The acquisition of competence to respond to ecdysone in Drosophila is transcript specific

The steroid hormone ecdysone induces a precise sequence of gene activity in Drosophila melanogaster salivary glands in late third larval instar larvae. The acquisition of competence for this response does not result from a single event or pathway but requires factors that accumulate throughout the instar. Individual transcripts become competent to respond at different times and their expression is differentially affected in ecd1, dor22 and BR-C mutants. The induction of early-late transcripts, originally assumed to necessarily follow early transcripts, is partially independent of early transcript activation. Attempts to inhibit the synthesis of regulatory proteins reveal transcript-specific superinduction effects. Furthermore these inhibitors lead to the induction of betaFTZ-F1 and E93 transcripts at levels normally found in prepupal glands. These studies reveal the complexity of the processes underlying the establishment of a hormonal response.