Lecture 6 Flashcards
What recognizes the 5’ cap structure in RNA?
Eukaryotic initiation factor 4E (elF4E)
Recruits ribosome to mRNA
What is IRES?
Internal Ribosome Entry Site
How does viral RNA get translated without a cap?
Through the IRES that is found in the 5’ UTR that allows internal initiation
The ribosome binding on the RNA depends on …
The secondary structure of the IRES
What are other mechanisms used by viruses to manipulate translation machinery
Ribosome shunting
Methionine-Independent initiation
Leaky scanning
Re-initiation
Suppression of termination
Ribosomal frameshifting
What is ribosome shunting?
Ribosomes bypass parts of the 5’ untranslated region to reach the start codon or to bypass a stable secondary structure in the mRNA
Reduces the need for mRNA unwinding of secondary structure
What is methionine-independent initiation?
Translation initiation at a non-AUG codon
Ex: CRPV IRES mimics the met-initiator tRNA (tRNAi) and can recruit 80S ribosomes without any ElFs or Met-tRNAi
Ex: Valine binds to the tRNA-like structure of turnip yellow mosaic virus (TYMV)
What is leaky scanning?
Encode several proteins on the same mRNA
Initiate at non-optimal codons (instead of AUG)
What is re-initiation of RNA?
Ribosomal re-initiation allows expression of a downstream open reading frame in a bicistronic mRNA
Ribosomes translate the upstream ORF but following termination, a proportion of 40S subunits remain tethered to the mRNA and go on to reinitiate at the start codon of the downstream ORF
What is suppression of termination?
Stop codons can sometime be misread or charged with suppressor tRNAs
In HIV, a UAG codon is suppressed approx 10% of time to allow for the Gag-Pol fusion protein to be synthesized
A pseudoknot causes the ribosome to pause, thereby allowing more time for a suppressor tRNA to be charged
What is ribosomal frameshifting?
Ribosome pauses, slips back and shifts into a different reading frame (-1 or +1)
Used to synthesized a protein encoded in a different reading frame and/or to bypass a stop codon
Requires a sequence that promotes ribosome slippage and the tRNA to move by one base
True or false: Many viruses turn off host protein synthesis to prevent infected cells from mounting an antiviral response
True
What does poliovirus 2A protease do?
Cleaves elF4G to inhibit host protein synthesis
Why can poliovirus protein synthesis continue even when the poliovirus 2A protease cleaves elF4G?
Poliovirus protein synthesis depends on IRES and not cap structure
