We have isolated an ecdysone-inducible gene, E74, from the early puff at position 74EF in the Drosophila polytene chromosomes. We show that E74 consists of three nested transcription units that derive from unique promoters but share a single polyadenylation site. The 90 kb E74A unit is directly induced by ecdysone and leads to the synthesis of a 9.0 kb mRNA that contains an unusually long 5’leader (1891 nucleotides) with 17 short ORFs. Within the fifth of the seven E74A introns are two E74B promoters that direct the synthesis of 4.9 and 5.1 kb mRNAs. The nested arrangement of these transcription units leads to the formation of two E74 proteins, each with a unique N-terminal domain joined to a common C-terminal domain. The unique N-terminal domains contain regions rich in acidic amino acids while the C-terminal domain is rich in basic amino acids and is very similar to proteins encoded by the efs proto-oncogene superfamily. introduction

The larval-to-adult metamorphosis of Drosophila melanogaster is initiated by a pulse of the steroid hormone 2O-OH ecdysone (hereafter referred to as ecdysone) that occurs at the end of the third larval instar. This pulse triggers a coordinated change in the developmental pathways of both the imaginal tissues that yield the adult structures and the strictly larval tissues, most of which eventually