New components of the spliced leader RNP required for nematode trans-splicing

Pre-messenger-RNA maturation in nematodes and in several other lower eukaryotic phyla involves spliced leader (SL) addition trans -splicing 1 , 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 ( M r 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 M r 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.