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cap. Finally, the 3
0
end of the snRNA is trimmed, and the molecule is transported
back into the nucleus as a mature snRNP. It is then processed with other snRNPs in
the Cajal bodies, from which it can be released to participate in splicing.
2. The SL snRNP
Evidence indicates that the SL RNA molecules also mature by this process. The
SL1 RNA (the first C. elegans SL RNA to be characterized) is transcribed by RNA
polymerase II from a gene cluster (rrs-1) on chromosome V, containing
110
tandem repeats of this gene and the gene encoding 5S rRNA in the opposite orien-
tation (
Ferguson et al., 1996; Krause and Hirsh, 1987
). The SL1 RNA primary
transcript is 105 nt long. Early characterization of the SL1 RNA locus showed that
it contains the same distal and proximal transcriptional promoter elements seen at
the U1, U2, U4, and U5 snRNA loci (
Thomas et al., 1990
). Although SL1 RNA
nuclear export and Sm assembly have never been examined, C. elegans does have an
ortholog of the SMN gene (
Miguel-Aliaga et al., 1999
). Furthermore SL1 RNA
contains a TMG cap and is bound by Sm proteins (
Bruzik et al., 1988; Thomas et al.,
1988; Van Doren and Hirsh, 1988
), features also found in most spliceosomal
snRNPs. Finally, Sm-containing SL1 snRNPs have been observed in the nucleus
(MacMorris and Blumenthal, unpublished observations). These similarities sug-
gested that the SL1 snRNP could play a role in trans-splicing analogous to the roles
played by the spliceosomal U snRNPs in cis-splicing (
Bruzik et al., 1988
). A
subsequent in vitro study employing extract made from the nematode Ascaris lum-
bricoides (which also uses an SL snRNP to trans-splice its pre-mRNA) showed that
synthetic SL RNAworked for trans-splicing following extract-mediated addition of
the TMG cap and the Sm proteins (
Maroney et al., 1990
).
In its mature form, an SL snRNP is thought to be composed of three stem-loops
and an Sm-binding domain (
Bruzik et al., 1988
)(
Fig. 3a
). The first stem-loop
contains the leader sequence, which is spliced onto an RNA molecule during
trans-splicing. The nucleotide sequence at this splice site (AG/GUAAAC) closely
mirrors the 5
0
splice site consensus sequence in introns (AG/GURAGU). Several
studies have examined the effects of mutations incorporated into this leader
sequence. Initial in vitro studies of trans-splicing in Ascaris extract indicated that
the leader sequence could be extensively modified without abolishing trans-splicing
(
Maroney et al., 1991
). Later, it was discovered that the expression of a synthetic SL1
RNA transgene could rescue the embryonic lethality induced by deletion of the
native rrs-1 SL RNA gene locus (
Ferguson et al., 1996
). Most deletions engineered
into the leader sequence of this SL1 RNA transgene resulted in constructs incapable
of rescuing this lethality (
Ferguson et al., 1996; Xie and Hirsh, 1998
). The severity of
these deletions was probably due to removal of a necessary promoter element from
the leader sequence (
Hannon et al., 1990
), because no transgenic SL1 RNA was
transcribed from these constructs in vivo. Very small deletions or substitutions just
upstream of the splice site were less detrimental. When SL1 RNA transcription was
driven by the U2 promoter, essential features of the spliced leader could be more
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