Promiscuous RNA binding by WDR5 remodels the KMT2A (MLL1) histone methyltransferase complex to an inactive state

Chromatin-modifying complexes are critical in gene regulation, yet their proposed interactions with RNA remain poorly understood. Here, we show that WDR5, an essential subunit of the MLL1 (KMT2A) histone methyltransferase complex, binds RNA with high affinity but without sequence specificity. Using a stringent approach, we demonstrate that WDR5 directly engages a diverse pool of RNAs in cells, predominantly as a function of RNA abundance rather than specific motifs. Equilibrium binding assays further show that RNA length, rather than sequence, dictates WDR5 affinity. We map multiple RNA-binding regions on WDR5, some of which overlap with surfaces used in MLL1 complex subunit interfaces. Strikingly, we find that RNA binding disrupts the MLL1 complex by competitively displacing WDR5 from these critical protein-protein interactions, leading to a marked inhibition of MLL1 catalytic activity. This regulatory mechanism provides a layer of control over histone methylation, potentially integrating transcriptional activity with chromatin state.