EST Cloning and Expression of 17β-Hydroxysteroid Dehydrogenase in the Ascidian, Ciona intestinalis Testis

Ascidians have been employed as model organisms in investigating spermatogenesis. 17β-hydroxysteroid dehydrogenase (HSD) is a steroidogenic enzyme essential for invertebrate spermatogenesis. A homologue of HSD was found in the EST database of Ciona intestinalis and cloned. Sequence analysis showed significant homology to zebra fish, sea urchin and human l7β-HSD. The gene has an open reading frame (ORF) of 918 nucleotides coding for a polypeptide of 306 amino acids and a calculated mass of 35-kDa. Immunoblotting with an antibody raised against HSD recognized a 35-kDa protein purified from the C. intestinalis testis. The HSD protein was localized in steroidogenic cells in the Ciona testis. These results suggest that C. intestinalis 17β-HSD is equivalent to the enzyme of vertebrate Leydig cells and that 17β-HSD could be a phylogenetic marker for organisms producing steroids.