A GENE THAT IS RELATED TO SRY AND IS EXPRESSED IN THE TESTES ENCODES A LEUCINE ZIPPER-CONTAINING PROTEIN

SRY-related cDNA encoding a protein with a high-mobility-group (HMG) b ox and a leucine zipper motif, which was designated SOX-LZ, was isolat ed from a rainbow trout testis cDNA library. Comparison of this cDNA w ith the mouse homologous cDNA isolated from a testis cDNA library exhi bits an overall amino acid sequence identity of 77%, which is in strik ing contrast to the abrupt loss of amino acid sequence homology outsid e the HMG box found among mammalian SRY genes. In both rainbow trout a nd mice, Northern (RNA) blot analyses have revealed the presence of a testis-specific 3-kb-long SOX-LZ mRNA, and this transcript appeared co incidentally with the protamine mRNA, suggesting its expression in the germ line. A recombinant HMG box region protein encoded by SOX-LZ cou ld bind strongly with an oligonucleotide containing an AACAAT sequence , which is also recognized by mouse Sry and Sox-5. Upon cotransfection into CHO cells, SOX-LZ transactivated transcription through its bindi ng motif when the region including the leucine zipper motif was delete d [SOX-LZ (D105-356)]; however, the intact SOX-LZ failed to transactiv ate. The intact SOX-LZ could form homodimers through the leucine zippe r, which resulted in inhibition of DNA binding by the HMG box, while S OX-LZ (D105-356), which was incapable of dimerization, showed specific binding with the AACAAT sequence. Thus, the repressed transactivation of the intact SOX-LZ in CHO cells was primarily attributable to the l ow level of DNA binding of SOX-LZ homodimers.