Characterization of core promoter elements for ecdysone receptor isoforms of the silkworm, Bombyx mori

Two ecdysone receptor (EcR) isoforms, EcR-A and EcR-B1, are expressed in a tissue- and stage-specific manner, although the details of their transcription mechanisms are unknown. We determined the transcription start sites of EcR-A and EcR-B1 isoforms of Bombyx mori and found that both core promoter regions consist of initiator (Inr) and downstream promoter elements (DPE) but not TATA boxes. Promoter truncation analysis performed using the luciferase reporter assays and BmN cells showed that, in both isoforms, the regions -296 to -74 for BmEcR-B1, -104 to -61 for BmEcR-A and downstream regions of +1 are essential for basal transcriptional activity. Mutation experiments revealed that both DPE and its 5'-flanking CGCGCG sequence are crucial but DPE of BmEcR-B1 is not important for BmEcR-A transcription. These results indicate that the basal promoter activities differ between the two BmEcR isoforms.