THE ROLE OF ECDYSONE IN THE INDUCTION AND MAINTENANCE OF HSP27 TRANSCRIPTS DURING LARVAL AND PREPUPAL DEVELOPMENT OF DROSOPHILA

The Drosophila melanogaster small heat-shock hsp27 carries the canonic ecdysone response element (EcRE) at -537. This EcRE has been used ext ensively both in cultured cells and in vitro for studies of the ecdyso ne response, We have characterised the developmental expression of hsp 27 in wild-type larval and prepupal salivary glands in parallel with t hat of the E74B and E74A primary ecdysone response transcripts, which are induced in the mid and late third larval instar by a minor and maj or increase in ecdysone titre respectively. The induction of hsp27 occ urs between these two events in larvae and in parallel with that of E7 4A in prepupae. Transcript levels are severely reduced in ecd1 and dor (22) (deep orange) mutant larvae but are only moderately reduced in la rvae mutant for the Broad-Complex allele br(1435). By culturing saliva ry glands of different ages with low (1.8 x 10(-8) M) or high (1.8 x 1 0(-6) M) concentrations of hormone, we show that the response of an Ec RE varies during development and that the timing of the response canno t be predicted solely from its apparent strength in cell line analyses .