Repression is overcome as the levels of both ecdysone and EcR decrease during early-prepupal development. In late prepupae, the midprepupal puffs regress and the early puffs are reinduced. In addition, a few stage-specific early puffs, typified by E93, are directly induced by ecdysone in late prepupae, but not in late larvae. The early puffs cannot be induced by ecdysone in early-prepupal salivary glands. Rather, a preceding period of protein synthesis and low ecdysone concentration is required before these puffs become competent to respond to hormone.
Negative regulation is required in molting, as much as positive effects.
The complexity of the insect molting hierarchy serves as a warning for a generation of scientists who would unravel the hierarchies of mammalian development. These proteins differ in their amino-terminal sequences but contain identical DNA binding domains and ligand binding domains. The A and B1 isoforms are encoded by overlapping transcription units that have different promotors and can be separately controlled. There are two conserved domains characteristic of steroid receptor superfamily members. A comparative tree of DNA-binding domain amino acid sequences reveals the evolutionary affinities of Drosophila nuclear receptor proteins.