Polycistronic expression and RNA-binding specificity of the C. elegans homologue of the spliceosome-associated protein SAP49.

Splicing of mRNA precursors (pre-mRNAs) occurs in a multimolecular complex, termed spliceosome, which is comprised of pre-mRNA, small nuclear ribonucleoprotein particles (snRNPs), and other protein factors including spliceosome-associated proteins (SAPs). SAP49 is thought to be a subunit of the essential splicing factor SF3b and is involved in U2 snRNP function in mammalian cells. We have isolated a Caenorhabditis elegans cDNA encoding an RNA-binding protein with two RNA recognition motifs (RRMs) which shows extensive similarity to the human SAP49. The primary transcript for this C. elegans SAP49 homologue (cSAP49) seems to contain at least two additional cistrons and can be processed into three different mature mRNAs by trans-splicing. The cSAP49 mRNA, like other mRNAs in the same polycistronic unit, is expressed in most of the developmental stages, consistent with its putative essential function for mRNA splicing. By means of an in vitro RNA selection system, we demonstrate that cSAP49 possesses specific RNA-binding activity which resides in its second RRM.