Coordination of larval and prepupal gene expression by the DHR3 orphan receptor during Drosophila metamorphosis.

The DHR3 orphan receptor gene is induced directly by the steroid hormone ecdysone at the onset of Drosophila metamorphosis. DHR3 expression peaks in early prepupae, as the early puff genes are repressed and betaFTZ-F1 is induced. Here we provide evidence that DHR3 directly contributes to both of these regulatory responses. DHR3 protein is bound to many ecdysone-induced puffs in the polytene chromosomes, including the early puffs that encode the BR-C and E74 regulatory genes, as well as the E75, E78 and betaFTZ-F1 orphan receptor loci. Three DHR3 binding sites were identified downstream from the start site of betaFTZ-F1 transcription, further indicating that this gene is a direct target of DHR3 regulation. Ectopic expression of DHR3 revealed that the polytene chromosome binding pattern is of functional significance. DHR3 is sufficient to repress BR-C, E74A, E75A and E78B transcription as well as induce betaFTZ-F1. DHR3 thus appears to function as a switch that defines the larval-prepupal transition by arresting the early regulatory response to ecdysone at puparium formation and facilitating the induction of the betaFTZ-F1 competence factor in mid-prepupae. This study also provides evidence for direct cross-regulation among orphan members of the nuclear receptor superfamily and further implicates these genes as critical transducers of the hormonal signal during the onset of Drosophila metamorphosis.