FIG. 3.

Gal4 is partially blocked by the PcG. (A to E) Enhancer trap patterns in the P insertion lines. RNA in situ hybridizations showing LacZ expression (in the absence of Gal4) in germ band retracted embryos. (A) A control embryo containing the UAS-LacZ cassette on the 3rd chromosome outside of the BX-C shows no LacZ expression. (B to E) Trimmed-down insertions p[bx], p[iab-4], p[Mcp], and p[Abd-B], respectively. The most anterior parasegment in which transcription occurs is marked with a bracket. (F to J) Gal4 activation of LacZ expression. (F) Gal4 strongly activates LacZ expression throughout the control embryo. (G to J) Gal4 activation in p[bx], p[iab-4], p[Mcp], and p[Abd-B], respectively. LacZ expression is much weaker or absent in PcG-repressed segments. The anterior-most parasegment in which strong activation of LacZ transcription occurs is marked with a bracket. All embryos shown in this and subsequent figures are dissected along the dorsal midline and are displayed as “pelts,” with the anterior oriented toward the top of the page.