The vitellogenin gene of the mosquito Aedes aegypti is a direct target of ecdysteroid receptor

In the female mosquito Aedes aegypti, vitellogenin (Vg), the major YPP, is activated by 20-hydroxyecdysone (20E) at the transcriptional level. We used cell transfection assays in the Drosophila S2 cells to investigate whether 20E acts directly on the Vg gene via its functional receptor, the heterodimer composed of the ecdysteroid receptor (EcR) and the ultraspiracle (USP) proteins. We demonstrated that the Vg 5′-regulatory region contains a functional ecdysteroid-responsive element (VgEcRE1) that is necessary to confer responsiveness to 20E. VgEcRE binds directly to EcR-USP produced in vitro and extracted from the vitellogenic fat body nuclei. The binding intensity of the EcR-USP-EcRE1 complex from nuclear extracts corresponds to the levels of ecdysteroids and of the Vg transcript during the vitellogenic cycle. Given the modest level of 20E-dependent activation, it is likely that the EcR-USP receptor acts synergistically with other transcription factors to bring about the high level of Vg gene expression.