Spz1, a novel bHLH-Zip protein, is specifically expressed in testis.

We isolated a novel bHLH-Zip gene designated Spz1 from a mouse testis cDNA library. Spz1 is expressed specifically in the testis and epididymis. Immunofluorescence staining detected Spz1 protein in the nuclei of LFG6 Leydig cells. The ability of Spz1 protein to bind to the bHLH consensus-binding site, the E-box, was confirmed by EMSA, and a 9-bp asymmetric target site was identified by random selection and PCR amplification. Hormonal regulation of Spz1 was investigated and downregulation of Spz1 expression by testosterone and retinoic acid was found. This nuclear transcription factor may play a crucial role in spermatogenesis by regulating cell proliferation or differentiation through binding to specific DNA sequences like other bHLH-Zip molecules.