THE BROAD-COMPLEX GENE IS A TISSUE-SPECIFIC MODULATOR OF THE ECDYSONERESPONSE OF THE DROSOPHILA HSP23 GENE

The steroid hormone ecdysone causes dramatic changes in the genetic pr ograms leading to the pupariation of Drosophila melanogaster, and the Broad-Complex (BR-C) gene is known to play a key role in this process, Previously we showed that BR-C regulates developmental changes in tra nscription and chromatin structure of the 67B heat shock gene cluster, which contains four small hsp genes. Importantly, the downregulation of the ksp23 gene in the BR-C mutants correlates with the absence of a DNase I-hypersensitive site (DAS) at position -1400, To study the fun ctional importance of the DHS-1400, we have introduced genomic fragmen ts containing a modified hsp23 gene into the Drosophila germ line, Our analysis shows that the ecdysone response element is necessary but no t sufficient for full developmental expression of hsp23 in the late th ird instar and that there is, indeed, another regulatory element, in t he vicinity of DHS-1400, We also show that hsp23 developmental express ion is not tissue specific. A construct lacking the ecdysone response element is unable to direct normal hsp23 expression in all tissues exc ept the brain, Similarly, brain-specific expression is BR-C independen t, although in the other tissues we find different requirements for BR -C genetic functions, The effect of the br mutations is restricted to ming imaginal discs and midgut tissue, while that of 2Bc is restricted to the fat body and Malpighian tubules, and mutations in the rbp, gro up have no effect in any of the tissues studied. Thus, BR-C regulatory action is mediated through different genetic functions in a tissue-sp ecific manner.