20 Questions
What is the primary function of tmRNA in trans-translation?
To recycle ribosomes stuck on truncated mRNAs
What is the main reason for ribosomal stalling?
Lack of tRNA in the A site
What is the consequence of not resolving a stalled ribosome?
Production of truncated proteins
In eukaryotes, how is a stalled ribosome typically resolved?
By degrading the mRNA and the ribosome
What is unique about the tmRNA molecule?
It has both tRNA and mRNA properties
What is the main function of tmRNA in the process of trans-translation?
To release the stalled ribosome and add a tag for protein degradation
What is the structure of the tmRNA molecule?
It has a tRNA-like structure at one end and a long extension with a reading frame
What happens when the ribosome encounters a triplet codon out of place?
The ribosome releases the mRNA and the tmRNA takes over
What is the reason for the majority of stalled ribosomes?
The absence of a tRNA in the A site
What is the function of the Ala residue attached to the tmRNA?
To tag the incomplete protein for degradation
What is the function of the tmRNA in prokaryotes?
To degrade truncated proteins and release stuck ribosomes
What is the purpose of the stringent response in prokaryotes?
To restrict protein synthesis when amino acids are limited
What is the structure of the tmRNA that allows it to simulate tRNA?
A secondary structure that resembles tRNA, but lacks an anticodon
What is the role of the alarmones in the stringent response?
To inhibit protein synthesis when amino acids are limited
What is the characteristic of the alarmones that allows them to act as a signal?
They are unstable and have a short half-life
What happens to the tRNA molecules when there is a lack of amino acids?
They form a complex with FTU
What is the effect of RelA activation on RNA polymerase?
It pauses the elongation of RNApol
What is the function of RelA?
To generate alarmones
What happens to the ribosome when a tRNA without an amino acid binds to it?
It pauses the translation
What is the result of RelA activation on protein synthesis?
It reduces protein synthesis
Study Notes
Trans-Translation
- Trans-translation is a process where a tmRNA (transfer-messenger RNA) molecule is used, which is simultaneously a tRNA and an mRNA.
- This process occurs when ribosomes stall, which can lead to a problem of producing a tail of stalled ribosomes, resulting in truncated proteins.
- In eukaryotes, the solution to this problem is to remove the ribosome and eliminate the truncated proteins, and if the messenger is incorrect, degrade it.
- In prokaryotes, the tmRNA molecule acts in place of the next aminoacyl-tRNA, freeing the stalled ribosome.
- The tmRNA molecule has a tRNA-like structure at one end with an attached Ala, and a large extension with a tertiary structure and a reading frame marked in pink.
- The tmRNA provides an alanine and a reading frame to continue, and the Ala binds the peptidyl to the tmRNA, causing the ribosome to shift.
- This leads to the ribosome leaving the real mRNA and translating the pink side of the tmRNA, allowing normal tRNAs to enter.
- The resulting proteins are tagged with Ala and the tmRNA sequence, which is a signal for proteolysis, similar to ubiquitin in eukaryotes.
- The tmRNA competes and displaces the rest of the mRNA if the stall is not caused by the end of the truncated messenger.
Stringent Response
- A more important response to amino acid scarcity is the stringent response, which occurs when cells detect a lack of amino acids.
- Prokaryotes, such as E. coli, can synthesize their own amino acids but will stop producing them if they are present in the culture medium or environment.
- When amino acids are absent, the cells change their metabolism to produce them, and if they are then removed, the cells will respond by restricting growth and protein synthesis.
- This response is triggered by alarmone molecules, which are derived from nucleotides and have a phosphate group at the 5' and 3' ends.
- Alarmone molecules are unstable and have a short half-life, ensuring that the response is temporary and only active while the signal is present.
- The absence of amino acids leads to tRNAs without amino acids, which can form a complex with EF-Tu, but not with aminoacyl-tRNAs.
- This complex activates the RelA enzyme, which produces alarmone molecules and triggers the stringent response.
- The stringent response reduces the synthesis of rRNA, tRNA, and mRNA, and increases proteolysis to generate amino acids.
Learn about trans-translation, a process where tmRNA acts as both tRNA and mRNA, resolving stalled ribosomes and preventing truncated proteins. Discover how this process works in eukaryotes.
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