Abstract
Maturation of a fraction of mRNAs in nematodes involves the acquisition of a common 5' terminal spliced leader sequence derived from a nonpolyadenylylated spliced leader RNA by trans splicing. We have developed a cell-free system prepared from Ascaris lumbricoides embryos that accurately and efficiently synthesized the spliced leader RNA of A. lumbricoides. Transcription of the spliced leader RNA was catalyzed by RNA polymerase II, and the majority of the spliced leader RNAs synthesized in vitro possessed a trimethylguanosine cap structure identical to that found on in vivo-synthesized spliced leader RNA.