A subset of Mer1p-dependent introns requires Bud13p for splicing activation and nuclear retention

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Abstract
In the yeast Saccharomyces cerevisiae, Mer1p is expressed only during meiosis, and its expression is linked to the splicing of at least three mRNAs: MER2, MER3, and AMA1. Previous evidence suggests that Mer1p activates splicing by directly recruiting snRNPs or stabilizing intermediate splicing complexes formed on pre-mRNA that contains an intronic Mer1p enhancer element. However, some splicing factors, especially accessory/non-snRNP factors, have critical roles in retaining unspliced pre-mRNAs in the nucleus. We tested if Mer1p may indirectly regulate splicing by preventing the export of pre-mRNAs to the cytoplasm and also demonstrated that a second subunit of the Retention and Splicing (RES) complex, Bud13p, has transcript-specific effects on Mer1p-activated splicing. The results indicated that Mer1p can retain unspliced pre-mRNA in the nucleus; however, nuclear retention could not be uncoupled from splicing activation. In the absence of Mer1p, the AMA1 pre-mRNA is exported to the cytoplasm, translated, but not subjected to nonsense-mediated decay (NMD) despite a premature stop codon in the intron. These data imply that Mer1p can retain pre-mRNAs in the nucleus only by facilitating their interaction with the spliceosome and that two subunits of the RES complex modulate Mer1p function on two of the three Mer1p-dependent introns. The results also support models for cytoplasmic degradation of unspliced pre-mRNAs that fail to assemble into spliceosomes in yeast.