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 beta FTZ-F1, a nuclear hormone receptor superfamily member derived from the 75CD mid-prepupal puff. We show that beta FTZ-F1 represses its own transcription and is repressed by ecdysone, explaining its brief expression in mid-prepupae. We further show that ectopic beta FTZ-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 beta FTZ-F1 plays a central role in the prepupal genetic response to ecdysone and provide a molecular mechanism for stage-specific responses to steroid hormones.