lecture 10 Flashcards
RNA degredation
damaged mRNA
incorrectly transcribed/ processed mRNA
control gene expression
casein mRNA
mRNA increases 70 fold by prolactin stimulation
but transcription only increases 2 fold
how?
increase in the half life of the RNA in response to prolactin meaning mRNA is no longer degraded as fast
polyA tail is increased (decreases in the polyA tail decreased the stability of the mRNA)
3’ UTR of RNA binds proteins which aid in this stabilisation
circular mRNA also helps protect the mRNA from turn over as the cap and the polyA tail are the points where nucleases can get in to turn over an mRNA
mRNA wont be circular if we lose the cap or the polyA tail which allows the exonucleases to gain access
phase 1 of degradation
bulk of mRNA is degraded by
exonucleases
decapping enzymes give an end for the exonucleases to access e.g.
DCP1 and DCP2
they remove the cap and open up the 5’ end of the mRNA
deadenylases also give an access point for exonucleases e.g.
Ccr/Not complex
they remove the polyA tail from the 3’ end
endonucleases cut the mRNA in the middle giving two open ends for the exonucleases e.g.
argonaute, Swt1 and Smg6
phase II of degradation
we need exonucleases to degrade from the open ends
they either work from 5’ to 3’ or 3’ to 5’
the exosome
the main 3’ to 5’ exonuclease in the cell
multisubunit complex
degrades to single nucleotides
XRN1
5’ to 3’ exonuclease
deadenylation-dependant decay
as mRNA gets older its polyA tail shortens
if it gets to short then the mRNA is signalled for degradation
we can go straight to exosome or can go from 5’ to 3’ RNA if its signalled to get its cap removed
there are signals that can target is fro premature degradation e.g
- AU-rich elements in 3’ UTR which promote deadenylation gives mRNA shorter half life
- nonsense codon in front of the stop codon, promotes deadenylation
- c-fos major coding determinant, in the mRNA that promotes degradation
- miRNA, bind miRNA recognition site and promote degradation
deadenylation - independent decay
- Rps28B bids to mRNA and it recruits Edc3 which promotes decapping
- endonucleases cut mRNA in the middle which allows the exosome or XRN1 in to degrade the mRNA without removing the cap or polyA tail
NMD (nonsense mediated decay)
mutation can cause a stop codon in the mRNA
cell recognises if the stop codon is in the right place due to the distance between it and the splice site (if the greater than 55 nucleotides then its a NMD target)
exon junction complexes are deposited onto the exons after splicing
these proteins remain on the mRNA until the first round on translation when they are removed by the ribosome and displaces them
when ribosome comes to a normal stop codon all the EJCs are removed but if its a premature stop codon EJCs remain downstream
UPF proteins which are part of the EJC are recruited to interact with the RNA degradation machinery (its turned over)
NMD monitors mRNA - the process is known as surveillance
