Abstract

Developmental decisions rely on cells making accurate transcriptional responses to signals they receive. For example, Notch pathway activity results in rapid transcriptional outputs in the absence of any amplification steps. Local condensates or transcription factor hubs are proposed to facilitate recruitment of key nuclear complexes and their co-factors to promote gene activation. To investigate whether transcription hubs are formed under conditions of endogenous Notch signalling, we combined real-time measurements of Notch transcription-complex enrichments relative to a fluorescently tagged gene locus with quantitative live imaging of gene transcription from two linked loci. An enriched hub containing the co-activator Mastermind (Mam) was formed in a signalling-dependent manner during developmental stages when transcription occurs. Tracking hubs in real time revealed that they are highly dynamic and, when imaged together with transcription in the same nucleus, Mam condensation consistently correlates with the onset and profile of transcription. Manipulations affecting signalling levels had concordant effects on hub intensities and transcriptional profiles, altering the probability and amplitude of transcription. Together the results support a model in which signalling induces the formation of transcription hubs whose properties are instrumental in the quantitative gene expression response to Notch activation.