Control of vitellogenin genes expression by sequences derived from transposable elements in rainbow trout

In most of oviparous animals, vitellogenins (VTG) are the major egg yolk precursors. They are produced in the liver under the control of estrogens. In rainbow trout ( Oncorhynchus mykiss), the vtg genes cluster contains an unusually large number of almost identical gene copies. In order to identify the regulatory elements in their promoters, we used a combination of reporter plasmids containing genomic sequences including putative estrogen response elements (EREs) and we performed transient transfection assays in MCF-7 and yeast cells. We found a functional ERE corresponding to the sequence GGG GCAnnnT AACCT ( rtvtgERE), which differs from the consensus ERE (ERE cs) by three base pairs. This non-palindromic ERE is located in the env gene of a retrotransposon relic, 180 base pairs upstream of the transcriptional start site. Fluorescence anisotropy experiments confirmed that the purified human estrogen receptor α (hERα) can specifically bind to rtvtgERE. Furthermore, we observe that the stability of hERα-ERE cs and hERα- rtvtgERE complexes is similar with equilibrium dissociation constants of 3.0 nM and 6.2 nM respectively, under our experimental conditions. Additionally, this rtvtgERE sequence displays a high E2-responsiveness through ER activation in cellulo. In the rainbow trout, the functional ERE ( rtvtgERE) lies within promoter sequences which are mostly composed of sequences derived from transposable elements (TEs), which therefore may have acted as an evolutionary buffer to secure the proper expression of these genes. ►“Control of vitellogenin genes expression by sequences derived from transposable elements in rainbow trout” by Anthony Bouter, Nicolas Buisine, Adélaïde Le Grand, Nathalie Mouchel, Jacques Wolff, Olivier Sire and myself. ► In this work, we used a set of in vitro and in cellulo experiments to identify and characterize the estrogen responsive element (ERE) driving the expression of vitellogenin genes. Our key findings are that this ERE significantly differs from the canonical element and that it is located in sequences derived from transposable elements. These have important functional, physiological and evolutionary implications. ► Functionally, we found that the rainbow trout estrogen receptor, a transcription factor belonging to the super family of nuclear receptors, induces a strong estrogen-independent transcriptional activity with canonical EREs. In contrast, the receptor displays a nice estrogen-dependent activity when acting upon