Coordinated active repression operates via transcription factor cooperativity and multiple inactive promoter states in a developing organism

Refining transcriptional levels via active repression in a euchromatic context represents a critical regulatory process. While the molecular players of active repression are well described, their dynamics remain obscure. Here, we used snail expression dynamics as a paradigm to uncover how repression, mediated by the Snail (Sna) repressor, can be imposed within a developing tissue. Combining live imaging and mathematical modeling, we show that Sna-mediated repression is cooperative and that cooperativity is primarily mediated by the distal enhancer. Repression shifts transcription bursting dynamics from a two-state ON/OFF regime to a three-state repressed regime with two temporally distinct OFF states. Mutating Sna binding sites suggests that repression introduces the long-lasting inactive state, which is stabilized by cooperativity. Our approach offers quantitative insights into the dynamics of repression and how transcription factor cooperativity coordinates cell fate decisions within a tissue.