Stuxnet fine-tunes Notch dose during development using a functional Polycomb response element

Evolutionarily conserved Notch signaling is highly sensitive to changes in Notch receptor dose caused by intrinsic and environmental fluctuations. It is well known that epigenetic regulation responds dynamically to genetic, cellular and environmental stresses. However, it is unclear whether the Notch receptor dose is directly regulated at the epigenetic level. Here, by studying the role of the upstream epigenetic regulator Stuxnet (Stx) in Drosophila developmental signaling, we find that Stx promotes Notch receptor mRNA expression by counteracting the activity of Polycomb repressive complex 1 (PRC1). In addition, we provide evidence that Notch is a direct PRC1 target by identifying and validating in vivo the only bona fide Polycomb response element (PRE) among the seven Polycomb group (PcG)-binding sites revealed by DamID-seq and ChIP-seq analysis. Importantly, in situ deletion of this PRE results in increased Notch expression and phenotypes resembling Notch hyperactivation in cell fate specification. These results not only underscore the importance of epigenetic regulation in fine-tuning the Notch activity dose, but also the need to assess the physiological significance of omics-based PcG binding in development.