Myogenic transcriptional activation of MyoD mediated by replication-independent histone deposition

In mammals, the canonical histone H3 and the variant H3.3 are assembled into chromatin through replication-coupled and replication-independent (RI) histone deposition pathways, respectively, to play distinct roles in chromatin function. H3.3 is largely associated with transcriptionally active regions via the activity of RI histone chaperone, HIRA. However, the precise role of the RI pathway and HIRA in active transcription and the mechanisms by which H3.3 affects gene activity are not known. In this study, we show that HIRA is an essential factor for muscle development by establishing MyoD activation in myotubes. HIRA and Asf1a, but not CHD1 or Asf1b, mediate H3.3 incorporation in the promoter and the critical upstream regulatory regions of the MyoD gene. HIRA and H3.3 are required for epigenetic transition into the more permissive chromatin structure for polymerase II recruitment to the promoter, regardless of transcription-associated covalent modification of histones. Our results suggest distinct epigenetic management of the master regulator with RI pathway components for cellular differentiation.