Spatial and temporal patterns of E74 transcription during Drosophila development.

The E74 gene occupies one of the early puff loci (74EF) central to the Ashburner model for the ecdysone-induced puffing pattern in Drosophila. In support of this model, we show that the E74A promoter is directly activated by ecdysone and is subsequently repressed by ecdysone-induced proteins. Further support derives from the correspondence observed between 74EF puff size and the accumulation of nascent transcripts on the E74A unit. These transcripts elongate at 1.1 kb/min so that this 60 kb unit acts as a timer, delaying the appearance of its mRNA by 1 hr. E74A transcription is induced in a variety of ecdysone target tissues in late third instar larvae and during each of the ecdysone pulses that mark the six stages of Drosophila development. These results support an extension of the Ashburner model in which ecdysone pulses coordinate tissue development. The temporal pattern of E74B transcription overlaps but is distinct from that of E74A.