Low-level repressive histone marks fine-tune gene transcription in neural stem cells.

Coordinated regulation of gene activity by transcriptional and translational mechanisms 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 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 Fruitless (Fru) binds -regulatory elements of most genes uniquely transcribed in neuroblasts. Loss of function alone has no effect on INP commitment but drives INP dedifferentiation when translational control is reduced. Fru negatively regulates gene expression by promoting low-level enrichment of the repressive histone mark H3K27me3 in gene -regulatory regions. Identical to loss-of-function, reducing Polycomb Repressive Complex 2 activity increases stemness gene activity. We propose low-level H3K27me3 enrichment fine-tunes gene transcription in stem cells, a mechanism likely conserved from flies to humans.