Fig. 1.

Two gypsy insulators trigger the formation of a chromatin loop. (A) Illustration of the transgenic system used in this study and of the distribution of PcG proteins along the transgene in different conditions (14). Transgene DNA is shown by green lines, and the edges of the transgenes are indicated by green triangles. Corresponding Drosophila line names are indicated above each transposon. Reporter genes, gypsy insulators, and PREs are represented, respectively, by salmon-pink boxes, orange diamonds, and blue ovals. Lox and FRT sites allowing, respectively, the excision of insulators and PREs are indicated by vertical black lines. Active promoters are shown by arrows and the Polycomb protein (PC) binding profiles are indicated by blue curves. The question mark in the bottom indicates the putative interaction between the two insulators. (B) H3C profile of the (P)(S)YSW-22E line and its derivative without the proximal insulator or the PRE, analyzed at adult stage on a 30-kb region centered on an anchor fragment located just downstream of the distal insulator. Transgenes and surrounding genomic regions are drawn to scale. DpnII fragments selected for this analysis are indicated by light gray bands. The anchor fragment is highlighted in dark gray. Fragments whose distances to the anchor changed due to excision of the proximal insulator and the PRE are indicated, respectively, in light orange and in light blue. Error bars represent the SD of the means of two independent experiments.