REGULATION OF THE TRANSCRIPTION FACTOR E75 BY 20-HYDROXYECDYSONE AND JUVENILE-HORMONE IN THE EPIDERMIS OF THE TOBACCO HORNWORM, MANDUCA-SEXTA, DURING LARVAL MOLTING AND METAMORPHOSIS

The homolog of the ecdysteroid-induced transcription factor E75A in Dr osophila melanogaster was cloned from the tobacco hornworm, Manduca se xta, and its developmental expression and hormonal regulation were ana lyzed. Both E75A and E75B mRNAs were found in the abdominal epidermis during both the larval and the pupal molts, with E75A appearing before E75B, coincident with the rise of ecdysteroid. Exposure of either fou rth or fifth instar epidermis to 20E in vitro caused the rapid, transi ent induction of E75A RNA with a peak at 6 and 3 h, respectively, foll owed by maintenance at low levels until 24 h. Epidermis from fourth in star larvae with high endogenous juvenile hormone (TH) showed a 10-fol d higher sensitivity to 20E (EC50 = 2 x 10(-8) M for fourth instar and 2 x 10(-7) M for fifth instar epidermis). The presence of the protein synthesis inhibitor anisomycin had no effect on the induction but pre vented the decline, indicating that E75A RNA was directly induced by 2 0E, but its down-regulation depended on protein synthesis. Exposure of day 2 fifth instar epidermis to 20E in the presence of TH I, which pr events the 20E-induced pupal commitment, caused an increased accumulat ion of E75A RNA throughout the culture period although the temporal pa ttern was unaffected. These findings show for the first time that III plays a role in 20E-induced early gene expression and suggest that the higher levels of E75A may be required for maintenance of larval commi tment of this epidermis. (C) 1998 Academic Press.