The ensemble of tissue-specific changes that drivesDrosophilametamorphosis is initiated by the steroid hormone ecdysone and proceeds through a transcriptional cascade comprised of primary response transcriptional regulators and secondary response structural genes. TheBroad-Complex(BR-C) primary response early gene is composed of several distinct genetic functions and encodes a family of related transcription factor isoforms. Our objective in this study was to determine whether individual BR-C isoforms directly regulate secondary response target genes. A cluster of 10 salivary gland-specific secondary responseL71late genes are dependent on theBR-C rbp+genetic function. Transgenic animals expressing individual BR-C isoforms were tested for their ability to provide theBR-C rbp+genetic function by monitoring the transcriptional activation of theL71genes. We found that the BR-C Z1 isoforms could complement the transcriptional defects seen inrbpmutants but the Z2, Z3, and Z4 isoforms could not. We conclude that theBR-C rbp+function is provided by the BR-C Z1 isoform in prepupal salivary glands.L71gene rescue was restricted to the prepupal salivary gland, suggesting the involvement of additional factors inL71gene regulation. Interestingly, we found that the overexpression of Z3 or Z4 isoforms inBR-C+salivary glands repressedL71expression, indicating that BR-C proteins might also function as transcriptional repressors. Molecular mapping and characterization of the regulatory sequences that controlL71-6expression revealed several Z1 isoform binding sites. Mutagenesis of these Z1 binding sites resulted in the failure to activate late gene expressionin vivowhen measured by transgenic reporter genes. We conclude that theBR-Cearly gene directly activates late gene transcription by interacting with late genecis-acting regulatory elements and that this interaction is responsible for the temporal linkage of early and late ecdysone-induced gene expression.

During metamorphosis, the reorganization of the nervous system of Drosophila melanogaster proceeds in part through remodeling of larval neurons. In this study, we used in-vitro imaging techniques and immunocytochemistry to track the remodeling of the thoracic ventral neurosecretory cells. Axons of these neurons prune their larval arbors early in metamorphosis and a larger, more extensive adult arbor is established via branch outgrowth. Expression of EcR dominant negative constructs and an EcR inverted repeat construct resulted in pruning defects of larval axon arbors and a lack of filopodia during pruning, but showed variable effects on outgrowth depending on the construct expressed. Cells expressing either UAS-EcR-B1W650A or UAS-EcR-AW650A lacked filopodia during the outgrowth period and formed a poorly branched, larval-like arbor in the adult. Cells expressing UAS-EcR-B1F645A, UAS-EcR-B2W650A or UAS-IR-EcR (core) showed moderate filopodial activity and normal, albeit reduced, adult-like branching during outgrowth. These results are consistent with the role of activation versus derepression via EcR for successive phases of neuronal remodeling and suggest that functional ecdysone receptor is necessary for some, but not all, remodeling events. 10.1242/dev.02191