SAFB associates with nascent RNAs to promote gene expression in mouse embryonic stem cells

Scaffold Attachment Factor B (SAFB) is a conserved RNA Binding Protein (RBP) that is essential for early mammalian development. However, the RNAs that associate with SAFB in mouse embryonic stem cells have not been characterized. Here, we addressed this unknown using RNA-seq and SAFB RNA immunoprecipitation followed by RNA-seq (RIP-seq) in wild-type ESCs and in ESCs in which SAFB and SAFB2 were knocked out. SAFB predominantly associated with introns of protein-coding genes through purine-rich motifs. The transcript most enriched in SAFB association was the lncRNA Malat1, which also contains a purine-rich region in its 5' end. Knockout of SAFB/2 led to down- and upregulation of approximately 1,000 genes associated with multiple biological processes, including genes that are regulated by Polycomb and genes involved in apoptosis, cell division, and cell migration. The spliced and nascent transcripts of many downregulated genes associated with high levels of SAFB in wild-type cells, implying that SAFB binding promotes their expression. Reintroduction of SAFB into double-knockout cells restored gene expression towards wild-type levels, an effect that was again observable at the level of spliced and nascent transcripts. Proteomics analysis revealed a significant enrichment of nuclear speckle-associated and RS-domain containing proteins among SAFB interactors. Our findings suggest that among other potential functions in mouse embryonic stem cells, SAFB promotes the expression of a subset of genes through its ability to bind purine regions in nascent RNA.