The hormone-binding domain of oreochromis aureus estrogen receptor gei homology comparison with other steroid binding receptors

Although the estrogen receptor gene of Oreochromrs aureus (OaER) shows 85% homology to rainbow trout ER (rtER), the molecular organization of its exons and introns in the hormone-binding E domain is more closely related to the human ER gene. Comparison with other vertebrates yielded reduced homologies of 64-67%, probably due to evolutionary speciation. The E₁ and E₂ exons of OaER are interspersed by a short intron of 1.3 kb which is flanked by consensus splice sites. This is in sharp contrast to the 11 kb intron separating E₁ and E₂ 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-binding pocket were found in OaER. However, the 4th conserved cys is replaced by a ser. This substitution which is the result of a single base mutation probably suggests different affinity for estrogen or transactivation 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 vertebrates, such that they form a separate subgroup in the UPCMA tree of steroid receptors.