RNA Processing in C. elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 195

Because there is no contiguous upstream 5 0 splice site, U2 cannot associate with a
bound U1 snRNP. Instead, it interacts with the 5 0 splice site on the SL snRNP to
initiate spliceosome formation and trans-splicing ( Maroney et al., 2000 ).
The 5 0 splice site is located in the first stem-loop of the SL RNA and is conse-
quently base-paired in a manner similar to the pairing that occurs between the 5 0
splice site of an intron and the U1 snRNP during cis-splicing. In this case, the leader
sequence of the SL1 snRNP was proposed to act as a chimeric molecule, with the
upstream exon capable of interacting with downstream sequence in the same RNA
molecule to initiate its own splicing ( Bruzik et al., 1988 ). However, it was subse-
quently shown that this base pairing is not required for trans-splicing

in vitro

( Maroney et al., 1991 ).
Trans-splicing is also dependent on the U4-U5-U6 tri-snRNP ( Maroney et al.,
1996, 2000 ). This complex interacts with the SL snRNP and the U2 snRNP at the 3 0
splice site to form an active spliceosome, although the mechanism of this process is
poorly understood. Presumably the trans-splicing reaction occurs analogously to the
cis-splicing reaction, so a branch-point adenosine upstream of the 3 0 splice site on
the pre-mRNA attacks the 5 0 splice site of SL snRNP, forming a Y-branched molecule
and freeing the leader sequence ( Fig. 4 ). Such a Y-branched structure has been

Fig. 4 Products of the trans-splicing reaction. The spliced leader of the SL1 snRNP is in red. Sequences
found in the outron of the pre-mRNA (from rsp-3, in this case) are in blue. During the trans-splicing
reaction, an upstream adenosine in the outron attacks the 5 0 splice site of the SL1 snRNP. The 3 0 end of the
SL1 spliced leader then attacks the 3 0 splice site on the pre-mRNA to form a capped, trans-spliced RNA
molecule. The outron is freed from the pre-mRNA but remains attached to the remainder of the snRNP by
the 2 0 -5 0 phosphodiester bond at its branch-point adenosine, forming a Y-branched molecule. (For interpre-
tation of the references to color in this figure legend, the reader is referred to the web version of this topic.)