Lecture 11 - Post-Transcriptional Regulation of Gene Expression Flashcards
What are post-transcriptional controls?
Controls that act later in the pathway from DNA to protein to modulate the amount of gene product that is made - and in some cases, to determine the exact amino acid sequence of the protein product
When do post-transcriptional controls operate?
After RNA polymerase has bound to the gene’s promoter and has begun RNA synthesis
What can RNA splicing produce?
Different forms of a protein from the same gene
How does RNA splicing shorten the transcripts of many eukaryotic genes?
By removing the intron sequences from the mRNA precursor
What is alternative RNA splicing?
A process that splices an RNA transcript differently and thereby make different polypeptide chains from the same gene
How does alternative splicing get carried out?
Spliceosome
1) U1 snRNP base pairs with 5’ splice junction
2) U2 snRNP base pairs with the branch point
3) Other spliceosome subunits are recruited
4) Adenine in branch point “attacks” 5’ site
5) Lariat forms and frees the 3’ OH
How is alternative RNA splicing negatively controlled?
A repressor protein binds to a specific sequence in the pre-mRNA transcript and blocks access of the splicing machinery to a splice junction
What does negatively controlled alternative RNA splicing result in?
The use of a secondary splice site, thereby producing an altered pattern of splicing
How is alternative RNA splicing positively controlled?
The splicing machinery is unable to remove a particular intron sequence efficiently without assistance from an activator protein. Because RNA is flexible, the nucleotide sequences that bind these activators can be located many nucleotide pairs from the splice junctions they control, and they are often called splicing enhancers.
How is alternative RNA splicing involved in Drosophila sex determination?
XX –> Sxl on –> Tra on –> female
XY –> Sxl off –> Tra off –> male
- Transcription of Sxl is carried out in females and promotes alternative splicing of Tra on RNA
How does Sxl promote alternative splicing of tra RNA?
1) Sxl binds to splice acceptor site for exon 2 on tra RNA which prevents spliceosome interaction with the splice acceptor site
2) Spliceosome interacts with the alternative splice acceptor site (distal site)
3) Female tra mRNA is translated into a protein that acts as a splice factor for dsx and fru RNAs
What happens in the absence of Sxl?
1) Default splicing of tra using proximal splice acceptor site
2) mRNA produces a truncated non-functional protein
3) Default splicing of dsx and fru
What mechanism deliberately overrides the delayed export of RNA molecules from the nucleus?
Regulated nuclear transport of mRNA
How is alternative splicing and nuclear export carried out for HIV?
1) RNA genome enters the host cell
2) Reverse trascriptase –> dsDNA
3) dsDNA imported into the nucleus and integrates into the host genome
4) Transcription produces a single pre-mRNA
5) Alternative splicing generates 10 distinct mRNAs from a single RNA
6) Nuclear export
7) Translation to make 10 distinct proteins
8) Non-spliced RNA is exported from the nucleus and packaged into new virus particles
What is Rev?
An encoded protein in a virus that binds to a specific RNA sequence located within a viral intron
- Made in cytosol and gets imported into nucleus
What does Rev do?
- Binds specific sequences found in introns of HIV transcripts and binds the nuclear export receptor
- Brings unspliced HIV virus of RNA out of nucleus to be packaged
What consequences for HIV growth and pathogenesis does the regulation of nuclear export by Rev create?
- Ensures the nuclear export of specific unspliced RNAs
- Divides the viral infection into an early phase and a late phase
- HIV achieves latency (HIV genome has become integrated into host-cell genome but production of viral proteins has temporarily ceased)
How does the Rev protein from HIV work?
1) Makes DNA copy and integrates it into the genome
2) Transcription and splicing so that mRNA is exported from the nucleus and translated
3) Unspliced transcript is not exported, it is degraded
What is RNA localization?
mRNA get directed to specific intracellular locations before their efficient translation begins, allowing the cell to position its mRNAs close to the sites where the encoded protein is needed
What common cell mechanism might RNA localization be used to do?
Concentrate high-level production of proteins at specific sites
What do the mechanisms for mRNA localization require?
Signals in the mRNA itself that are usually concentrated in the 3’ untranslated region (UTR)
