Sequence and expression of IMP-L1, an ecdysone-inducible gene expressed during Drosophila imaginal disc morphogenesis.

Drosophila imaginal discs are induced by the steroid hormone 20-hydroxyecdysone to initiate morphogenesis leading to formation of the adult appendages and thoracic epidermis at the end of the third larval instar. Ecdysone-dependent transcriptional activation of a set of genes that encode imaginal disc transcripts found on membrane-bound polysomes precedes and may be responsible for some aspects of the cellular changes that mediate epithelial morphogenesis in this system. A 1.35 kb transcript from one of these genes, IMP-L1, is first observed in vivo at or just prior to pupariation, as ecdysone titers are peaking and beginning to decline. Expression is initiated in proximal areas of the antennal disc, later spreading to a more widespread but nonuniform distribution throughout other thoracic imaginal discs. IMP-L1 is not, however, expressed in other ecdysone target tissues such as salivary glands or fat body. The IMP-L1 gene encodes a novel protein product containing a signal peptide, a possible transmembrane domain, two highly charged domains and a proline rich C-terminal domain. We suggest that the delayed timing of expression of this secondary response gene is necessary for proper ordering of cellular events associated with disc morphogenesis.