[2] The outron is an intron-like sequence possessing similar characteristics such as the G+C content[3] and a splice acceptor site that is the signal for trans-splicing.
In eukaryotes such as euglenozoans, dinoflagellates, sponges, nematodes, cnidarians, ctenophores, flatworms, crustaceans, chaetognaths, rotifers, and tunicates, the length of spliced leader (SL) outrons range from 30 to 102 nucleotides (nt), with the SL exon length ranging from 16 to 51 nt, and the full SL RNA length ranging from 46 to 141 nt.
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.
[3] 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.
[2] 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.