Transcription commonly occurs in bursts resulting from alternating productive
(ON) and quiescent (OFF) periods. Yet how transcriptional bursts are regulated
to determine spatiotemporal transcriptional activity remains unclear. Here we
perform live transcription imaging of key developmental genes in the fly
embryo, with single polymerase sensitivity. Quantification of single allele
transcription rates and multi-polymerase bursts reveals shared bursting
relationships among all genes, across time and space, as well as cis- and
trans-perturbations. We identify the allele's ON-probability as the main
determinant of the transcription rate, while changes in the transcription
initiation rate are limited. Any given ON-probability determines a specific
combination of mean ON and OFF times, preserving a constant characteristic
bursting time scale. Our findings point to a convergence of various regulatory
processes that predominantly affect the ON-probability, thereby controlling
mRNA production rather than mechanism-specific modulation of ON and OFF times.
Our results thus motivate and guide new investigations into the mechanisms
implementing these bursting rules and governing transcriptional regulation