2 FUNCTIONAL FORMS OF THE XENOPUS-LAEVIS ESTROGEN-RECEPTOR TRANSLATEDFROM A SINGLE MESSENGER-RNA SPECIES

Steroid receptors are nuclear proteins that regulate gene transcriptio n in a ligand-dependent manner. Overexpression of the Xenopus estrogen receptor in a vaccinia virus derived expression system revealed that the receptor localized exclusively in the nucleus of the infected cell s, irrespective of the presence or absence of the ligand. Furthermore, two forms of the receptor were produced, a full-length and a N-termin al truncated version, which are translated from a single mRNA species by the use of two AUG within the same reading frame. These 66- and 61- kDa receptors were also observed after in vitro translation of the mRN A as well as in primary Xenopus hepatocytes. Both forms are potent est rogen dependent transcriptional activators in transient transfection e xperiments, as well as in in vitro transcription assays.