THE ORPHAN RECEPTOR BMHNF-4 OF THE SILKMOTH BOMBYX-MORI - OVARIAN ANDZYGOTIC EXPRESSION OF 2 MESSENGER-RNA ISOFORMS ENCODING POLYPEPTIDES WITH DIFFERENT ACTIVATING DOMAINS

Two silkmoth nuclear receptor isoforms, BmHNF-4a and BmHNF-4b, that ar e related to the mammalian orphan receptor HNF-4, were characterized. Their characterization revealed that they differ from each other only in their 5' UTR and N-terminus of the predicted polypeptides. In ovari an tissue, the two receptors are expressed as a delayed response to 20 -hydroxy-ecdysone and their expression increases during vitellogenesis . BmHNF-4 mRNA is localized in the cytoplasm of follicular cells and a binding activity that recognizes a mammalian HNF-4 response element i s present in follicular cell nuclear extracts. BmHNF-4 mRNA is also pr esent in the oocyte, the unfertilized egg and the early embryo, thus d isplaying a behavior reminiscent of maternal mRNA. Both mRNA isoforms are found in the embryo following fertilization and their abundance is modulated during ensuing embryogenesis. In contrast to the rather lim ited distribution of HNF-4 in mammalian tissues, BmHNF-4 is expressed in most larval and pharate adult tissues of the silkmoth. (C) 1998 Els evier Science Ireland Ltd.