Project description:Coordinated regulation of stemness gene activity by transcriptional and translational controls poise stem cells for a timely cell-state transition during differentiation. Although important for all stemness-to-differentiation transitions, mechanistic understanding of the fine-tuning of stemness gene transcription is lacking due to the compensatory effect of translational control. We used intermediate neural progenitor (INP) identity commitment to define the mechanisms that fine-tune stemness gene transcription in fly neural stem cells (neuroblasts). We demonstrate that the transcription factor FruitlessC (FruC) binds cis-regulatory elements of most genes uniquely transcribed in neuroblasts. Loss of fruC function alone has no effect on INP commitment but drives INP dedifferentiation when translational control is reduced. FruC negatively regulates gene expression by promoting low-level enrichment of the repressive histone mark H3K27me3 in gene cis-regulatory regions. Identical to fruC loss-of-function, reducing Polycomb Repressive Complex 2 activity increases stemness gene activity. We propose low-level H3K27me3 enrichment fine-tunes stemness gene transcription in stem cells, a mechanism likely conserved from flies to humans.
Project description:Coordinated regulation of stemness gene activity by transcriptional and translational controls poise stem cells for a timely cell-state transition during differentiation. Although important for all stemness-to-differentiation transitions, mechanistic understanding of the fine-tuning of stemness gene transcription is lacking due to the compensatory effect of translational control. We used intermediate neural progenitor (INP) identity commitment to define the mechanisms that fine-tune stemness gene transcription in fly neural stem cells (neuroblasts). We demonstrate that the transcription factor FruitlessC (FruC) binds cis-regulatory elements of most genes uniquely transcribed in neuroblasts. Loss of fruC function alone has no effect on INP commitment but drives INP dedifferentiation when translational control is reduced. FruC negatively regulates gene expression by promoting low-level enrichment of the repressive histone mark H3K27me3 in gene cis-regulatory regions. Identical to fruC loss-of-function, reducing Polycomb Repressive Complex 2 activity increases stemness gene activity. We propose low-level H3K27me3 enrichment fine-tunes stemness gene transcription in stem cells, a mechanism likely conserved from flies to humans.
Project description:Bivalency, the paradoxical juxtaposition of transcriptionally activating trimethylation of histone H3 lysine 4 (H3K4me3) and the repressive trimethylation of histone H3 lysine 27 (H3K27me3), has been proposed to decorate developmental genes poised for gene expression regulation. Here, we report development of sequential internally calibrated chromatin immunoprecipitation (Re-ICeChIP-seq), capable of measuring absolute quantities of nucleosomal patterns of histone marks in a genome-wide fashion, combined with in situ control of antibody specificity. Re-ICeChIP-seq of H3K4me3/H3K27me3 in mESC reveals that bivalent genes can be delineated into two classes, distinguished by the nucleosomal ratio of H3K4me3 to H3K27me3. Consistent with the canonical role of bivalency, H3K27me3-rich bivalent nucleosomes demarcate promoters of poorly expressed developmental genes that may be poised for activation or repression. Yet our measurements reveal surprisingly widespread presence of bivalency at promoters of highly-expressed housekeeping genes, characterized by H3K4me3-rich bivalent nucleosomes. Moreover, the ratio of H3K4me3 to H3K27me3 at transcription start sites better correlates with gene expression than H3K4me3 or H3K27me3 alone, suggesting cooperation between opposing marks to fine-tune gene expression. Finally, we report that major H3K4 methyltransferases exhibit wide acceptance of various H3K27me3 substrates.