Biology Reference
In-Depth Information
protein, factor VII. Three positive regulatory elements (
FXP3
,
FXP2,
and
FXP1
)
have been characterized upstream of the
F10
gene.
FXP1
and
FXP2
act in an ori-
entation- and position-independent fashion whilst
FXP1
and
FXP3
are responsi-
ble for directing liver-specific gene expression. The putative boundary element
just upstream of the
FXP3
sequence is thought to prevent transcriptional activa-
tion of the
F7
gene by the
F10
gene enhancers.
Sequence motifs involved in mRNA splicing and processing.
One of the
characteristics of eukaryotic genes that distinguishes them from their prokary-
otic counterparts is the production of large pre-mRNAs which contain interven-
ing non-coding sequences (introns) that are removed by a highly accurate
cleavage/ligation reaction known as splicing before the mRNA is transported to
the cytoplasm for translation (reviewed by Green, 1986; Padgett
et al
., 1986).
Splicing not only permits the removal of introns from the primary transcript but
also allows the generation of different mRNAs from the same gene by
alternative
splicing
, an important mechanism for tissue-specific or developmental regulation
of gene expression and a very economical means of generating biological diver-
sity (Nadal-Ginard
et al
., 1987; Norton, 1994). Alternative splicing may be regu-
lated by variation in the intracellular levels of antagonistic splicing factors
(Caceres
et al
., 1994).
The splicing of a eukaryotic mRNA appears to occur as a two-stage process. In
the case of a simple two exon gene, the pre-mRNA is first cleaved at the 5
(donor)
splice site to generate two splicing intermediates, an exon-containing RNA
species and a lariat RNA species containing the second exon plus intervening
intron. Cleavage at the 3
(acceptor) splice site and ligation of the exons then
occurs resulting in the excision of the intervening intron in the form of a lariat.
Splicing efficiency is critically dependent upon the accuracy of cleavage and
rejoining. This accuracy appears to be determined, at least in part, by the virtually
invariant GT and AG dinucleotides present at the 5
exon/intron junctions
respectively. However, more extensive consensus sequences spanning the 5
and 3
and
3
splice junctions are evident (Mount, 1982; Padgett
et al
., 1986) and the coding
sequence flanking intron junctions exhibits some degree of conservation (Long
et
al
., 1998). More recently, Zhang (1998) has proposed AG
GTRAGT as a consen-
sus sequence for donor splice sites and (Y)
n
NCAG
G (where
n
has a mean of
nine) as a consensus for acceptor splice sites. Stephens and Schneider (1992)
noted the similarity between the donor and acceptor sites and suggested that these
junctions may have been derived from a common 'proto-splice site' ancestor
(
Figure 1.5
). During evolution, the emphasis of the sequence information at each
site has shifted to the intronic side of the junction (
Figure 1.5
).
A few human nuclear genes possess introns with noncanonical terminal dinu-
cleotide sequences (e.g. 'AT-AC introns'; Tarn and Steitz, 1997); these include the
transcription factor
E2F1
(20q11.2) gene, the cartilage matrix protein (
MATN1
;
1p25) gene, the Hermansky-Pudlak syndrome (
HPS
; 10q23) gene and the paralo-
gous sodium channel
-subunit genes (
SCN4A
, 17q23.1-q25.3;
SCN5A
, 3p24-p21;
SCN8A
, 12q13). The removal of AT-AC introns requires two low-abundance
snRNAs, U4atac and U6atac, not found in the major spliceosome (Tarn and Steitz,
1996). There is now good evidence for the existence of two distinct splicing systems
