Transcription regulatory signals in the 5' and 3' regions of Oreochromis aureus ER gene

The Oreochromis aureus estrogen receptor (OaER) 5' region is 62.2% AT-rich and does not display well-positioned consensus TATA and CAAT boxes. A functional imperfect 13 bp ERE, TGTTAtggTGACC, deviating at 2 bases is located in the leader exon. Transient transfection assays indicate that this ERE confers a 5-fold increase in SEAP reporter gene activity at 20 h post E₂-induction. The continued elevation of transcription after the initial peak could be responsible for the 'memory' effect of E₂-induced vitellogenesis. The 4.4 kb OaER 3' region is AT-rich and has a high representation of, EREs and GREs. There are a 10 copies of the destabilizing pentamer ATTTA which, in transient transfection experiments strongly suppressed SEAP activity. The 3' EREs are functional, and on their own, induce a bimodal increase in SEAP activity of 9-fold at 6 h and 11-fold at 18 h post-E₂ induction. A recombinant construct of SEAP gene flanked by the 5' upstream and 3' flanking regions of the OaER gene allowed these regulatory signals to 'cross-talk' to achieve a 5- and 25-fold increase for the first and second peaks, respectively. This bimodal response to E₂-induction is attributed to both transcriptional and translational controls over the SEAP gene. This work represents a novel illustration of a synergistic interaction between the 5' and 3' regulatory elements of a steroid receptor in autoregulation.