(26b) Dynamic Control of Gene Expression in Developing Embryos

Past studies on gene regulation in development have emphasized the spatial limits of gene expression. However, it is not well understood whether the kinetics of gene expression also plays an important role in mediating normal development. Newly developed quantitative live imaging methods offer the first opportunity to explore the temporal limits of gene expression. We have characterized the transcriptional dynamics of even-skipped (eve) and fushi tarazu (ftz) expression in living Drosophila embryos. The endogenous loci were tagged with MS2 and PP7 stem-loop RNAs using CRISPR-Cas9 genome editing methods. Highly dynamic patterning of eve and ftz is observed, whereby nascent transcripts are initially expressed throughout the embryo and rapidly refined into sequential stripes. Surprisingly, the strongest early sites of eve and ftz expression correspond to the inter-stripe regions of the mature pattern, suggesting a substantial reorganization in expression. Subsequently, we deleted the stripe 1 enhancer for both eve and ftz and examined the changes in transcriptional dynamics. Both eve and ftz display transcriptional bursting, and spatial refinement appears to occur by silencing gene expression during the refractory period between bursts. This implies that transcriptional bursting can be used as a mechanism to facilitate dynamic repression of gene expression. In summary, we propose that transcriptional bursting facilitates dynamic gene control in development.