Gene Structure 6 Flashcards
What does RNA editing do?
Adds nucleotides/changes the information encoded by a gene. Generates protein diversity.
How is RNA editing regulated?
Developmentally
What is Trypanosoma brucei and what can it cause?
A parasite that can cause sleeping sickness in humans.
What is a kinetoplast?
Genome inside the mitochondria of Trypanosoma brucei. Consists of 50 maxicircles and 10,000 minicircles of DNA.
What do maxicircles code for?
Proteins involved in the mitochondrial oxidative phosphorylation pathway- including cytochrome complexes and ATP synthase.
How is cytochrome oxidase II mRNA edited?
Insertion of uridine so needed prior to a premature stop codon in order to put the stop codon out of frame and produce the functional protein.
How is cytochrome oxidase III mRNA edited?
Many uridine insertions are required throughout the mRNA sequence in order to produce the functional protein. Without uridine insertion, only a small region of the protein resembles cytochrome oxidase III in other organisms.
What can insertion of uridines result in?
Formation of start codons, correction of frameshift mutations, removal of premature stop codons to form the appropriate stop codon.
What is the role of minicircles?
Code for guide RNAs which associate with maxicircle mRNA and accessory proteins to form the editosome.
What two types of domain are found within the editosome?
Insertion and deletion domains.
Describe guide RNAs that associate with the editosome.
Consist of an anchor sequence (9-13nts) which base pairs with pre-mRNA and a guiding sequence (20-30nts) which directs the insertion or deletion of uridines.
What is the mechanism of RNA editing in Trypanosoma brucei?
- Poly(U) tail added to gRNA transcripts- by TUTase.
- gRNA base pairs with pre-mRNA.
- Endonuclease cleavage at mismatched base.
- Uridine insertion- TUTase or deletion- exonuclease.
- Ligation to complete mRNA
How does RNA editing allow developmental regulation in Trypanosoma brucei?
Kinetoplast repressed during transfer to host cell, and active in the quiescent form (once parasite is in blood) and the procyclic form (once in gut of host).
Describe RNA editing in Physarum.
Additions of cytosines, uridines, CU and GU. Editing seen in mitochondrial mRNAs, rRNAs and tRNAs. Occurs cotranscriptionally.
Describe RNA editing in paramyxovirus.
Transcription dependent RNA editing. Polymerase shifts on template DNA, extra G added after polymerase realignment.
Describe RNA editing in mitochondria.
Cytosine deamination. Seen in mRNA, rRNA and tRNA. Occurs post-transcriptionally.
Describe RNA editing in plastids.
Cytosine deamination. Seen in mRNA. Occurs post-transcriptionally.
What changes occur in mammalian RNA editing?
C to U (deamination) and A to I.
How does A to I editing change the protein sequence?
Inosine acts as a guanosine.
How is Apolipoprotein B edited in the intestine?
C to U change at a specific position produces a smaller protein. APOBEC1 associates with ACF and binds to secondary structures within the mRNA. APOBEC1 then edits the mRNA.
When does RNA editing occur?
Post-transcriptionally.
What is the most common form of RNA editing in eukaryotes?
Editing by adenosine deaminases- ADARs. Changes A to I.
What types of RNA can be edited by ADARs?
mRNA, tRNA, ncRNA and viral RNAs.
How do ADARs associate with mRNA?
Bind to secondary structures that form within the editing site complementary sequence (within intron near editing site).
What codons are most frequently edited by ADARs?
Stop codons.
How many ADAR enzymes have been shown to exist in mammals and which of these edit RNA?
3 ADARs, ADAR 1 and ADAR 2 have been shown to edit RNA.
What genes are often recoded by ADARs?
Neuronal genes- e.g. Editing of Alu repeats found in neuronal genes, editing of LINEs to only transpose within nervous system.
What happens if there is incorrect editing of the serotonin receptor?
Causes depression as cells cannot detect serotonin.
What happens if the Q->R change is not made in the AMPA glutamate receptor?
Channel is constantly open, excess influx of calcium ions into neurones. Associated with major depressive disorder and schizophrenia.
How many recoding events are there in the human nervous system?
Approximately 100.
Why has it been suggested that squids have high amounts of recoding events in their nervous system?
May be because squid are cold blooded and may require extensive editing mechanisms to respond to temperature changes.
