Abstract
The functional insect ecdysteroid receptor is comprised of two nuclear receptors, the ecdysteroid receptor (EcR) and the RXR homologue, ultraspiracle (USP), which form a heterodimer. The dimer recognizes various hormone response elements and the effect of these elements on transcriptional activity of EcR isoforms was determined in vertebrate cells transfected with EcR and USP. Only constitutive activity mediated by the core response elements was preserved after elimination of nonspecific binding sites on the DNA of the vector. The constitutive transcriptional activity was regulated in a complex manner by the N-termini of both EcR and USP, the DBD of USP and the type and number of hormone response elements (HRE). Cooperative effects at oligomeric response elements particularly DR1 depended on the type of ecdysteroid response element and the N-termini of EcR and USP. The DBD of USP abolishes or attenuates synergistic effects. The data show that in the absence of hormone, transcriptional activity is regulated in a complex manner that offers additional possibilities for ecdysteroid receptor mediated gene regulation during development.
