Transcription factor handover reshapes hub dynamics to coordinate temporal progression of Notch response

To decipher information in space and time, cells require mechanisms to sustain gene expression programmes once an initial signal decays. Localised condensation of transcription complexes into hubs is emerging as a key step in gene regulation and, by combining live imaging of transcription and of transcription complexes at target loci, we have investigated how these evolve to enable a prolonged transcription outcome in response to Notch signalling. Measuring the dynamics of hindsight (hnt) transcription reveals two phases. The first, a probabilistic response similar to E(spl)-C genes, is directly controlled by Notch and the second, a sustained response, is indirectly controlled through the BTB transcription factor Broad. The two phases correlate with a transition in transcription factor enrichment at hnt: complexes of Notch coactivator Mastermind and of Broad are successively enriched. Notably, the lifetime of the hubs formed by each are distinct and correlate with the change in transcription profiles. These reflect the binding dynamics of each complex, with Broad having significantly more stable association with chromatin, forming longer lasting hubs and yielding sustained transcription. Furthermore at intermediate stages we detect hubs containing both Broad and Mam implying an evolution in hub composition. Together our data support that the dynamic properties of transcription hubs are crucial for the probability and dynamics of transcription and point towards a ''hub handover'' in enabling temporal progression in gene expression responses.