A yeast intronic splicing enhancer and Nam8p ave required for Mer1p-activated splicing

Three introns whose splicing is activated during meiosis in S. cerevisiae c ontain a Mer1p-dependent splicing enhancer. The enhancer can impose Mer1p-a ctivated splicing upon the constitutively spliced actin intron provided the basal splicing efficiency of actin is first reduced. Of several nonessenti al splicing factors tested, only the U1 snRNP protein Nam8p is indispensabl e for Mer1p-activated splicing. We show that Mer1p associates with the U1 s nRNP even in the absence of Nam8p or pre-mRNA. This work defines a yeast sp licing enhancer and shows that constitutively expressed and cell type-speci fic factors combine to regulate splicing of a specific subset of pre-mRNAs including SPO70, MER2, and MER3.