Pre-messenger-RNA maturation in nematodes and in several other lower eukaryotic phyla involves spliced leader (SL) addition trans-splicing1,2. In this unusual RNA processing reaction, a short common 5′ exon, the SL, is affixed to the 5′-most exon of multiple pre-mRNAs. The nematode SL is derived from a trans-splicing-specific ∼100-nucleotide RNA (SL RNA) that bears striking similarities to the cis-spliceosomal U small nuclear RNAs U1, U2, U4 and U5 (refs 3, 4); for example, the SL RNA functions only if it is assembled into an Sm small nuclear ribonucleoprotein (snRNP)5. Here we have purified and characterized the SL RNP and show that it contains two proteins (relative molecular masses 175,000 and 30,000 (Mr 175K and 30K)) in addition to core Sm proteins. Immunodepletion and reconstitution with recombinant proteins demonstrates that both proteins are essential for SL trans-splicing; however, neither protein is required either for conventional cis-splicing or for bimolecular (trans-) splicing of fragmented cis constructs. The Mr 175K and 30K SL RNP proteins are the first factors identified that are involved uniquely in SL trans-splicing. Several lines of evidence indicate that the SL RNP proteins function by participating in a trans-splicing specific network of protein–protein interactions analogous to the U1 snRNP–SF1/BBP–U2AF complex that comprises the cross-intron bridge in cis-splicing.
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We thank J. Bruzik, J. A. Wise and M. Caprara for comments on the manuscript, and A. M. Micenmacher for help with figures and manuscript preparation. The work was supported by grants from the N.I.H.
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