Molecular cloning of ecdysone 20-hydroxylase and expression pattern of the enzyme during embryonic development of silkworm Bombyx mori

In various insects, 20-hydroxyecdysone (20E) is indispensable for embryonic development. In eggs of the silkworm Bombyx mori, 20E has been demonstrated to be produced by two metabolic pathways: de novo synthesis from cholesterol and dephosphorylation of ovary-derived physiologically inactive ecdysteroid phosphates. In the former, ecdysone 20-hydroxylase (E20OHase) has been suggested to be a key enzyme. In the latter, it has been demonstrated that the dephosphorylation of ecdysteroid phosphates is catalyzed by a specific enzyme, ecdysteroid-phosphate phosphatase (EPPase). In this study, a cDNA encoding E20OHase was cloned from 3-day-old nondiapause eggs of B. mori and sequenced using PCR techniques. The protein exhibited the signature sequences characteristic of P450 enzymes, and mediated the conversion of ecdysone to 20E using the baculovirus expression system. Semi-quantitative analysis revealed that the E20OHase mRNA is expressed predominantly during gastrulation and organogenesis in nondiapause eggs, but is scarcely detected in diapause eggs whose development is arrested at the late gastrula stage. The developmental changes in the expression patterns of E20OHase and EPPase suggest that both enzyme activities are regulated at the transcription level, and both enzymes contribute cooperatively to 20E formation during embryonic development.