Regulation of transcription factors MHR4 and betaFTZ-F1 by 20-hydroxyecdysone during a larval molt in the tobacco hornworm, Manduca sexta.

During the last larval molt in Manduca sexta, a number of transcription factors are sequentially expressed. Unlike E75A and MHR3, whose mRNAs are induced when the ecdysteroid titer increases, the expression of MHR4 mRNA occurs transiently at the onset of the decline of ecdysteroid titer followed by betaFTZ-F1 mRNA expression when the ecdysteroid titer becomes low. When day 2 fourth epidermis was exposed to 20-hydroxyecdysone (20E) in vitro, MHR4 mRNA appeared between 12 and 21 h, peaked at 24 h, and then declined. Using the protein synthesis inhibitors cycloheximide and anisomycin both in vivo and in vitro, we found that the MHR4 transcript was directly induced by 20E and required the presence of 20E for its expression. The accumulation of MHR4 mRNA, however, did not occur until a 20E-induced inhibitory protein(s) disappeared. This control of MHR4 expression is unique among the ecdysone-induced transcription factors. When the epidermis was cultured with 20E, betaFTZ-F1 mRNA was not induced until after the removal of 20E as previously found for Drosophila and the silkworm Bombyx mori. The presence of juvenile hormone had no effect on accumulation of either transcript. Copyright 2001 Academic Press.