The MLLT3 YEATS domain binds histone marks (H3K9/18/27ac/cr) and ncRNA (7SK), linking transcription and RNA signaling to regulate hematopoiesis.

MLLT3 (AF9) is a critical regulator of hematopoiesis. The YEATS domain of MLLT3 binds to acylated histones (H3K9ac/cr). We showed that the YEATS domain also binds to RNA. PAR-CLIP data demonstrated binding to the snRNA 7SK. Biochemical studies show high-affinity binding to stem loop 4 (SL4) of 7SK. NMR chemical shift perturbation mapping of the binding site for 7SK SL4 on the YEATS domain was used to generate a model of the complex. Based on the model, mutations were identified that disrupt 7SK binding without affecting H3K9ac binding. Introduction of these mutations into full-length MLLT3 skewed hematopoiesis toward the lymphoid lineage, abrogated the transforming properties of NES-AF9, and reduced the self-association of MLLT3. PRO-Seq and RNA-Seq identified genes driving the observed phenotype and showed a block in transcriptional initiation with the loss of RNA binding. Our results demonstrate a critical role for RNA binding in the function of MLLT3.