THE HORMONE-BINDING DOMAIN OF OREOCHROMIS-AUREUS ESTROGEN-RECEPTOR GENE - HOMOLOGY COMPARISON WITH OTHER STEROID-BINDING RECEPTORS

Although the estrogen receptor gene of Oreochromis aureus (OaER) shows 85% homology to rainbow trout ER (rtER), the molecular organization o f its exons and introns in the hormone-binding E domain is more closel y related to the human ER gene. Comparison with other vertebrates yiel ded reduced homologies of 64-67%, probably due to evolutionary speciat ion. The E(1) and E(2) exons of OaER are interspersed by a short intro n of 1.3 kb which is flanked by consensus splice sites. This is in sha rp contrast to the 11 kb intron separating E(1) and E(2) exons of rtER which also displayed a rare GC donor junction. Three conserved cys at 83, 112 and 195, which are important for formation of 3-D ligand-bind ing pocket were found in OaER. However, the 4th conserved cys is repla ced by a ser. This substitution which is the result of a single base m utation probably suggests different affinity for estrogen or transacti vation of the OaER gene. Two overlapping steroid binding and receptor dimerization domains have been observed. The E domain of OaER and rtER has diversified significantly from that of other non-piscine vertebra tes, such that they form a separate subgroup in the UPGMA tree of ster oid receptors.