Worm Breeder's Gazette 10(2): 9
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Maturation of some mRNAs in C. elegans involves the acquisition of a 22 nt leader spliced on the 5' end. This is believed to occur through a trans-splicing mechanism. The discovery of trans-splicing on mRNAs that also mature through cis-splicing of introns raises interesting questions. What is the machinery responsible for trans- splicing? Are the same components used in both cis- and trans- splicing? Are there components unique to trans-splicing and to cis- splicing? How are the two mechanisms kept separate from one another to prevent splicing of the leader sequence to an inappropriate 3' splice acceptor site within the body of an mRNA? We began to address some of these questions by examining the spliced leader precursor (preSL). Computer analysis of the preSL sequence generated a striking secondary structure. PreSL RNA is folded so that the 22 nt that comprise the spliced leader form a stem-loop structure with the cleavage point being the first base pair of the stem following the loop. Also striking was a sequence with perfect conservation to the consensus Sm binding site (AA(U3-6)GG) found in snRNAs. This suggested that preSL might be found in an snRNP-like particle. To test the functionality of the Sm binding sequence, we assayed material that was Sm precipitable. Cell free extracts were prepared from sodium hypochlorite treated embryos. PreSL RNA is precipitable by four different patient sera recognizing snRNPs. However, a monoclonal antibody directed against the Sm determinant fails to precipitate preSL RNA as does patient sera specific for U1 or U2 snRNPs. PreSL can also associate with proteins necessary to form an Sm precipitable particle in a heterologous system. Using an in vitro transcribed preSL RNA and either Hela nuclear or cytoplasmic extracts, we have found that the preSL RNA becomes precipitable by the patient sera described above. Using monoclonal antibodies generated against m(3) (2,2,7)G we have shown that the preSL RNA has a trimethyl cap, another feature characteristic of eukaryotic snRNAs. PreSL RNA in C. elegans has features similar to the snRNAs of eukaryotes that participate in cis-splicing. It is bound by Sm proteins and possesses a trimethyl cap structure. PreSL RNA is unique in that it contains the 5' exon to be spliced to the pre-mRNA during the trans-splicing reaction. We suggest that preSL may serve two functions during trans-splicing. First, it provides the 5' exon to be spliced to the pre-mRNA. In addition, preSL may substitute for U1 in the trans-splicing reaction.