THE DROSOPHILA BROAD-COMPLEX EARLY GENE DIRECTLY REGULATES LATE GENE-TRANSCRIPTION DURING THE ECDYSONE-INDUCED PUFFING CASCADE

The ensemble of tissue-specific changes that drives Drosophila metamor phosis is initiated by the steroid hormone ecdysone and proceeds throu gh a transcriptional cascade comprised of primary response transcripti onal regulators and secondary response structural genes. The Broad-Com plex (BR-C) primary response early gene is composed of several distinc t genetic functions and encodes a family of related transcription fact or isoforms. Our objective in this study was to determine whether indi vidual BR-C isoforms directly regulate secondary response target genes . A cluster of 10 salivary gland-specific secondary response L71 late genes are dependent on the BR-C rbp(+) genetic function. Transgenic an imals expressing individual BR-C isoforms were tested for their abilit y to provide the BR-C rbp(+) genetic function by monitoring the transc riptional activation of the L71 genes. We found that the BR-C Z1 isofo rms could complement the transcriptional defects seen in rbp mutants b ut the Z2, Z3, and Z4 isoforms could not. We conclude that the BR-C rb p(+) function is provided by the BR-C Z1 isoform in prepupal salivary glands. L71 gene rescue was restricted to the prepupal salivary gland, suggesting the involvement of additional factors in L71 gene regulati on. Interestingly, we found that the overexpression of Z3 or Z4 isofor ms in BR-C+ salivary glands repressed L71 expression, indicating that BR-C proteins might also function as transcriptional repressors. Molec ular mapping and characterization of the regulatory sequences that con trol L71-6 expression revealed several Z1 isoform binding sites. Mutag enesis of these Z1 binding sites resulted in the failure to activate l ate gene expression in vivo when measured by transgenic reporter genes . We conclude that the BR-C early gene directly activates late gene tr anscription by interacting with late gene cis-acting regulatory elemen ts and that this interaction is responsible for the temporal linkage o f early and late ecdysone-induced gene expression. (C) 1996 Academic P ress, Inc.