Rules of transcriptional bursting revealed by absolute quantification of nascent RNA in living fly embryos*

Transcriptional bursting is a common regulatory strategy for controlling gene outputs in various organisms. Over the past 15 years, experiments that dissected transcriptional regulation into its smaller parts have helped us identify many molecular processes associated with burst modulation under various physiological conditions, such as enhancer regulation, transcription factor regulation, epigenetics modifications, etc. However, these reductionist insights have not yet been successful in adding up to a coherent biophysical model that links molecular mechanisms to empirical bursting dynamics. In this work, we provide a unified perspective on this problem by quantifying transcriptional activity in live Drosophila embryos with single RNA detection sensitivity. We reveal a common regulatory strategy that displays surprising constraints on transcriptional bursting under a whole variety of different experimental conditions, including cis- and trans- perturbations in mutant fly strains. We observe a massive data collapse across many conditions, hinting at general rules underlying the dynamics of transcriptional bursting that could be linked to evolutionarily conserved biophysical properties of the core eukaryotic transcriptional machinery.