The C. elegans trans-spliced leader RNA is bound to Sm and has a trimethylguanosine cap.

mRNA splicing in C. elegans is unusual: most introns are very short (approximately 50 bases), and many mRNAs receive a leader by trans-splicing. The donor in trans-splicing is a 94 nucleotide molecule, termed the leader RNA, that contributes its 5' 22 nucleotides to a variety of mRNAs. We show here that C. elegans has the usual snRNAs, which presumably catalyze the splicing reactions. As expected, they are bound to the Sm antigen and have 2,2,7-methylguanosine caps. Remarkably, the trans-spliced leader RNA is also Sm-associated and has this special cap. Hence, a molecule discovered as a substate of splicing has properties of molecules heretofore known only to facilitate splicing of other RNAs. Mature mRNAs that have received the leader evidently lack 2,2,7-methylguanosine caps, suggesting that these caps are removed or altered during processing.