mRNA
Pre-mRNA Processing
There are 3 major processing steps of making mRNA: capping, splicing and polyadenylation.
Wissink et al., Nat Rev Genet (2019)
Capping
Spt5 can interact/recruit capping enzyme. Cet1 and Ceg1 bind the Pol II phosphorylated CTD (Ser5-P interacts with Ceg1) and cap nascent RNA (GpppN). Then Abd1 bind to catalyse N7 methylation.
Cet1: 5' RNA triphosphatase
Ceg1: RNA guanylyltransferase
Abd1: Methyl Transferase
Capped RNAs that start with adenosine can be methylated(m6Am). m7Gpppm6Am. In this case, methylase interacts with Ser5-P whrough WW domain. This m6Am might affect RNA stability (hard to remove cap) and translation initiation.
Akichika et al, Science (2019)
Capping is specific to transcripts made by RNA Pol II, transcripts made by RNA Pol I and Pol III are not capped.
Splicing
Two parts of splicing: Branching and Exon Ligation.
Splice site:
5': (exon)...AG-GURAGU...(intron)
3': (intron)...NCAG-N...(exon)
In the exon: ...UACUAAC...
UACUAAC sequence is 20-40 nucleotides upstream of the downstream exon.
Wilkinson et al, Annu Rev Biochem (2020)
