The Elongation Factor and Histone Chaperone Spt6 Maintains ESC Pluripotency by Controlling Super-Enhancers and Counteracting Polycomb Repressive Complex 2

Spt6 coordinates nucleosome dis- and re-assembly, transcriptional elongation and mRNA processing. Here, we report that depleting Spt6 in ESCs reduced expression of pluripotency factors, increased expression of cell lineage-affiliated developmental regulators, and induced cell morphological and biochemical changes indicative of ESC differentiation. Selective down-regulation of pluripotency factors upon Spt6 depletion may be mechanistically explained by its enrichment at ESC super-enhancers where Spt6 controlled H3K27 acetylation and methylation, and super-enhancer RNA transcription. In ESCs, Spt6 interacted with the PRC2 core subunit Suz12 and prevented H3K27me3 accumulation at ESC super-enhancers and associated promoters. Biochemical as well as functional experiments revealed that Spt6 could compete for binding of the PRC2 methyltransferase Ezh2 to Suz12 and reduce PRC2 chromatin engagement. Thus, in addition to serving as histone chaperone and transcription elongation factor, Spt6 counteracts repression by opposing H3K27me3 deposition at critical genomic regulatory regions. Examination of genome-wide distribution of Spt6 in mouse embryonic stem cells and 3 different histone modifications in control or Spt6 KD ESCs.