MOLECULAR-ORGANIZATION OF A DROSOPHILA PUFF SITE THAT RESPONDS TO ECDYSONE

The 68C locus in the polytene chromosomes of D. melanogaster salivary glands is puffed during the last half of the 3rd larval instar and harbors the structural gene for sgs3, one of the glue polypeptides synthesized by the glands during this period. This puff regresses in response to the steroid hormone ecdysone. A set of overlapping cloned segments were isolated that define approximately 50 kb [kilobases] of DNA at the 68C puff locus. Three polysomal poly(A)+ RNA that are abundant in the salivary glands during the intermolt-puff stage are transcribed from 3 genes (II, III and IV) that map near the center of the cloned DNA in a 5 kb cluster region. These are the only transcripts from the 50 kb of 68C DNA detectable in these glands, and they are undetectable in other larval tissues at this stage, or in whole animals at other stages of development. Gene IV, which yields an RNA of 1.1 kb, is evidently the structural gene for sgs3. Genes II and III, which yield RNA of 0.36 kb and 0.32 kb, respectively, are oppositely oriented so that their promoters are adjacent, suggesting that this pair may form a single regulated unit, a suggestion that is enhanced by the fact that the pair is bounded by an inverted repeat of 0.3 kb elements. The possible, but as yet unidentified, functions of this gene pair are discussed. A 9.2 kb element belonging to a family of transposable elements called roo is inserted adjacent to the 5 kb cluster region in some but not other D. melanogaster strains. This insertion has no obvious effect on the transcription of genes II, III and IV. Although roo elements yield a 9 kb poly(A)+ RNA in embryos, no roo transcripts were detected in intermolt salivary glands, whether they do or do not contain an element at the 68C puff site.