The ecdysone-induced puffing cascade in Drosophila salivary glands: a Broad-Complex early gene regulates intermolt and late gene transcription.

The steroid hormone 20-OH ecdysone triggers a classic and well-defined program of chromosome puffing that is assumed to reflect changes in transcriptional activity in Drosophila salivary glands. Mutations in each of four Broad-Complex locus (BR-C) complementation groups were analyzed for their effects on the expression of other genes that reside in several major salivary gland puffs. RNA blot analysis showed that the rbp function of the BR-C is required for the transcription of six genes in the 71E late puff and is the first demonstration that an ecdysone-induced early gene controls the transcription of late genes within the puffing cascade. In addition, the rbp function is required for the transcription of four intermolt genes (Sgs-3, Sgs-4, Sgs-5 and 71E gene VII). Mutations in the broad, l(1)2Bc and l(1)2Bd functions of the BR-C had no effect on the expression of the genes examined. We propose that the BR-C functions to control transcription at many other salivary gland loci at the beginning of metamorphosis.