IDENTIFICATION OF POLYCOMB AND TRITHORAX GROUP RESPONSIVE ELEMENTS INTHE REGULATORY REGION OF THE DROSOPHILA HOMEOTIC GENE SEX COMBS REDUCED

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 segment ation gene products, as well as other transcription factors. Once esta blished, the Polycomb group (Pc-G) and trithorax group (trx-G) gene pr oducts maintain the spatial pattern of Scr expression for the remainde r of development. We report the identification of DNA fragments in the Scr regulatory region that may be important for its regulation by Pol ycomb and trithorax group gene products. When DNA fragments containing these regulatory sequences are subcloned into P-element vectors conta ining a white minigene, transformants containing these constructs exhi bit 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 sugges ts that the event determining whether a cell in the eye anlagen will e xpress 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 o f trithorax group loci. The repressor activity of one fragment, normal ly located in Scr Intron 2, is increased when it is able to homologous ly pair, a property consistent with genetic data suggesting that Scr e xhibits transvection. Another Scr regulatory fragment, normally locate d 40 kb upstream of the Scr promoter, silences ectopic expression of a n Scr-lacZ fusion gene in the embryo and does so in a Polycomb-depende nt manner. We propose that the regulatory sequences located within the se DNA fragments may normally mediate the regulation of Scr by protein s encoded by members of the Polycomb and trithorax group loci.