The constitutively active estrogen receptor (ER) binds and activates the promoter of the vitellogenin (Vtg) gene in the Sydney rock oyster, Saccostrea glomerata.

Vitellogenin (Vtg) is a well-established biomarker of estrogenic exposure in aquatic animals. In vertebrates, Vtg gene transcription is controlled by the estrogen receptors (ERs). Although an ER ortholog is present in molluscs, its role as a transcriptional regulator remains elusive. Here, we tested the hypothesis that in the Sydney rock oyster, Saccostrea glomerata, the ER ortholog activates Vtg gene transcription through specific interaction with its promoter. Luciferase reporter assays indicated that sgER activated both a minimal promoter containing the consensus estrogen-responsive elements (EREs) and the sgVtg promoter in an estrogen-independent manner. The sgVtg promoter-luciferase activation was significantly reduced when any of three putative ERE half sites (½EREs) in the promoter were mutated. Electrophoretic mobility shift assay (EMSA) confirmed that sgER binds specifically to a 68-bp promoter sequence where these ½EREs reside. Overall, the results suggest that sgER is a constitutively active transcription factor that binds and activates the sgVtg promoter.