The Hox transcription factor Ubx ensures somatic myogenesis by suppressing the mesodermal master regulator Twist

Early determination factors and lineage-specific master regulators are essential for the specification of cell and tissue types. However, once a cell has committed to a specific fate, it is equally critical to restrict the activity of such factors to enable proper differentiation. In many studies the functional network for master regulators are under constant investigations. Yet, how these factors are silenced remains unclear. Using the Drosophila mesoderm as a model and a comparative genomic approach, we identified the Hox transcription factor (TF) Ultrabithorax (Ubx) to be critical for the repression of the mesodermal master regulator Twist (Twi). Mesoderm-specific Ubx loss-of-function experiments using CRISPR/Cas9 as well as overexpression experiments demonstrated that Ubx majorly impacts twi transcription. A detailed mechanistic analysis revealed that Ubx requires the function of the NK-homeodomain protein Tinman (Tin) but not the muscle differentiation factor Myocyte enhancer factor 2 (Mef2) to bind to the twi promoter. Furthermore, we found these TF interactions to be critical for silencing of the twi promoter region by recruiting the Polycomb DNA binding protein Pleiohomeotic (Pho). In sum, our study demonstrates that the Hox TF Ubx is a critical player in mediating the silencing of the mesodermal master regulator Twi, which is crucial for coordinated muscle differentiation.