RESUMEN
To ensure that the embryo can package exponentially increasing amounts of DNA, replication-dependent histones are some of the earliest transcribed genes from the zygotic genome. However, how the histone genes are identified is not known. The pioneer factors Zelda and CLAMP collaborate at a subset of genes to regulate zygotic genome activation in Drosophila melanogaster and target early activated genes to induce transcription. CLAMP also regulates the embryonic histone genes and helps establish the histone locus body, a suite of factors that controls histone mRNA biosynthesis. The relationship between Zelda and CLAMP led us to hypothesize that Zelda helps identify histone genes for early embryonic expression. We found that Zelda targets the histone locus early during embryogenesis, prior to histone gene expression. However, depletion of zelda in the early embryo does not affect histone mRNA levels or histone locus body formation. While surprising, these results concur with other investigations into Zelda's role in the early embryo, suggesting the earliest factors responsible for specifying the zygotic histone genes remain undiscovered.
RESUMEN
Cell type-specific enhancers are activated by coordinated actions of lineage-determining transcription factors (LDTFs) and chromatin regulators. The SWI/SNF chromatin remodeling complex BAF and the histone H3K4 methyltransferase MLL4 (KMT2D) are both implicated in enhancer activation. However, the interplay between BAF and MLL4 in enhancer activation remains unclear. Using adipogenesis as a model system, we identify BAF as the major SWI/SNF complex that colocalizes with MLL4 and LDTFs on active enhancers and is required for cell differentiation. In contrast, the promoter enriched SWI/SNF complex PBAF is dispensable for adipogenesis. By depleting BAF subunits SMARCA4 (BRG1) and SMARCB1 (SNF5) as well as MLL4 in cells, we show that BAF and MLL4 reciprocally regulate each other's binding on active enhancers before and during adipogenesis. By focusing on enhancer activation by the adipogenic pioneer transcription factor C/EBPß without inducing cell differentiation, we provide direct evidence for an interdependent relationship between BAF and MLL4 in activating cell type-specific enhancers. Together, these findings reveal a positive feedback between BAF and MLL4 in promoting LDTF-dependent activation of cell type-specific enhancers.
