xGCNF, a nuclear orphan receptor is expressed during neurulation in Xenopus laevis

Nuclear orphan receptors are DNA binding proteins that share the domain structure of the nuclear hormone receptor superfamily, although ligands are unknown. We have identified an orphan receptor in Xenopus laevis 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 translated 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 (DRO). Transcripts of xGCNF are found in oocytes and in much smaller amounts in the testes. In developmental Northern blots and RNase protection using RNA from different embryonic stages, zygotic expression of xGCNF peaks at midneurula. From late gastrula to midneurula stages, an anterior to posterior concentration gradient of the RNA was observed in whole mount in situ analysis. This antero-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. Transcription of xGCNF in animal cap explants occurred independent of mesoderm induction.