Cotranscriptional recruitment of yeast TRAMP complex to intronic sequences promotes optimal pre-mRNA splicing

Most unwanted RNA transcripts in the nucleus of eukaryotic cells, such as splicing-defective pre-mRNAs and spliced-out introns, are rapidly degraded by the nuclear exosome. In budding yeast, a number of these unwanted RNA transcripts, including spliced-out introns, are first recognized by the nuclear exosome cofactor Trf4/5p-Air1/2p-Mtr4p polyadenylation (TRAMP) complex before subsequent nuclear-exosome-mediated degradation. However, it remains unclear when spliced-out introns are recognized by TRAMP, and whether TRAMP may have any potential roles in pre-mRNA splicing. Here, we demonstrated that TRAMP is cotranscriptionally recruited to nascent RNA transcripts, with particular enrichment at intronic sequences. Deletion of TRAMP components led to further accumulation of unspliced pre-mRNAs even in a yeast strain defective in nuclear exosome activity, suggesting a novel stimulatory role of TRAMP in splicing. We also uncovered new genetic and physical interactions between TRAMP and several splicing factors, and further showed that TRAMP is required for optimal recruitment of the splicing factor Msl5p. Our study provided the first evidence that TRAMP facilitates pre-mRNA splicing, and we interpreted this as a fail-safe mechanism to ensure the cotranscriptional recruitment of TRAMP before or during splicing to prepare for the subsequent targeting of spliced-out introns to rapid degradation by the nuclear exosome.