XGCNF, A NUCLEAR ORPHAN RECEPTOR IS EXPRESSED DURING NEURULATION IN XENOPUS-LAEVIS

Nuclear orphan receptors are DNA binding proteins that share the domai n structure of the nuclear hormone receptor superfamily, although liga nds are unknown. We have identified an orphan receptor in Xenopus laev is and named it xGCNF based on its high degree of sequence homology to the previously described murine germ cell nuclear factor (mGCNF). In gel-electrophoresis mobility shift analysis experiments in vitro trans lated xGCNF and mGCNF proteins both bind specifically as homodimers to the same response element, a direct repeat of the half-site consensus AGGTCA with zero spacing (DR0). Transcripts of xGCNF are found in ooc ytes and in much smaller amounts in the testes. In developmental North ern blots and RNase protection using RNA from different embryonic. sta ges, zygotic expression of xGCNF peaks at midneurula. From late gastru la to midneurula stages, an anterior to posterior concentration gradie nt of the RNA was observed in whole mount in situ analysis. This anter o-posterior gradient of expression was also observed in exogastrulae, both in the ectoderm and mesoderm. In the midneurula embryo, the mRNA was predominantly found in the neural plate and neural crest. Transcri ption of xGCNF in animal cap explants occurred independent of mesoderm induction.