The major goal of this research is to understand aspects of the molecular and biological significance of trans-splicing through an evolutionary comparison and analysis of trans-splicing in lower invertebrates, with particular attention to parasitic flatworms of medical and veterinary importance. The primary research emphasis will use several strategies to identify and characterize spliced leader RNAs in parasitic flatworms to determine the distribution, organization, and characteristics of spliced leader RNAs and genes. Such an evolutionary comparison will allow Dr. Davis to identify and focus on the important, conserved aspects of trans-splicing and will provide information on the evolution of spliced leader RNAs, their genes, and the role of spliced leaders in the evolution of invertebrate gene expression. These studies are also likely to contribute to our understanding of the significance of spliced leader addition and could provide insight into the evolution of RNA processing in general. The applicant has shown that a subset of mRNAs in Schistosoma mansoni contain a 36-nucleotide spliced leader (SL) at their 5' termini. The schistosome spliced leader RNA (SL RNA) is unique, exhibiting no sequence identity with trypanosome or nematode SL RNAs, and contains only two computer-predicted stem loops compared to three observed in trypanosome and nematode SL RNAs. Both cis- and trans-splicing occur within the same transcripts, and the applicant and his co-workers have shown that an internal exon can act as a splice acceptor for either cis- or trans-splicing in schistosomes. The unique properties of spliced leader RNA in schistosomes, as well as, the similarities with other SL RNAs, may provide insights into the process of trans-splicing that may not be apparent in other systems. Furthermore, aspects of trans-splicing may not be identical among different invertebrate groups. Consequently, a second area of research, although of lesser emphasis, will be to identify and characterize selected schistosome mRNAs and genes which undergo trans-splicing with respect to organization, expression, and processing of these genes. Such studies may provide insight into the function and biological significance of the SL in the expression of these genes as well as the determinants, mechanism and regulation of trans-splicing in schistosomes. This research program will address fundamental questions concerning gene expression, RNA processing, and the evolution of RNA processing and gene expression in invertebrates, particularly in schistosomes and other parasitic flatworms. Information from these studies will further our understanding of the molecular basis of parasitism and may lead to the development of novel therapeutic strategies against several parasites of medical and veterinary importance.