lec 10 Flashcards
(20 cards)
RNA processing and regulation
- 5’ cap
- poly A tail
- alternative splicing
what does a transcript have to undergo to be considered a “matured mRNA” and be exported from the nucleus?
- addition of 5’ cap
- addition of 3’ poly a tail
- splicing
co/post-transcriptional RNA mod
can affect the amount and type of protein expressed
- 5’ capping and 3’ polyadenylation determine how pre-mRNA/mRNA will be handled
- creation of different mRNAs through differential splicing of the same nRNA allows cells to generate different proteins from the same pre-mRNA transcript
are RNA processing proteins attached to the CTD tail of the RNA Pol II?
yes
formation of the 5’ cap
prokaryotic mRNA = unmod
eukaryotic mRNA = capped and polyadenylated
- by enzymes on C terminal domain of RNA pol II
1st mod
- added to pre-mRNA by 3 different enzymes right after it begins to be translated from the DNA
func of 5’ cap
all mediated via interactions with the cap binding complex (proof of completion)
1. req for splicing of pre-mRNA (1st intron)
2. req for nuclear export of mRNA
3. promotes mRNA translation in cytsol
4. prevents RNA degradation by exonucleases
5’-5’ (reverse linkage)
creates a structure that is not recognized by 5’-3’ exonucleases
- protects RNA transcript from degradation in nucleus
- 5’ cap binds to CBC
> helps RNA to be properly spliced and exported in translation of mRNAs in cytosol
addition of poly-A tail at 3’ end of transcript
enzymes involved
1. Cstf: cleavage stimulation factor
2. CPSF: cleavage and polyadenylation specificity factor
3. PAP: poly-A-polymer
cleavage factors recognize consensus req in mRNA transcripts
func of 3’ poly-A tail
nucleotide seq in pre-mRNA = binding sites for enzymes that form 3’ poly-A tail of eukaryotic mRNA
binding sites = encoded in genome, poly-A tail is not!
proteins recognize the binding sites to mediate polyadenylation after pre-mRNA is formed
*enzymes hitch a ride on RNA Pol II CTD
cleavage factor enzymes
different CF recognizes different consensus cleavage seq on mRNA
- mRNA could have different sites of 3’ end cleavage and poly-A tail addition
alternative poly-A addition sites
many genes have alternative poly-A sites which can
- change the length of 3’ UTR
- can alter translational regulation - change the length of the last exon
- alter protein seq
choice of poly A sites can be regulated by external sites
synthesized poly-A tail
poly-A binding proteins bind to it
- determines the final length of poly-A tail
as soon as mRNA is exported to the cytosol
- 3’ end = vulnerable to degradation by 3’-5’ exonucleases
poly-A shortening (degradation) acts as a “timer” that counts down lifetime of each mRNA
once poly-A tail reaches to ~ 25 nucleotides
- 5’ cap is removed
- transcript is rapidly degraded
poly-A tail func
- causes transcription to terminate
- promotes transport out of nucleus after being bound by poly-A binding proteins
- can regulate initiation of translation
- facilitates circularization of mRNA - helps control total amount of translation
- decaying degradation by exonucleases
RNA splicing
removes introns and joins exons
- makes mRNA with continuous coding seq
splice sites: consensus seq at 5’ and 3’ ends of introns/exons
spliceosome
machinery that catalyzes pre-mRNA splicing
- deposits exon junction complexes (EJC) at splice junctions
> req for nuclear export
> In nonsense-mediated decay in the cytosol
protein/snRNA complex that binds to “splice sites” at 5’ and 3’ ends of introns/exons and mediates splicing
- recruited/blocked by recognition factors
different combo of recognition factors in different cells
- different mRNA transcripts
Alternative RNA splicing
sites can make different gene products
- splicing isoforms with the help of spliceosomes
mechanism that allow for increased varaibility of genes without increasing teh number of genes
down syndrome cell adhesion molecule
example of alternative splicing of nRNA
splicing enhancers
RNA enhancers that bind recognition factors
recognition factors
proteins that recruit/block spliceosome formation
many of recognition factors that bind splicing enhancers are
cell-type-specific
