Patterns of MHR3 expression in the epidermis during a larval molt of the tobacco hornworm Manduca sexta

MHR3, an ecdysone-induced transcription factor, was shown to appear in the abdominal epidermis of the tobacco hornworm Manduca sexta in a pattern-spec ific manner as the 20-hydroxyecdysone (20E) titer rises for the larval molt . The crochet epidermis that forms the hooked setae on the proleg is first to show MHR3 mRNA and protein followed sequentially by the spiracle, the do rsal intrasegmental annuli, the interannular regions, and finally the trich ogen and tormogen cells. The protein appears in the nuclei about 8 h before the onset of cuticle formation, is present during the outgrowth of the set ae, and disappears after epicuticle formation. In vitro studies showed that MHR3 mRNA induction in the crochet epidermis by 20E was more sensitive (EC 50 = 10(-6) M; 50% induction by 2 h exposure to 4 x 10(-6) M 20E) and did n ot require protein synthesis for maximal accumulation compared to the dorsa l epidermis. The ecdysone receptor complex is present in both tissues at th e outset of the molt and therefore is not a determining factor in these res ponses. Thus, in addition to the ecdysone receptor complex, region-specific factors govern both sensitivity and timing of responsiveness of MHR3 to 20 E to ensure that this transcription factor will be present when needed for its differentiative role. (C) 2000 Academic Press.