Cloning and expression of the zebrafish germ cell nuclear factor

Nuclear orphan receptors are DNA-binding proteins that share the domain structure of the nuclear hormone receptor superfamily, although their ligands are unknown. Members of the nuclear receptor family are involved in the regulation of various developmental and reproductive processes. We have identified such a nuclear orphan receptor in the zebrafish and named it zebrafish germ cell nuclear factor (zfGCNF) based on its high sequence homology to previously described mouse, human, and Xenopus laevis GCNF. Detailed sequence comparison of zfGCNF with mouse, human, and frog GCNF revealed high homologies in the domains conserved in the nuclear receptor family. Homology in the DNA-binding domain is 97% for frog and even 98.5% for mouse and human when compared to the zebrafish sequence. Homology in the E III subdomain of the transactivation/ligand-binding E domain is 100% when compared to the mouse and human sequences. Transcripts of different size were detected by Northern blot analysis in the zebrafish ovary, whereas, in the testis only one transcript was present. In situ hybridization revealed that zfGCNF was predominantly expressed in previtellogenic oocytes in the ovary and in spermatocytes in the testis.