RNA Processing in C. elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 198

Analysis of Ascaris extract has identified two proteins, SL95p and SL30p, which
specifically associate with the SL snRNP and are necessary for in vitro trans-splicing
( Denker et al., 1996 ). Additional examination of their function has shown that SL95p
binds both the SL snRNP and SL30p, and that this complex can associate with SF1/
BBP, effectively bringing the SL snRNP 5 0 splice site into proximity with the pre-
mRNA 3 0 splice site ( Denker et al., 2002 ). Recently, orthologs of SL95p and SL30p
have been identified in C. elegans ( MacMorris et al., 2007 ). SNA-2 and SNA-1 have
also been shown to associate with the SL1 snRNP. RNAi-mediated knockdown of
sna-2 is lethal, as is the deletion of the sna-2 gene. Deletion of sna-1 produces cold-
sensitive sterility. However, the sna-1 mutant animals are still capable of SL1 trans-
splicing, indicating that these proteins may not perform the function ascribed to them
in Ascaris.
Furthermore, some SNA-2 is found bound to a SNA-1 paralog, SUT-1. Deletion of
SUT-1 also leads to cold-sensitive sterility. This heterodimer does not associate with
any SL snRNP, but can associate with one of the family of several recently discovered
nematode-specific snRNPs, called SmY 1-12 ( Jones et al., 2009; MacMorris et al.,
2007 ). These SmY snRNPs are thought to fold into two stem-loops, flanking an Sm-
binding site ( Fig. 3b ). It has been proposed that stem-loop nucleotides of these SmY
snRNPs might base pair with complementary sequences in the stem-loops of the SL
snRNPs. These interactions may aid in recycling Sm proteins from spent SL snRNPs
after trans-splicing ( MacMorris et al., 2007 ).
6. Trans-Splicing in Other Species
Trans-splicing is not restricted to C. elegans. Soon after the discovery of the
SL1 spliced leader and trans-splicing in C. elegans, RNA containing identical
spliced leader sequence was discovered in nematodes from several other genera
( Bektesh et al., 1988 ). Indeed, further analysis showed that closely related
variants of this spliced leader can be found in all but one of the five major
clades of the nematode phylum ( Guiliano and Blaxter, 2006; Pettitt et al., 2008 ).
The high degree of conservation observed throughout most of the phylum indi-
cates that trans-splicing in nematodes probably arose in a common ancestor.
Interestingly, however, a representative of the most basal nematode clade has
trans-splicing, but its multiple spliced leaders appear unrelated to SL1 of the
other four clades.
SL-type trans-splicing has also been observed in several other phyla, including
platyhelminthes ( Davis, 1997 ), rotifers ( Pouchkina-Stantcheva and Tunnacliffe,
2005 ), cnidaria ( Stover and Steele, 2001 ), the primative chordates Ciona intestinalis
( Vandenberghe et al., 2001 ) and oikopleura ( Ganot et al., 2004 ), and trypanosomes
( Sutton and Boothroyd, 1986 ), as well as several others including some but not all
arthropods ( Douris et al., 2010 ). Although all of these organisms attach a spliced
leader to their RNA, the leader sequences, as well as the SL RNAs that donate them,
are unrelated to those found in nematodes. Additionally, these widely divergent
phyla are evolutionarily separated from each other by phyla in which trans-splicing