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. Counter to prior models of specific recruitment, we find that WDR5, an essential subunit of the MLL1/KMT2A histone methyltransferase complex, binds RNA promiscuously. Using a stringent approach, we demonstrate that WDR5 directly engages a diverse pool of cellular RNAs, dictated primarily by RNA abundance rather than specific motifs. Equilibrium binding assays show that RNA length, rather than sequence, governs high affinity. WDR5 binds to RNA through multiple surfaces, some of which overlap with 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. Further experiments in human cell lines suggest that this disassembly mechanism may operate to homeostatically downregulate MLL1 complex activity, thereby integrating transcriptional activity with chromatin state.