EPIGENETIC REGULATION OF GENE EXPRESSION BY ECDYSONE HORMONE SIGNALING

The development of a multicellular organism from a single progenitor is a complex process that involves the establishment of gene expression profiles specific to each stage and tissue. In Drosophila, pulses of the steroid hormone ecdysone play a central role in coordinating development events throughout the life cycle, including the larval molts and the onset of the larval-to-adult transition. The initial response to ecdysone is mediated by its receptor, EcR, which activates a series of downstream transcription factors that amplify and temporally progress the cascade. The genetic response has tissue- and temporal-specificity; each successive pulse initiates different developmental events in tissues throughout the animal. However, our understanding of how this specificity is achieved remains incomplete. In this work, we examined how EcR and one of its downstream transcription factors, E93, coordinate changes in gene expression. We found that changes in chromatin accessibility are a central means by which specificity to the response is achieved but that EcR and E93 have different roles. E93 is required to temporally progress the accessibility profile and binds many sites dependent on E93 for their accessibility, indicating that it may have pioneer-like activity. In contrast, EcR appears to be a passive factor in which tissue-specific differences in open chromatin direct its binding to different sites between the two tissues. Since EcR functions at the top of a transcriptional hierarchy, we further investigated its direct role in promoting changes in gene expression. We found that EcR plays a broad role in coordinating the response to ecdysone and that changes in its binding profile are an important means by which specificity is achieved. To determine the function of EcR binding, we investigated its role in regulating enhancer activity. As expected, we found that it regulates the temporal activity of enhancers. Unexpectedly, however, we also found that it regulates the spatial pattern of enhancers. This indicates that EcR may regulate gene expression differences that occur within tissues, as well as between them. Collectively, this work has provided new insights into how tissue- and temporal-specific gene expression responses can be generated by a single, extrinsic signal.Doctor of Philosoph