Trans-spliced leader RNA exists as small nuclear ribonucleoprotein particles in Caenorhabditis elegans

Maturation of some messenger RNAs in the nematode Caenorhabditis elegans involves the acquisition of a 22-base leader at their 5' ends. This 22-base leader, called the spliced leader (SL), is derived from the 5' end of a precursor RNA of 90-100 bases, called spliced leader RNA (SL RNA). SL RNA is transcribed from a 1-kilobase DNA repeat which also encodes the 5S ribosomal RNA. A subset of mRNAs in C. elegans acquire SL from SL RNA by a trans-splicing mechanism. SL behaves as a 5' exon in the trans-splicing reaction. Using antisera against the Sm antigen that is associated with small nuclear ribonucleoprotein particles (snRNPs), we precipitated SL RNA from extracts of C. elegans, indicating that it is bound by the Sm antigen in vivo. SL RNA also possesses the unique trimethylguanosine (m32,2,7G) cap characteristic of most small nuclear RNAs. Therefore, SL RNA is a chimaeric molecule, made up of an snRNA attached to a 5' exon and is a constituent of a snRNP.