RAINBOW-TROUT ESTROGEN-RECEPTOR PRESENTS AN EQUAL SPECIFICITY BUT A DIFFERENTIAL SENSITIVITY FOR ESTROGENS THAN HUMAN ESTROGEN-RECEPTOR

The estrogen receptor is a transcription factor that mediates the acti ons of estrogens which plays a major role in salmonid vitellogenesis. Previously, we cloned and described the estradiol receptor in rainbow trout. To study the transactivation characteristics, the full length c DNA was inserted in an expression vector and tested by transfection on an estrogen-dependent promoter (pERE-TK-CAT), For the first time, dir ect comparison between the trout receptor (rtER) and human receptor (h ER) in an analogous system has been possible. We demonstrate that rtER can, like hER, transactivate transcription in a strictly hormone-depe ndent manner. The specificity and sensitivity of the rtER response to different steroids have been studied. With rtER, the E2 concentration needed for half maximal activation is 10 times higher than with hER. I n addition, we show that rtER has a weak affinity for androgens and tr ansactivation could be induced using high testosterone concentration. Experiments show that both rtER and hER have an equal specificity for estrogens but that the rtER binds its ligand with a lower sensitivity than hER. Several hypotheses concerning the role of main amino acids w ithin the receptor protein are proposed based on the different propert ies of the receptors and comparison of sequences.