40
2 Nucleic Acids and Nuclear Proteins
from the Kinetoplastida transcripts [10, 341,
493].
A spliced leader of 22 nt has also been found in
10-20 % of the mRNAs of various free-living and
parasitic nematode species. As in the Kinetoplastida, the SL here also stems from an SL
RNA coded for by a family of more than 100
genes. Anyone pre-mRNA can be spliced either
cis- or trans-. Trans-splicing requires a particular
type of spliceosome. In contrast to the Kinetoplastida, the SL sequences of all nematode species are identical. However, in Caenorhabditis
elegans there are two SLs that differ in 7 of the
22 nt and are distributed gene-specifically
amongst the mRNAs. Thus, for example, the
mRNAs of three out of four glyceraldehyde-3phosphate dehydrogenase (GAPDH) genes, and
many other mRNAs, carry SL1, whereas the
fourth GAPDH gene carries SL2. Trans-splicing
has also been found in trematodes [10, 101,
117, 161,205].
2.7.4 Production of Multiple 'ftanscripts
ofa Gene
There are many exceptions to the rule of "one
gene, one polypeptide". Alternative initiation,
polyadenylation or splicing signals lead to the
formation of multiple mRNAs on a gene, and
possibly also to the production of multiple proteins; these alternatives may be regulated in a
tissue- or stage-specific manner. Thus, in addition
to the diversification of duplicate genes and posttranslational protein modification, this gives a
further mechanism for the creation of variant proteins which can be adapted to the specific requirements of different cells or developmental stages.
Alternative splicing has now been shown for
many pre-mRNAs. In many cases, the alternatively spliced exons stem from the duplication of
a common precursor; this mechanism for producing multiple proteins apparently arose early in the
evolution of the eukaryotes. As yet, very little is
known about the regulation mechanisms that
determine the splicing processes preferred by different tissues or developmental stages [403].
Examples are known for all the possibilities for
alternative splicing. The alternatives may depend
upon the retention in the mature mRNA of a particular exon (Fig. 2.11A), as in the tropomyosin I
gene of Drosophila or the troponin T gene of the
chicken. In many cases, more than one exon is
involved in alternative splicing. The random combination of n exons produces 2D mRNAs. Four
variants of the basic myelin protein of the mouse
are known to arise from the random combination
of two exons (Fig. 2.12c). In the troponin T gene
of the rat, each of the five exons 4-8 can be
excised during mRNA maturation, producing 32
tissue-specific and development-dependent isoforms. In many cases, particular exons may
alternate but they never lie in tandem
(Fig. 2.11B). This is true for exons 16 and 17 of
the 18 exons in the rat troponin T gene; in this
way, the number of possible isoforms is increased
to 64 [46]. The different C-terminal sequences of
the membrane-bound and secreted immunoglobulins arise because the mRNAs are endowed
with different exons, following differential splicing of the 3' end; each has its own poly(A) signal
(see Fig. 6.4, p. 224).
Facultative splicing signals within an exon
result in differences between mature mRNAs due
to the presence or absence of parts of exons
(Fig. 2.11 C and D); for example, the fibronectin
variants of the rat are produced in this way
(Fig.2.12a). If an intron is not excised
(Fig. 2.11 E), then the encoded protein gains an
extra domain. This is true, for example, for the yfibrinogen molecules of the rat, 10 % of which
contain an amino acid sequence coded for by
B ."'.' ~ '"---./'.
',/
...................... "",' ' ' ' .'
o .'~'.~'.'. ' .' . ' .
' ... '
",,'
.... /
........
F TATA
_---_
~
r m ------ A ----- . ' ' .
TATA
-".-,'.'
E .:~:.f).:~:.
G
,',
_----- --.-- -_ AATtAA
~ , , - ' . , ' t
•
- AATAAA
Fig. 2.11 A-G. Different mRNAs and proteins can be produced from the same gene
by alternative splicing [46]. A Particular
exons are spliced or excised; B alternating
exons are excluded from the finished
mRNA; C exons include several acceptor
sequences; D exons include several donor
sequences; E introns are not excised; F several different promoters are available;
G several different poly(A) signals are
available
2 Nucleic Acids and Nuclear Proteins
from the Kinetoplastida transcripts [10, 341,
493].
