text: In standard cis-splicing, the donor splice site in upstream position is required together with an acceptor site located on downstream position on the same pre-RNA molecule.
  By contrast, the SL trans-splicing relies on a 3 acceptor splice site on the outron, and a 5 donor splice site (GU dinucleotide) located on a separate RNA molecule, the SL RNA.
  Moreover, the outron of the premature mRNA contains a branchpoint adenosine — followed by a downstream polypyrimidine tract — which interacts with the intron-like portion of the SL RNA to form a Y branched byproduct, reminiscent of the lasso structure formed during intron splicing. Nuclear machinery then resolves this Y branching structure by trans-splicing the SL RNA sequence to the 3′ trans-splice acceptor site (AG dinucleotide) of the pre-mRNA.
  When outrons are processed, the SL exon is trans-spliced to distinct, unpaired, downstream acceptor sites adjacent to each open reading frame of the polycistronic pre-mRNA, leading to distinct mature capped transcripts.