibit weak translation in optimal conditions?
The secondary structure of the mRNA folds on itself blocking the Shine-Delgarno sequence and inhibiting access to the start codon.
How is the rpoS gene activated?
- Non-optimal conditions
- DsrA RNA base pairs to the 5' UTR of σs
- The DsrA binding "reveals" the Shine-Delgarno Sequence.
- Strong Translation is allowed
What is DsrA RNA?
RNA used in regulation of the RpoS gene
DsrA RNA is an example of...?
What are three ways of controlling access to the Shine-Delgarno sequence?
- Translation (Ribosome)
- Temperature (Conditions)
- Trans-Acting RNA
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