The Xenopus Sox3 gene expressed in oocytes of early stages.

We have isolated an SRY-type HMG box (Sox) cDNA, XSox3, from a Xenopus immature ovary cDNA library. The XSox3 cDNA contains an open reading frame (ORF) of 309 amino-acid residues, showing 62.7 and 77.0% homology with human and chicken Sox3 proteins, respectively. We also showed that the XSox3 gene is composed of a single exon by sequence analysis of the genomic clone and the determination of the transcription start site of the XSox3. The XSox3 mRNA was detected only in ovary and was at a higher level in immature ovary than in mature ovary. During oocyte maturation, the XSox3 mRNA was most abundant in stage I oocytes, and the XSox3 protein was detected in stage I and II oocytes. Recombinant XSox3 protein produced in Escherichia coli bound specifically to sequences containing the binding motif for the HMG box of SRY or SOX proteins, AACAAT or AACAAAG, demonstrating its sequence-specific DNA binding property. Taken together, these results indicate that the XSox3 protein may participate in early oogenesis of Xenopus as a transcription factor.