Identification and characterization of a Sox2 homolog in the Japanese flounder Paralichthys olivaceus.

Sox2 plays an essential role in maintaining the pluripotency of embryonic and neural stem cells as well as in the neurogenesis. While it has been well studied in mammals, information from lower invertebrate especially marine fish is still limited. In this study, we cloned and sequenced the full-length cDNA and partial 5'-flanking region of the Japanese flounder Sox2. Phylogenetic, gene structure, and protein comparison analyses revealed that Paralichthys olivaceus Sox2 (Po-Sox2) was homologous to mammalian Sox2. Quantitative real-time RT-PCR results showed that Po-Sox2 was not maternal inherited, and the transcripts were present from high blastula-stage onwards, with the highest level at the mid-gastrula stage. Tissue distribution analysis revealed that Po-Sox2 was present not only in neural tissues, but also in gonadal and gill tissues. In addition, we analyzed the Po-Sox2 promoter region for several species-conserved motifs as well as various transcription factor binding sites. The overall hypomethylation status of the identified CpG sites in the 5'-regulatory region revealed that it was not involved in the transcriptional modulation of Po-Sox2. All these results suggest that Po-Sox2 may have a conserved function in neurogenesis and early embryonic development.