mRNA 5 '-leader trans-splicing in the chordates

We report the discovery of mRNA 5'-leader trans-splicing (SL trans-splicing ) in the chordates. In the ascidian protochordate Ciona intestinalis, the m RNAs of at least seven genes undergo trans-splicing of a 16-nucleotide 5'-l eader apparently derived from a 46-nucleotide RNA that shares features with previously characterized splice donor SL RNAs. SL trans-splicing was known previously to occur in several protist and metazoan phyla, however, this i s the first report of SL trans-splicing within the deuterostome division of the metazoa. SL trans-splicing is not known to occur in the vertebrates. H owever, because ascidians are primitive chordates related to vertebrate anc estors, our findings raise the possibility of ancestral SL trans-splicing i n the vertebrate lineage.