Dissecting tissue-specific transcriptional response to 20E in Drosophila by E23 expression
Transcriptional activation by 20-hydroxyecdysone (20E) in Drosophila provides an excellent model for studying tissue-specific response to steroids. An increase in 20E titer controls degradation of larval and proliferation of adult tissues during metamorphosis. To study 20E-dependent transcription, we used the natural system for controlling the 20E titer – E23 membrane transporter, which exports 20E from the cell. We artificially expressed the E23 in tissues to suppress the first wave of 20E-inducible transcription at metamorphosis.  Expression of E23 revealed a plethora of 20H dependent genes in salivary glands, but mildly affected the transcription in brain/eye-antennal discs. We described the mechanisms controlling the transcriptional activation by 20E in Drosophila salivary glands. 20E depletion caused a decreased binding of Pol II and TFIID subunit, TBP, to target promoters, demonstrating 20E role in the stimulation of transcription initiation. At the target loci 20E depletion resulted in a malfunctioning of sites co-bound with EcR and CBP/Nejire and enriched with H3K27Ac histone mark inherent to active enhancers. At these sites, 20E titer was found to control chromatin accessibility and acetylation. We presume that the activity of these 'active' ecdysone-sensitive elements – (ESEs) is responsible for an active status of 20E targets in the salivary glands of wandering larvae. Overall design: RNA-Seq and ChIP-Seq analysis were performed on tissues (salivary glands and brains/eye-antennal discs) of wandering larvae (4-6h before puparium formation). All experiments were done using hsp-e23 larvae, treated with the double heat shock (with 1-hour rest) 20-22 h before puparium formation, and in control conditions, without heat shocks