RNA trans-splicing in Fasciola hepatica. Identification of a spliced leader (SL) RNA and SL sequences on mRNAs.

We have identified a new spliced leader (SL) in Fasciola hepatica by characterizing the 5'-terminal sequences of its enolase mRNA, an mRNA also trans-spliced in the flatworm Schistosoma mansoni (Rajkovic, A., Davis, R.E., Simonsen, J.N., and Rottman, F.M. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 8879-8883). This 37-nucleotide spliced leader is at the 5' ends of multiple Fasciola mRNAs and is likely to be derived from the 5' terminus of a nonpolyadenylated, 108-nucleotide RNA with a trimethylguanosine cap. The SL RNA gene is present in approximately 100 copies within a 1.1-kilobase genomic tandem repeat. Secondary structure predictions indicate that the Fasciola SL RNA contains three stem loops in contrast to two previously observed in S. mansoni. Fasciola and S. mansoni SLs are likely to be evolutionarily related although their sequence identity is only 65%. In contrast with nematodes, absolute conservation of SL sequences and secondary structure does not occur in trematodes. A spliced leader in Fasciola indicates that trans-splicing is likely to be a common feature in other trematodes and perhaps other flatworms.