FIG. 5.

Transcription by T7RNAP is partially blocked by the PcG at multiple sites within the BX-C. RNA in situ hybridizations in germ band retracted embryos. All embryos contained a source of T7RNAP, which is expressed in the CNS. (A) The control is an embryo containing a P element with the T7 promoter/UAS-LacZ cassette inserted on the X chromosome. T7RNAP produces a transcript in the cells of the CNS from PS3 to -14, mirroring the expression pattern of the polymerase. (B to F) Transcription by T7RNAP in the BX-C insertions. In panels C and D, the initial full-length P elements were used, and in panels B, E, and F, the trimmed-down forms were used. The anterior-most parasegment in which the control region is active is indicated for each embryo. In the anterior segments repressed by the PcG, T7RNAP produces a transcript in only a subset of the polymerase-producing cells.