Meyerowitz E M, Hogness D S
Cell. 1982 Jan;28(1):165-76. doi: 10.1016/0092-8674(82)90386-5.
The 68C locus in the polytene chromosomes of Drosophila melanogaster salivary glands is puffed during the last half of the third 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. We have isolated a set of overlapping cloned segments that define approximately 50 kb of DNA at the 68C puff locus. Three polysomal poly(A)+ RNAs that are abundant in the salivary glands during the intermolt-puff stage are transcribed from three 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. Correlative criteria indicate that gene IV, which yields an RNA of 1.1 kb, is the structural gene for sgs3. Genes II and III, which yield RNAs 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.
在黑腹果蝇唾液腺的多线染色体中，68C位点在第三龄幼虫的后半期出现胀泡，该位点包含sgs3的结构基因，sgs3是唾液腺在此期间合成的一种胶状多肽。这个胀泡会因类固醇激素蜕皮激素而消退。我们分离出了一组重叠的克隆片段，这些片段在68C胀泡位点定义了约50 kb的DNA。在蜕皮间期-胀泡阶段唾液腺中丰富的三种多聚核糖体多聚腺苷酸加尾RNA是从三个基因（II、III和IV）转录而来的，这三个基因位于克隆DNA中心附近的一个5 kb簇区域内。这些是在这些腺体中可检测到的来自68C DNA的50 kb区域的仅有的转录本，在这个阶段的其他幼虫组织或其他阶段的整个动物中均未检测到。相关标准表明，产生1.1 kb RNA的基因IV是sgs3的结构基因。分别产生0.36 kb和0.32 kb RNA的基因II和III方向相反，因此它们的启动子相邻，这表明这一对基因可能形成一个单一的调控单元，由于这一对基因由0.3 kb元件的反向重复序列界定，这一推测得到了进一步支持。讨论了这对基因可能但尚未确定身份的功能。一种属于名为roo的转座子家族的9.2 kb元件在一些但不是所有的黑腹果蝇品系中插入到5 kb簇区域附近。这种插入对基因II、III和IV的转录没有明显影响。尽管roo元件在胚胎中产生9 kb的多聚腺苷酸加尾RNA，但在蜕皮间期唾液腺中未检测到roo转录本，无论这些唾液腺在68C胀泡位点是否含有该元件。
