Alternative splicing

1
Alternative splicing With a few genes,
alternative splicing generates more than one
mRNA from the primary transcript.
Exons, or parts of exons, may be skipped.
2
In SV40 virus, T antigen protein functions in
initiation of viral DNA replication. Large and
small forms of T antigen are made by alternative
splicing.
3
Proteins bias the use of alternative splice sites.
Small t is made in host cells with high
concentrations of alternative splicing factor
ASF protein. This is a SER-ARG rich SR protein.
It favours the use of the downstream 5 splice
site.
In general, it is not well understood how
alternative splicing is controlled.
4
Group I and group II introns are found in genes
within organelles.
Group I introns are more common. They also occur
in nuclear genes of lower eukaryotes.
Group I and group II introns are not related,
but both can autosplice.
Secondary structure in the intron RNA forms the
catalytic site.
5
Group II introns
-have splice sites that resemble nuclear splice
sites
-form a lariat
-have a characteristic secondary structure.
6
This structure resembles the complex of U2 and U6
snRNAs with the branch site in spliceosome-catalys
ed RNA splicing.
Nuclear splicing probably evolved from group II
intron splicing.
snRNAs of the spliceosome compensate for lack of
sequence information in nuclear introns.
7
Group I introns
are found in
1. Nuclear rRNA genes of lower eukaryotes e. g.
the protozoan Tetrahymena thermophila.
2. Genes of fungal mitochondria.
3. A few genes in phage T4 and bacteria.
8
Group I introns can also excise themselves from
RNA transcripts.
The splicing reaction only requires precursor
RNA, cations and a guanine nucleotide cofactor
(GTP, GDP, GMP or G).
The reaction is similar to group II intron
splicing except that
1. Free G is used to break the exon-intron
junction.
2. No lariat is formed.
3. The excised intron undergoes further
autocatalytic reactions.
9
Group I introns fold into a characteristic
structure.
This includes a G-binding site
and an IGS that pairs with the end of the 5 exon.
The 3 hydroxyl group of the free G breaks the
exon-intron junction.
10
The exposed exon then attacks the 3 splice site.
11
The excised intron undergoes further
self-catalysed reactions.
12
The terminal guanosine-414 enters the G-binding
site.
The 3 hydroxyl attacks the phosphate on the 5
side of A16.