THE DROSOPHILA BROAD-COMPLEX REGULATES DEVELOPMENTAL-CHANGES IN TRANSCRIPTION AND CHROMATIN STRUCTURE OF THE 67B HEAT-SHOCK GENE-CLUSTER

At the end of the third larval instar of Drosophila, ecdysone triggers the tissue-specific activation of gene expression. In cytogenetic and molecular studies, the Broad-complex (BR-C) locus has been defined as a key element in the hierarchy of hormonal regulation of gene activit y. Here we show that BR-C function is required for the regulation of t he small hsp genes, which are clustered in the 67B puff and are known to be activated by heat shock and by ecdysone during development. We h ave found that the genes of the 67B cluster are expressed differential ly in the salivary glands. While hsp23 and. hsp27 transcripts accumula te at relatively high levels, those of hsp22 and hsp,26 are present at low and intermediate levels, respectively. The complete BR-C deficien cy as well as mutations of the npr class reduce the expression of gene s hsp23 and hsp27 by 95 to 99%. The analysis of mutants representing t wo subfunctions of the BR-C-l(1)2Bab and l(1)2Bc, has shown that the l atter is principally required for complete hsp induction. As sites of DNase I hypersensitivity in chromatin are believed to correspond to ge ne regulatory sequences, we have studied the changes of chromatin stru cture in the 67B region at different states of hsp gene activity. Upon hormonal induction, at the onset of metamorphosis, additional DNase I hypersensitive sites (DHS) appear in the 5' regions, four DHSs are as sociated with hsp23 and two with hsp27. We suggest that they are due t o the binding of the hormone-receptor complex and/or transcription fac tors. related to ecdysone action. Finally, two DHSs (at - 1400 of hsp2 3 and at - 1200 of hsp27) are absent in the mutant nuclei, and thus ma y correspond to the target sequences for the BR-C-dependent regulatory protein(s).