U1 snRNP regulates alternative promoter activity by inhibiting premature polyadenylation

Transcription and splicing are intricately linked processes, with emerging evidence highlighting the involvement of splicing factors mediating their coupling. U1 small nuclear ribonucleoprotein particle (U1 snRNP), a key splicing factor, not only serves as the initial component of the spliceosome but also plays a crucial role in preventing premature cleavage and polyadenylation, facilitating long-distance transcription elongation. Here, we show that U1 snRNP regulates alternative promoter usage through inhibition of premature polyadenylation. Inhibiting U1 snRNP with antisense oligonucleotides or introducing a premature polyadenylation leads to a significant decrease in downstream promoter activity in genes with premature polyadenylation sites in between two promoters. Conversely, restoring U1 snRNP activity or inhibiting premature polyadenylation sites using a gene-specific U1 snRNP rescues downstream promoter activity. Mechanistically, U1 snRNP inhibition correlates with reduced chromatin accessibility and serine 5 phosphorylation levels of RNA polymerase II at downstream promoters. Our findings propose a model where U1 snRNP facilitates productive transcription from upstream promoters, triggering downstream promoter activation by destabilizing nucleosomes and promoting promoter escape. Six samples with two distinct experimental conditions were investigated by Precision Run-On Sequencing (PRO-seq): 3 biological replicate sets of cells treated with a Control antisense morpholino oligonucleotides (Control ASO) and 3 biological replicate sets of cells treated with a U1 antisense morpholino oligonucleotides (U1 ASO).