Translational regulation of Sf1 integrates alternative splicing and hematopoietic stem cell fate.
The transition of hematopoietic stem cells (HSCs) from quiescence to lineage commitment requires precise posttranscriptional control, yet the contribution of mRNA isoform regulation remains poorly defined. Here, we identify a translationally controlled splicing program that contributes to HSC fate decisions. Using activity-based signatures of 305 splicing regulators, we uncover widespread posttranscriptional modulation of the spliceosome in stem and progenitor cells. The branch point recognition factor Sf1 emerges as a key node, regulated by a conserved structured 5' untranslated region (UTR) that cooperates with the RNA-binding protein Igf2bp2 to control its translation. Disrupting this cis-trans module reduces Sf1 protein synthesis and skews differentiation toward stem and erythroid programs. Mechanistically, Sf1-dependent alternative splicing remodels 5' UTRs of hematopoietic and DNA damage response genes, altering their translation and modulating DNA damage resolution. Together, these findings reveal an unrecognized translational layer controlling spliceosome activity and link RNA regulons, alternative splicing, and HSC fate determination.