Dynamic regulation of anterior-posterior patterning genes in living Drosophila embryos

Abstract

Expression of the gap and pair-rule genes plays an essential role in body segmentation during Drosophila embryogenesis [1-5]. However, it remains unclear how precise expression patterns of these key developmental genes arise from stochastic transcriptional activation at the single cell level. Here, I employed genome editing and live imaging approaches to comprehensively visualize regulation of the gap and pair-rule genes at the endogenous loci. Quantitative image analysis revealed that the total duration of active transcription (transcription period) is a major determinant of spatial patterning of gene expression in early embryos. The length of transcription period is regulated by the continuity of bursting activities in individual nuclei, with core expression domain producing more bursts than boundary region. Each gene exhibits distinct rate of nascent RNA production during transcriptional bursting, which contributes to gene-to-gene variability in the total output. I also provide evidence for "enhancer competition", wherein a distal weak enhancer interferes with transcriptional activation by a strong proximal enhancer to downregulate the length of transcription period without changing the transcription rate. Analysis of endogenous hunchback (hb) locus revealed that the removal of distal shadow enhancer induces strong ectopic transcriptional activation, which suppresses refinement of broad expression domain into narrower stripe pattern at the anterior part of embryos. This study provides key insights into the link between transcriptional bursting, enhancer-promoter interaction and spatiotemporal patterning of gene expression during animal development.