142
Alternatively Spliced Genes
two reaction steps, the stereoselectivity of
the transesteriFcation reactions and the
difference in the metal-ion dependence
support
the
existence
of
two
reaction
sites.
Accumulating evidence suggests that
pre-mRNA splicing may be RNA-catalyzed
with U6snRNA at the center for cataly-
sis and protein factors including PRP8
as
components
of
the
catalytic
core.
U6snRNA plays a critical role in catal-
ysis and in interacting with pre-mRNA
and other spliceosomal snRNAs. In yeast,
U6snRNA
contributes
to
spliceosomal
catalysis by coordinating catalytic metals.
Precise catalytic mechanisms of mam-
malian pre-mRNA splicing remain to be
elucidated.
2
Alternative pre-mRNA Splicing
2.1
Alternative Splicing and its Role in
Regulating Gene Activities and Generating
Genetic Diversity
The vast majority of vertebrate genes con-
tains at least one intron. It is estimated
that more than 50% of human genes un-
dergo
alternative splicing
, the process that
generates distinct splicing products from
the same pre-mRNA transcript by using
different splice sites (±ig. 5). Alternative
splicing is an important mechanism for
regulating gene activities in eukaryotic
species. Alternative splicing events can
Mutually exclusive exons
Exon inclusion or skipping
Alternative 3
splice site
Alternative 3
splicing coupled with
alternative polyadenylation
Pairwise mutually exclusive
Alternative 5
splice site
Stop
p
(A)
n
p
(A)
n
Alternative splicing coupled
with differential promoter usage
Intron retention or removal
Trans-splicing
Fig. 5
Diverse patterns of alternative splicing. Alternatively spliced
regions or exons are illustrated as shaded boxes.
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