Characteristics of trans-splicing in Schistosoma mansoni were examined to explore the significance and determinants of spliced leader (SL) addition in flatworms. Only a small subset of mRNAs acquire the SL. Analysis of 30 trans-spliced mRNAs and four genes revealed no discernable patterns or common characteristics in the genes, mRNAs, or their encoded proteins that might explain the functional significance of SL addition. While the mRNA encoding the glycolytic enzyme enolase is trans-spliced, mRNAs encoding four other glycolytic enzymes are not, indicating trans-splicing is not prevalent throughout this metabolic pathway. Although the 3 end of flatworm SLs contribute an AUG to mRNAs, the SL AUG does not typically serve to provide a methionine for translation initiation of reading frames in recipient mRNAs. SL RNA expression exhibits no apparent sex, tissue, or cell specificity. Trans-spliced genes undergo both cis-and trans-splicing, and the sequence contexts for these respective acceptor sites are very similar. These results suggest trans-splicing in flatworms is most likely associated either with some property conferred on recipient mRNAs by SL addition or related to some characteristic of the primary transcripts or transcription of trans-spliced genes.Trans-splicing is an RNA processing event that accurately joins sequences derived from independently transcribed RNAs. In one form of trans-splicing, a leader sequence (the spliced leader, SL) 1 is donated from the 5Ј end of a small, non-polyadenylated RNA (the spliced leader RNA, SL RNA) to pre-mRNAs to form the 5Ј-terminal exon of mature mRNAs (for recent reviews see Refs. 1-6). This form of RNA maturation was first described in trypanosomes (7,8) and subsequently in other kinetoplastida and the flagellated protozoan Euglena (9). The identification of trans-splicing in two divergent invertebrate phyla, first in nematodes (10) and then in flatworms (11), suggests that this particular form of RNA processing may be an important form of gene expression common in early metazoa.The general distribution of trans-splicing and its origin in metazoa is currently not known. Furthermore, both the origin of early metazoan groups and the phylogenetic relationships between flatworms, nematodes, and other early invertebrates have been difficult to delineate (12, 13). Trans-splicing may have arisen independently in several invertebrate lineages (6) and, if true, the characteristics and functional significance of spliced leader addition might also be different in diverse metazoan groups. Trans-splicing is of particular interest in flatworms (Phylum Platyhelminthes) as these metazoa may represent the earliest bilateral animals, and one possible evolutionary tree places a flatworm-like ancestor as the progenitor of a number of other early invertebrate groups (12, 13). We have recently shown that trans-splicing is present in diverse trematode flatworms and in a predominantly free-living group generally considered to represent primitive flatworms (14). 2 This suggests that spliced ...

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