Transcription termination promotes splicing efficiency and fidelity in a compact genome.

Splicing of terminal introns is coupled to 3'-end processing by cleavage and polyadenylation (CPA) of mRNAs in mammalian genes. Whether this functional coupling is universally conserved across eukaryotes is unclear. Here we show using long read RNA sequencing in S . cerevisiae that splicing inactivation does not result in widespread CPA impairment. We also show that inactivation of CPA has limited impact on splicing efficiency. The negative impact of CPA inactivation on splicing is mainly due to transcription termination defects that promote readthrough transcription, leading to splicing inhibition for downstream intron-containing genes. Splicing impairment due to 5' extensions is length-dependent and can be detected independently from CPA inactivation for endogenous or synthetic genes, and is likely due to an increased distance of splicing signals to the 5' cap. Finally, we found that deficient termination can promote novel intragenic and long-range intergenic splicing events. These results argue against a broad coupling between splicing and CPA in S . cerevisiae but show that efficient CPA-mediated transcription termination is critical for splicing fidelity and efficiency in a compact genome.

Significance statement: Accurate gene expression requires that the enzyme that polymerizes RNA stops at the proper site (termination). In addition multiple RNA processing reactions, including removal of intervening sequences are necessary to produce mature mRNAs. How these different steps in the RNA biogenesis pathways influence each other is not well understood. In this study, we show that inactivation of termination induces mature RNA formation defects, including long RNAs that retain intervening sequences, or chimeric RNAs containing sequences from genes located next to each other on the genome. This study underscores the importance of proper termination to ensure accurate and efficient splicing of adjacent genes, which is particularly critical for compact genomes in which genes are located close to each other.