Regulation of transcription factors MHR4 and beta FTZ-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 fac tors are sequentially expressed. Unlike E75A and MHR3, whose mRNAs are indu ced when the ecdysteroid titer increases, the expression of MHR4 mRNA occur s transiently at the onset of the decline of ecdysteroid titer followed by beta FTZ-F1 mRNA expression when the ecdysteroid titer becomes low. When da y 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. Usin g the protein synthesis inhibitors cycloheximide and anisomycin both in viv o and in vitro, we found that the MHR4 transcript was directly induced by 2 0E 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, b eta FTZ-F1 mRNA was not induced until after the removal of 20E as previousl y found for Drosophila and the silkworm Bombyx mori. The presence of juveni le hormone had no effect on accumulation of either transcript. (C) 2001 Aca dcmic Press.