A spliced leader of 22 nt has also been found in
10-20 % of the mRNAs of various free-living and
parasitic nematode species. As in the Kinetoplastida, the SL here also stems from an SL
RNA coded for by a family of more than 100
genes. Anyone pre-mRNA can be spliced either
cis- or trans-. Trans-splicing requires a particular
type of spliceosome. In contrast to the Kinetoplastida, the SL sequences of all nematode species are identical. However, in Caenorhabditis
elegans there are two SLs that differ in 7 of the
22 nt and are distributed gene-specifically
amongst the mRNAs. Thus, for example, the
mRNAs of three out of four glyceraldehyde-3phosphate dehydrogenase (GAPDH) genes, and
many other mRNAs, carry SL1, whereas the
fourth GAPDH gene carries SL2. Trans-splicing
has also been found in trematodes [10, 101,
117, 161,205].
2.7.4 Production of Multiple 'ftanscripts
ofa Gene
There are many exceptions to the rule of "one
gene, one polypeptide". Alternative initiation,
polyadenylation or splicing signals lead to the
formation of multiple mRNAs on a gene, and
possibly also to the production of multiple proteins; these alternatives may be regulated in a
tissue- or stage-specific manner. Thus, in addition
to the diversification of duplicate genes and posttranslational protein modification, this gives a
further mechanism for the creation of variant proteins which can be adapted to the specific requirements of different cells or developmental stages.
Alternative splicing has now been shown for
many pre-mRNAs. In many cases, the alternatively spliced exons stem from the duplication of
a common precursor; this mechanism for producing multiple proteins apparently arose early in the
evolution of the eukaryotes. As yet, very little is
known about the regulation mechanisms that
determine the splicing processes preferred by different tissues or developmental stages [403].
Examples are known for all the possibilities for
alternative splicing. The alternatives may depend
upon the retention in the mature mRNA of a particular exon (Fig. 2.11A), as in the tropomyosin I
gene of Drosophila or the troponin T gene of the
chicken. In many cases, more than one exon is
involved in alternative splicing. The random combination of n exons produces 2D mRNAs. Four
variants of the basic myelin protein of the mouse
are known to arise from the random combination
of two exons (Fig. 2.12c). In the troponin T gene
of the rat, each of the five exons 4-8 can be
excised during mRNA maturation, producing 32
tissue-specific and development-dependent isoforms. In many cases, particular exons may
alternate but they never lie in tandem
(Fig. 2.11B). This is true for exons 16 and 17 of
the 18 exons in the rat troponin T gene; in this
way, the number of possible isoforms is increased
to 64 [46]. The different C-terminal sequences of
the membrane-bound and secreted immunoglobulins arise because the mRNAs are endowed
with different exons, following differential splicing of the 3' end; each has its own poly(A) signal
(see Fig. 6.4, p. 224).
Facultative splicing signals within an exon
result in differences between mature mRNAs due
to the presence or absence of parts of exons
(Fig. 2.11 C and D); for example, the fibronectin
variants of the rat are produced in this way
(Fig.2.12a). If an intron is not excised
(Fig. 2.11 E), then the encoded protein gains an
extra domain. This is true, for example, for the yfibrinogen molecules of the rat, 10 % of which
contain an amino acid sequence coded for by
B ."'.' ~ '"---./'.
',/
...................... "",' ' ' ' .'
o .'~'.~'.'. ' .' . ' .
' ... '
",,'
.... /
........
F TATA
_---_
~
r m ------ A ----- . ' ' .
TATA
-".-,'.'
E .:~:.f).:~:.
G
,',
_----- --.-- -_ AATtAA
~ , , - ' . , ' t
•
- AATAAA
Fig. 2.11 A-G. Different mRNAs and proteins can be produced from the same gene
by alternative splicing [46]. A Particular
exons are spliced or excised; B alternating
exons are excluded from the finished
mRNA; C exons include several acceptor
sequences; D exons include several donor
sequences; E introns are not excised; F several different promoters are available;
G several different poly(A) signals are
available