Splicing of mRNA precursors is essential in the regulation of gene expression. U2AF65 recognizes the poly-pyrimidine tract and helps in the recognition of the branch point. Inactivation of fission yeast U2AF65 (
Prp2) blocks splicing of most, but not all, pre-mRNAs, for reasons that are not understood. Here, we have determined genome-wide the splicing efficiency of fission yeast cells as they progress into synchronous meiosis in the presence or absence of functional Prp2. Our data indicate that in addition to the splicing elements at the 3\' end of any intron, the nucleotides immediately upstream the intron will determine whether
Prp2 is required or dispensable for splicing. By changing those nucleotides in any given intron, we regulate its
Prp2 dependency. Our results suggest a model in which
Prp2 is required for the coordinated recognition of both intronic ends, placing
Prp2 as a key regulatory element in the determination of the exon-intron boundaries.