Ecdysone coordinates the timing and amounts of E74A and E74B transcription in Drosophila

Pulses of the steroid hormone ecdysone function as temporal signals to coordinate the development of both larval and adult tissues in Drosophila. Ecdysone acts by triggering a genetic regulatory hierarchy that can be visualized as puffs in the larval polytene chromosomes. In an effort to understand how the ecdysone signal is transduced to result in sequential gene activation, we are studying the transcriptional control of E74, an early gene that appears to play a regulatory role in the hierarchy. Northern blot analysis of RNA isolated from staged animals or cultured organs was used to characterize the effects of ecdysone on E74 transcription. Ecdysone directly activates both E74A and E74B promoters. E74B mRNA precedes that of E74A, each mRNA appearing with delay times that agree with their primary transcript lengths and our previous transcription elongation rate measurement of ∼1.1 kb/min. The earlier appearance of E74B transcripts is enhanced by its activation at an ∼25-fold lower ecdysone concentration than E74A. E74B is further distinguished from E74.4 by its repression at a significantly higher ecdysone concentration than that required for its induction, close to the concentration required for E74.4 activation. These regulatory properties lead to an ecdysone-induced switch in E74 expression, with an initial burst of E74.8 transcription followed by a burst of E74A transcription. We also show that the patterns of ecdysone-induced E74.4 and E74B transcription vary in four ecdysone target tissues. These studies provide a means to translate the profile of a hormone pulse into different amounts and times of regulatory gene expression that, in turn, could direct different developmental responses in a temporally and spatially regulated manner.