Abstract
Pulses of ecdysteroids induce dramatic changes in gene expression that direct the early stages of Drosophila metamorphosis. This gene activity is reflected by the appearance of early and late puffs in the salivary gland polytene chromosomes. Curiously, the early puff genes that have been studied to date are induced by both the late larval and prepupal pulses of ecdysteroids and are expressed in many ecdysteroid target tissues, raising the question of how the hormone directs the complex stage- and tissue-specific responses associated with metamorphosis. In an effort to address this question, we have isolated and characterized the E93 gene responsible for the stage-specific 93F early puff. The E93 mRNA displays no response to ecdysteroids in late larval salivary glands but is directly induced 12 hr later by the prepupal ecdysteroid pulse, identical to the response of the 93F puff. In tissues other than the salivary gland, however, E93 displays complex spatial and temporal regulation. E93 spans at least 55 kb of genomic DNA and encodes a 146-kDa protein that has no matches in the sequence databases, but displays several characteristics of known Drosophila transcription factors. We propose that E93 acts in a stage-specific regulatory hierarchy in the salivary gland to direct its histolysis in response to the prepupal ecdysteroid pulse.
