Post-Transcriptional & Translational Regulation Flashcards
What specific elements in the 3’ UTR of cells is responsible for regulating synthesis of the transferrin receptor?
Iron Responsive Elements (IRE)
Describe the mechanism of miRNA.
Pre-miRNA bases pairs with itself in the nucleus before moving to the cytosol. A Dicer RNase processes the pre-miRNA to produce double-stranded miRNA. One strand binds with an argonaute protein and forms a mature RNA-induced silencing complex (RISC). The miRNA acts as a guide that brings the nuclease into contact with the target mRNA by complementary base pairing. mRNA is degraded and protein synthesis limited. Inhibition of protein synthesis often requires 2 miRNAs binding.
What splice sites conform less to consensus sequences and are often not bound unless sites with greater affinity to consensus sequences cannot be found?
Cryptic Spice Sites
Describe the default mechanism for mRNA degradation, when there are no regulatory sequences present.
mRNA degradation begins with slow deadenylation of the poly-A tail by a deadenylase. When the poly-A tail becomes very short, deadenylase is replaced by an exosome, which rapidly degrades the poly-A tail. From the 5’ end, a decapping enzyme and nuclease degrade the mRNA.
Describe the mechanism of global translational inhibition.
eIF2+GTP is required to initiate translation. eIF2B is a mediator that replaces GDP with GTP. When eIF2 is phosphorylated, eIF2B binds but is unable to unbind thus stopping most translation.
How does translational regulation differ from transcriptional regulation?
Translational regulation effects protein levels much more quickly.
Where does translation take place?
Cytoplasm
What enzyme is responsible for breaking down the poly-A tail?
Deadenylase
Describe the mechanism of RNAi.
A dicer cuts the RNA into small interfering RNAs (siRNA). A siRNA strand combines with argonaute proteins to form a RISC. The siRNA acts as a guide to locate and destroy foreign RNA.
What are the types of alternative splicing?
Constitutive & Regulated
Describe negative and positive control of regulated alternative splicing.
Negative control - Involves a repressor, which prevents recognition of splicing sites by snRNPs
Positive control - Involves an activator, which activates recognition of a splicing site so that it is removed by a spliceosome
What protein recognizes splice sites in pre-mRNA?
snRNP (small nuclear ribonucleoproteins)
Where does RNA editing take place?
Nucleus
Note, this process is very rare
What is alternative polyadenylation?
Many transcripts have alternative cleavage and polyadenylation sites that can generate different mRNA isoforms with different 3’ UTRs (untranslated regions). These different UTR sequences allow for inclusion or exclusion of regulatory elements, without changing the amino acid sequence.
What is the relationship between mRNA stability and protein synthesis?
The more stable the mRNA, the longer it takes to be degraded, and the more protein is translated. mRNA degradation is often regulated by sequences in the 3’ UTR.
What is an exon junction complex?
EJCs mark the location of removed introns and play a role in regulation.
Describe the mechanism of alternative mRNA degradation.
Some mRNA contain recognition sites for an endonuclease in the 3’ UTR. The endonuclease cleaves the poly-A tail, allowing the nuclease to degrade mRNA from the 5’ end and deacetylase from the 3’ end. This process is more rapid.
How does stabilization of IREs in the 5’ UTR of ferritin affect protein synthesis?
When IREs are destabilized, the 5’ cap is accessible, translation occurs, and protein is synthesized. When iron levels are low, a protein binds and stabilizes IRE, but blocks access to the 5’ cap, thus preventing translation.
This process is the opposite of IRE binding in the 3’ UTR and translation of the transferrin receptor.
What is nonsense-mediated mRNA decay?
mRNA is “tested” in a pioneer round of translation immediately outside the nucleus. If there is an error in the mRNA and a stop codon is reached before all exon junction complexes are reached, this indicates a nonsense mutation and triggers mRNA degradation.
What nucleotide changes occur during RNA editing?
C to U - creates a stop codon
A to Inosine - changes the amino acid (very important in the brain)
Note, this process is rare and only takes place in the nucleus
What is the effect of iron levels on synthesis of the transferrin receptor?
When iron concentration is high in the cell, IREs are unbound by proteins and results in mRNA degradation. When iron concentrations are low, proteins bind IREs to stabilize the mRNA and promote translation of the transferrin receptor.
What are the features of regulated alternative splicing?
Regulated splicing is more intentional and involves addition or removal of functional domains with assistance from activators or repressors. Regulated splicing is tissue- or development stage-specific, with only one version of the protein expressed in the cell.
