Gindhart J G, Kaufman T C
Howard Hughes Medical Institute, Indiana University, Bloomington 47405, USA.
Genetics. 1995 Feb;139(2):797-814. doi: 10.1093/genetics/139.2.797.
The Drosophila homeotic gene Sex combs reduced (Scr) is necessary for the establishment and maintenance of the morphological identity of the labial and prothoracic segments. In the early embryo, its expression pattern is established through the activity of several gap and segmentation gene products, as well as other transcription factors. Once established, the Polycomb group (Pc-G) and trithorax group (trx-G) gene products maintain the spatial pattern of Scr expression for the remainder of development. We report the identification of DNA fragments in the Scr regulatory region that may be important for its regulation by Polycomb and trithorax group gene products. When DNA fragments containing these regulatory sequences are subcloned into P-element vectors containing a white minigene, transformants containing these constructs exhibit mosaic patterns of pigmentation in the adult eye, indicating that white minigene expression is repressed in a clonally heritable manner. The size of pigmented and nonpigmented clones in the adult eye suggests that the event determining whether a cell in the eye anlagen will express white occurs at least as early as the first larval instar. The amount of white minigene repression is reduced in some Polycomb group mutants, whereas repression is enhanced in flies mutant for a subset of trithorax group loci. The repressor activity of one fragment, normally located in Scr Intron 2, is increased when it is able to homologously pair, a property consistent with genetic data suggesting that Scr exhibits transvection. Another Scr regulatory fragment, normally located 40 kb upstream of the Scr promoter, silences ectopic expression of an Scr-lacZ fusion gene in the embryo and does so in a Polycomb-dependent manner. We propose that the regulatory sequences located within these DNA fragments may normally mediate the regulation of Scr by proteins encoded by members of the Polycomb and trithorax group loci.
果蝇同源异型基因性梳减少（Scr）对于唇节和前胸节形态特征的建立和维持是必需的。在早期胚胎中，其表达模式通过几种间隙基因和分节基因产物以及其他转录因子的活性来建立。一旦建立，多梳家族（Pc-G）和三胸节家族（trx-G）基因产物在发育的剩余阶段维持Scr表达的空间模式。我们报告了在Scr调控区域中鉴定出的DNA片段，这些片段可能对其受多梳和三胸节家族基因产物的调控很重要。当含有这些调控序列的DNA片段亚克隆到含有白色小基因的P因子载体中时，含有这些构建体的转化体在成年眼中表现出色素沉着的镶嵌模式，这表明白色小基因的表达以克隆可遗传的方式受到抑制。成年眼中色素沉着和无色素克隆的大小表明，决定眼原基中的细胞是否会表达白色的事件至少早在第一龄幼虫期就发生了。在一些多梳家族突变体中，白色小基因的抑制程度降低，而在三胸节家族位点的一个子集的突变体果蝇中，抑制作用增强。一个通常位于Scr内含子2中的片段，当它能够同源配对时，其抑制活性会增加，这一特性与遗传数据一致，表明Scr表现出转座效应。另一个通常位于Scr启动子上游40 kb处的Scr调控片段，以多梳依赖的方式沉默胚胎中Scr-lacZ融合基因的异位表达。我们提出，这些DNA片段内的调控序列可能通常介导多梳和三胸节家族位点成员编码的蛋白质对Scr的调控。
