(619e) Enhancer-Mediated Dynamic Gene Control

Past studies on gene regulation have emphasized the spatial limit of gene expression. Indeed, mis-regulation of gene expression pattern often leads to developmental defects. However, the kinetics of gene expression is not as well characterized and hence its role in mediating normal development is not well understood. Moreover, recent studies revealed significant cell-to-cell variability, where neighboring cells that express the same gene exhibit different transcriptional dynamics. Such noisy transcriptional activity, however, still leads to robust cellular processes so that organisms develop, metabolize, and reproduce in a stable manner. It is of great importance to understand how a gene is regulated over time, and how such inherently stochastic transcriptional dynamics are “tamed” to produced reliable cellular outcomes.

We employ quantitative live imaging methods to visualize the transcriptional dynamics of two key components of the Rho signaling pathway in living Drosophila embryos, T48 and Fog. Both genes display dorsoventral gradients of expression due to differential timing of transcription activation. Transcription begins as a narrow stripe of two or three cells along the ventral midline, followed by progressive expansions into more lateral regions. Quantitative image analyses suggest that these temporal gradients produce differential spatial accumulations of t48 and fog mRNA along the dorsoventral axis, and this is correlated with myosin activity that mediates downstream morphogenetic processes. Then we examine changes in transcriptional dynamics by either deleting or optimizing a binding site for the transcriptional activator, Dorsal, within the enhancer sequence. Changes in a single binding site lead to changes in both the spatial boundary of a gene as well as the transcription activation kinetics. Moreover, higher and lower cell-to-cell stochasticity is observed upon deletion and enhancement of a binding site, respectively. In summary, we propose that enhancers control the timing of gene activation and in this way mediates downstream patterning and morphogenetic processes.