A molecular mechanism for the stage specificity of the Drosophila prepupal genetic response to ecdysone

Two successive pulses of ecdysone signal the ends of larval and prepupal development in Drosophila, inducing early and late puffs in the salivary gland polytene chromosomes. Early puff induction in prepupae is dependent on a preceding period of protein synthesis and low ecdysone concentration. We demonstrate here that the competence acquired during this interval can be provided by βFTZ-F1, a nuclear hormone receptor superfamily member derived from the 75CD midprepupal puff. We show that βFTZ-F1 represses its own transcription and is repressed by ecdysone, explaining its bried expression in mid-prepupae. We further show that ectopic βFTZ-F1 expression leads to enhanced levels of ecdysone-induced BR-C, E74, and E75 early gene transcription and premature induction of the stage-specific 93F early puff and E93 transcription. These findings indicate that βFTZ-F1 plays a central role in the prepupal genetic response to ecdysone and provide a molecular mechanism for stage-specific responses to steroid hormones.