| Description |
The onset of metamorphosis in Drosophila melanogaster is controlled by pulses of the steroid hormone 20-hydroxyecdysone. The striking tissue reorganization associated with this developmental event is attributed to complex and temporally coordinated programs of ecdysone-induced gene expression that are reflected in the reproducible pattern of puffs in the giant salivary gland polytene chromosomes of late third instar larvae and prepupae. A hierarchical model to account for these patterns suggests that ecdysone, acting in concert with a specific receptor protein, directly activates a small set of 'early' regulatory genes, the products of which down-regulate their own expression and activate a large set of 'late' effector genes. The ecdysone-inducible E74 gene, from the 74EF early puff, provides a paradigm for studies of early gene function. E74 is directly induced by ecdysone at the onset of metamorphosis and encodes two related proteins, E74A and E74B, that share an ETS DNA-binding domain characteristic of the ets-related protein superfamily. The consensus recognition sequence for E74A protein, determined by random oligonucleotide selection, contains a 5'-GGAA-3' core motif similar to the binding site of other ets proteins. In accordance with the hypothesis that the early ecdysone-inducible proteins function as transcriptional regulators and activate a set of downstream genes, the E74A protein was localized to a subset of ecdysone-inducible puffs, predominantly late puffs, by staining salivary gland polytene chromosomes with anti-E74A antibodies. To complement these data and specifically define the genomic sites bound by the protein, a number of genomic DNA fragments corresponding to bound puff loci were scanned for E74A binding sites. Of particular interest were a cluster of four strong binding sites located within the regulatory region of the L71-6 late gene. These sites were shown to be critical for proper L71-6 induction in newly formed prepupae, providing evidence that E74A protein directly regulates late gene transcription. These studies indicate that E74A functions as a transcription factor and demonstrate a direct molecular link between a primary ecdysone-inducible protein and a target late gene in the regulatory hierarchies that direct the onset of Drosophila metamorphosis. |
