TheXenopus laevisEstrogen Receptor: Sequence Homology with Human and Avian Receptors and Identification of Multiple Estrogen Receptor Messenger Ribonucleic Acids*

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Abstract
We have isolated and sequenced a cDNA clone encompassing the entire protein coding region of the Xenopus laevis estrogen receptor (xER). The Xenopus ER, the first steroid hormone receptor to be sequenced from a cold-blooded organism, exhibits two regions of striking amino acid homology with the human and avian ERs. In the putative DNA binding region, the amino acid sequence of the xER differs from those of the human and avian ERs at only one of 83 amino acids. The putative hormone binding region contains 44 and 46 amino acid blocks in which the sequence is identical in the Xenopus and human ERs. Blot hybridizations of Xenopus liver RNA suggest that the xER is encoded by four mRNAs with lengths of approximately 9, 6.5, 2.8, and 2.5 kilobases. In contrast, hybridization of human RNA to a human ER cDNA clone reveals only a single major ER RNA, approximately 6.7 kilobases in length.