The role of the BR-C locus on the expression of genes located at the ecdysone-regulated 3C puff of Drosophila melanogaster.

During the third larval instar, the steroid moulting hormone ecdysone activates three temporally distinct puff sets on the D. melanogaster salivary gland polytene chromosome: the so-called intermoult, early and late puffs. Hormonal regulation of intermoult puffs is quite complex and, so far, largely not understood. In order to further investigate this aspect, we have analysed the effects of mutations in a key regulator of the ecdysone response at the onset of metamorphosis, the Broad-Complex (BR-C) locus, on the expression of genes mapping at the 3C intermoult puff. On the basis of an accurate examination of 3C intermoult gene activity in single, carefully staged, third instar larvae of wild-type and BR-C mutant strains, we were able to subdivide these genes into two groups. Each group is characterised by a different temporal expression profile, so that at the beginning of the wandering stage the transcription of the first group declines as group II transcription is induced. Interestingly, the BR-C locus appears to play a regulatory role in establishing this transcriptional switch. By using mutants of each of the three lethal complementation groups, we precisely defined the role of BR-C functions in this developmental transition and we show that this locus also plays an essential role in the early pre-metamorphic hormonal response.